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Carbocationic Rearrangements of Silphinane Derivatives

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journal contribution
posted on 19.11.1998, 00:00 by Robert M. Coates, Jonathan Z. Ho, Michael Klobus, Lijuan Zhu
Solvolysis of silphin-1α and -1β-yl mesylates (18α-OMs and 18β-OMs) gave rise to mixtures of silphinene (4), bridgehead alcohol 22-OH (or its acetate), and α-terrecyclene (5) accompanied by trace amounts of isocomene (1) and modhephene (2). The 103 higher solvolysis rate determined for 18α-OMs over its epimer signifies a concerted rearrangement to a more stable tertiary bridgehead carbocation (36) which undergoes a second rearrangement and elimination to α-terrecyclene (5) (see Scheme in the paper). Isocomene and modhephene presumably arise from a minor competing pathway resulting from 7→1 hydride shift to the silphin-7-yl ion (38 11) which partitions between methyl and cyclopentane ring rearrangements. Acetolysis of secosilphinyl nosylate 21 (X = ONs) is accompanied by π participation leading directly to 38 and from there to a 2:1 mixture (6%) of isocomene and modhephene. TiCl4-mediated heterolysis of silphin-1α-yl trifluoroacetate (18α-O2CCF3) initiates a complex rearrangement pathway to 3-chloro-1,4,4,11-tetramethyltricyclo[,8]undecane (24). α-Terrecyclene (5) was converted to various oxygenated terrecyclane derivatives by dihydroxylation, hydroboration, and epoxidation (see Scheme in the paper) and to its exocyclic isomer β-terrecyclene (34, see Scheme in the paper). The observed rearrangements of the silphinyl mesylates (see Scheme in the paper) afford chemical precedent for a biogenetic pathway that links terrecyclanes (e.g. quadrone), isocomene, and modhephene to the silphinane family of cyclopentanoid sesquiterpenes formally derivable from caryophyllene (see Scheme in the paper).