Terpenoid butenolides

Several butenolide natural products of terpenoid origin have been synthesized from furan precursors. On the other hand furan natural products are synthesized from butenolide precursors, e.g. ancistrofuran (133) (Scheme 75). The synthesis of two members of the drimane class of sesquiterpenes containing a butenolide ring have been reported. The first synthesis provides a route to racemic cinnamodial, and the second describes the synthesis of optically active confertifolin (134) from the natural product manool. A rather doubtful mechanism is proposed for the most important reaction in this sequence (Scheme 76). ... 

Corey and his co-workers have described a synthesis of the six-ring terpenoid isopulegone (150) from citronellol which proceeds via ene transformation of the intermediate citronellal (149), and Schultz and Godfrey have demonstrated that the butenolide (151) is a versatile annelating agent for the construction of linear tricyclic y-lactone systems (152) found amongst sesquiterpenes (Scheme 29). 

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