A novel, efficient and stereospecific synthesis of the marine
natural product capnellene from p-cresol
is described. Generation of
4-methyl-6,6-spiroepoxycyclohexa-2,4-dienone (9) from
5-methylsalicyl
alcohol (8), its in situ cycloaddition with cyclopentadiene
(in situ), and the photochemical oxa-di-π-methane reaction of an endo
tricyclo[5.2.2.02,6]undecenone are the key
features of our strategy.
An efficient synthetic route to appropriately designed endo
tricyclo[5.2.2.02,6]undecenones
(compounds 6, 11−13) endowed with most of the
structural and stereochemical features of capnellene,
from the keto epoxide 7, are described. The
photochemical reaction of 6, and 11b,d upon
sensitized
irradiation readily gave the oxa-di-π-methane products 5,
14, and 15 respectively. The
tetracyclic
compound 5 was elaborated to capnellene after cleavage of
the peripheral cyclopropane bond,
Barton's deoxygenation, deprotection of the carbonyl group, and Wittig
reaction.
Formation of the title compound 3, via an unusual Prins-type reaction on isoeugenol with formaldehyde in alkaline medium, and its conversion into a tricyclo[5.2.2.0 2,6 ]undecane system 4 is described.
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