Novel Michael-Wittig reactions of methyl 3-oxo-4-(triphenylarsoranylidene)butanoate and substituted 2H-pyran-5-carboxylates; The synthesis of highly functionalised 2-cyclohexenonedicarboxylates

Novel Michael-Wittig condensations of methyl 3-oxo-4-(triphenylarsoranylidene)-butanoate 6 and substituted 2,2-dimethyl-2H-pyran-5-carboxylates 5a to 5e gave substituted 4-alkyl-6-oxo-4-cyclohexene-1,3-dicarboxylates 8a to 8e in a mixture of three keto diastereomers and two enol diastereomers. Apart from substituted 4-alkyl-6-oxo-4-cyclohexene-1,3-dicarboxylates 8f to 8g, less hindered substituted 2-methyl-2H-pyran-5-carboxylates 5f and 5g gave the substituted tetrahydrobenzofurans 9f and 9g. 4-Halomethyl-6-oxo-4-cyclohexene-1,3-dicarboxylates 8 were mostly in the enol diastereomeric form. A plausible mechanism is given for the unique formation for these products.