A new synthetic approach to bruceantin via an intramolecular Diels–Alder reaction: stereoselective construction of the pentacyclic model system

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Abstract
An efficient synthesis of the pentacyclic system (2) as a model for bruceantin (1) is described. The key feature of the synthesis was a stereoselective intramolecular Diels–Alder reaction of the benzocyclobutene derivatives (15) and (16), which were conveniently derived from the aldehyde (6) in 7 steps. The tetracyclic cycloadducts (17) and (19) were converted, via two separate routes, into the alcohol (26), which was then transformed stereoselectively to the pentacyclic lactone (2).