Synthesis of four sesquiterpenoid lactone skeletons, germacranolide, elemanolide, cadinanolide, and guaianolide, from a single photoadduct

Article abstract of DOI:10.1021/jo00379a001

The syntheses of four different sesquiterpenoid carbon skeletons, germacranolide, elemanolide, cadinanolide, and guaianolide, from the photoadduct 2 are described. Thermolysis of 3 and 9, which both were derived from 2a, gave germacranolides 4 and 10, respectively, in which the lactone ring bridged C(4) and C(6) while thermolysis of 16 (also derived from 2a) gave a mixture of isomeric germacranolides 17 and 18 in which the γ-lactone moiety bridged C(6) and C(7). A number of factors involved in the critical cycloreversion step to give the ten-membered ring of the germacranolides are discussed and MM1 calculations on the adduct precursors are used to support some of the suggestions. Thermolysis of 3 also gave the elemanolide 5 in addition to 4. Thermolysis of 2b gave the cadinanolide 26 after a remarkable sequence of thermal transformations. Finally, sensitized irradiation of 4 gave the guaianolide 30. The conversion of a number of the products to more functionalized derivatives, some of which are related to known sesquiterpenoids, is also described.