Vinylphosphonium salts, ring

Vinylphosphonium salts, ring closures with — 19, 904 2-Vinylpyridine s. Pyridylethylation... 

The photocycloaddition of arylazirines with a variety of multiple bonds proceeds in high yield and provides a convenient route for the synthesis of five-membered heterocyclic rings. Some of the dipolarophiles include azodicarboxylates, acid chlorides, vinylphosphonium salts and p-quinones. 

The reaction of 4-(dichloromethylene)-2-phenyl-5(477)-oxazolone 400 and tri-phenylphosphine affords the vinylphosphonium salt 401, from which Al-benzoyl (triphenylphosphoranylidene)ketenimine 403 is obtained by hydrolytic ring opening of 401 and subsequent treatment of 402 with triethylamine (Scheme 7.129). 

New cyclization reactions leading to thiophens have been discovered. In particular, the reaction between a-mercapto-ketones and vinylphosphonium salts appears very promising. Many useful ring-closures leading to thiophens have been extended to selenophens and tellurophens. 

Synthesis of Thiophens by Ring-closure Reactions.—A new general method for the preparation of alkylated 2,5-dihydrothiophens (3), through the reaction of o -mercapto-ketones (1) or a-mercapto-aldehydes with vinylphosphonium salts... 

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