Cycloadditions in syntheses. Part 28. 2-Azabicyclo[2.2.0]hexane-3,5-dione and its derivatives: synthesis and transformation to azetidin-2-ones

2-Azabicyclo[2.2.0]hexane-3,5-dione has been synthesized via photopyridone formation from 4-(dimethyl-t-butylsiloxy)- or 4-alkoxy-2-pyridones followed by mild acid hydrolysis. The title compound and its derivatives react with a variety of nucleophiles (ROH, RR′NH, RSH, or RMgBr) to give azetidin-2-ones having an acetic acid or acylmethyl side-chain at the 4-position, and hence serve as new building blocks for carbapenem nuclei.