Carbocationic Rearrangements of Silphinane Derivatives

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 10 3 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. TiCl 4 -mediated heterolysis of silphin-1α-yl trifluoroacetate ( 18α -O 2 CCF 3 ) initiates a complex rearrangement pathway to 3-chloro-1,4,4,11-tetramethyltricyclo[5.3.1.0 3,8 ]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).