Reaction Between Alkyl Isocyanides and Cyclic 1,3-Diketones: A Convenient Synthesis of Functionalized 4 H-Pyrans

Alkyl isocyanides react with dialkyl acetylendicarboxylates in the presence of CH-acids such as cyclopentane-1,3-dione, cyclohexane-1,3-dione, or 5,5-dimethylcyclohexane-1,3-dione to afford highly functionalized 4H-pyrans in fairly high yields. In the case of reaction between dimethyl acetylenedicarboxylate and 5,5-dimethylcyclohexane-1,3-dione in the presence of cyclohexyl isocyanide or benzyl isocyanide tetrahydro-cyclopentab]pyran derivatives were isolated in addition to the 4H-pyran system. The free energy barrier (96.9kJmol^–1) for restricted rotation around the polarized double bond of the enaminone moiety in dimethyl 2-cyclohexylamino-(4,4-dimethyl-2,6-dioxocyclohexylidene)methyl]but-2-enedioate was determined by dynamic NMR spectroscopy.