FACILE SYNTHESIS OF STABLE ANALOGS OF 2-OXOCYCLOBUTANECARBOXYLATES - 2-[(DIPHENYLMETHYLENE)AMINO]CYCLOBUTENECARBOXYLATES, DERIVATIVES, AND REACTIONS

An efficient two-step synthesis of 2-[(diphenylmethylene)amino]cyclobutenecarboxylate (4a) and some analogous derivatives from 2-chloro-2-cyclopropylideneacetates 2, 17, 22, and 25 and nonenolizable ketimines, especially diphenylmethyleneamine (DPMA-H), is described. A likely mechanism for the formation of 4a from the primary Michael adduct 3 of DPMA-H to 2 and its substituted analogues is presented. The unique neighboring group effect of the DPMA moiety to allow formation of an azaspiropentane intermediate and its regioselective rearrangement to cyclobutenamine derivatives is discussed and further exemplified by an extremely facile SET alpha-chlorination. Compound 4a and derivatives undergo a thermal ring-opening reaction to the corresponding butadienes with subsequent formation of 1,3-disubstituted 3,4-dihydroisoquinolines 39. Further transformations of 4a and some derivatives include transesterification, hydrolysis to methyl 2-oxocyclobutanecarboxylates, and addition of N-phenyltriazolinedione.