First General Approach to Cyclohex-3-ene-1,1-bis(phosphonates) by Diels−Alder Cycloaddition of Tetraethyl Vinylidenebis(phosphonate) to 1,3-Dienes

Tetraethyl vinylidenebis(phosphonate) (VBP) reacts smoothly with substituted 1,3-dienes at 90−110 °C without solvent to give the corresponding cyclohex-3-ene-1,1-bis(phosphonates) in good yields (60−85%). With nonsymmetrically substituted dienes, mixtures of regioisomers are obtained, the regioisomeric ratio being exclusively controlled by electronic effects. Danishefsky's diene allows tetraethyl 4-oxocyclohex-2-ene-1,1-bis(phosphonate) to be obtained in an 81% overall yield after the acid-catalyzed hydrolysis of the Diels−Alder cycloadduct. With 2,3-dimethoxy-1,3-butadiene, a mixture of regioisomeric dimethoxycyclohexene-1,1-bis(phosphonates) is formed by the VBP-catalyzed isomerization of the normal Diels−Alder cycloadduct. The mixture converges into tetraethyl 3,4-dimethoxycyclohex-2-ene-1,1-bis(phosphonate) at prolonged reaction times.