Synthesis of new thiazolium betaines and the ring expansion reaction via 1,4‐dipolar cycloaddition

Abstract

AbstractThe 1,4‐dipolar cycloaddition of 3‐phenyl‐7‐[N‐pheny(carbamoyl)]‐5,6‐dihydroimidazo[2,1‐b]thia‐zolium betaine (7d) with a series of aliphatic alkylating agents such as ethyl bromoacetate, α‐chloroacetone, and ethyl 4‐chloroacetoacetate gave a variety of new ring‐expanded cycloadducts 10a‐c instead of ring transformation compounds 9. This result was derived from the difference of reactivity between N‐ and S‐alkylations of thiazolium betaines 7a,d. The advantage of our method is to prepare the triheterocyclic compounds 10a‐c of complicated structure using reactive thiazolium betaine 7d in a one‐pot without isolation of intermediates. Treatment of N‐bridged thiazolo compounds 1a‐c with benzoyl isothiocyanate led to new thiazolium betaines 2a‐c, which were reacted with methyl iodide to afford the S‐alkylated quarternary ammonium salts 3a‐c. Synthesis of new 2‐iminothiazolinium betaines 5a,b also was carried out.