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

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.