Chemoselectivity of the Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine

The reactions of 5‐benzylidene‐3‐phenylrhodanine ( 2 ; rhodanine=2‐thioxo‐1,3‐thiazolidin‐4‐one) with diazomethane ( 7a ) and phenyldiazomethane ( 7b ) occurred chemoselectively at the exocyclic CC bond to give the spirocyclopropane derivatives 9 and, in the case of 7a , also the C ‐methylated products 8 ( Scheme 1 ). In contrast, diphenyldiazomethane ( 7c ) reacted exclusively with the CS group leading to the 2‐(diphenylmethylidene)‐1,3‐thiazolidine 11 via [2+3] cycloaddition and a ‘two‐fold extrusion reaction’. Treatment of 8 or 9b with an excess of 7a in refluxing CH 2 Cl 2 and in THF at room temperature in the presence of [Rh 2 (OAc) 4 ], respectively, led to the 1,3‐thiazolidine‐2,4‐diones 15 and 20 , respectively, i.e. , the products of the hydrolysis of the intermediate thiocarbonyl ylide. On the other hand, the reactions with 7b and 7c in boiling toluene yielded the corresponding 2‐methylidene derivatives 16, 21a , and 21b . Finally, the reaction of 11 with 7a occurred exclusively at the electron‐poor CC bond, which is conjugated with the CO group. In addition to the spirocyclopropane 23 , the C ‐methylated 22 was formed as a minor product. The structures of the products ( Z )‐ 8, 9a, 9b, 11 , and 23 were established by X‐ray crystallography.