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 α,β-unsaturated lactones, the title compound 2 is shown to be a reactive dienophile. Under stannic chloride catalysis, it undergoes Diels–Alder reaction region- and stereoselectively with a variety of dienes at room temperature. The synthesis of (±)-o-mentha-1,3-dien-1→8-olide (20) has been achieved by a two-step sequence using lactone ester 2. Addition of 2 to 1-acetoxy-1,3-butadiene afforded epimeric acetoxy lactones 12 and 13 which were converted to the monoterpenoid lactone 20 by treatment with lithium iodide dihydrate in collidine.