Enantiospecific Synthesis of (+)-Byssochlamic Acid, a Nonadride from the Ascomycete <i>Byssochlamys </i><i>f</i><i>ulva</i>

A photoaddition−cycloreversion strategy applicable to enantiospecific synthesis of natural byssochlamic acid and its enantiomer was developed in which porcine liver esterase (PLE) catalyzed hydrolysis of 24 was used to desymmetrize this dimethyl ester to give (R)-25. Analogous PLE catalyzed hydrolysis of dimethyl ester 32 and bis(methylthio)methyl ester 48, while highly regioselective, gave racemic half acids 33 and 49, respectively. Stepwise coupling of (±)-49 and (R)-46, the latter derived from 25, afforded diolide 52, which upon irradiation gave exo,exo and exo,endo [2 + 2] photoadducts 53 and 54, respectively. The photoadducts underwent thermal cycloreversion to produce nine-membered bislactones 55 and 56. Conversion of these lactones via 1,5-cyclononadienes 57 and 58 to (+)-byssochlamic acid (3) was accompanied by acid-catalyzed epimerization of the n-propyl substituent. Stepwise coupling of (S)-60 (the enantiomer of 46) with (±)-49 led to diolide 63, which upon photoaddition−cycloreversion gave bis-γ-lactones 66 and 67. These 1,5-cyclononadienes were transformed into (−)-68, the enantiomer of natural byssochlamic acid.