Unusual [8+2] annelation in the reactions of allenic ester/ketone-derived 1,3-dipoles with tropone

All-carbon 1,3-dipoles derived from allenic esters/ketones, by catalytic interaction with triphenylphosphine, undergo an unusual [8 + 2] annelation with tropone, leading to 8-oxa-9-(ethoxycarbonyl/acylalkylidene)bicyclo[5.3.0]deca-1,3,5-trienes. Dipoles derived from allenic ketones as well as an alpha-methyl-substituted allenic ester display high reactivity and selectivity.