ANTI SELECTIVE SPIROCARBOMERCURATION - SYNTHESIS AND STEREOCHEMISTRY OF THE SPIROBICYCLIC SESQUITERPENES SPIROJATAMOL AND ERYTHRODIENE

The stereoselectivity of mercuric salt-induced spirocarbannulation reactions of substituted and unsubstituted 2-(4-pentynyl)-1-[(trimethylsilyl)oxy]cyclohexenes has been examined. This intramolecular alkyne carbomercuration reaction has been applied to the first enantioselective syntheses of the natural products erythrodiene and spirojatamol. Treatment of 2-(4-pentynyl)-1-[(trimethylsilyl)oxy]cyclohexene with HgCl2 gave 7(E)-mercuriomethylenespiro[4.5]decan-6-one, the product of anti selective carbomercuration. Similar intramolecular carbomercuration of (S)-4-isopropyl-2-(4-pentynyl)-1-[(trimethylsilyl)oxy]cyclohexene, derived from (S)-(-)-perillyl alcohol, gave diastereomeric axial and equatorial spirocarbannulation products in a 7:3 ratio and high yield. Both C-vinylation products, (1S and 1R,5S,7E)-5-isopropyl-7-mercuriomethylenespiro[5.4]decan-2-one), bear (E)-exocyclic vinyl mercurials. The major (1S,5S)-C-vinylation product was converted into both (-)-erythrodiene and (-)-spirojatamol in one and three steps, respectively. Thus, (-)-erythrodiene, a spirobicyclic sesquiterpene hydrocarbon from the Caribbean gorgonian coral Erythropodium caribaeorum, was synthesized from (S)-(-)-perillyl alcohol in 10 steps and approximately 35% overall yield using the mercuric salt-induced spirocarbannulation reaction as the key step. (-)-Spirojatamol, the enantiomer of the natural product isolated from the Indian plant Nardostachys jatamansi, was similarly prepared from (S)-(-)-perillyl alcohol in 12 steps and ca. 20% overall yield. This work indicates that the absolute configuration of (-)-erythrodiene ((1S,5S)-(-)-5-isopropyl-2,7-dimethylenespiro[5.4]decane) is enantiomeric to the carbon skeleton of (1S,5R,8S)-(+)-spirojatamol, the only other naturally occurring sesquiterpene known to posses the 5-isopropyl-2-methylenespiro[5.4]decane skeleton.