Studies in the cycloproparene series: chemistry of 1-acyl-1H-cyclopropa[b]naphthalenes and synthesis of cyclopropa[b]naphthalenylidene enol ethers

The 1H-cyclopropa[b]naphthalenyl anion reacts with N,N-dimethylamides to provide C-1 acyl derivatives in good yield in what are rare examples of mono-substitution in the cycloproparene series. The availability of these acylcycloproparenes provides for subsequent benzylic proton abstraction and enolate ion interception to give O-silyl and O-phosphinate derivatives. This reaction sequence leads to the first cyclopropa[b]naphthalenylidene enol ethers and provides the only example of exocyclic alkene formation taking place in one step from the benzylic centre. Protonation of the acyl derivatives by mineral acid triggers three-membered ring opening to 2,3-disubstituted naphthalenes rather than 2-monosubstituted analogues that typically arise from protonation at the aromatic ring. Upon thermolysis ring expansion to a naphthofuran occurs.