DOI: 10.1002/chem.201500050
Communication
& Enantioselective Synthesis
Enantioselective Allylation of (2E,4E)-2,4-Dimethylhexadienal: Synthesis of (5R,6S)-(+ +)-Pteroenone Petr Koukal and Martin Kotora*[a] Abstract: Allylation, trans- and cis-crotylation of (2E,4E)2,4-dimethylhexadienal, a representative a,b,g,d-unsaturated aldehyde, was carried out under different catalytic and stoichiometric conditions. The reactions catalyzed by organocatalysts TRIP-PA and N,N’-dioxides gave the best results with respect to yields, asymmetric induction, and catalyst load in comparison to other procedures. The developed methodology was applied in the enantioselective synthesis of (5R,6S)-(+ +)-pteroenone, a defensive metabolite (ichthyodeterrent) of the Antarctic pteropod Clione antarctica.
Enantioselective allylation (and related processes such as crotylation) of various aldehydes (aryl, heteroaryl, alkenyl, and alkyl), which provides chiral homoallylic alcohols, has been intensively studied in past decades and has become a standard tool of synthetic organic chemistry.[1] Allylation can be carried out under catalytic conditions utilizing Lewis acid or base activation to promote the reaction or by using stoichiometric chiral reagents. However, allylation of a,b,g,d-unsaturated aldehydes has received little attention from organic chemists and, hence, only a handful of reports are known. Moreover, most of these reports do not deal with the subject exclusively, but rather as a part of other studies. Typical examples are exemplified by the groups of: a) Denmark, who studied allylation of (S)(2E,4E)-2,8-dimethyldecadienal and related substances under different catalytic and stoichiometric conditions as a part of their synthetic efforts to prepare papulacandin D;[2] b) Morken, who performed a nickel complex catalyzed allylboration of several a,b,g,d-unsaturated aldehydes within the framework of their reaction mechanism studies;[3] c) Kocˇovsky´, who tested his METHOX catalyst on a couple of examples;[4] and d) Campagne, who studied AgX/BINAP (2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) catalyzed allylations.[5] As far as synthetic applications of enantioselective allylation of a,b,g,d-unsaturated aldehydes is concerned, there are, to the best of our knowledge, only four examples. Two of them were based on catalytic [a] P. Koukal, Prof. Dr. M. Kotora Department of Organic Chemistry, Faculty of Science Charles University in Prague Hlavova 8, 123 43, Praha 2 (Czech Republic) Fax: (+ 420) 221-951-236 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500050. Chem. Eur. J. 2015, 21, 7408 – 7412
enantioselective allylations and served as the key steps in the syntheses of papulacandin D[2] and macrolactin A.[5] The remaining two syntheses were based on diastereoselective allylations using a chiral borane and a chiral titanium complex. They were used in the syntheses of macrolactin A[6] and natural (5’-oxoheptene-1’E,3’E-dienyl)-5,6-dihydro-2H-pyran-2-one.[7] In view of the aforementioned, along with the fact that enantioselective allylation could be an important step in syntheses of various natural compounds such as pteroenone— a defensive metabolite (ichthyodeterrent) isolated from the Antarctic pteropod Clione antarctica,[8] tiacumicin antibiotics isolated from Dactylosporangium aurantiacum,[9] antillatoxin isolated from marine cyanobacterium Lyngbya majuscula[10] (Figure 1) and other compounds[11]—we decided to screen vari-
Figure 1. Pteroenone 1, tiacumicin, and antillatoxin—natural compounds with highlighted sections showing how they could be potentially prepared by allylation of a,b,g,d-unsaturated aldehydes.
ous reaction conditions to assess the scope of allylation with respect to catalytic conditions. We chose (2E,4E)-2,4-dimethylhexadienal (2), prepared by using the previously published procedures from ethyl 2-bromopropionate, as a model compound because its structural feature, represented by the substituted diene moiety, can be found in the aforementioned natural compounds. At the outset we decided to screen several enantioselective allylation procedures based on Lewis acid, Brønsted acid, or Lewis base catalytic or stoichiometric protocols (Figure 2 and Table 1). First of all, the AgOTf/BINAP/KF/ [18]crown-6 catalyzed allylation with allyltrimethoxysilane developed by Yamamoto was tested.[12] It furnished the desired product 3 in a rather low yield (27 %) and with mediocre enantioselectivity of 77 % ee (Entry 1). The Keck allylation using a stoichiometric amount of Ti(OiPr)4/BINOL (1,1’-bi-2-naphthol) and allyltributylstannane[13] proceeded in a similar manner, giving 3 with 63 % ee
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Communication yields (78–99 %) and with good optical purity (83–87 % ee) even at low catalyst loadings (Entries 4–7). The allylation with allyltrichlorosilane in the presence of diastereoisomeric (R,Ra)and (R,Sa)-N,N’-dioxide catalysts (Figure 2) proceeded somewhat disappointingly with regard to asymmetric induction, especially as excellent results have been achieved in the case of allylation of a,b-unsaturated aldehydes.[17] However, these conditions provided 3 with only mediocre optical purity in the range of 55–69 % ee (Entries 8–10). Finally, diastereoselective allylation of 2 by applying a stoichiometric amount of Brown’s allylation reagent (+ +)-Ipc2B(allyl) (Figure 2)[18] also yielded 3 with insufficient asymmetric induction (63 % ee; Entry 11). Next, we focused our attention on enantioselective crotylation. We decided to check the use of TRIP-PA and N,N’-dioxide Figure 2. Structures of the selected catalysts and the allylation and crotylacatalysts because they showed the most promising results retion reagents. garding asymmetric allylation, yields, and catalysts loads. First, crotylation using trans-crotyl reagents that were supposed Table 1. Allylations of 2 to 3 under various conditions. to provide predominantly the anti-diastereoisomer was studied (Table 2). Thus, crotylations with the trans-crotylboronic acid pinaEntry XnM Equiv Catalytic system mol % Solvent T t Yield ee col ester catalyzed by (R)- or (S)[8C] [days] [%][a] [%][b] TRIP-PA (10 mol %) in toluene 1 (MeO)3Si 3 (S)-BINAP/AgOTf/KF/[18]crown-6 5/5/5/5 THF ¢20 3 27 77 (S) proceeded with high asymmetric 2 Bu3Sn 1.1 (S)-BINOL/Ti(O-iPr)4 100/100 CH2Cl2 ¢20 3 21 63 (S) induction (94 and 93 % ee, re1.1 (S)-BINAP/AgOTf/4 æ molecular sieves 30/30 THF ¢20 3 40 89 (S) 3 Bu3Sn 1.2 (S)-TRIP-PA 15 toluene ¢20 3 90 85 (S) 4 (Pin)B[c] spectively), reasonable yields of 1.2 (S)-TRIP-PA 5 toluene ¢20 1 78 87 (S) 5 (Pin)B[c] anti-4, and excellent anti/syn [c] 1.2 (S)-TRIP-PA 2 toluene ¢20 1 99 85 (S) 6 (Pin)B ratios (> 30:1; Entries 1 and 2). 1.2 (S)-TRIP-PA 1 toluene ¢20 1 94 83 (S) 7 (Pin)B[c] The use of dichloromethane or 8 Cl3Si 2 (R,Ra)-dioxide 5 THF ¢40 1 33 60 (R) 2 (R,Sa)-dioxide 5 THF ¢40 1 57 69 (S) 9 Cl3Si THF as the reaction medium 2 (R,Sa)-dioxide 5 toluene ¢40 3 30 55 (S) 10 Cl3Si yielded the product with low +)-Ipc2B 1.5 – Et2O ¢114 3 67 63 (R) 11 (+ enantioselectivities of 64 and [a] Isolated yields. [b] ee values were determined by Mosher ester method; see reference [21]. The racemic ho47 % ee, respectively, and remoallyl alcohol 3 that served as a standard was obtained by the addition of allylmagnesium bromide to 2 in versed absolute configurations Et2O at 20 8C. The configuration assignment was based on analysis of the corresponding Mosher esters.[21] (Entries 3 and 4). Surprisingly, [c] (Pin)B = pinacol borane. the use of (R,Ra)-N,N’-dioxide catalyst and trans-crotyltrichlorosiand 21 % yield (Entry 2). Attempts to carry out the Keck allylalane[19] provided anti-4 with a miserably low asymmetric induction (15 % ee; Entry 5). On the other hand, the use of (R,Sa)tion under catalytic conditions were not met with success. A much better result, with respect to asymmetric induction, was Table 2. Crotylations of 2 with trans-crotyl reagents to anti-4 under various conditions. obtained in the case of addition of allyltributylstannane, catalyzed by AgOTf/BINAP,[5, 14] which provided 3 with a very good syn/anti Entry XnM Equiv Catalyst mol % Solvent T t Yield ee enantioselectivity of 89 % ee and [8C] [days] [%][a] [%][b] 40 % yield (Entry 3). The low 2 (R)-TRIP-PA 10 toluene ¢20 1 44 94 (3R,4R) > 30:1 1 (Pin)B[c] yields of all three reactions could 2 (S)-TRIP-PA 10 toluene ¢20 1 64 93 (3S,4S) > 30:1 2 (Pin)B[c] be attributed to a low reaction 2 (S)-TRIP-PA 10 CH2Cl2 ¢20 1 53 64 (3R,4R) 28:1 3 (Pin)B[c] rate as, even after three days, 2 (S)-TRIP-PA 10 THF ¢20 1 61 47 (3R,4R) 24:1 4 (Pin)B[c] the unreacted starting material 2 (R,Ra)-dioxide 2.5 THF ¢40 1 52 15 (3R,4R) 19:1 5 Cl3Si 2 (R,Sa)-dioxide 2.5 THF ¢40 1 68 96 (3S,4S) 19:1 6 Cl3Si was detected. The use of (S)7 (S,S)-transEZ-CrotylMix 1.2 – CH2Cl2 20 1 18 89 (3R,4R) 12:1 [15] TRIP-PA (Figure 2), a chiral Cl ¢10 3 36 80 (3R,4R) 11:1 8 (S,S)-transEZ-CrotylMix 1.1 – CH 2 2 Brønsted acid,[16] to catalyze ad[a] Isolated yields. [b] ee values were determined by Mosher ester method. The configuration assignment was dition of the allylboronic acid pibased on analysis of the corresponding Mosher esters; see reference [21]. [c] (Pin)B = pinacol borane. nacol ester gave rise to 3 in high Chem. Eur. J. 2015, 21, 7408 – 7412
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Communication N,N’-dioxide catalyst (5 mol %) gave the desired product anti-4 with the best enantioselectivity achieved yet (96 % ee), good isolated yield (68 %), and a very good anti/syn ratio of 19:1 (Entry 6). Finally, the reaction was also run with a stoichiometric amount of (S,S)-trans EZ-CrotylMix at two different temperatures; however, the asymmetric induction remained below 90 % ee and only mediocre diastereoselectivities of 11:1 and 12:1 were achieved (Entries 7 and 8). It is worth mentioning that in all cases (except Entries 3 and 4) of the TRIP-PA and N,N’-dioxide catalyzed reactions the (S)-catalysts gave the (4S)products and vice versa, which is in agreement with the previously observed facial selectivity.[15, 17] Additionally, crotylations utilizing cis-crotyl reagents that were supposed to provide predominantly the syn-diastereoisomer were studied (Table 3). Thus, crotylations with the cis-crotylboronic acid pinacol ester catalyzed by (R)- or (S)-TRIP-PA in toluene proceeded with asymmetric inductions of 82 and 80 % ee and anti/syn ratios of 1:14 and 1:19, respectively (Entries 1 and 2). The use of (R,Ra)-N,N’-dioxide catalyst and cis-crotyltrichlorosilane[19] provided syn-4 with a very low asymmetric in-
Scheme 1. Racemic synthesis of pteroenone 1. Reagents and conditions: a) iPr2NH (2 equiv), nBuLi (2 equiv), EtCN (2 equiv), THF, ¢78 8C, 2 h; b) TBSCl (2.1 equiv), imidazole (4 equiv), 4-dimethylaminopyridine (DMAP; 1.1 equiv), DMF, 60 8C, 3 days; c) 1) tBuLi (10 equiv), nPrI (5 equiv), Et2O, ¢78 8C, 0.5 h, 2) NH4Cl (aq.); d) HF (16 equiv), TBAF (20 equiv), THF, 3 days, 0 8C to ca. 45 8C.
oenone, we developed a new and alternative pathway based on condensation of aldehyde 2 with propionitrile (Scheme 1). The straightforward racemic synthesis started with addition Table 3. Crotylations of 2 with cis-crotyl reagents to syn-4 under various conditions. of the a-lithiated propionitrile (prepared by using the freshly prepared lithium diisopropylamide (LDA)) to aldehyde 2 furnishing b-hydroxynitrile 5 as syn/anti Entry XnM Equiv Catalyst mol % Solvent T t Yield ee [8C] [days] [%][a] [%][b] a 1.3:1 mixture of anti/syn diastereomers in 59 % yield. Protec(R)-TRIP-PA 10 toluene ¢20 1 55 82 (3S,4R) 1:14 1 (Pin)B[c] 2 (S)-TRIP-PA 10 toluene ¢20 1 64 80 (3R,4S) 1:19 2 (Pin)B[c] 2 tion of the hydroxyl group fol2 (R,Ra)-dioxide 2.5 THF ¢40 1 31 33 (3R,4S) 1:30 < 3 Cl3Si lowed, giving rise to 6 in 60 % 2 (R,Sa)-dioxide 2.5 THF ¢40 1 49 85 (3R,4S) 1:30 < 4 Cl3Si yield and with a 1.6:1 anti/syn 2 (R,Sa)-dioxide 1.25 THF ¢40 1 33 74 (3R,4S) 1:10 5 Cl3Si ratio. Then, addition of nPrLi, [a] Isolated yields. [b] ee values were determined by Mosher ester method. The configuration assignment was prepared by reaction of nPrI and based on analysis of the corresponding Mosher esters; see reference [21]. [c] (Pin)B = pinacol borane. tBuLi, with subsequent acidic work-up provided, after isolation, the protected b-hydroxyketone 7 in a good yield of 84 % (1.8:1 anti/syn). Finally, deprotection duction (33 % ee; Entry 3). The use of (R,Sa)-N,N’-dioxide catalyst of the TBS group by a mixture of HF/tetra-n-butylammonium was tested at two catalyst loadings (2.5 and 1.25 mol %): the fluoride (TBAF) quantitatively gave a mixture of racemic antihigher loading gave rise to syn-4 with the best enantioselectiv1 (pteroenone) and syn-1. Determination of suitable deprotecity achieved for this reaction (85 % ee) and a anti/syn ratio of tion conditions required a considerable amount of experimen1:30 (Entry 4), whereas, the lower loading provided the prodtal work and the reaction conditions were tuned on a model uct with diminished asymmetric induction (74 % ee; Entry 5). substrate; see the Supporting Information for details. With these results in hand, we decided to proceed with synFinally, the enantioselective synthesis of pteroenone based thesis of pteroenone[20] by enantioselective crotylation. Howevon asymmetric crotylation was initiated (Scheme 2). Enantioseer, prior to the enantioselective synthesis, the racemic synthelective crotylation of 2 with trans-crotylboronic acid pinacol sis of pteroenone was carried out to check feasibility of the ester catalyzed by (S)-TRIP-PA (5 mol %) at ¢30 8C provided chosen synthetic pathway and also to get the racemic stan(3S,4S)-4 with an isolated yield of 86 % and with very good dard of the final compound for evaluation of ee. The initial attempt to synthesize racemic pteroenone was based on the enantioselectivity (> 95 % ee). Protection of the hydroxyl group crotylation reaction and we could successfully prepare the tertby using TBSCl, which provided (3S,4S)-8 in 86 % yield, was folbutyldimethylsilyl (TBS)-protected pteroenone. Moreover, by lowed by Wacker oxidation of the double bond to the correfollowing this synthetic route we could solve a number of synsponding ketone (3R,4S)-9. The oxidation proved to be a rather thetic problems (Wacker oxidation, protection, etc.) that were troublesome step and required a lot of experimental work on crucial for successful development of the enantioselective synthe racemic version to assess the best reaction conditions (see thesis of pteroenone 1 (for details see the Supporting Informathe Supporting Information). The use of PdCl2/CuCl/O2 catalytic tion). Nonetheless, for straightforward access to racemic ptersystem in DMF/H2O furnished (3R,4S)-9 in 30 % overall yield,
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Communication Acknowledgements This work was supported by grants from the Ministry of Education, Youth, and Sports (MSM0021620857) and the Czech Science Foundation (P207/11/0587). The authors also would thank Lach-Ners.r.o. for the generous gift of chemicals as a part of the award given to M.K. Keywords: aldehyde · allylation · asymmetric synthesis · Brønsted acid · Lewis base
Scheme 2. Enantioselective synthesis of (5R,6S)-(+ +)-pteroenone 1. Reagents and conditions: a) trans-crotylboronic acid pinacol ester (1.2 equiv), (S)-TRIPPA (5 mol %), toluene, ¢30 8C, 3 days; b) TBSCl (1.1 equiv), Im (2 equiv), DMAP (10 mol %), DMF, 60 8C, 3 days; c) PdCl2 (25 mol %), CuCl (1.5 equiv), DMF/H2O (4:1), 40 8C, 3 days; d) iPr2NH (2.5 equiv), nBuLi (2.5 equiv), EtI (3.5 equiv), THF, ¢78 to 20 8C, 3 h; e) HF (16 equiv), TBAF (20 equiv), THF, 40 8C, 3 days.
which included one repetition of the reaction with the recovered unreacted starting material (3S,4S)-8. The low yield could be attributed to low conversion at ambient temperature or to the formation of intractable side-products at higher reaction temperatures (40 8C). The side-products probably result from concomitant oxidation of the diene moiety. The propyl chain was introduced by a-lithiation of ketone (3R,4S)-9 followed by reaction with ethyl iodide, which provided (5R,6S)-7 in 32 % isolated yield. Also in this case, the rather low isolated yield of (5R,6S)-7 could be accounted for by the formation of intractable side-products and difficulties associated with purification. Finally, deprotection of the hydroxyl group by the mixture of HF/TBAF as used previously yielded (5R,6S)-(+ +)-pteroenone 1 in 54 % isolated yield and enantioselectivity over 95 %. The spectral and physical characteristics were in agreement with the previously published data.[19] In conclusion, we have shown that enantioselective allylation and crotylation of (2E,4E)-2,4-dimethylhexadienal 2, a representative a,b,g,d-unsaturated aldehyde, which leads to a valuable synthetic building block, can be carried out under various conditions. The best results regarding asymmetric induction, product yields, and catalyst loads were achieved with chiral TRIP-PA and (R,Sa)-N,N’-dioxide catalysts. It should be also noted that the best ee values for the crotylations were obtained with the dioxide catalyst at low catalyst loadings. The developed enantioselective crotylation was applied in the enantioselective synthesis of natural (5R,6S)-(+ +)-pteroenone 1. In addition, the racemic synthesis of pteroenone based on the aldol reaction of a-lithiated propionitrile could serve, in principle, as an inspiration for practical applications of the recently developed enantioselective addition of nitriles to aldehydes catalyzed by a chiral Rh-complex.[22]
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[1] For reviews, see: a) S. E. Denmark, J. Fu, Chem. Rev. 2003, 103, 2763 – 2793; b) M. Yus, J. C. Gonzlez-Gûmez, F. Foubelo, Chem. Rev. 2011, 111, 7774 – 7854; c) M. Yus, J. C. Gonzlez-Gûmez, F. Foubelo, Chem. Rev. 2013, 113, 5595 – 5698. [2] a) S. E. Denmark, C. S. Regens, T. Kobayashi, J. Am. Chem. Soc. 2007, 129, 2774 – 2776; b) S. E. Denmark, T. Kobayashi, C. S. Regens, Tetrahedron 2010, 66, 4745 – 4759. [3] P. Zhang, J. P. Morken, J. Am. Chem. Soc. 2009, 131, 12550 – 12551. [4] A. V. Malkov, M. Barzog, Y. Jewkes, J. Mikusˇek, P. Kocˇovsky´, J. Org. Chem. 2011, 76, 4800 – 4804. [5] M. Georgy, P. Lesot, J.-M. Campagne, J. Org. Chem. 2007, 72, 3543 – 3549. [6] a) V. Prahlad, A.-A. S. El-Ahl, W. A. Donaldson, Tetrahedron: Asymmetry 2000, 11, 3091 – 3102; b) V. Prahlad, W. A. Donaldson, Tetrahedron Lett. 1996, 37, 9169 – 9172. [7] S. Bouzbouz, E. de Lemos, J. Cossy, J. Saez, X. Franck, B. FigadÀre, Tetrahedron Lett. 2004, 45, 2615 – 2617. [8] W. Y. Yoshida, P. J. Bryan, B. J. Baker, J. B. McClintock, J. Org. Chem. 1995, 60, 780 – 782. [9] W. Erb, J. Zhu, Nat. Prod. Rep. 2013, 30, 161 – 174. [10] J. Orlaja, D. G. Nagle, V. L. Hsu, W. H. Gerwick, J. Am. Chem. Soc. 1995, 117, 8281 – 8282. [11] Among other representatives, there are thuggacins (H. Steinmetz, H. Irschik, B. Kunze, H. Reichenbach, G. Hçfle, R. Jansen, Chem. Eur. J. 2007, 13, 5822 – 5832) and several compounds found in a review (J. A. Marco, M. Carda, J. Murga, E. Falomir, Tetrahedron 2007, 63, 2929 – 2958). [12] M. Wadamoto, N. Ozasa, A. Yanagisawa, H. Yamamoto, J. Org. Chem. 2003, 68, 5593 – 5601; N. Ozasa, A. Yanagisawa, H. Yamamoto, J. Org. Chem. 2003, 68, 5593 – 5601. [13] a) G. E. Keck, K. H. Tarbet, L. S. Geraci, J. Am. Chem. Soc. 1993, 115, 8467 – 8468; b) G. E. Keck, D. Krishnamurthy, M. C. Grier, J. Org. Chem. 1993, 58, 6543 – 6544; c) G. E. Keck, L. S. Geraci, Tetrahedron Lett. 1993, 34, 7827 – 7828. [14] A. Yanagisawa, H. Nakashima, A. Ishiba, H. Yamamoto, J. Am. Chem. Soc. 1996, 118, 4723 – 4724. [15] a) P. Jain, J. C. Antilla, J. Am. Chem. Soc. 2010, 132, 11884 – 11886; b) S. Fustero, E. Rodrguez, R. Lzaro, L. Herrera, S. Cataln, P. Barrio, Adv. Synth. Catal. 2013, 355, 1058 – 1064. [16] For other examples of chiral Brønsted acid catalysis, see: a) M. Mahlau, P. Garca-Garca, B. List, Chem. Eur. J. 2012, 18, 16283 – 16287; b) C.-H. Xing, Y.-X. Liao, Y. Zhang, D. Sabarova, M. Bassous, Q.-S. Hu, Eur. J. Org. Chem. 2012, 1115 – 1111; c) D. Kampen, C. M. Reisinger, B. List, Top. Curr. Chem. 2009, 291, 395 – 456; d) M. Terada, Chem. Commun. 2008, 4097 – 4112; e) T. Akiyama, Chem. Rev. 2007, 107, 5744 – 5758. [17] A. Kadlcˇkova, I. Valterov, L. Duchcˇkov, J. Roithov, M. Kotora, Chem. Eur. J. 2010, 16, 9442 – 9445. For our other representative examples regarding Lewis base catalyzed allylation, see: a) T. Cadart, P. Koukal, M. Kotora, Eur. J. Org. Chem. 2014, 7556 – 7560; b) F. Hessler, A. Korotvicˇka, D. Necˇas, I. Valterov, M. Kotora, Eur. J. Org. Chem. 2014, 2543 – 2548; c) K. Vlasˇan, R. Hrdina, I. Valterov, M. Kotora, Eur. J. Org. Chem. 2010, 7040 – 7044; d) A. Kadlcˇkov, R. Hrdina, I. Valterov, M. Kotora, Adv. Synth. Catal. 2009, 351, 1279 – 1283; e) R. Hrdina, M. Dracˇnsky´, I. Valterov, J. Hodacˇov, I. Csarˇov, M. Kotora, Adv. Synth. Catal. 2008, 350, 1449 – 1456. [18] a) H. C. Brown, M. C. Desai, P. K. Jadhav, J. Org. Chem. 1982, 47, 5065 – 5069; b) H. C. Brown, B. Singaram, J. Org. Chem. 1984, 49, 945 – 947;
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Communication c) P. K. Jadhav, K. S. Bhat, P. T. Perumal, H. C. Brown, J. Org. Chem. 1986, 51, 432 – 439. [19] For preparation of trans- and cis-crotyltrichlorosilanes, see: K. Iseki, Y. Kuroki, M. Takahashi, S. Kishimoto, Y. Kobayashi, Tetrahedron 1997, 53, 3513 – 3526 and references therein as well as the Supporting Information. [20] So far only three reports regarding syntheses of pteroenone have been reported: a) Y. Nakamura, H. Kiyota, B. J. Baker, S. Kuwahara, Synlett 2005, 635 – 636; b) H. Kiyota, Biosci. Biotechnol. Biochem. 2006, 70, 317 – 324; c) H. Asao, Y. Nakamura, Y. Furuya, S. Kuwahara, B. J. Baker, H. Kiyota, Helv. Chim. Acta 2010, 93, 1933 – 1944. [21] The absolute configurations of the products obtained from enantioselective allylations were determined by comparison of the 1H NMR
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chemical shifts of the relevant esters with both enantiomers of Mosher acid, which allows stereochemical assignment based on the anisotropic effect induced by the phenyl ring present in the ester moiety on the two different substituents of the alcohol. The ee values were also determined from the corresponding Mosher esters. J. M. Seco, E. QuiÇo, R. Riguera, Chem. Rev. 2004, 104, 17 – 117. [22] D. Sureshkumar, V. Ganesh, N. Kumagai, M. Shibasaki, Chem. Eur. J. 2014, 20, 15723 – 15726.
Received: January 6, 2015 Published online on March 26, 2015
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& Enantioselective Synthesis
Enantioselective Allylation of (2E,4E)-2,4-Dimethylhexadienal: Synthesis of (5R,6S)-(+ +)-Pteroenone Petr Koukal and Martin Kotora*[a] Abstract: Allylation, trans- and cis-crotylation of (2E,4E)2,4-dimethylhexadienal, a representative a,b,g,d-unsaturated aldehyde, was carried out under different catalytic and stoichiometric conditions. The reactions catalyzed by organocatalysts TRIP-PA and N,N’-dioxides gave the best results with respect to yields, asymmetric induction, and catalyst load in comparison to other procedures. The developed methodology was applied in the enantioselective synthesis of (5R,6S)-(+ +)-pteroenone, a defensive metabolite (ichthyodeterrent) of the Antarctic pteropod Clione antarctica.
Enantioselective allylation (and related processes such as crotylation) of various aldehydes (aryl, heteroaryl, alkenyl, and alkyl), which provides chiral homoallylic alcohols, has been intensively studied in past decades and has become a standard tool of synthetic organic chemistry.[1] Allylation can be carried out under catalytic conditions utilizing Lewis acid or base activation to promote the reaction or by using stoichiometric chiral reagents. However, allylation of a,b,g,d-unsaturated aldehydes has received little attention from organic chemists and, hence, only a handful of reports are known. Moreover, most of these reports do not deal with the subject exclusively, but rather as a part of other studies. Typical examples are exemplified by the groups of: a) Denmark, who studied allylation of (S)(2E,4E)-2,8-dimethyldecadienal and related substances under different catalytic and stoichiometric conditions as a part of their synthetic efforts to prepare papulacandin D;[2] b) Morken, who performed a nickel complex catalyzed allylboration of several a,b,g,d-unsaturated aldehydes within the framework of their reaction mechanism studies;[3] c) Kocˇovsky´, who tested his METHOX catalyst on a couple of examples;[4] and d) Campagne, who studied AgX/BINAP (2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) catalyzed allylations.[5] As far as synthetic applications of enantioselective allylation of a,b,g,d-unsaturated aldehydes is concerned, there are, to the best of our knowledge, only four examples. Two of them were based on catalytic [a] P. Koukal, Prof. Dr. M. Kotora Department of Organic Chemistry, Faculty of Science Charles University in Prague Hlavova 8, 123 43, Praha 2 (Czech Republic) Fax: (+ 420) 221-951-236 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500050. Chem. Eur. J. 2015, 21, 7408 – 7412
enantioselective allylations and served as the key steps in the syntheses of papulacandin D[2] and macrolactin A.[5] The remaining two syntheses were based on diastereoselective allylations using a chiral borane and a chiral titanium complex. They were used in the syntheses of macrolactin A[6] and natural (5’-oxoheptene-1’E,3’E-dienyl)-5,6-dihydro-2H-pyran-2-one.[7] In view of the aforementioned, along with the fact that enantioselective allylation could be an important step in syntheses of various natural compounds such as pteroenone— a defensive metabolite (ichthyodeterrent) isolated from the Antarctic pteropod Clione antarctica,[8] tiacumicin antibiotics isolated from Dactylosporangium aurantiacum,[9] antillatoxin isolated from marine cyanobacterium Lyngbya majuscula[10] (Figure 1) and other compounds[11]—we decided to screen vari-
Figure 1. Pteroenone 1, tiacumicin, and antillatoxin—natural compounds with highlighted sections showing how they could be potentially prepared by allylation of a,b,g,d-unsaturated aldehydes.
ous reaction conditions to assess the scope of allylation with respect to catalytic conditions. We chose (2E,4E)-2,4-dimethylhexadienal (2), prepared by using the previously published procedures from ethyl 2-bromopropionate, as a model compound because its structural feature, represented by the substituted diene moiety, can be found in the aforementioned natural compounds. At the outset we decided to screen several enantioselective allylation procedures based on Lewis acid, Brønsted acid, or Lewis base catalytic or stoichiometric protocols (Figure 2 and Table 1). First of all, the AgOTf/BINAP/KF/ [18]crown-6 catalyzed allylation with allyltrimethoxysilane developed by Yamamoto was tested.[12] It furnished the desired product 3 in a rather low yield (27 %) and with mediocre enantioselectivity of 77 % ee (Entry 1). The Keck allylation using a stoichiometric amount of Ti(OiPr)4/BINOL (1,1’-bi-2-naphthol) and allyltributylstannane[13] proceeded in a similar manner, giving 3 with 63 % ee
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Communication yields (78–99 %) and with good optical purity (83–87 % ee) even at low catalyst loadings (Entries 4–7). The allylation with allyltrichlorosilane in the presence of diastereoisomeric (R,Ra)and (R,Sa)-N,N’-dioxide catalysts (Figure 2) proceeded somewhat disappointingly with regard to asymmetric induction, especially as excellent results have been achieved in the case of allylation of a,b-unsaturated aldehydes.[17] However, these conditions provided 3 with only mediocre optical purity in the range of 55–69 % ee (Entries 8–10). Finally, diastereoselective allylation of 2 by applying a stoichiometric amount of Brown’s allylation reagent (+ +)-Ipc2B(allyl) (Figure 2)[18] also yielded 3 with insufficient asymmetric induction (63 % ee; Entry 11). Next, we focused our attention on enantioselective crotylation. We decided to check the use of TRIP-PA and N,N’-dioxide Figure 2. Structures of the selected catalysts and the allylation and crotylacatalysts because they showed the most promising results retion reagents. garding asymmetric allylation, yields, and catalysts loads. First, crotylation using trans-crotyl reagents that were supposed Table 1. Allylations of 2 to 3 under various conditions. to provide predominantly the anti-diastereoisomer was studied (Table 2). Thus, crotylations with the trans-crotylboronic acid pinaEntry XnM Equiv Catalytic system mol % Solvent T t Yield ee col ester catalyzed by (R)- or (S)[8C] [days] [%][a] [%][b] TRIP-PA (10 mol %) in toluene 1 (MeO)3Si 3 (S)-BINAP/AgOTf/KF/[18]crown-6 5/5/5/5 THF ¢20 3 27 77 (S) proceeded with high asymmetric 2 Bu3Sn 1.1 (S)-BINOL/Ti(O-iPr)4 100/100 CH2Cl2 ¢20 3 21 63 (S) induction (94 and 93 % ee, re1.1 (S)-BINAP/AgOTf/4 æ molecular sieves 30/30 THF ¢20 3 40 89 (S) 3 Bu3Sn 1.2 (S)-TRIP-PA 15 toluene ¢20 3 90 85 (S) 4 (Pin)B[c] spectively), reasonable yields of 1.2 (S)-TRIP-PA 5 toluene ¢20 1 78 87 (S) 5 (Pin)B[c] anti-4, and excellent anti/syn [c] 1.2 (S)-TRIP-PA 2 toluene ¢20 1 99 85 (S) 6 (Pin)B ratios (> 30:1; Entries 1 and 2). 1.2 (S)-TRIP-PA 1 toluene ¢20 1 94 83 (S) 7 (Pin)B[c] The use of dichloromethane or 8 Cl3Si 2 (R,Ra)-dioxide 5 THF ¢40 1 33 60 (R) 2 (R,Sa)-dioxide 5 THF ¢40 1 57 69 (S) 9 Cl3Si THF as the reaction medium 2 (R,Sa)-dioxide 5 toluene ¢40 3 30 55 (S) 10 Cl3Si yielded the product with low +)-Ipc2B 1.5 – Et2O ¢114 3 67 63 (R) 11 (+ enantioselectivities of 64 and [a] Isolated yields. [b] ee values were determined by Mosher ester method; see reference [21]. The racemic ho47 % ee, respectively, and remoallyl alcohol 3 that served as a standard was obtained by the addition of allylmagnesium bromide to 2 in versed absolute configurations Et2O at 20 8C. The configuration assignment was based on analysis of the corresponding Mosher esters.[21] (Entries 3 and 4). Surprisingly, [c] (Pin)B = pinacol borane. the use of (R,Ra)-N,N’-dioxide catalyst and trans-crotyltrichlorosiand 21 % yield (Entry 2). Attempts to carry out the Keck allylalane[19] provided anti-4 with a miserably low asymmetric induction (15 % ee; Entry 5). On the other hand, the use of (R,Sa)tion under catalytic conditions were not met with success. A much better result, with respect to asymmetric induction, was Table 2. Crotylations of 2 with trans-crotyl reagents to anti-4 under various conditions. obtained in the case of addition of allyltributylstannane, catalyzed by AgOTf/BINAP,[5, 14] which provided 3 with a very good syn/anti Entry XnM Equiv Catalyst mol % Solvent T t Yield ee enantioselectivity of 89 % ee and [8C] [days] [%][a] [%][b] 40 % yield (Entry 3). The low 2 (R)-TRIP-PA 10 toluene ¢20 1 44 94 (3R,4R) > 30:1 1 (Pin)B[c] yields of all three reactions could 2 (S)-TRIP-PA 10 toluene ¢20 1 64 93 (3S,4S) > 30:1 2 (Pin)B[c] be attributed to a low reaction 2 (S)-TRIP-PA 10 CH2Cl2 ¢20 1 53 64 (3R,4R) 28:1 3 (Pin)B[c] rate as, even after three days, 2 (S)-TRIP-PA 10 THF ¢20 1 61 47 (3R,4R) 24:1 4 (Pin)B[c] the unreacted starting material 2 (R,Ra)-dioxide 2.5 THF ¢40 1 52 15 (3R,4R) 19:1 5 Cl3Si 2 (R,Sa)-dioxide 2.5 THF ¢40 1 68 96 (3S,4S) 19:1 6 Cl3Si was detected. The use of (S)7 (S,S)-transEZ-CrotylMix 1.2 – CH2Cl2 20 1 18 89 (3R,4R) 12:1 [15] TRIP-PA (Figure 2), a chiral Cl ¢10 3 36 80 (3R,4R) 11:1 8 (S,S)-transEZ-CrotylMix 1.1 – CH 2 2 Brønsted acid,[16] to catalyze ad[a] Isolated yields. [b] ee values were determined by Mosher ester method. The configuration assignment was dition of the allylboronic acid pibased on analysis of the corresponding Mosher esters; see reference [21]. [c] (Pin)B = pinacol borane. nacol ester gave rise to 3 in high Chem. Eur. J. 2015, 21, 7408 – 7412
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Communication N,N’-dioxide catalyst (5 mol %) gave the desired product anti-4 with the best enantioselectivity achieved yet (96 % ee), good isolated yield (68 %), and a very good anti/syn ratio of 19:1 (Entry 6). Finally, the reaction was also run with a stoichiometric amount of (S,S)-trans EZ-CrotylMix at two different temperatures; however, the asymmetric induction remained below 90 % ee and only mediocre diastereoselectivities of 11:1 and 12:1 were achieved (Entries 7 and 8). It is worth mentioning that in all cases (except Entries 3 and 4) of the TRIP-PA and N,N’-dioxide catalyzed reactions the (S)-catalysts gave the (4S)products and vice versa, which is in agreement with the previously observed facial selectivity.[15, 17] Additionally, crotylations utilizing cis-crotyl reagents that were supposed to provide predominantly the syn-diastereoisomer were studied (Table 3). Thus, crotylations with the cis-crotylboronic acid pinacol ester catalyzed by (R)- or (S)-TRIP-PA in toluene proceeded with asymmetric inductions of 82 and 80 % ee and anti/syn ratios of 1:14 and 1:19, respectively (Entries 1 and 2). The use of (R,Ra)-N,N’-dioxide catalyst and cis-crotyltrichlorosilane[19] provided syn-4 with a very low asymmetric in-
Scheme 1. Racemic synthesis of pteroenone 1. Reagents and conditions: a) iPr2NH (2 equiv), nBuLi (2 equiv), EtCN (2 equiv), THF, ¢78 8C, 2 h; b) TBSCl (2.1 equiv), imidazole (4 equiv), 4-dimethylaminopyridine (DMAP; 1.1 equiv), DMF, 60 8C, 3 days; c) 1) tBuLi (10 equiv), nPrI (5 equiv), Et2O, ¢78 8C, 0.5 h, 2) NH4Cl (aq.); d) HF (16 equiv), TBAF (20 equiv), THF, 3 days, 0 8C to ca. 45 8C.
oenone, we developed a new and alternative pathway based on condensation of aldehyde 2 with propionitrile (Scheme 1). The straightforward racemic synthesis started with addition Table 3. Crotylations of 2 with cis-crotyl reagents to syn-4 under various conditions. of the a-lithiated propionitrile (prepared by using the freshly prepared lithium diisopropylamide (LDA)) to aldehyde 2 furnishing b-hydroxynitrile 5 as syn/anti Entry XnM Equiv Catalyst mol % Solvent T t Yield ee [8C] [days] [%][a] [%][b] a 1.3:1 mixture of anti/syn diastereomers in 59 % yield. Protec(R)-TRIP-PA 10 toluene ¢20 1 55 82 (3S,4R) 1:14 1 (Pin)B[c] 2 (S)-TRIP-PA 10 toluene ¢20 1 64 80 (3R,4S) 1:19 2 (Pin)B[c] 2 tion of the hydroxyl group fol2 (R,Ra)-dioxide 2.5 THF ¢40 1 31 33 (3R,4S) 1:30 < 3 Cl3Si lowed, giving rise to 6 in 60 % 2 (R,Sa)-dioxide 2.5 THF ¢40 1 49 85 (3R,4S) 1:30 < 4 Cl3Si yield and with a 1.6:1 anti/syn 2 (R,Sa)-dioxide 1.25 THF ¢40 1 33 74 (3R,4S) 1:10 5 Cl3Si ratio. Then, addition of nPrLi, [a] Isolated yields. [b] ee values were determined by Mosher ester method. The configuration assignment was prepared by reaction of nPrI and based on analysis of the corresponding Mosher esters; see reference [21]. [c] (Pin)B = pinacol borane. tBuLi, with subsequent acidic work-up provided, after isolation, the protected b-hydroxyketone 7 in a good yield of 84 % (1.8:1 anti/syn). Finally, deprotection duction (33 % ee; Entry 3). The use of (R,Sa)-N,N’-dioxide catalyst of the TBS group by a mixture of HF/tetra-n-butylammonium was tested at two catalyst loadings (2.5 and 1.25 mol %): the fluoride (TBAF) quantitatively gave a mixture of racemic antihigher loading gave rise to syn-4 with the best enantioselectiv1 (pteroenone) and syn-1. Determination of suitable deprotecity achieved for this reaction (85 % ee) and a anti/syn ratio of tion conditions required a considerable amount of experimen1:30 (Entry 4), whereas, the lower loading provided the prodtal work and the reaction conditions were tuned on a model uct with diminished asymmetric induction (74 % ee; Entry 5). substrate; see the Supporting Information for details. With these results in hand, we decided to proceed with synFinally, the enantioselective synthesis of pteroenone based thesis of pteroenone[20] by enantioselective crotylation. Howevon asymmetric crotylation was initiated (Scheme 2). Enantioseer, prior to the enantioselective synthesis, the racemic synthelective crotylation of 2 with trans-crotylboronic acid pinacol sis of pteroenone was carried out to check feasibility of the ester catalyzed by (S)-TRIP-PA (5 mol %) at ¢30 8C provided chosen synthetic pathway and also to get the racemic stan(3S,4S)-4 with an isolated yield of 86 % and with very good dard of the final compound for evaluation of ee. The initial attempt to synthesize racemic pteroenone was based on the enantioselectivity (> 95 % ee). Protection of the hydroxyl group crotylation reaction and we could successfully prepare the tertby using TBSCl, which provided (3S,4S)-8 in 86 % yield, was folbutyldimethylsilyl (TBS)-protected pteroenone. Moreover, by lowed by Wacker oxidation of the double bond to the correfollowing this synthetic route we could solve a number of synsponding ketone (3R,4S)-9. The oxidation proved to be a rather thetic problems (Wacker oxidation, protection, etc.) that were troublesome step and required a lot of experimental work on crucial for successful development of the enantioselective synthe racemic version to assess the best reaction conditions (see thesis of pteroenone 1 (for details see the Supporting Informathe Supporting Information). The use of PdCl2/CuCl/O2 catalytic tion). Nonetheless, for straightforward access to racemic ptersystem in DMF/H2O furnished (3R,4S)-9 in 30 % overall yield,
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Communication Acknowledgements This work was supported by grants from the Ministry of Education, Youth, and Sports (MSM0021620857) and the Czech Science Foundation (P207/11/0587). The authors also would thank Lach-Ners.r.o. for the generous gift of chemicals as a part of the award given to M.K. Keywords: aldehyde · allylation · asymmetric synthesis · Brønsted acid · Lewis base
Scheme 2. Enantioselective synthesis of (5R,6S)-(+ +)-pteroenone 1. Reagents and conditions: a) trans-crotylboronic acid pinacol ester (1.2 equiv), (S)-TRIPPA (5 mol %), toluene, ¢30 8C, 3 days; b) TBSCl (1.1 equiv), Im (2 equiv), DMAP (10 mol %), DMF, 60 8C, 3 days; c) PdCl2 (25 mol %), CuCl (1.5 equiv), DMF/H2O (4:1), 40 8C, 3 days; d) iPr2NH (2.5 equiv), nBuLi (2.5 equiv), EtI (3.5 equiv), THF, ¢78 to 20 8C, 3 h; e) HF (16 equiv), TBAF (20 equiv), THF, 40 8C, 3 days.
which included one repetition of the reaction with the recovered unreacted starting material (3S,4S)-8. The low yield could be attributed to low conversion at ambient temperature or to the formation of intractable side-products at higher reaction temperatures (40 8C). The side-products probably result from concomitant oxidation of the diene moiety. The propyl chain was introduced by a-lithiation of ketone (3R,4S)-9 followed by reaction with ethyl iodide, which provided (5R,6S)-7 in 32 % isolated yield. Also in this case, the rather low isolated yield of (5R,6S)-7 could be accounted for by the formation of intractable side-products and difficulties associated with purification. Finally, deprotection of the hydroxyl group by the mixture of HF/TBAF as used previously yielded (5R,6S)-(+ +)-pteroenone 1 in 54 % isolated yield and enantioselectivity over 95 %. The spectral and physical characteristics were in agreement with the previously published data.[19] In conclusion, we have shown that enantioselective allylation and crotylation of (2E,4E)-2,4-dimethylhexadienal 2, a representative a,b,g,d-unsaturated aldehyde, which leads to a valuable synthetic building block, can be carried out under various conditions. The best results regarding asymmetric induction, product yields, and catalyst loads were achieved with chiral TRIP-PA and (R,Sa)-N,N’-dioxide catalysts. It should be also noted that the best ee values for the crotylations were obtained with the dioxide catalyst at low catalyst loadings. The developed enantioselective crotylation was applied in the enantioselective synthesis of natural (5R,6S)-(+ +)-pteroenone 1. In addition, the racemic synthesis of pteroenone based on the aldol reaction of a-lithiated propionitrile could serve, in principle, as an inspiration for practical applications of the recently developed enantioselective addition of nitriles to aldehydes catalyzed by a chiral Rh-complex.[22]
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[1] For reviews, see: a) S. E. Denmark, J. Fu, Chem. Rev. 2003, 103, 2763 – 2793; b) M. Yus, J. C. Gonzlez-Gûmez, F. Foubelo, Chem. Rev. 2011, 111, 7774 – 7854; c) M. Yus, J. C. Gonzlez-Gûmez, F. Foubelo, Chem. Rev. 2013, 113, 5595 – 5698. [2] a) S. E. Denmark, C. S. Regens, T. Kobayashi, J. Am. Chem. Soc. 2007, 129, 2774 – 2776; b) S. E. Denmark, T. Kobayashi, C. S. Regens, Tetrahedron 2010, 66, 4745 – 4759. [3] P. Zhang, J. P. Morken, J. Am. Chem. Soc. 2009, 131, 12550 – 12551. [4] A. V. Malkov, M. Barzog, Y. Jewkes, J. Mikusˇek, P. Kocˇovsky´, J. Org. Chem. 2011, 76, 4800 – 4804. [5] M. Georgy, P. Lesot, J.-M. Campagne, J. Org. Chem. 2007, 72, 3543 – 3549. [6] a) V. Prahlad, A.-A. S. El-Ahl, W. A. Donaldson, Tetrahedron: Asymmetry 2000, 11, 3091 – 3102; b) V. Prahlad, W. A. Donaldson, Tetrahedron Lett. 1996, 37, 9169 – 9172. [7] S. Bouzbouz, E. de Lemos, J. Cossy, J. Saez, X. Franck, B. FigadÀre, Tetrahedron Lett. 2004, 45, 2615 – 2617. [8] W. Y. Yoshida, P. J. Bryan, B. J. Baker, J. B. McClintock, J. Org. Chem. 1995, 60, 780 – 782. [9] W. Erb, J. Zhu, Nat. Prod. Rep. 2013, 30, 161 – 174. [10] J. Orlaja, D. G. Nagle, V. L. Hsu, W. H. Gerwick, J. Am. Chem. Soc. 1995, 117, 8281 – 8282. [11] Among other representatives, there are thuggacins (H. Steinmetz, H. Irschik, B. Kunze, H. Reichenbach, G. Hçfle, R. Jansen, Chem. Eur. J. 2007, 13, 5822 – 5832) and several compounds found in a review (J. A. Marco, M. Carda, J. Murga, E. Falomir, Tetrahedron 2007, 63, 2929 – 2958). [12] M. Wadamoto, N. Ozasa, A. Yanagisawa, H. Yamamoto, J. Org. Chem. 2003, 68, 5593 – 5601; N. Ozasa, A. Yanagisawa, H. Yamamoto, J. Org. Chem. 2003, 68, 5593 – 5601. [13] a) G. E. Keck, K. H. Tarbet, L. S. Geraci, J. Am. Chem. Soc. 1993, 115, 8467 – 8468; b) G. E. Keck, D. Krishnamurthy, M. C. Grier, J. Org. Chem. 1993, 58, 6543 – 6544; c) G. E. Keck, L. S. Geraci, Tetrahedron Lett. 1993, 34, 7827 – 7828. [14] A. Yanagisawa, H. Nakashima, A. Ishiba, H. Yamamoto, J. Am. Chem. Soc. 1996, 118, 4723 – 4724. [15] a) P. Jain, J. C. Antilla, J. Am. Chem. Soc. 2010, 132, 11884 – 11886; b) S. Fustero, E. Rodrguez, R. Lzaro, L. Herrera, S. Cataln, P. Barrio, Adv. Synth. Catal. 2013, 355, 1058 – 1064. [16] For other examples of chiral Brønsted acid catalysis, see: a) M. Mahlau, P. Garca-Garca, B. List, Chem. Eur. J. 2012, 18, 16283 – 16287; b) C.-H. Xing, Y.-X. Liao, Y. Zhang, D. Sabarova, M. Bassous, Q.-S. Hu, Eur. J. Org. Chem. 2012, 1115 – 1111; c) D. Kampen, C. M. Reisinger, B. List, Top. Curr. Chem. 2009, 291, 395 – 456; d) M. Terada, Chem. Commun. 2008, 4097 – 4112; e) T. Akiyama, Chem. Rev. 2007, 107, 5744 – 5758. [17] A. Kadlcˇkova, I. Valterov, L. Duchcˇkov, J. Roithov, M. Kotora, Chem. Eur. J. 2010, 16, 9442 – 9445. For our other representative examples regarding Lewis base catalyzed allylation, see: a) T. Cadart, P. Koukal, M. Kotora, Eur. J. Org. Chem. 2014, 7556 – 7560; b) F. Hessler, A. Korotvicˇka, D. Necˇas, I. Valterov, M. Kotora, Eur. J. Org. Chem. 2014, 2543 – 2548; c) K. Vlasˇan, R. Hrdina, I. Valterov, M. Kotora, Eur. J. Org. Chem. 2010, 7040 – 7044; d) A. Kadlcˇkov, R. Hrdina, I. Valterov, M. Kotora, Adv. Synth. Catal. 2009, 351, 1279 – 1283; e) R. Hrdina, M. Dracˇnsky´, I. Valterov, J. Hodacˇov, I. Csarˇov, M. Kotora, Adv. Synth. Catal. 2008, 350, 1449 – 1456. [18] a) H. C. Brown, M. C. Desai, P. K. Jadhav, J. Org. Chem. 1982, 47, 5065 – 5069; b) H. C. Brown, B. Singaram, J. Org. Chem. 1984, 49, 945 – 947;
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Communication c) P. K. Jadhav, K. S. Bhat, P. T. Perumal, H. C. Brown, J. Org. Chem. 1986, 51, 432 – 439. [19] For preparation of trans- and cis-crotyltrichlorosilanes, see: K. Iseki, Y. Kuroki, M. Takahashi, S. Kishimoto, Y. Kobayashi, Tetrahedron 1997, 53, 3513 – 3526 and references therein as well as the Supporting Information. [20] So far only three reports regarding syntheses of pteroenone have been reported: a) Y. Nakamura, H. Kiyota, B. J. Baker, S. Kuwahara, Synlett 2005, 635 – 636; b) H. Kiyota, Biosci. Biotechnol. Biochem. 2006, 70, 317 – 324; c) H. Asao, Y. Nakamura, Y. Furuya, S. Kuwahara, B. J. Baker, H. Kiyota, Helv. Chim. Acta 2010, 93, 1933 – 1944. [21] The absolute configurations of the products obtained from enantioselective allylations were determined by comparison of the 1H NMR
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chemical shifts of the relevant esters with both enantiomers of Mosher acid, which allows stereochemical assignment based on the anisotropic effect induced by the phenyl ring present in the ester moiety on the two different substituents of the alcohol. The ee values were also determined from the corresponding Mosher esters. J. M. Seco, E. QuiÇo, R. Riguera, Chem. Rev. 2004, 104, 17 – 117. [22] D. Sureshkumar, V. Ganesh, N. Kumagai, M. Shibasaki, Chem. Eur. J. 2014, 20, 15723 – 15726.
Received: January 6, 2015 Published online on March 26, 2015
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