SYNTHESIS OF 2,9,10-TRIOXATRICYCLO[4.3.1.0(3,8)]DECANE ANALOGS OF RESINIFERATOXIN

Structurally simplified analogues of the daphnane diterpene resiniferatoxin (RTX) 1, possessing the unusual 2,9,10-trioxatricyclo[4.3.1.0 3,8]decane system have been synthesised stereoselectively from cyclohexa-1,3-diene: functionalisation of the diene afforded the anti-epoxide, 1,4-di-O-benzyl-t-2,t-3-epoxycyclohexane-r-1,c-4-diol 4, the ring-opening of which was examined using various organometallic reagents; organoaluminium species were found to be the most efficient to effect this reaction. When trimethylsilyl (in place of benzyl) ethers were used to protect the diol, selective deprotection of 1,4-di-O-trimethylsilyl-2-O-(p-tolylsulfonyl)-c-3-[3-(tert-butyldiphenylsilyloxy)-prop-1-ynyl]cyclohexane-r-1,t-2,c-4-triol 16 was achieved using citric acid in methanol - the equatorially disposed trimethylsilyl ether was found to be more easily cleaved than the axially orientated one. Formation of the tricyclic orthoester was achieved by the generation of a dioxolenium ion from 1-O-phenylacetyl-2-O-(p-tolylsulfonyl)-c-3-[3-(tert-butyldiphenylsilyloxy)-prop-1-ynyl]cyclohexane-r-1,t-2,c-4-triol 19, by heating in 2,4,6-trimethylpyridine, with in situ intramolecular trapping by the suitably orientated hydroxy group to give 1-benzyl-7-(3-tert-butyldiphenylsilyloxyprop-1-ynyl)-2,9,10-trioxatricyclo[4.3.1.0 3,8]decane 20.