Ketene-S,S,-acetals-VI: synthesis of 3,3-bis-(methylthio)-2-propene-2-alkyl-1-aryl-1-ones and their reaction with guanidine: a novel route for pyrimidine synthesis

Abstract

The α-ketoketene-S,S,-acetals 2a-h have been synthesized from the appropriate ketones 1a-f in good yields by extending the established method. The acetals 2a-d react with guanidine in the presence of sodium ethoxide to give pyrimidines 4a-d in 35-59% overall yields (Scheme 1). Treatment of 2e with guanidine similarly gave 5a in 23% yield. However, the reaction of 2e with guanidine in the presence of sodium n-propoxide gave a mixture of three products from which 5a was isolated as major product (20%) with 6a (7%) and 7a (5%). Under similar reaction conditions, 2f gave 5b, 6b and 7b in identical yields. Treatment of 2g-h with guanidine in the presence of sodium n-propoxide gave only 8a (28%) and 8b (23%) respectively. The formation of these pyrimidines involves base induced 1,3-proton migration in 2a-h to give intermediate olefins 10a-h.