Ck. Sha et al., TOTAL SYNTHESIS OF (-)-DENDROBINE VIA ALPHA-CARBONYL RADICAL CYCLIZATION, Journal of the American Chemical Society, 119(18), 1997, pp. 4130-4135
An efficient total synthesis of enantiomerically pure (-)-dendrobine (
1) is accomplished based on an ol-carbonyl radical cyclization reactio
n. (S)-carvotanacetone (6) was converted to bicyclic acetal-enone 4 in
five steps. CuI-mediated conjugate addition of 4-(trimethylsilyl)-3-b
utynylmagnesium chloride to 4 followed by treatment with TMSC1 afforde
d trimethylsilyl enol ether 10. Iodination of 10 with NaI and m-CPBA g
ave iodoketone 3. Intramolecular radical cyclization of 3, effected wi
th Bu3SnH and AIBN, furnished tricyclic ketone 2. Removal of the TMS g
roup of 2 and oxidation of the resulting cyclic acetal 12 with m-CPBA
and BF3 etherate gave peroxy compound 13. Compound 13 was treated with
DBU to yield lactone 14. Hydroboration of 13 with basic H2O2 oxidatio
n gave diol 17. Conversion of 17 to the corresponding azido alcohol 19
followed by Jones oxidation furnished azido ketone 20. Treatment of 2
0 with PPh3 followed by reduction with NaBH3CN afforded amine 21. Crud
e amine 21 was methylated to give enantiomerically pure (-)-dendrobine
(1).