Carbocationic rearrangements of silphinane derivatives

Solvolysis of silphin-1 alpha and -1 beta-yl mesylates (18 alpha-OMs and 18 beta-OMs) gave rise to mixtures of silphinene (4), bridgehead alcohol 22-O H (or its acetate), and alpha-terrecyclene (5) accompanied by trace amounts of isocomene (1) and modhephene (2). The 10(3) higher solvolysis rate dete rmined for 18 alpha-OMs over its epimer signifies a concerted rearrangement to a more stable tertiary bridgehead carbocation (36) which undergoes a se cond rearrangement and elimination to alpha-terrecyclene (5) (see Scheme 5 in the paper). Isocomene and modhephene presumably arise from a minor compe ting 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 pi pa rticipation leading directly to 38 and from there to a 2:1 mixture (6%) of isocomene and modhephene. TiCl4-mediated heterolysis of silphin-1 alpha-yl trifluoroacetate (18 alpha-O2CCF3) initiates a complex rearrangement pathwa y to 3-chloro-1,4,4,11-tetramethyltricyclo[5.3.1.0(3,8)]undecane (24). alph a-Terrecyclene (5) was converted to various oxygenated terrecyclane derivat ives by dihydroxylation, hydroboration, and epoxidation (see Scheme 3 in th e paper) and to its exocyclic isomer beta-terrecyclene (34, see Scheme 4 in the paper). The observed rearrangements of the silphinyl mesylates (see Sc heme 5 in the paper) afford chemical precedent for a biogenetic pathway tha t links terrecyclanes (e.g. quadrone), isocomene, and modhephene to the sil phinane family of cyclopentanoid sesquiterpenes formally derivable from car yophyllene (see Scheme 1 in the paper).