A reactive lactonic dienophile – 3-carbethoxy-5,5-dimethyl-2(5H)-furanone. Application to the synthesis of a monoterpenoid lactone

Abstract
In contrast to the low dienophilicity generally observed for .alpha.,.beta.-unsaturated lactones, the title compound 2 is a reactive dienophile. Under stannic chloride catalysis, it undergoes Diels-Alder reaction regio- and stereoselectively with a variety of dienes at room temperature. The synthesis of (.+-.)-o-mentha-1,3-dien-1 .fwdarw. 8-olide (20) [found in Phascolarctos cinereus urine] was achieved by a 2-step sequence using lactone ester 2. Addition of 2 to 1-acetoxy-1,3-butadiene afforded epimeric acetoxy lactones which were converted to the monoterpenoid lactone 20 by treatment with lithium iodide dihydrate in collidine.

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