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Chiral sulfur ylides for the synthesis of bengamide E …
Intrigued by this report, we embarked upon an aziridination study using our technology involving the decarboxylation of carboxymethylsulfonium betaine functionality to generate a sulfur ylide in situ. One of our goals was to address the question if alternative reagents and solvent systems could be used without significantly compromising the levels of conversion while maintaining high levels of stereoselectivity. For comparative purposes, we chose to test a series of aryl substituted chiral non-racemic tert-butylsulfinyl imines against our sulfonium salt 1. As we found in our epoxidation studies, the incorporation of S-aryl functionality offers added stability of the sulfur ylide (vide infra) and thus we hypothesized that this would as well result in high levels of conversion when subjected to imine functionality. Except for this single modification, use of imine as opposed to aldehyde functionality, all other reaction conditions were kept the same as in our previously optimized work with aldehyde π-acceptors. A total of seven sulfinimine derivatives were surveyed. The data from the study is presented below in .
Reaction of sulfur ylide with a series of aryl substituted chiral non-racemic sulfinyl imines afforded the corresponding aziridines in high yield and good stereoselection. The sulfur ylides were generated by the thermally induced decarboxylation of carboxymethylsulfonium betaines. A drop in the diastereomeric ratio was observed when going from electron deficient to electron releasing aryl substituted imines. Sulfonium methylidene aziridinations involving the decarboxylation of carboxymethylsulfonium betaine functionality compliments existing technologies with the advantages of the reaction protocol, levels of conversion and scope.
Data on the use of sulfonium ylides in the synthesis of ..
Stockman and co-workers have several reports on the Corey-Chaykovsky reaction of chiral sulfinyl imines (). Using methylidene transfer agents derived from trimethyl sulfonium iodide, a series of aliphatic, aromatic, and heterocyclic tert-butylsulfinyl aziridines were prepared. Reaction times ranging between three and ten hours using NaH as base, DMSO as solvent at 20°C lead to the highest levels of conversion and diastereoselectivity.
The results for our decarboxylation approach show extremely high levels of conversions while operating a much warmer reaction conditions when compared to aziridination protocols found in the literature. It is interesting that a comparison of reaction of sulfonium salt 1 with trimethylsulfonium iodide resulted in nearly identical levels of diastereocontrol. To explain the above results we propose that at high temperatures, the decarboxylation rate is high providing relatively high levels of ylide that can be trapped by suitably modified π-acceptors. Attempts to react sulfonium salt 1 with unactivated imines resulted in recovery of the starting imine. As we noted in prior reports, as the electrophilicity of the carbonyl compound decreases, the forward rate of reaction slows down and fails to compete effectively with ylide fragmentation and other side reactions. These observations and the data presented herein documents the clear relationship between electronic control and diastereoselectivity. Research in our group continues in the areas of 1) understanding the electrostatic relationships of this process, 2) increasing the scope of substrate using modified sulfonium salts, and 3) the preparation and use of a chiral, non-racemic sulfide promoter. The results from these studies will be reported in due course.
by means of the use of chiral sulfur ylides.
Methylidene transfer to imine functionality activated and directed with a tert-butyl sulfinyl moiety using decarboxylation of arylcarboxymethylsulfonium betaines has been achieved. While extremely high levels of imine conversion were observed, the levels of diastereocontrol are at best considered to be moderate. The levels of conversion were significant especially when working with electron releasing aryl imines functionality. While higher levels of diastereocontrol could be achieved when operating at lower reaction temperatures, lower temperatures resulted in lower levels of conversion given that the protocol has as its key step the thermal decarboxylation of carboxymethyl betaine functionality. Notwithstanding the levels of diastereocontrol nor the thermal requirements when considering conversion, the ease by which the sulfur ylide is generated, the scope and levels of conversion obtained are, together, the key advantages of this method over other existing technologies.
Nucleophilic alkylidene transfer agents have been shown to be extremely useful for the introduction of functionality into organic molecules. Once generated, they can and do react with a large variety of π-acceptors. A representative list of processes involving a π-acceptor would include formation of oxiranes from aldehydes and ketones, aziridines from imines, and cyclopropanes from electron deficient olefins. The direct and stereospecific assembly of small ring carbo- and heterocyclic building blocks represents a very attractive synthetic transformation, especially when considering the assembly of materials of biological and medicinal significance., Previous communications from our lab reported on a novel protocol for the preparation of nucleophilic alkylidene transfer agents. The protocol had as its key step the decarboxylation of carboxymethyl betaine functionality which generated the requisite sulfur ylides in situ (). Sulfur ylide intermediate ii is best described as a semi-stabilized ylide when compared to thioacetate scaffolds consisting of not alkyl aryl substitution but alkyl alkyl substitution on sulfur. While traditional points of stabilization would focus on the alkylidene carbon, attachments onto the sulfur moiety and even solvation of the intermediate itself have also been shown to improve to the half-lives of sulfur ylides. When ylide ii is used in excess quantities, clean conversion of aldehyde to oxirane functionality is observed. The ease by which the sulfur ylide is generated from sulfonium salt 1 is the key advantage of this method over other existing technologies. That is, the generation of the ylides by decarboxylation has distinct advantages over more traditional methods. First, the process does not require the use of strong and often pyrophoric bases such as butyl lithium or sodium hydride. Second, the method does not require expensive metal catalysts. Third this approach is compatible with ecologically benign solvent systems. And finally, fourth, through fine-tuning of the electronics of the betaine, a range of activity and lifetime of methylidene transfer agent can be “dialled in.”
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Chemical Reactivity - Michigan State University
Abstract: The ring opening of NH-aziridines with thiols was found to proceed in good yield at room temperature in the presence of an ionic liquid---butyl-3-methylimidazolium chloride (BMIM chloride). This mild methodology was applied to the synthesis of a camphor-derived chiral aminosulfide. The sulfide was used to generate a sulfur ylide, which effected an asymmetric epoxidation of benzaldehyde (e.r. 85:15, trans:cis 90:10, 87% yield). The amino group enabled easy recovery of the sulfide (98% yield) after the reaction by a simple acid/base extraction.
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