Secondary mould metabolites. Part 58. Modifications in basic conditions and Michael additions of the protoilludane sesquiterpene tsugicoline A; some implications for the biogenesis of other sesquiterpenoids produced by Basidiomycetes

Abstract

The protoilludane sesquiterpene tsugicoline A 1 rearranges at pH 13 into compound 8a with a structure very similar to isolactarane sesquiterpenes and into compound 9a similar to the natural furosesquiterpene tsugicoline H 9c; under different basic conditions compounds 4a, 5, 6a and 7a,b, are formed. In particular, 4a shows the same skeleton of the natural tsugicoline E 4c. Compound 1 reacts with some thiols to give a Michael addition to α,β-conjugated carbonyl system, giving the adducts 10–13. In the case of compounds 12a,b a new tetrahydrothiophene ring is formed. The structures and stereochemistry of the products are discussed with the aid of NMR data; compounds 4a,c, 10 and 12a,b are correlated also with tandem MS studies. The implications of these results on current opinions on the biogenetic pathways of sesquiterpenes of Basidiomycetes are discussed.