Lewis Acid-Catalyzed Intramolecular [2 + 2] Cycloaddition of alpha-Ester-Substituted Conjugated Dienyl- and Trienylphosphonates. New Synthesis of Functionalized Cyclic Terpenoids.

alpha-Ester-substituted 1,3-dienylphosphonates 7 and 8, prepared by the Knoevenagel condensation, underwent intramolecular [2 + 2] cycloaddition in the presence of Lewis acid to form bicyclo[4.2.0] (26-57% yield) and bicyclo[3.2.0]skeleton (14-38% yield), respectively. Similar treatment of homologous 1,3-dienylphosphonate 11 and 1,3,5-trienylphosphonate 12 resulted in the formation of ionone derivatives (30-94% yield). The intramolecular cycloaddition reaction was applicable to several conjugated dienes bearing an ester group.