Stereochemistry and Reactions of Presilphiperfolanol: A Branch Point Marker in Triquinane Sesquiterpene Biogenesis

The structure and stereochemistry of the tricyclic sesquiterpene
alcohol (−)-presilphiperfolanol, a structural
precursor for the angular and propellane triquinane sesquiterpenes, are
established as
(1S,4S,7S,8S,11S)-2,2,4,8-tetramethyltricyclo[5.3.1.04,11]undecan-11-ol
[[2aS-(2aα,4aβ,5β,7aβ,7bβ)]-decahydro-1,1,2a,5-tetramethyl-7bH-cyclopenta[cd]inden-7b-ol, 1] by an
X-ray crystallographic analysis of its p-nitrobenzoate
derivative 8a and by correlation
with (−)-silphiperfol-6-ene (6). Dehydration of
1 afforded a 2:1 mixture of presilphiperfol-1(8)- and 7-enes
(9 +
10) which underwent epoxidation on both α and β faces.
Acid-catalyzed dehydration with
H2SO4−silica gel lead
to competing ring contractions by migrations of C11 and C9 to C8 to
give silphiperfolene (6) and α-terrecyclene
(12) in a 98:1 ratio. Solvolysis of 8a in
60% aqueous acetone provided a mixture of olefins (9 +
10 + 6) and 1
(9%) as the sole alcohol product. The similarity of the
solvolysis rate (5.2 × 10-5
s-1 at 50 °C) to the
literature
value for trans-bicyclo[3.3.0]octan-1-yl
p-nitrobenzoate indicates comparable levels of strain relief
in the respective
transition states.