Base-promoted cyclization of ortho -hydroxyacetophenones with in situ generated cyclopropenes: diastereoselective access to spirobenzo[ b ]oxepines and related precursors.

An unprecedented [5 + 2] spirocyclization route to obtain a vital class of functionalized spirobenzo[ b ]oxepine-cyclopropanes in good to high yields with excellent diastereoselectivities is reported. This domino reaction proceeds through a regioselective oxa-Michael addition of ortho -hydroxyacetophenones as 1,5-binucleophiles to in situ produced highly reactive cyclopropenes from 2-aroyl-1-chlorocyclopropanecarboxylates triggered by Cs 2 CO 3 and the subsequent intramolecular aldol reaction under heating conditions, enabling the formation of new C-O and C-C bonds for benzo[ b ]oxepine ring synthesis. Moreover, at ambient temperature, the above C-O/C-C bond-forming event takes place preferentially via a [4 + 2] annulation path over a spirocyclization route, leading to substituted fused-cyclopropanes with good diastereoselectivities. Gratifyingly, further alterations of the obtained spirobenzo[ b ]oxepines and tetrahydrocyclopropa[ b ]chromenes afford fascinating classes of 4 H -chromen-4-ones and cyclopenta[ c ]chromenes, respectively, under metal-free conditions.