Lithiation α to heteroatoms: eliminative ring fission of heterocycles

Abstract

The behaviour of some heterocycles, having two heteroatoms in a 1,4-relationship, towards lithium di-isopropylamide (LDA) has been examined. The dihydrobenzothiazines (2a–c) and (3b), morpholine (12), benzoxazine (14), oxathiane (16), piperazine (18), tetrahydroquinoxaline (20), 1,4-dithiane (25), and dihydrodioxine (27) undergo ready ‘eliminative ring fission.’ Very marked differences have been observed for unsubstituted and substituted derivatives. Whereas the dibenzoyltetrahydroquinoxaline (20) and the dioxine (27) readily react to give ring-opened products (21) and (30) respectively, substituted derivatives (22), (23), and (32) were found to be completely unreactive.