Journal of the American Chemical Society, Vol.116, No.8, 3367-3374, 1994
Total Synthesis of the Cembranoid Diterpene Lactone (+)-Cleomeolide - Some Remarkable Conformational Features of 9-Membered Belts Linked in 2,6-Fashion to a Methylenecyclohexane Core
The total synthesis of (+)-cleomeolide (1) has been accomplished. The key construction elements of this cembranoid lactone were (i) improved conversion of optically pure Wieland-Miescher ketone into dienol ether 12 and oxidative cleavage of the latter to aldehydo ester 13; (ii) avoidance of complications arising from steric blockade of C-15 for introduction of the methylene group at that site; (iii) exploitation of an intramolecular Wadsworth-Emmons cyclization for macrocyclic ring construction; (iv) modulation of the conformation adopted by the medium ring by diastereofacial control of epoxidation of the C-3/C-4 double bond; and (v) intramolecular cyclization of the epoxy acid derived from 27 by nucleophilic capture at the more substituted oxiranyl carbon to deliver the target molecule. The deep-seated topographical change that accompanies the formation of 29 projects the macrocyclic ring quasi-axially from the methylenecyclohexane subunit in the manner known by X-ray crystallography to be adopted by the target compound. The intramolecular cyclization to set the bridgehead double bond was thereby facilitated.
Keywords:CLAISEN REARRANGEMENT;STEREOSPECIFIC SYNTHESIS;VINYL ETHERS;DEOXYGENATION;ANNELATION;ANNULATION;ALDEHYDES;KETONES;ESTERS;ACID