Carbocationic Rearrangements of Silphinane Derivatives

Abstract

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 5 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[5.3.1.03,8]undecane (24). α-Terrecyclene (5) was converted to various oxygenated terrecyclane derivatives by dihydroxylation, hydroboration, and epoxidation (see Scheme 3 in the paper) and to its exocyclic isomer β-terrecyclene (34, see Scheme 4 in the paper). The observed rearrangements of the silphinyl mesylates (see Scheme 5 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 1 in the paper).