Acylimonium ions

Chiral 2-thiazolidinethiones, e.g., 40, prepared from cysteine esters by the reaction with carbon disulfide23, are used for the enantiotopic differentiation of methylene groups (Section C.). Similarly, carbon disulfide reacts with amino alcohols such as (5)-2-amino-J-butanol or (S)-2-amino-3-methyl-1-butanol (valinol) to give the 2-thiazolidinethiones (S)-ETT 42 and (S)-IPTT 4124 which have been used as auxiliaries (after Y-acylation with carboxylic acids and dicyclo-hexylcarbodiimide) in acylimonium ion cyclizations (Section D.I.4.5.). 

Schoemaker, H. E. Speckamp, W. N. "Stereocontrolled Synthesis of Functionalized 1-Azaspirans. Efficient Synthesis of Perhydrohistrionicotoxin" Tetrahedron 9 0, 36, 951-958. For a related approach see Evans, D. A. Thomas, E. W. "A formal Synthesis of c//-Perhydrohistrionicotoxin via a-Acylimonium Ion-Olefin Cyclizations" Tetrahedron Lett. 1979, 411-414. 

The first example of this fragmentation reaction to be described is the conversion of the /V-oxide (63) to the 17-oxa-D-homoandrostane derivative (65 Scheme 12). The crucial step in this transformation involves the participation of the lone pair of electrons on oxygen in the departure of trifluoroacetate ion from the 0-acylimonium salt interm ate (64). 

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