Total synthesis of some marasmane and lactarane sesquiterpenes

A general and efficient synthetic route to the marasmane sequiterpenes (±)-isovelleral (2) and (±)-stearoylvelutinal (1b) is described. Total syntheses of two other naturally occurring sesquiterpenes, deconjugated anhydrolactarorufin A (5) and lactarorufin A (6), were achieved using an acid-catalyzed ring expansion. All four syntheses are highly stereoselective and do not require the use of any protecting groups. Finally, the protic acid-catalyzed degradation of velutinal (1a) was investigated in an effort to chemically induce the biologically important conversion of velutinal (1a) to isovelleral (2). The experimental results thus obtained indicate that an enzymatic mechanism for the key transformation of velutinal(1a) into isovelleral(2) is more plausible than one that is acid-catalyzed