Ylide compounds aziridine precursors

A major improvement addressing the issue of practicability and safety by avoidance of the direct use of (potentially) explosive diazo compounds was recently reported by Aggarwal and co-workers [82, 83], The direct addition of diazo compounds was replaced by use of suitable precursors which form the desired diazo compound in situ. The Aggarwal group developed this concept for the corresponding sulfur ylide type epoxidation (see Section 6.8) [82], and successfully extended it to aziridination [83]. Starting from the tosylhydrazone salt 66 the diazo compound is formed in situ under conditions (phase-transfer-catalysis at 40 °C) which were found to be compatible with the sulfur ylide type aziridination [82, 83], The concept of this improved method, for which sulfide 67 (Scheme 5.41) is the most efficient catalyst, is shown in Scheme 5.40. 

Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [1320-1325], aziridines [1321,1326,1327] or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the... 

This methodology yields various heterocyclic compounds in reactions using internal perfluoroolefins or systems with conjugated multiple bonds as precursors. Hopefully, this approach will find wide use and new information will permit more profound generalizations. On thermolysis or photolysis aziridines are converted into azomethine ylides, reacting with perfluoro olefins (for example, hexafluoropropylene and perfluorobut-2-ene) to give pyrrole derivatives (autoclave, 160 °C) (76CJC218). 

An intramolecular allyl silane/N-sulfonyl iminium ion cyclization has also been used as a pivotal step in an approach to the tricyclic core of the unique marine alkaloid sarain A [46]. The starting material was aziridine ester 129 (Scheme 25) which was elaborated to amide 130. An important step in the synthetic strategy was thermolysis of 130 to an azomethine ylide, which underwent stereospecific intramolecular 1,3-dipolar cycloaddition with the Z-alkene to produce bicyclic lactam 131 [47]. This compound was then elaborated into allyl silane 132. It was then possible to replace the lactam N-benzyl functionality with a tosyl moiety, leading to 133, and subsequent reduction of the carbonyl group afforded the desired cyclization precursor a-hydroxy sulfonamide 134. Exposure of 134 to ferric chloride promoted cyclization to a single stereoisomeric tricyclic amino alkene 136 having the requisite sarain A nucleus. It is believed that the intermediate N-sulfonyl iminium ion cyclizes via the conformation shown in 135. 

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