Olivanic acid analogues. Part 4. Cycloaddition reactions of p-nitrobenzyl 7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate, and synthesis of the 8-oxo-1-azatricyclo[4.2.0.02,4]octane-2-carboxylate system

Abstract
p-Nitrobenzyl 7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (1) participated in Diels–Alder and 1,3-dipolar cycloaddition reactions, yielding a range of novel polycyclic azetidinone structures. The two 8-oxo-1-azatricyclo [4.2.0.02,4]octane-2-carboxylates (21) and (24) were prepared, and their cyclopropane stereochemistries assigned using nuclear Overhauser difference spectroscopy. Sodium (1′RS,2SR,4RS,6RS,7SR)-7-(1-hydroxyethyl)-8-oxo-l-azatricyclo[4.2.0.0.2,4]octane-2-carboxylate (37) was synthesised from compound (1)via p-nitrobenzyl (1′RS,5RS,6SR)-6-[1-(p-nitrobenzyloxycarbonyloxy)ethyl]-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (32).