Mercaptoacetic acid 5,6-dihydro

A method involving the facile addition of mercaptoacetic acid across the double bond of an azomethine for the one-step preparation of 4-thiazolidinones has been used in the synthesis of thiazolo[2,3-c][l,4]thiazine-3(2//)-one. Thus, 2f/-3,4-dihydro-1,4-thiazines (138) react with alkyl mercaptoacetic acid at 100°C under nitrogen to give thiazolo[2,3-c][l,4]thiazin-3(2//)-ones (139) Similarly, 7/f-hexahydro-l,4-benzothia-zine[138, RR = (CH2)4] condenses with mercaptoacetic acid to yield 139 [RR = (CH2)4, R = R = H] (64M1335) (Scheme 30). 

Two different routes to 5,7-dimethoxy-6,9-dimethyl-l,2-dihydro-3H-pyrrolo[l,2-a]indol-3-one (137) were developed by Vice and co-workers [48, 49]. The first route (Scheme 19) was based on addition of a methylthio group at C-3 of 5,7-dimethoxy-2,3,6-trimethylindole (134) using methylsulfenyl chloride [48]. Treatment of the product (135) with lithium diethylamide and methyl iodoacetate gave ester 136. Reduction with mercaptoacetic acid followed by base-catalyzed cyclization then gave 137. 

Diimino-thietane-2-carboxylic acid derivatives has been detected as an intermediate in synthesis of 4-amino-5-thioxo-l,5-dihydro-pyrrol-2-ones from ethyl mercaptoacetate and oxalodiimidoyl dichloride <2006JOC2332> (Scheme 29). 

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