Diterpenoids synthesis

The intramolecular cyclopropanation of appropriate y,(5-unsaturated a-diazoketones following a stereoselective catalytic reduction of the cyclopropyl ketone group provides a useful approach in diterpenoid synthesis. Some examples of the use of the cyclopropanation-reductive cleavage approach in synthesis are shown in equations 67 and 68l0f103. 

The synthesis of the tetracyclic amide 437 has been reported by Meyer and co-workers (196) in work directed toward a general diterpenoid synthesis. The enantiomer of this amide has been prepared earlier by Tahara and colleagues (197) and converted to atisine, veatchine, and garryine. 

Homoallylic alcohols are also suitable for controlling the introduction of an alkyl chain, as shown by the synthesis of the diterpenoid atractyligenin61, The starting tricyclic alcohol is converted to the selenocarbonate, a suitable radical precursor. Treatment with tributyltin hydride initiates the stereoselective cyclization which affords the bridged lactone system, an intermediate in the diterpenoid synthesis. Due to the low cyclization rate, tributyltin hydride is added slowly to suppress the formation of noncyclized reduction products. 

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