Phosphonium ylide catalysis: a divergent diastereoselective approach to synthesize cyclic ketene acetals [thia(zolidines/zinanes)] from β-ketothioamides and dihaloalkanes

Abstract

Phosphonium ylides are being reported here as a catalyst for the formation of thiazolidines and 1,3-thiazinanes from β-ketothioamides (which act as a three atom N, C, and S synthon) with dihaloalkanes via [3 + 2] and [3 + 3] annulations under metal-free conditions. An N,C,S-centred chemoselective dihaloalkane-controlled cascade process has been identified for the preparation of cyclic N,S-heterocycles (thiazolidines and 1,3-thiazinanes) from identical β-ketothioamides. The reaction proceeds via consecutive sulfur and nitrogen nucleophilic attack of the thioamide on dihaloalkanes enabling the formation of S–C and N–C bonds. The ring size of the skeletally distinct N,S-heterocycles has been efficiently tuned by switching the use of 1,2- and 1,3-dihaloalkanes as α,β- and α,γ-dielectrophiles. It is noteworthy that the products possess Z-stereochemistry with regard to the exocyclic CC double bond at the 2-position of the ring, revealing exclusive diastereoselectivity. Since phosphorus ylides have found limited use as catalysts, control experiments revealed their behaviour as a catalyst, which not only increase the catalyst tool box, but also would contribute to the field of ylide chemistry.