π-Facial Stereoselectivity in Diels−Alder Cycloadditions to 1-Oxaspiro[4.5]deca-6,9-dien-8-one. The Strong Directive Effect of Ether Oxygen in a Cross-Conjugated Ketone Setting

The title compound 1, prepared from 1,4-cyclohexanedione monoethylene ketal, was treated with several reactive dienes, including diphenylisobenzofuran and 9,10-dihydro-11,12-dimethylene-9,10-ethanoanthracene. These [4 + 2] cycloadditions proceed with a strong kinetic bias for bonding to the dienophile from the direction syn to the tetrahydrofuranyl oxygen and consequently hold value in stereoselective synthesis.