FUNGAL EXTRACTIVES .12. CONSTRUCTION OF VELLERANE SKELETON WITH TOTAL SYNTHESES OF RACEMIC VELLERAL, VELLEROLACTONE, AND PYROVELLEROLACTONE - REVISED STRUCTURES

Velleral, vellerolactone, and pyrovellerolactone, three hydroazulenic sesquiterpenoids from basidiomycetes of the genus Lactarius, have been prepared by cycloaddition of enamines of bicyclo[3.3.0]octanones to 4-substituted tetrolic esters, followed by fission of the four-membered ring and subsequent hydrogenolytic deamination with diborane. Further functional group manipulation gave the natural compounds. X-ray crystallographic analyses of key synthetic intermediates, together with spectroscopic data on synthetic stereoisomers of the natural compounds, necessitate a revision of the earlier suggested structures for all three title compounds. Molecular mechanics calculations revealed that the configuration of a methyl group on carbon atom 3 in the seven-membered ring has a profound effect on the conformation of the hydroazulenic ring system. These findings are corroborated by the X-ray crystallographic results. © 1978, American Chemical Society. All rights reserved.