Pentacyclic furanosteroids: the synthesis of potential kinase inhibitors related to viridin and wortmannolone.

A regiocontrolled intermolecular Diels-Alder reaction of an o-benzoquinone followed by an intramolecular nitrile oxide cyclization is employed to prepare the BCD fragment of viridin. The AE segment is attached to it by means of an intramolecular Diels-Alder reaction of an o-benzoquinone monoketal generated in situ from tricycle 15 and 5-trimethylsilyl-2E,4E-pentadienol 20. The silyl substituent at C-1 of the pentacyclic product directs the dihydroxylation of the C2-C3 double bond to its beta-face. Various transformations of the 1alpha-trimethylsilyl-2beta,3beta-dihydroxy pentacycle into several others with oxygen substituents in ring A are described. One of these products 40 possesses the same structure and relative stereochemistry in rings A, B, and E as that of the natural product wortmannolone 3.