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Recent developments of direct rhenium-catalyzed [1,3]-transpositions of allylic alcohols and their silyl ethers Ivan Volchkov and Daesung Lee* The direct metal-catalyzed [1,3]-transposition of allylic alcohols and allylic silyl ethers is a synthetically useful isomerization process that occurs via [3,3]-sigmatropic rearrangement induced by high oxidation state oxometal complexes. The isomerization requires only a catalytic amount of promoter, and high chirality transfer can be achieved. Thus, it bears a significant potential to become a powerful tool in multistep synthesis. Although [1,3]-transposition of allylic alcohols has been known since the late 1960s,

Received 20th January 2014 DOI: 10.1039/c4cs00036f

the development of synthetically useful protocols that allow for a high level of regio- and stereoselectivity control and their synthetic applications have emerged only recently. This tutorial review summarizes recently developed regioselective [1,3]-transpositions of allylic alcohols and silyl ethers and

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their applications to natural product synthesis.

Key learning points (1) (2) (3) (4) (5)

The latest development of transition metal-catalyzed [1,3]-transposition of allylic alcohols and their silyl ethers. Current view on the mechanism of transition metal-catalyzed allylic [1,3]-transposition. Chirality transfer in [1,3]-transposition. Existing strategies to control regio- and stereoselectivity of the transposition. Recent applications of allylic transpositions to the synthesis of complex natural products.

1. Transition metal-catalyzed [1,3]-transposition: synthetic utility and current trends The direct catalytic [1,3]-transposition of functional groups is a powerful atom-1,2 and step-economical3,4 synthetic method. It allows for the preparation of a less accessible isomer from a more readily available precursor in a single operation without the derivatization of the substrate. Isomerization of allylic alcohols and silyl ethers through metal-catalyzed [1,3]-transposition of the oxygen moieties is a prototypical example of this transformation (Fig. 1). The [1,3]-transposition catalyzed by oxometal complexes is a reversible process, and thus it gives a thermodynamic mixture of isomeric products as long as no regiochemistry-controlling element exists in the system. This transformation was first developed for the isomerization of terpene-based alcohols over four decades ago5 and was applied commercially in the

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois, 60607, USA. E-mail: [email protected]

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Fig. 1 Transition metal-catalyzed [1,3]-transposition of allylic alcohols and allylic silyl ethers.

fragrance industry. However, its utility as a valuable synthetic tool was not demonstrated until recently when a number of complex molecule syntheses were achieved employing this method. The limited use of [1,3]-allylic transposition in preparative organic chemistry is most likely caused by the difficulty in controlling regio- and stereoselectivity in the reaction as well as the relatively harsh conditions the catalysts operate in. The reversible nature of [1,3]-transposition is not an inherent limitation because in dynamic kinetic resolution of secondary allylic alcohols,6–8 the oxovanadium complex-mediated reversible [1,3]-transposition plays a crucial role in racemization. Since the initial discovery of vanadium-catalyzed allylic [1,3]-transposition by Chabardes in the late 1960s,5 several high oxidation state transition metal oxo-complexes of V,5,9–13 W,14 Mo,15–17 and Re18–24 have been

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identified to be suitable catalysts for the isomerization. However, most of these complexes require either high reaction temperatures to achieve optimal catalyst performance or display limited stability and functional group compatibility. Thus, V- and W-based complexes such as trialkyl orthovanadates VO(OR)35,9–11 and WO(OSiEt3)3Py14 exhibit high catalytic activity only at temperatures between 140–200 1C although a low catalyst loading of 0.05–2 wt% is enough to achieve the equilibrium. Activation of vanadium acetylacetonate (VO(acac)2) with TMSOOTMS generates an ill-defined catalyst system12,13 that exhibits high activity at room temperature in the selective isomerization of primary allylic alcohols to the corresponding tertiary ones although side reactions were also observed. The combination of MoO2(acac)2–TMSOOTMS,13 however, was shown to be less effective. Other Mo-based systems such as MoO2(Ot-Bu)215 and oxo-peroxo molybdenum complexes17 were reported to be competent catalysts and their catalytic behavior was explored. Due to the high oxidation states of metal oxo-complexes, all available catalysts possess significant Lewis acidity, which is more evident as the number of oxo-ligands increases. Because of this Lewis acidic nature, side reactions such as dehydration, isomerization, condensation, and racemization of substrates compete with the desired [1,3]-transposition. In addition, oxocomplexes of Mo(VI), such as MoO2(Ot-Bu)2,15 slowly oxidize alcohols, which ultimately reduces the catalyst lifetime. In comparison with high oxidation state oxo-complexes of Mo and W, oxochromium(VI) complexes are powerful oxidizing agents. Therefore, they are not employed for the [1,3]-transposition of primary and secondary allylic alcohols. Nevertheless, they have been used for the oxidative [1,3]-transposition of tertiary allylic alcohols to a,b-unsaturated carbonyl compounds. Several reviews have been written on oxidative [1,3]-transposition:25–28 for detailed discussions readers are referred to these articles. Compared to these complexes, oxorhenium-based catalytic systems were found to be superior for the allylic [1,3]transposition. Initial discovery of [1,3]-transposition with Bu4NReO4/pTsOHH2O18,19 was followed by that with MeReO3,20–22

which have been applied industrially for the equilibration between secondary and primary alcohols with low catalyst loading.20 Ultimately, triphenylsilyl perrhenate, Ph3SiOReO3,29 the most efficient catalyst for the [1,3]-transposition of allylic alcohols and allylic silyl ethers has evolved through early investigations and mechanistic studies. Ph3SiOReO3 possesses high catalytic activity at or below room temperature without competing with oxidation of substrates, which outcompetes the reactivity of previously known catalysts.23,24 Development of this superior catalytic system has elicited further exploration for regio- and stereoselective allylic [1,3]-transpositions. Through subsequent investigations30,31 the scope of the reaction, structural features of substrates favoring the position of equilibrium, and the degree of chirality transfer have been revealed. Based on these studies [1,3]-rearrangements lacking a strong thermodynamic driving force has been coupled with kinetically or thermodynamically favorable downstream events to achieve high regio- and stereocontrol. For example, preferential trapping of sterically less hindered regioisomeric alcohol was achieved employing N,O-bis(trimethylsilyl)acetamide30 as well as trapping of the equilibrating siloxy-moiety by a tethered boronate functionality.32,33 Alternatively, the trapping of a [1,3]-transposed hydroxyl group with an external or internal electrophile followed by thermodynamically controlled postequilibration was demonstrated.34–36

Ivan Volchkov was born in Obninsk, Russia. He received his MS in organometallic chemistry at the Lomonosov Moscow State University in 2008. He then moved to the U.S. to pursue PhD studies at the University of Illinois at Chicago under the guidance of Prof. Daesung Lee. Ivan’s PhD research focused on the development of regioselective allylic transposition of silyl ethers and its application to the total Ivan Volchkov synthesis of natural products. After obtaining his PhD in 2014 he joined Prof. Michael J. Krische’s group at the University of Texas at Austin as a postdoctoral research associate.

Daesung Lee, born in Korea, received his BSc and MSc in chemistry from Seoul National University. After completion of his doctorate at Stanford University in 1998 and two years of postdoctoral training at Harvard, he joined the Chemistry Department at the University of Wisconsin-Madison in 2000 and in 2007 he moved to the University of Illinois at Chicago. His current research interests Daesung Lee include the development of new synthetic methods and their application to the synthesis of biologically active natural compounds.

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2. Mechanism of transition metal-catalyzed [1,3]-transposition of allylic alcohols The mechanism of metal-catalyzed [1,3]-transposition of allylic alcohols was initially proposed by Chabardes for oxovanadium complexes.9 The reaction starts with the exchange of one of the alkoxy ligands of the catalyst with substrate alcohol 1-1 to generate vanadate ester 1-2 (Scheme 1). The subsequent

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Scheme 1 Mechanism of [1,3]-transposition of allylic alcohols proposed by Chabardes.

isomerization of an allylic alkoxide via [3,3]-sigmatropic rearrangement forms intermediate ester 1-4, which upon ligand exchange with substrate 1-1, releases product 1-5 and returns vanadate ester 1-2 into the catalytic cycle. All steps of the mechanism are reversible and the rearrangement generates a thermodynamic mixture of isomeric alcohols. This mechanism was later proved both by experiments23,24 and DFT-calculations,37 and the details of charge distribution in the transition state and its effects on selectivity were revealed. Kinetic studies on Ph3SiOReO3-catalyzed [1,3]-transposition performed by Osborn indicated that this reaction is first-order with respect to both the catalyst and the substrate alcohol. Isomerization of hex-1-en-3-ol 2-1 to (E)-hex-2-en-1-ol (E)-2-2 has DHa = 13.3  0.3 kcal mol1 and DSa = 14.8  1.0 e.u., and for the reverse reaction, DHa = 12.4  0.6 kcal mol1 and DSa = 18.3  2.2 e.u. were found (Scheme 2). The large negative activation entropy suggests that the reaction proceeds via a [3,3]-sigmatropic pathway with highly ordered polarized transition state 2-4. In chair-like transition state 2-4, a partially positive allyl group migrates intramolecularly across a partially negatively charged perrhenate moiety. However, for the formation of (Z)-2-2, the enthalpy of activation is 20.9  0.4 kcal mol1 and the activation entropy equals 3.1  1.5 e.u. For its reverse

Scheme 2

Mechanism of allylic [1,3]-transposition proposed by Osborn.

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reaction, the parameters are DHa = 24.9  0.7 kcal mol1 and DSa = 19.3  2.2 e.u. The positive entropies of activation indicate that the rearrangement to (Z)-2-2 takes place via an ionization–recombination pathway with the formation of a solvent-separated positively charged allylic cation and a negatively charged perrhenate moiety in the transition state 2-5. Moreover, the formation of the Z-isomer has a low rate of 0.07 turnovers per minute compared to that for (E)-hexenol (9.00 turnovers per minute). A similar rate of 0.16 turnovers per minute was found for the conversion of (Z)-2-2 to other isomers. The authors explained that the observed selectivity for (E)-2-2 is the consequence of the minimized diaxial interactions between the oxo-ligand and a propyl substituent in the chairlike transition structure. Based on the proposed mechanism, the greater activity of Ph3SiOReO3 compared to other oxo-metal catalysts is attributed to the effective stabilization of a partially negatively charged perrhenate moiety by the increased number of spectator oxo-ligands on the metal center. Based on thorough investigations of the scope and limitations of [1,3]-transposition of allylic alcohols, Grubbs and coworkers suggested a more detailed mechanistic rationale.31 Similar to Osborn’s mechanism, they proposed that the isomerization occurs through an asynchronous [3,3]-sigmatropic rearrangement with polarized chair-like transition structure 3-4 (Scheme 3). In this transition state the C–O bond cleavage precedes the other C–O bond formation, which results in a partial positive charge character on the allyl fragment and a partial negative charge character on the perrhenate moiety. Due to the strong polarization of the C–O bond in 3-4, the formation of ion pair intermediate 3-6 competes with a sigmatropic rearrangement. The existence of ion pair 3-6 mainly accounts for the formation of undesired side products via elimination (3-7) and condensation (3-8), as well as for racemization of enantiomerically enriched substrates.

Scheme 3 Mechanism of allylic [1,3]-transposition proposed by Grubbs and coworkers.

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The selective formation of E-isomers from both (E)- and (Z)-allylic alcohols is explained by the greater extent of diaxial interactions in the transition state leading to the (Z)-configured alcohol in accordance with the Osborn’s rationale. Because of high polarization in transition state 3-4, the electronic effect of substituents have a strong influence on the reaction rate and the partitioning of rhenate ester intermediates between two manifolds of the reaction. Accordingly, functional groups capable of stabilizing a positive charge character on the allylic moiety, such as electron-donating groups, or more substituted alkenes would increase the rate of isomerization. However, they would also decrease the regio- and stereoselectivity due to facile formation of a free allylic cation. Transposition of alcohols bearing electron-withdrawing substituents or less substituted double bonds would have a more concerted nature, thereby showing an increased selectivity for the reaction. For the reactions of electron-rich substrates, low reaction temperatures of 78 to 50 1C are required to suppress the formation of ion pairs, which would lead to the decomposition of the substrate, whereas reactions of electron-poor allylic alcohols can be performed at room temperature. Similarly, the degree of chirality transfer is affected by the electronic nature of the substrate; the higher electron-withdrawing capacity of substituents results in the higher degree of chirality transfer.

3. Overview of regioselective Re-catalyzed [1,3]-transposition of allylic alcohols and their silyl ethers 3.1. Investigation of the scope and limitations of Re-catalyzed transposition of allylic alcohols and the chirality transfer In 1997, Osborn and coworkers disclosed the isomerization of hex-1-en-3-ol 4-1a to a mixture of regioisomers with 2 mol % of trimethylsilyl perrhenate (Me3SiOReO3) in acetonitrile at 25 1C which was completed in less than 10 minutes (Scheme 4).23 The catalytic activity of this new catalyst was much higher than that of MoO2(OtBu)2.15 The analogous complex Ph3SiOReO3 displayed even higher reactivity and afforded an equilibrium mixture of hexenols within 5 minutes at 0 1C. The lower catalytic activity of Me3SiOReO3 compared to Ph3SiOReO3 is due to the formation of Me3SiOSiMe3 and water from Me3SiOH. Concomitant reaction of water with the catalyst gives perrhenic acid causing a loss of catalytic activity over time. This report

Scheme 4 Discovery of R3SiOReO3-catalyzed isomerization of alcohols and silyl ethers.

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sets the starting point for the emergence of the most efficient catalysts in the field of [1,3]-transposition of allylic alcohols and their silyl ethers. By taking advantage of the Wilkinson’s38 synthesis of perrhenate esters (ROReO3) from silyl ethers ROSiMe3 and Me3SiOReO3, Osborn expanded the scope of the [1,3]transposition of allylic alcohols to their silyl ether derivatives.24 The isomerization of trimethylsilyl ether 4-2b in the presence of 1 mol % of Me3SiOReO3 reached equilibrium in less than 60 minutes at room temperature in dichloromethane. When Ph3SiOReO3 was employed as a catalyst, a small induction period was observed. The position of equilibrium between two isomeric silyl ethers depends on the silyl group of the substrate. In the case of unprotected alcohol 4-2a, an equilibrium mixture in a 40 : 60 ratio was obtained favoring secondary alcohol. Trimethylsilyl derivative 4-2b afforded a mixture with an increased content of primary ether by 15% (55 : 45 ratio was recorded). Increasing the steric bulk of the silyl ether resulted in an even greater proportion. The rearrangement of TES- and TBS-derivatives (4-2c and 4-2d) afforded primary and secondary hexenyl ethers in 62 : 38 and 57 : 43 ratios, respectively. These catalytic processes, however, were relatively slow requiring several hours to reach an equilibrium state. In a more coordinating solvent such as acetonitrile or THF, the rate of reaction became significantly lower. Although Re2O7 could catalyze this [1,3]-transposition, the rate of isomerization was not measured because of its limited solubility in dichloromethane. After this major breakthrough in catalyst development, several approaches for regioselective [1,3]-transposition of allylic alcohols and their silyl ethers have been developed and successfully applied to natural product synthesis. Although some strategies to obtain high product selectivity, based on the introduction of substituents for increased conjugation21,22 and trapping transposed alcohols with borate esters,11 were reported earlier, these reactions were not systematically investigated. In 2005 Grubbs and coworkers undertook their seminal studies30,31 to define the scope and limitations of Ph3SiOReO3-catalyzed transposition and investigated the chirality transfer with enantiomerically enriched substrate alcohols. Grubbs demonstrated that a variety of secondary benzylic and heteroaromatic allylic alcohols selectively rearranged to their conjugated isomers in the presence of Ph3SiOReO3 in ethereal solvents at low temperatures (78 to 50 1C) within a short period of time. Similar reactions at room temperature or in other solvents such as CH2Cl2, CH3CN, and toluene resulted in extensive side reactions including dehydration and condensation. The reactivity of allylic alcohols strongly depends on their electronic properties (Scheme 5). Thus, rearrangement of 5-1 at 50 1C afforded the conjugated product in 98% yield and high E-selectivity. Substrate 5-3 containing an electron-poor allylic moiety underwent efficient [1,3]-transposition only at room temperature, whereas substrate 5-5 possessing an electron-rich allylic functionality readily isomerized at 78 1C yet exhibited extensive side product formation at higher temperatures. Substrates possessing more substituted allylic moieties were more

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Scheme 5 Ph3SiOReO3-catalyzed transposition of allylic alcohols driven by conjugation.

reactive than those bearing less substituted alkene functionality. Thus, transposition of benzylic alcohol 5-7 containing terminal alkene occurred within a time comparable to that of 5-1 bearing disubstituted alkene at a higher reaction temperature of 0 1C. Allylic alcohols bearing heteroaromatic substituents displayed similar reactivity. Thiophenyl-substituted alcohol 5-9 afforded a good yield of isomeric product at 50 1C. However, the analogous furylcontaining substrate rapidly decomposed. Isomerization of indole-containing compounds was possible only after introducing an electron-withdrawing N-tosyl substituent (5-11). These observations are consistent with the formation of a partially cationic allylic moiety in the transition state. Side products resulting from competing reaction pathways involving an allylic cation diminishes at lower reaction temperatures. The chirality transfer in [1,3]-transposition was examined with enantiomerically enriched substrates (S,E)-6-1 and (S,Z)-61 (Scheme 6). Isomerization of (S,E)-6-1 under optimal reaction conditions afforded product (S,E)-6-2 with a considerable loss of enantiopurity. Similarly, the rearrangement of (S,Z)-6-1

Scheme 6 Isomerization of enantiomerically enriched allylic alcohols catalyzed by Ph3SiOReO3.

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(498% ee) furnished conjugated isomer (R,E)-6-2 in 72% ee. The absolute configuration of (S,E)-6-2 and (R,E)-6-2 is controlled by the double bond geometry of their precursors. The [1,3]-transposition of (S,E)-6-1 occurs via chair-like transition state 6-3, in which the E-alkene geometry results in an equatorial position of the alkyl substituent that leads to (S)-stereochemistry of the product. For (S,Z)-6-1, the rearrangement proceeds through transition state 6-4, which gives the (R)-configured product due to an axially oriented alkyl substituent. The influence of electronic effects of an aromatic substituent on chirality transfer was also investigated.31 Accordingly, the rearrangement of substrate 7-1 bearing an electrondonating methoxy substituent at 78 1C delivered racemic product 7-2 (Scheme 7). In contrast, the isomerization of electron-deficient substrate 7-3 at 50 1C afforded enantioenriched alcohol 7-4. Allylic alcohols bearing trisubstituted allylic moieties displayed dramatically decreased efficiency of chirality transfer compared to those possessing disubstituted alkene. Thus, substrate 7-5 delivered tertiary alcohol 7-6 in only 9% ee. Therefore, synthesis of enantioenriched tertiary allylic alcohols via allylic [1,3]-transposition is only possible for the compounds bearing functional groups that are able to compensate for the electron-donating effects of additional alkyl substituents of the alkene moiety. The transpositions of 6-1, 7-1, 7-3, and 7-5 indicate that the loss of enantiopurity is caused by the formation of a cationic intermediate, which is suppressed by electron-withdrawing substituents on substrate alcohols. In the allylic transposition of enantiomerically enriched cyanohydrin 8-1, high chirality transfer was observed at room temperature affording alcohol 8-2 in excellent yield and enantiopurity (Scheme 8).31 The observed high enantiopurity is most likely the consequence of low reactivity of the cyanohydrins due

Scheme 7

Influence of electronic effects on [1,3]-chirality transfer.

Scheme 8

Isomerization of allylic cyanohydrins catalyzed by Ph3SiOReO3.

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to an electron-withdrawing nature of the cyano group. Only cyanohydrins bearing a trisubstituted double bond participate in allylic [1,3]-transposition. Cyanohydrins possessing a terminal double bond or a disubstituted double bond are completely inert toward the Ph3SiOReO3 catalyst. The first application of the Ph3SiOReO3-catalyzed allylic transposition in target-oriented synthesis is described in the enantioselective total synthesis of ()-galanthamine,39,40 an alkaloid possessing selective acetylcholinesterase inhibitory activity. This molecule was approved by FDA for the treatment of vascular dementia and Alzheimer’s diseases. Due to the failure of conventional direct allylic oxidation at the C3 position of advanced intermediate 9-1 an alternative sequence was designed to complete ()-galanthamine (Scheme 9). Thus, the reaction of a tosylammonium salt of tertiary amine 9-1 with dimethyldioxirane afforded epoxide 9-3 after concomitant treatment with DBU. Regioselective opening of epoxide 9-3 with sodium phenylselenide followed by chemoselective oxidation with NaIO4 and subsequent selenoxide elimination delivered [1,3]-transposition precursor 9-4. Treatment of 9-4 with p-toluenesulfonic acid to protonate basic nitrogen, which would otherwise deactivate the Re-catalyst, followed by exposure of the ammonium salt to a stoichiometric amount of Ph3SiReO3 afforded a 3 : 1 mixture of alcohols 9-2 and 9-4. After separation of regioisomers, ()-galanthamine was obtained in 50% yield. The integrity of the cyclic allylic alcohol was preserved during the [1,3]-transposition, indicating the involvement of a [3,3]-sigmatropic rearrangement. High product selectivity was observed in the [1,3]transposition of allylic alcohol 10-1, an early intermediate toward the synthesis of amphidinolide B1 (Scheme 10).41 When 10-1 was treated with 1 mol % of Ph3SiReO3 in ether at 60 1C, complete isomerization to regioisomeric allylic alcohol 10-2 was observed within 5 minutes. The origin of the high regioselectivity and chirality transfer during rearrangement is unclear. In the synthetic effort toward diterpenoid cladiell-11-ene-3,6,7triol, the MeReO3-catalyzed rearrangement of b-hydroxyketone to

Scheme 9 Ph3SiOReO3-catalyzed transposition in the synthesis of ()-galanthamine.

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Scheme 10 Ph3SiOReO3-catalyzed transposition in the synthesis of a precursor for ()-amphidinolide B1.

Scheme 11 MeReO3-catalyzed transposition in the synthesis of cladiell11-ene-3,6,7-triol.

a conjugated enone was employed at an advanced stage (Scheme 11).42 For the allylic transposition, substrate 11-1 was treated with 10 mol % of MeReO3 in benzene at room temperature for 48 h affording enone 11-2 in 99% yield with retention of alcohol stereochemistry. The efficient chirality transfer most likely is the consequence of a strong electron-withdrawing effect of the carbonyl group next to the tertiary alcohol, which makes 11-1 an electron-poor substrate. The reactivity of 11-1 is modulated by an electron-donating methyl substituent to compensate for the electron-withdrawing effect of the ketone, which allowed for the use of less reactive MeReO3 as a catalyst. In a recent total synthesis of laulimalide, a synthetic strategy relied on the installation of anti-epoxyalcohol functionality via the Payne rearrangement of precursor 12-2 (Scheme 12).43 In practice, the planned epoxide transposition was not successful under a variety of conditions. In an alternative approach, the Re-catalyzed [1,3]-transposition of 12-1 afforded [1,3]-transposed product 12-3 in good yield with complete chirality transfer. Employment of one equivalent of Ph3SiReO3 for 5 minutes in Et2O at 50 1C was found to be the optimum conditions, under which a thermodynamic mixture of isomeric alcohols 12-3 and 12-1 was obtained in a 4 : 1 ratio favoring the rearranged product. The alcohols were easily separated affording 12-3 in 78% yield.

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Scheme 12 Ph3SiOReO3-catalyzed allylic transposition in the synthesis of laulimalide.

Subsequent inversion of the hydroxy configuration via DMP oxidation and Corey–Bakshi–Shibata (CBS) reduction furnished 12-4 in 93% yield. The Sharpless asymmetric epoxidation followed by DDQmediated deprotection completed the synthesis of laulimalide. 3.2. Regiocontrol in allylic alcohol isomerization via the selective silylation of less hindered isomers Another tactic to obtain high product selectivity in the rearrangement of tertiary allylic alcohols is based on two factors: the preferential silylation of the sterically less hindered allylic alcohol isomer with N,O-bis(trimethylsilyl)acetamide (BSA), and the relatively slow rate of isomerization of the corresponding silyl ethers compared to their parent alcohols.30 The rearrangement of 13-1 (Scheme 13) in the presence of 1 equivalent of BSA and a catalytic amount of N-trimethylsilylacetamide (TMSA) as a promoter31 at 0 1C afforded primary alcohol 13-2 in 84% yield and 4.6 : 1 E/Z-selectivity upon removal of the silyl group. Interestingly, the same reaction in the absence of a silylating agent delivered a primary alcohol in only 30% yield, which indicates that 13-2 is a thermodynamically disfavored regioisomer. The degree of E-selectivity directly correlates with the steric pressure around the allylic moiety. Thus, [1,3]-transposition of the substrate 13-3 bearing a propyl substituent afforded product 13-4 as a 1.9 : 1 mixture of E/Z-isomers. On the other hand, the treatment of tert-butylsubstituted 13-5 with Ph3SiReO3 delivered isomeric alcohol in greater than 99 : 1 E/Z-selectivity. This protocol gives high regioselectivity control for the transposition of tertiary allylic alcohols to form primary allylic alcohols, however, the isomerizations of tertiary alcohols to form secondary alcohols are less efficient. Thus, [1,3]-transposition of 13-7 delivered 13-8 in only 26% yield. Additionally, the preferential trapping method cannot be extended to the isomerization of secondary allylic alcohols. Attempts to employ 13-9 as a substrate gave only silylated starting material. BSA-assisted regioselective [1,3]-transposition of tertiary allylic alcohols was employed for the preparation 14-2, a

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Scheme 13 BSA-assisted transposition of allylic alcohols catalyzed by Ph3SiOReO3.

building block in the total synthesis of 6 0 -hydroxyarenarol (Scheme 14).44 The direct installation of the required allylic silane moiety in this synthetic intermediate by Wittig olefination of ketone 14-1 was unsuccessful. Therefore, a five-step protocol was implemented involving the [1,3]-transposition of a tertiary allylic alcohol. The addition of vinylmagnesium bromide to form tertiary alcohol 14-3, followed by regioselective allylic transposition of a hydroxyl group in the presence of Ph3SiOReO3 and BSA, and subsequent hydrolysis of a silyl ether afforded primary alcohol 14-4 bearing a trisubstituted double bond in a 4 : 1 E/Z-ratio. Chlorination and displacement of the resulting chloride with TMSLi furnished the required allylic silane 14-2 with the concomitant cleavage of benzyl ether.

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Scheme 14 BSA-assisted allylic transposition in the synthesis of 6’-hydroxyarenarol. Scheme 16 Synthesis of cyclic boronic acid half esters via Re2O7catalyzed [1,3]-transposition.

Scheme 15 BSA-assisted allylic transposition in the synthesis of ()-apratoxin A.

The same strategy regarding regioselectivity control of [1,3]transposition rendered quick access to the advanced carboxylic acid subunit for the synthesis of a cytotoxic marine natural product ()-apratoxin A.45 The synthesis commenced with the rhenium-catalyzed isomerization of tertiary alcohol 15-1 in the presence of BSA with 3 mol % catalyst loading at 0 1C. Subsequent one-pot removal of the trimethylsilyl group from the primary silyl ether intermediates afforded a separable mixture of allylic alcohols (E)-15-2 and (Z)-15-2 in a 1 : 1 ratio (Scheme 15). After separation, the E-stereoisomer was elaborated into acid 15-3, which serves as an advanced intermediate for the total synthesis. 3.3. Lewis acid–base interactions as regiochemistry-controlling elements in the allylic [1,3]-transposition of silyl ethers The introduction of a tethered boronate functionality into the structures of allylic silyl ethers allows for the employment of Lewis acid–base interactions between the boronate and the [1,3]-transposed siloxy moieties to shift the transposition

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toward the desired direction.32 The required vinyl boronate for [1,3]-transposition was easily prepared by a rutheniumcatalyzed Alder-ene reaction (RCAER)46,47 of alkynyl boronate 16-2 with homoallylic silyl ethers 16-1 (Scheme 16). Several protecting groups for the homoallylic alcohol were tested and the TBS-group was found to be most suitable for both the Alder-ene reaction and the allylic transposition. The allylic transposition of vinyl boronate 16-3 proceeded at room temperature in the presence of 2.5 mol % of Re2O7 to generate cyclic boronic acid half ester 16-4 in 72% yield with complete chirality transfer. When the rearrangement was performed in dichloromethane, significant loss of stereochemical information was observed, and 16-4 was isolated in 80 : 20 er. Similarly, [1,3]-transposition of 16-5 obtained as an 80 : 20 mixture of stereoisomers afforded cyclic boronic acid half ester 16-6 with 80 : 20 er when the reaction was carried out in ether. However, the isomerization in dichloromethane delivered a racemic product. The formation of cyclic boronic acid half esters from allylic [1,3]-transposition can be explained by the mechanism depicted in Scheme 17. Upon activation of the Si–O bond of substrate 17-1 with Re2O7 via 17-2, the resultant intermediate 17-3 undergoes an allylic transposition to 17-4 through an asynchronous [3,3]-sigmatropic rearrangement. A ligand exchange via the expected Lewis acid–base interaction on 17-5 leads to a penultimate intermediate 17-6, which upon another ligand exchange with a substrate affords product 17-7 and regenerates a new initial intermediate 17-3. The resulting boronate 17-7 readily hydrolyzes during purification on silica gel providing the final product 17-8. The synthetic utility of RCAER46,47 and regioselective Re2O7-catalyzed [1,3]-transposition of allylic silyl ethers has been demonstrated in the total synthesis of ()-dactylolide (Scheme 18).33 RCAER of advanced intermediate 18-1 prepared as an 8 : 1 mixture of diastereomers and borylated alkyne 18-2 occurred selectively at the C7 position of the least hindered double bond, affording vinyl boronate 18-3 with the expected

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Scheme 17

Mechanism of boronate-directed allylic transposition of silyl ethers.

Scheme 18

Re2O7-catalyzed allylic transposition in the synthesis of ()-dactylolide.

(Z)-trisubstituted alkene (5 : 1 Z : E). Although the conversion of 18-1 was relatively low, an acceptable overall yield was obtained by recycling the alkene counterpart. The allylic transposition of 18-3 to introduce a hydroxyl group at the C7 position proceeded at room temperature in the presence of 5 mol % of Re2O7, which provided 18-4 in 65% yield with complete chirality transfer. 3.4. Regiocontrol in Re2O7-catalyzed [1,3]-transposition of allylic alcohols by reversible ketalization trapping More recently, the ketalization-based trapping strategy of the transposed hydroxyl group to obtain a high product and stereoselectivity was developed for the synthesis of the most thermodynamically stable 1,3-diol benzylidene acetals.34 In this approach, diol 19-1 was treated with benzaldehyde dimethyl acetal and a catalytic amount of Re2O7 in dichloromethane at room temperature affording 19-2 in a 98 : 2 diastereomeric ratio after 20 h (Scheme 19). This tandem [1,3]-transposition/ ketalization occurred via the rapid formation of the diastereomeric mixture of rearranged diol acetals followed by slow equilibration to give a thermodynamically more favorable 1,3-syn-product. Several other Re-based catalysts were tested for the reaction but they have demonstrated a slower equilibration rate. Among these catalysts, MeReO3 was the least reactive and gave a mixture of acetals in a 63 : 37 ratio after the same reaction time. On the other hand, Ph3SiOReO3 delivered 19-2 in a 96 : 4 dr, but higher catalyst loading was required. Although different solvents (Et2O, THF and toluene) can be

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employed for the rearrangement, dichloromethane is superior to other solvents affording the highest reaction rate. High regioand stereoselectivity can be obtained by employing other diol masking groups such as acetonide or p-methoxybenzaldehyde acetal. Intramolecular benzylidene transfer was also observed in the rearrangement of 19-3. The [1,3]-transposition of alcohol 19-5 in the presence of p-methoxybenzaldehyde dimethyl acetal afforded 19-6 in an 85 : 15 diastereomeric ratio with a concomitant silyl ether cleavage. In this case, the stereochemical information of the methyl substituent was translated to the transposed hydroxyl functionality of the product. Other acidsensitive protecting groups such as TBDPS and PMB underwent cleavage upon prolonged exposure to Re2O7. Thus, the allylic transposition of 19-7 delivered bis-benzylidene acetal 19-8 in 84% yield and 98 : 2 dr. Similarly, substrate 19-9 bearing opposite stereochemistry of the secondary hydroxyl group afforded diastereomeric syn-benzylidene acetal 19-10 with equally high stereoselectivity. 3.5. Cascade reactions employing [1,3]-transpositionintramolecular trapping with acetals, enones, and epoxides In 2011, an intramolecular version of ketalization trapping approach was implemented for the stereoselective synthesis of heterocycles and spiroketal structures.35 The degree of stereoselectivity in this reaction correlates with the stability differences in the diastereomeric products, which can be established by the reversibility of each individual step. Thus, treatment of ketal 20-1

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Scheme 19 Regioselectivity control in Re2O7-catalyzed allylic transposition/ ketalization trapping.

with Re2O7 delivered 20-2 as the sole product within 10 minutes (Scheme 20). The ring formation in 20-2 is reversible, leading to thermodynamic equilibration and generating the most

Scheme 20 synthesis.

Re2O7-catalyzed [1,3]-transposition/trapping in heterocycle

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stable 2,6-syn-disubstituted tetrahydropyran. However, no stereocontrol was observed when acetal 20-3 bearing a primary allylic alcohol was exposed to the reaction conditions. On the other hand, treatment of acetal 20-5 possessing a secondary allylic alcohol moiety afforded tetrahydropyran 20-6 in high diastereomeric excess. The higher stereoselectivity in the formation of ketals compared to that of acetals is attributed to the increased stability of the oxocarbenium ion, which promotes easier stereochemical editing via ring opening and closing. Stereoselectivity in the allylic transposition/cyclization of acetals bearing secondary allylic alcohols, however, arises from the cation-stabilizing effect of an additional alkyl group. This promotes thermodynamic equilibration via an ion-pair formation. The reversibility of the allylic transposition–electrophilic trapping sequence allowed for the synthesis of spiroketal and bis-spiroketal structures under thermodynamic control. Thus, spirocyclization of 20-7, possessing a stereochemically defined alcohol moiety, provided a single stereoisomer of anomeric spiroketal 20-8. In this cascade cyclization, the stereochemistry of a remote hydroxyl group controls two newly created stereogenic centers. Additionally, an oxa-Michael reaction acting as a trapping mechanism for transposing allylic alcohol functionality enabled the synthesis of syn-disubstituted tetrahydropyran 20-10 with high diastereoselectivity. In this case less thermodynamically stable products equilibrate through a retro-oxa-Michael addition. Subsequently, the allylic transposition/alcohol trapping strategy was expanded to variants where epoxides act as trapping agents.36 In this approach, rhenium oxide behaves as a transposition catalyst as well as a Lewis acid not only to activate the epoxide moiety but also to promote the product equilibration. Thus, the treatment of substrate 21-1 with Re2O7 in dichloromethane at room temperature provided bis(tetrahydropyran) 21-2 as a mixture of diastereomers within 12 h (Scheme 21). The cascade transformation, however, required 3.5 days at room temperature to reach complete equilibration to form a single

Scheme 21 Cascade reactions employing allylic transposition/trapping with epoxides.

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stereoisomeric product. Additionally, a ketone moiety can be introduced into the substrate structure between the epoxide and the allylic alcohol, thus allowing for an alternative strategy of chirality transfer. In this case, the ketone moiety behaves as a nucleophile to open the epoxide, which is activated by the Lewis acidic catalyst, with the formation of chiral oxocarbenium ion, which in turn acts as an electrophilic trap for the [1,3]transposed alcohol. By employing this strategy, spiroketal 21-4 was generated as a single stereoisomer from epoxyketone 21-3 in 93% yield. With a slight variation of the substrate an impressive level of stereochemical complexity can be achieved in a single reaction. The cascade cyclization of compound 21-5 bearing only stereochemically defined epoxide afforded tricyclic product 21-6 possessing five stereocenters. Similar efficiency of these cascade transformations was obtained employing a silica gel-supported Re2O7 catalyst (10% w/w).36 For the preparation of the immobilized catalyst, a slurry of Re2O7 and silica gel in Et2O was stirred for several hours and then dried under vacuum. The obtained freeflowing catalyst is easily weighed even in humid environments wherein the corresponding non-immobilized Re2O7 would readily liquefy. Because of the superior dispersion of a supported Re2O7 catalyst, reactions do not have induction periods that are often observed with free Re2O7. The favorable properties of the immobilized catalyst make it a better alternative to Ph3SiOReO3. A sequential allylic [1,3]-transposition/macrocyclization strategy has been developed as a key step in the formal synthesis of leucascandrolide A.48 Exposure of secondary allylic alcohol 22-1 to a catalytic amount of Re2O7 in ether at room temperature for 2.5 h afforded macrocyclic hemiacetal 22-2 in 69% yield (Scheme 22). Although 22-1 and its [1,3]-transposed isomer are both secondary allylic alcohols, the transposition occurred regioselectively due to hydrogen bonding between the transposed alcohol with a tetrahydropyranyl ether and due to the facile formation of an unexpectedly stable leucascandrolide macrolactol hemiketal moiety. Interestingly, subjection of the C19 epimer of 22-1 to the same reaction conditions provided identical macrolactol 22-2 in 49% yield. This result suggests that an epimerization takes place during the transposition. The authors rationalized the observed dynamic kinetic resolution based on a feasible, minor pathway of rearrangement through a boat-like transition state.

Scheme 22

Tutorial Review

3.6.

Ring-contractive allylic transposition of cyclic silyl ethers

Relief of ring strain of medium-sized rings was found to be an efficient regiochemistry-controlling element in the [1,3]transposition of cyclic silyl ethers.49 In this transformation eight-membered ring siloxadienes 23-3 undergo an effective ring contraction to form the corresponding six-membered siloxenes 23-4 bearing an endocyclic double bond and an alkenyl substituent (Scheme 23). Formation of the transdouble bond after the ring contraction from the internal cis-alkene provides an additional driving force for the transposition, thus ensuring perfect regiocontrol. Ring-contractive transposition is a facile process and generally completes in less than half an hour at room temperature in the presence of Re2O7 as a catalyst. For the preparation of 8-membered siloxadienes, the sequence of gold-catalyzed intramolecular allyl transfer–alcoholysis of alkynyl allyl silanes 23-1 followed by ring-closing metathesis (RCM) of intermediate silyl ethers 23-2 was employed. A variety of silacyclodienes derived from allylic 24-1 and 24-5 or from propargylic alcohols 24-7 and 24-9 underwent ringcontraction in the presence of 5 mol % of Re2O7, affording siloxenes 24-2, 24-6, 24-8, and 24-10 in good to high yield with perfect E-stereochemistry of the exocyclic double bond (Scheme 24). On the other hand, the rearrangement of substrate 24-3 derived from a simple allyl alcohol afforded product 24-4 with moderate efficiency presumably due to the formation of a thermodynamically less favorable terminal alkene from the internal one in the starting material. It has been found that rearrangement of enantiomerically enriched substrates 24-5 and 24-9 occurred with partial racemization. The extent of the racemization depends on the reaction time as longer reaction times resulted in lower ee values of the products.

Scheme 23

Synthesis and ring contraction of eight-membered siloxacycles.

Re2O7-catalyzed macrocyclization in the synthesis of leucascandrolide A.

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Scheme 24

Ring-contractive allylic transposition of cyclic silyl ethers.

Based on the allylic transposition of compounds 24-5 and 24-9, the following mechanistic rationale was proposed (Scheme 25). The reaction starts with the activation of the Si–O bond of substrate 25-1 with rhenium oxide via a s-bond metathesis-like process to form an open chain intermediate. Subsequent allylic transposition follows a [3,3]-sigmatropic

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rearrangement through a chair-like transition state 25-3 to give isomeric perrhenate ester 25-4. At this point, a second s-bond metathesis-like bond reorganization of 25-4 regenerates the catalyst and furnishes the 6-membered product 25-5 with a high ee value. Minimization of syn-pentane interactions in the transition state 25-3 leads to the E-stereochemistry of the alkenyl substituent in 25-4. Yet, due to the highly Lewis acidic nature of Re2O7, the allylic transposition may occur partially via an allylic cation intermediate 25-6, generating a product with low or no enantiomeric excess. The formation of an ion pair intermediate depends on the electronic nature of substituents R1 and R2. The utility of ring-contractive allylic transposition have been exploited in a recent total synthesis of amphidinolide V.50 Accordingly, allylic transposition precursor silacyclodiene 26-6 was initially envisioned to arise from allyl transfer–alcoholysis of 26-1 in the presence of allylic alcohol 26-2 and subsequent RCM (Scheme 26). However, due to the facile dehydration of 26-2 under the allyl transfer conditions, a different tactic to form the requisite silyl ether 26-3 was developed. The alkynyl silane 26-1 was first converted to siloxane 26-4 via allyl transfer– alcoholysis in the presence of phenol to introduce the Z-trisubstituted double bond. Subsequent phenoxide displacement by treating 26-4 with Red-Al delivered hydrosilane 26-5, which was then subjected to the reaction with allylic alcohol 26-2 under dehydrogenative coupling conditions to afford silyl

Scheme 25

Mechanism of silyl-directed allylic transposition/ring contraction.

Scheme 26

Ring strain-promoted allylic transposition in the synthesis of amphidinolide V.

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ether 26-3. RCM of 26-3 in the presence of the Grubbs secondgeneration catalyst (GII) together with benzoquinone delivered allylic transposition precursor 26-6 in 96% yield. The key allylic transposition was capricious, giving variable amounts of the inseparable Z-isomer of the allylic ether double bond. Thus, subjection of 26-6 to the standard reaction conditions for ring contraction (Re2O7, Et2O, r.t., 0.5 h) afforded 6-membered siloxene 26-7 as an inseparable mixture of E/Z-isomers in a 75 : 25 ratio. However, [1,3]-transposition carried out at 0 1C delivered 26-7 in an 85 : 15 E/Z-ratio after 16 h. Further attempts to decrease the amount of unwanted isomer by performing the reaction below 0 1C resulted in incomplete conversions due to the significantly decreased solubility of Re2O7 in ether at low temperatures. Interestingly, the ring contraction with more soluble Ph3SiOReO3 delivered 26-7 as a 1 : 1 mixture of double bond isomers. The undesired Z-isomer was separated at the later stage of the synthesis. Thus, ring contractive allylic transposition of silyl ethers enabled stereoselective construction of a functionalized 1,5-diene moiety, which rendered an efficient enantioselective total synthesis of amphidinolide V.

4. Conclusion In the years 2002–2013, creative strategies for regio- and stereoselective variants of Re-catalyzed [1,3]-transposition of allylic alcohols, which rely on structural features favoring the equilibrium toward the product, have been developed and successfully employed in complex natural product syntheses. Additionally, the direct regio- and stereoselective transpositions of allylic silyl ethers have been introduced and their utility has been demonstrated. These achievements significantly expanded the scope and utility of allylic [1,3]-transposition and have elicited the interest of the synthetic community. Despite these advances, several limitations of allylic transposition methods have to be addressed. Search for less acidic catalysts that would not cause undesired dehydration or isomerization, yet possess the same level of reactivity, and the development of different reaction strategies to obtain high selectivity are the main objectives for further research. Another open field for investigations is the development of selective direct interconversion of the hydroxyl group to other functionalities such as amino or halogen. Moreover, asymmetric versions of [1,3]-transposition remain undiscovered.

Acknowledgements Financial support from UIC through the Dean’s Scholar Award (I.V.) and the LAS Science Award (D.L.) is greatly acknowledged.

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