ACCESS TO PROTECTED 2-ALKYLIDENE 1,3-DIONES BY MODIFIED KNOEVENAGEL REACTION IN THE PRESENCE OF THIOPHENOL - A NEW APPROACH TO SPIROCYCLOPENTANOL CONSTRUCTION

A general procedure is described for trapping the products of Knoevenagel condensation involving 1,3-diketones and aliphatic aldehydes. Simple stirring of a three-component mixture consisting of each reactant and thiophenol in dichloromethane containing silica gel leads to products in which the 2-alkylidene 1,3-dione has been intercepted to give a (most often) crystalline Michael adduct. The yields are usually quite acceptable, especially if the beta-dicarbonyl compound is cyclic. Oxidation of these adducts with sodium periodate regenerates the conjugated enedione, which reacts rapidly with air to give a cyclic peroxide unless protected from the atmosphere. When a monoprotected succinaldehyde is utilized as starting material, hydrolysis of the resultant adduct in aqueous acid results in intramolecular aldolization to give a spirocyclic cyclopentanol.