Heteroatomic chains and their products of cyclisation. IV. t ‐butyl‐2‐phthalimido‐2‐(3,6‐dihydro‐1,3‐2 H ‐thiazine‐2‐yliden)‐acetates substituted in position 5 by a functional group

Substituted N ′‐thioacetylformamidines, readily enthiolized, exist in fact in the form of zwitterions 8 with the transfer of the mobile hydrogen on the nitrogen N ′. These zwitterions under (4 + 2) cyclocondensation with dienophiles. The dihydrothiazines formed ( 10 ) are possible interesting intermediates in the synthesis of cephems and cephalosporin analogs.