New Studies on [2+3] Cycloadditions of Thermally Generated N ‐Isopropyl‐ and N ‐(4‐Methoxyphenyl)‐Substituted Azomethine Ylides

The thermal reaction of 1‐substituted 2,3‐diphenylaziridines 2 with thiobenzophenone ( 6a ) and 9 H ‐fluorene‐9‐thione ( 6b ) led to the corresponding 1,3‐thiazolidines ( Scheme 2 ). Whereas the cis ‐disubstituted aziridines and 6a yielded only trans ‐2,4,5,5‐tetraphenyl‐1,3‐thiazolidines of type 7 , the analogous reaction with 6b gave a mixture of trans ‐ and cis ‐2,4‐diphenyl‐1,3‐thiazolidines 7 and 8 . During chromatography on SiO 2 , the trans ‐configured spiro[9 H ‐fluorene‐9,5′‐[1,3]thiazolidines] 7c and 7d isomerized to the cis ‐isomers. The substituent at N(1) of the aziridine influences the reaction rate significantly, i.e. , the more sterically demanding the substituent the slower the reaction. The reaction of cis ‐2,3‐diphenylaziridines 2 with dimethyl azodicarboxylate ( 9 ) and dimethyl acetylenedicarboxylate ( 11 ) gave the trans ‐cycloadducts 10 and 12 , respectively ( Schemes 3 and 4 ). In the latter case, a partial dehydrogenation led to the corresponding pyrroles. Two stereoisomeric cycloadducts, 15 and 16 , with a trans ‐relationship of the Ph groups were obtained from the reaction with dimethyl fumarate ( 14 ; Scheme 5 ); with dimethyl maleate ( 17 ), the expected cycloadduct 18 together with the 2,3‐dihydropyrrole 19 was obtained ( Scheme 6 ). The structures of the cycloadducts 7b, 8a, 15b , and 16b were established by X‐ray crystallography.