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Synthesis of cyclic alpha-diazo-beta-keto sulfoxides in batch and continuous flow Patrick G. McCaw, Naomi M. Buckley, Kevin S. Eccles, Simon E. Lawrence, Anita R. Maguire, and Stuart G. Collins J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b00172 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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Synthesis of cyclic α-diazo-β-keto sulfoxides in batch and continuous flow Patrick G. McCaw,a Naomi M. Buckley,a Kevin S. Eccles,a Simon E. Lawrence,a Anita R. Maguire,*b Stuart G. Collins.*a a

Department of Chemistry, Analytical and Biological Chemistry Research Facility, Synthesis and Solid State

Pharmaceutical Centre, University College Cork, Ireland. E-mail: [email protected] b

Department of Chemistry and School of Pharmacy, Analytical and Biological Chemistry Research Facility,

Synthesis and Solid State Pharmaceutical Centre, University College Cork, Ireland. Fax: +353(0)214274097; Tel: +353(0)214901694; E-mail: [email protected]

Abstract

Diazo transfer to β–keto sulfoxides to form stable isolable α-diazo-β-keto sulfoxides has been achieved for the first time. Both monocyclic and benzofused ketone derived β-keto sulfoxides were successfully explored as substrates for diazo transfer. Use of continuous flow leads to isolation of the desired compounds in enhanced yields relative to standard batch conditions, with short reaction times, increased safety profile and potential to scale up.

Keywords: α-diazo-β-keto-sulfoxide, diazo transfer, continuous flow, Amberlyst A21.

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1. Introduction

α-Diazocarbonyl compounds, first reported in 1883 by Curtius1 are a highly reactive class of compounds with a wide ranging spectrum of reactivity and broad uses in synthesis. Many reviews on the versatility and reactivity of these diazo compounds in organic synthesis exist, detailing among other transformations, C-H insertions, cyclopropanations, X-H insertions, dimerization and ylide formation.2-6 Successful generation of α-diazocarbonyl compounds via the Regitz methodology requires the use of a diazo transfer reagent. These reagents are renowned for their shock sensitivity, explosive nature and thermal decomposition properties 3,7,8

limiting potential use of diazo chemistry in large batch applications. Currently there is great interest in

continuous flow processing for handling diazo compounds and the associated increased safety profile. 9-11 Our research group has recently reported the ability to generate, use, and subsequently quench the explosive diazo transfer reagent tosyl azide in situ using continuous flow processing.12 This approach limits the amounts of hazardous compounds formed at any point in time, and is extremely advantageous to making diazo chemistry more acceptable for broader use.12-15 Additionally, by taking advantage of the process control enabled in continuous flow we have established a high yielding diazo transfer methodology for the synthesis of lactone derived α-diazosulfoxides using the Regitz diazo transfer methodology and the relatively safe diazo transfer reagent dodecylbenzenesulfonyl azide (DBSA) (Scheme 1). 16-19 Scheme 1: Solid supported base for the synthesis of α-diazosulfoxides in continuous flow.

R

O

R

S O

R

O

O

R

S O

N2

15

O

CH3CN

DBSA (2 eq) 0.039 M - 0.06M

Stable, isolable α-diazo-β-oxo sulfoxides have considerable synthetic potential, especially providing a mild and effective route to α-oxo sulfines. For α-diazocarbonyl compounds an intermediate carbene or carbenoid ylide has multiple possible reaction pathways, including dimerization, aromatic addition and Wolff rearrangement.2 The first attempts at generating α-diazosulfoxides were reported by Hodson and Holt in 1968 and did not lead to isolable compounds.20 This was followed by the successful generation of a series of cephalosporins containing α-diazosulfoxides by Rosati21 based on an earlier report by Campbell.22,23 The Maguire group rationalized that the unprecedented stability of these α-diazosulfoxides was the decreased availability of the sulfinyl lone pair for donation, facilitated by reduced conformational ACS Paragon Plus Environment

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mobility in the system. This led to a report on the design and synthesis of a range of stable lactone and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lactam derived α-diazosulfoxides for the first time (Figure 1).17

Figure 1: Structure of stable, isolable lactone and lactam derived α-diazosulfoxides.

17

On generation of the carbene from the α-diazosulfoxide, the predominant transformation observed is the hetero-Wolff rearrangement to an α-oxo sulfine (Scheme 2).17,24,25 The synthesis and synthetic versatility of sulfines has been recently reviewed in the literature. 26,27 Scheme 2: Generation of α-oxo sulfine from α-diazo-β-oxo sulfoxide via hetero-Wolff rearrangement.

The targets in this work were a newly designed class of novel α-diazosulfoxide derivatives containing αdiazoketone functionality as opposed to the diazolactone and diazolactam derivatives previously described (Scheme 3).17 The properties, stability and reactivity of α-diazoketones are distinctly different to those of α-diazoesters and diazo compounds derived from carboxylic acids in general. Typically, α-diazoketones are more labile than α-diazoesters, therefore opening the possibility of different reactivity and more diverse synthetic applications.4 Some recent examples of this complex reactivity of α-diazoketones include their highly regioselective cross-coupling with allylboronic acids under copper catalyzed conditions,28 and the rhodium catalyzed annulation of benzamides29 and benzylamine30 with diazoketones to form highly functionalized isoquinolinones and isoquinolines respectively.

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Herein, we report the successful design, synthesis and isolation of novel α-diazo-β-keto sulfoxides – both benzofused and monocyclic, and how the synthesis of this novel class of compounds is suited exceptionally well to generation in a continuous flow manner utilizing the Regitz diazo transfer methodology. Scheme 3: Comparison of previous work and this work.

Previous work:17,21-23,31

This work:

2. Results and Discussion. A preliminary investigation of diazo transfer to acyclic β-keto sulfoxides did not lead to stable isolable diazo derivatives; accordingly use of a cyclic system is necessary to reduce the conformational mobility around the sulfinyl lone pair and provide stability to the α-diazosulfoxide moiety.

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Based on our previous

experiences with the lactone and lactam derivatives a set of benzofused bicyclic (1 - 4) and monocyclic αdiazo-β-keto sulfoxides (5 - 7) were designed as novel synthetic targets (Figure 2), which have the potential to possess interesting reactivity. We envisaged the benzofused bicyclic compounds to be more rigid, and therefore more stable than the monocyclic systems.

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Figure 2 : The novel α-diazo-β-keto-sulfoxides designed as target molecules.

2.1 Synthesis of sulfides. The first step towards these novel α-diazosulfoxides is the synthesis of the cyclic sulfides. The β-keto sulfide 11 was prepared using a modified procedure from the literature, as described by Akkurt (Scheme 4).32

Scheme 4: Synthesis of the unsubstituted benzofused β-keto sulfide 11.

A similar cyclodehydration procedure has recently been described by Aitken for the synthesis of the cyclic sulfide 11 where a combination of phosphorus pentoxide and celite® are used to induce the transformation from the carboxylic acid 9.33 The other benzofused sulfides 18-20 were synthesized using a procedure described in the literature by Ramadas .34 The final step in the synthesis of the sulfides 18-20 is a direct cyclization in a cyclodehydration step, which involved an intramolecular Friedel-Crafts reaction of the acid directly to the sulfide (Scheme 5).

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Scheme 5: Cyclisation to the sulfides 18 - 20.

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HS R

O

CO2Me

K 2CO3 (2 eq) Cl

O R

MeOH 0°C - r.t., 1 h

AcOH/H 2O (50:50) O

S

∆, 20 h

OH

R

∆, Toluene, 5 h

S

79 - 81% 15 R = 2-Me 16 R = 4-Me 17 R = Naph

94 - 98% 1 2 R = 2-Me 13 R = 4-Me 14 R = Naph

O

O

O

S

S

S

18 31%

P2O 5 (5 eq)

O

R

S

19 26%

20 31%

The yields of these sulfides are not optimized; despite the harsh reaction conditions required for the transformation, the cyclisation step provided us with sufficient material for this synthetic study. These sulfides 18 - 20 were found to be relatively unstable and short lived at room temperature and were stored in the dark, in the freezer to successfully prevent decomposition to unidentified, highly coloured, intractable material. The sulfides were typically used within 24 hours of purification.

Synthesis of monocyclic β-keto-sulfides

Moving on to the monocyclic sulfides, as monocyclic lactone derived α-diazosulfoxides have been successfully prepared,17 we aimed to investigate whether the monocyclic α-diazo-β-keto sulfoxides would also be stable and isolable. The first target was the monocyclic sulfide 23 which was synthesized using a modification of a procedure previously reported by Kamenka (Scheme 6).35 Scheme 6 : Generation of the monocyclic sulfide 23 through cyclisation of the diester 21 followed by hydrolysis and decarboxylation of keto enol mixture 22

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The first step towards the sulfide 23 is generation of the diester 21, followed by treatment with potassium 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

t-butoxide in a Dieckmann condensation. Following hydrolysis of the keto enol mixture 22, and thermal decarboxylation, the sulfide 23 was obtained as a yellow oil in 50% yield after chromatography. With success in preparing the unsubstituted monocyclic sulfide 23, the next targets were alkyl substituted analogues 30 and 31, to investigate the impact of steric and electronic effects on both sulfoxidation and diazo transfer. The methyl substituted sulfide 30 is mentioned in the literature but the synthesis and characterization is not fully described.36 The same procedure, used for the synthesis of diester 21 above, is used for the synthesis of the substituted diesters 26 and 27 and cyclisation to the sulfides 30 and 31. The starting secondary alkyl chlorides 24 and 25 that were needed were not commercially available but were obtained through ring opening γ-lactones with thionyl chloride (Scheme 7). Scheme 7: Synthesis of the monocyclic sulfides 30 and 31.

2.2 Synthesis of sulfoxides. Having successfully obtained the sulfides using the procedures detailed above, exploration of the optimum conditions for oxidation to the corresponding novel β-keto sulfoxides was investigated. Using two oxidation methods, Oxone® in acetone or sodium periodate in methanol a range of novel monocyclic and benzofused β-keto sulfoxides were synthesized. The novel sulfoxides 32, 36 - 38 were each synthesized using sodium periodate as the oxidant following a standard procedure while sulfides 18 - 20 were oxidized to the corresponding sulfoxides 33 - 35 using Oxone® as oxidant (Figure 3). The exceptionally polar compounds 32 - 38 were obtained in good yield as white crystalline solids and did not require further purification after aqueous work up; interestingly there was no evidence for the formation of the sulfones at any point in this study. As with the lactone derived sulfoxides,17 the oxidant can approach the monocyclic sulfides 30 and 31 from either face with essentially equal ease, and in practise for sulfoxides 37 ACS Paragon Plus Environment

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and 38, two diastereomers were formed with no diastereoselectivity observed. The novel sulfoxide series 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

32 - 38 possess a similar stability profile as the sulfides and to prevent thermal or photochemical decomposition, they are kept in the freezer after preparation and used within 24 hours.

Figure 3: Oxidation of β-keto sulfides to novel β-keto sulfoxides.

2.3 Synthesis of α-diazosulfoxides in batch.

Diazo transfer to the sulfoxides was carried out using the Regitz diazo transfer methodology.37 Previously, the optimum conditions for diazo transfer to sulfoxides have been reported in the literature with tosyl azide being the preferred diazo transfer reagent, triethylamine as base and acetonitrile as solvent.17 Multiple reports exist in the literature on the safe generation and use of diazo transfer reagents,3,7,8,18,38,39 and although tosyl azide is known to be shock sensitive and highly explosive it can be used effectively when proper precautions are taken - such as the addition of the diazo transfer to the reaction at 0°C. Diazo transfer to the sulfoxide substrates 32 - 38 is generally complete in 9 hours or less, and the 1H NMR spectrum of the crude material shows the disappearance of the SOCH2 signal and is accompanied by the appearance of the diazo stretch between 2111 cm -1 and 2120 cm-1 in the infrared spectrum. The novel αdiazosulfoxide products 1 – 4 were isolated in moderate yields after purification by column chromatography (Figure 4). Interestingly, the naphthalene derived α-diazosulfoxide 4 was not successfully isolated as a pure compound after chromatography due to the presence of inseparable impurities caused by product decomposition, however the indicative signals were present in the 1H NMR spectrum of the crude material. Additionally the monocyclic α-diazosulfoxides 5-7 were challenging to isolate as pure ACS Paragon Plus Environment

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compounds but were present in low yields (

Synthesis of Cyclic α-Diazo-β-keto Sulfoxides in Batch and Continuous Flow.

Diazo transfer to β-keto sulfoxides to form stable isolable α-diazo-β-keto sulfoxides has been achieved for the first time. Both monocyclic and benzof...
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