Capnellenes Carbanions carbanionic

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

Kagechika, K. and Shibasaki, M. (1991) Asymmetric Heck reaction a catalytic asymmetric synthesis of the key intermediate for A -capneUene-3i3,8j8,10o -triol and A -capnellene-3/S,8/3,10a,14-tetrol. J. Org. Chem., 56,4093 (b) Kagechika, K., Ohshima, T. and Shibasaki, M. (1993) Asymmetric Heck reaction-anion capture process. A catalytic asymmetric synthesis of the key intermediates for the capneUenols. Tetrahedron, 49, 1773-82 (c) Ohshima, T., Kagechika, K., Adachi, M., et al. (1996) Asymmetric Heck reaction-carbanion capture process. Catalytic as5mmetric total synthesis of (—)-A LcapneUene. J. Am. Chem. Soc., 118, 7108-16 (d) Itano, W., Ohshima, T. and Shibasaki, M. (2006) Synthesis of the tricyclic core of 5a-capneUenols using asymmetric Heck reaction-carbanion capture process. Synlett, 3053-6. 

The use of carbanionic nucleophiles in the Mizoroki-Heck cyclization-/ -allyl nucleophilic trapping sequence allowed for streamlined access to the triquinane core common to various members of the capnellene family of natural products. For example, Shibasaki and coworkers obtained diquinane 57 in 77% yield and 87% ee by Mizoroki-Heck cy-clization of trienyl triflate 47 in the presence of malonate nucleophile 56 Scheme 16.14). It is notable that two new C-C bonds and three stereocentres are generated in this reaction. Eleven additional steps were used to convert intermediate 57 to ( )-A ( Ecapnellene (58). This first catalytic asymmetric total synthesis ( )-A d2). j pjjgjjgjjg achieved in 19 steps and 20% overall yield from commercially available materials. A related approach has recently been employed to prepare intermediates en route to capnellenols 53 and 54 (Scheme 16.12) [41]. 

Ohshima, T., Kagechika, K., Adachi, M. et al. (1996) Asymmetric Heck reaction-carbanion capture process. Catalytic asymmetric total synthesis of (—)-A -capnellene. J. Am. Chem. Soc., 118, 7108-16. 

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