Complete control of regioselectivity in the intramolecular [2 + 2] photocycloaddition of 2-alkenyl-3(2H)-furanones by the length of the side chain

The 2-(ω-alkenyl)-substituted 2-methyl-3(2H)-furanones 2a and 2b were prepared from biacetyl (3) in four reaction steps and in overall yields of 20% and 21%, respectively. They underwent a clean intramolecular [2 + 2] photocycloaddition upon irradiation at λ = 350 nm. Whereas compound 2a reacted in the expected manner and yielded 7-oxabicyclo[3.2.1.0^3,6] octane 7 (87% yield), the regioselectivity in the photocycloaddition of compound 2b was completely reversed. The reaction led to compound 8 (92% yield) with the unusual 9-oxabicyclo[4.2.1.0^3,8]nonane skeleton, the structure of which was established by single-crystal X-ray crystallography.