A novel [4 + 2] cycloaddition reaction involving Lawesson’s reagent. Structure and specific fragmentations of a new cyclic 1,2-thiaphosphinane-4-one

Abstract

A crude morpholine enamine of acetone treated with Lawesson’s reagent unexpectedly yielded a six-membered thiaphosphinane-4-one. This compound is the first example of a new class of heterocycles. It has been proven that it is formed from 4-methyl-2-morpholino-1,3-pentadiene which is usually present in crude morpholine enamine batches. A mechanism of this regioselective reaction was postulated and a characteristic chair-like conformation of the product was examined in detail. Additionally, some unusual primary fragmentations of the product with the loss of H2S and isobutylene were observed for positive and negative ESI ionization mode, respectively.