Intramolecular [2+2]-cycloaddition of propargylic 2,3-allenoates for the efficient synthesis of 3-oxabicyclo[4.2.0]octa-1(8),5-dien-4-ones: a dramatic substituent effect

[2+2]-Cycloaddition reactions of propargylic 2,3-allenoates under conventional heating conditions afforded 3-oxabicyclo[4.2.0]octa-1(8),5-dien-4-ones, which contain both the biologically important 5,6-dihydropyran-2-one and cyclobutene units.