Total synthesis of (+/-)-cameroonan-7alpha-ol and biomimetic rearrangements to related nopsane sesquiterpenes.
A total synthesis of the novel silphinane sesquiterpene alcohol (+/-)-cameroonanol (6-OH) from bicyclic enone 10 was accomplished by conjugate addition of crotylsilane, photochemical hydrobromination, intramolecular alkylation, and hydride reduction. The stereoisomers cameroonan-7beta-ol (18-OH) and 9-epicamerooonanols (19 and 20) were separated from isomer mixtures and the 9-desmethylcameroonanols (21-OH and 22-OH) were obtained by similar means. Solvolysis of 6-OMs and 18-OMs effected skeletal rearrangements to (+/-)-silphiperfol-6-ene (5), (+/-)-prenopsanol (7) and (+/-)-nopsanol (8), and (+/-)-silphiperfolan-7beta-ol (9) in parallel with biogenetic schemes proposed for these naturally occurring sesquiterpenes. The nor analogues 21-OMs and 22-OMs underwent solvolytic rearrangments to a similar set of nor products. The increase in solvolytic rates for the 7beta-mesylates 18-OMs and 22-OMs in comparison to the 7alpha epimers is attributed to concerted antiperiplanar Wagner-Meerwein rearrangements to the prenopsyl and norprenopsyl carbocations. Further analysis of the kinetic data and comparisons with solvolysis rates for the structurally related silphin-1beta-yl and silphin-1alpha-yl mesylates (28 and 29) are presented. The rearrangements observed afford chemical precedent for the biogenetic pathways in the literature for these silphinane sesquiterpenes.
Davis
Chad E
CE
Department of Chemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801, USA.
Duffy
Bryan C
BC
Coates
Robert M
RM
D002851:Chromatography, High Pressure Liquid; D007202:Indicators and Reagents; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D009822:Oils, Volatile / Q000737:chemistry; D010944:Plants / Q000737:chemistry; D012717:Sesquiterpenes / Q000138:chemical synthesis* / Q000737:chemistry*; D013237:Stereoisomerism
D007202:Indicators and Reagents; D009822:Oils, Volatile; D012717:Sesquiterpenes; C481923:cameroonan-7alpha-ol