Tosylaziridinations , asymmetric

Solladie-Cavallo has recently reported a two-step asymmetric synthesis of dis-ubstituted N-tosylaziridines from (R,R,R,Ss)-(-)-sulfonium salt 2 (derived from Eliel s oxathiane see Section 1.2.1.1) and N-tosyl imines with use of phosphazine base (EtP2) to generate the ylide (Scheme 1.42) [67], Although the diastereoselectiv-ity was highly substrate-dependent, the enantioselectivities obtained were very high (98.7-99.9%). The chiral auxiliary, although used in stoichiometric quantities, could be isolated and reused, but the practicality and scope of this procedure is limited by the use of the strong - as well as expensive and sensitive - phospha-zene base. 

A parallel development during this period was the identification of chiral bis(oxazolines) and bis(benzylidene)diaminocyclohexanes as useful ligands for copper-catalyzed, asymmetric tosylaziridination reactions with Phi = NTs (Scheme 64) [174,175]. Evidence for the likely formation of copper(III)-nitrene intermediates,cCu(III) = NTs , in such reactions was also reported [176]. 

Chiral (E)-crotylsilanes have been utilized with Phi = NTs for Cu(I) - catalyzed syntheses of olefinic dipeptide isosteres, examples of which are shown in Scheme 70 [191]. In this case, tosylamidation occurs with allylic inversion,probably via asymmetric tosylaziridination of the C,C-double bond. The diastereos-electivity of product formation is high (>30 1) and appears to be strongly influenced by the hydroxyl group in the starting compounds. 

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