Tetraethyl vinylidenebis(phosphonate) (VBP) reacts smoothly with substituted 1,3-dienes at 90-110 degrees 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.
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