Trapping of a thiocarbonyl ylide with imidazolethiones, pyrimidinethione, and thioamides

The reactions of 1,1,3,3-tetramethyl-8-thia-5,6-diazaspirol[3.4]oct-5-en-2- one (1a) with imidazole-2-thiones 3 and pyrimidine-2(1H)-thione (6) in CHCl 3 at 40-50 degrees yield 2,2,4,4-tetramethylcyclobutanone dithioacetals of type 4 and 7 respectively, by interception of the intermediate thiocarbonyl ylide 2a (Scheme 2).; product by 1,3-dipolar electrocyclization of 2a. Whe n thioacetamide (8a) and Thiirane 5 is formed as a minor thiobenzamide (8b) are used as trapping reagents. the primary adduct 10 undergoes a spontaneo us cyclization by intramolecular nucleophilic addition of the imino group a t the carbonyl group to yield bicyclic products of type 9. The structure of 9a has been established by X-ray crystallography.