Dicarboxylates of 3‐Methylidene‐β‐lactams: Addition Reactions to the Exocyclic Double Bond, Formation of Spiro‐β‐lactams, and Reductive Ring Opening by Hydrazines

O‐, S‐, and N‐Nucleophiles are added to the exocyclic double bond of the title compounds 1. The addition of O‐ or S‐nucleophiles yields stable products (Scheme 1), while addition of N‐nucleophiles results in thermally labile compounds (Scheme 2). The reaction is studied by spectroscopic methods. From hydrazine adducts, a spiro[azetidine‐3,3′‐pyrazolidine] 7 is obtained, and the addition products of methyl‐ and benzylhydrazine rearrange to pyrazol‐4‐carboxylates 6. Furthermore, the exocyclic double bond is used for the formation of spiro‐β‐lactams either by cyclopropane formation or by Diels‐Alder reactions (Scheme 4). The steric course of all reactions is studied, and it is shown that all reactions with the double bond occur from the side opposite to the bulkier substitutent at C(4) of the β‐lactam ring.