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Total Synthesis of (−)-Dendrobine via α-Carbonyl Radical Cyclization

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posted on 07.05.1997, 00:00 by Chin-Kang Sha, Rei-Torng Chiu, Cheng-Fen Yang, Nai-Tung Yao, Wei-Hong Tseng, Fen-Ling Liao, Sue-Lein Wang
An efficient total synthesis of enantiomerically pure (−)-dendrobine (1) is accomplished based on an α-carbonyl radical cyclization reaction. (S)-carvotanacetone (6) was converted to bicyclic acetal-enone 4 in five steps. CuI-mediated conjugate addition of 4-(trimethylsilyl)-3-butynylmagnesium chloride to 4 followed by treatment with TMSCl afforded trimethylsilyl enol ether 10. Iodination of 10 with NaI and m-CPBA gave iodoketone 3. Intramolecular radical cyclization of 3, effected with Bu3SnH and AIBN, furnished tricyclic ketone 2. Removal of the TMS group 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 14 with basic H2O2 oxidation gave diol 17. Conversion of 17 to the corresponding azido alcohol 19 followed by Jones oxidation furnished azido ketone 20. Treatment of 20 with PPh3 followed by reduction with NaBH3CN afforded amine 21. Crude amine 21 was methylated to give enantiomerically pure (−)-dendrobine (1).