23 Corynantheine型アルカロイドよりC-Mavacurine型アルカロイドへの化学変換

概要

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Pleiocarpamine 3, an interesting indole alkaloid of apocynaceae plants, has C-mavacurine skeleton. We wish to report that a partial synthesis of 20α-ethyl-19,20-dihydro-16-epi-pleiocarpamine 16 have been accomplished starting either from hirsutine 4 or dihydrocorynantheine 5. Treatment of desmethylhirsutine 7 (or desmethyldihydrocorynantheine 8) with BrCN/ 0.6% EtOH-CHCl_3 (or 20% EtOH-CHCl_3 for 8) then gave a mixture of 3-(R)-ethoxy- and 3-(S)-ethoxyderivatives 9a,b (sole C_3-(R), 9a from 8). The epimeric mixture 9a,b (or 9a itself) was oxidized to yield C_<16>-deformyl-chlorinated compound 11 with t-BuOCl at -78°. The new ring formation between Na and C_<16> of 11 was accomplished to give the C_3 epimeric mixture 14a,b (or a pure C_3-(R), 14a mp157-158°, from C_3-(R)- 11a) by treatment with NaH in DMSO. The final ring closing reaction of 14 (either from the C_3 epimeric mixture 14a,b or C_3-(R) 14a itself) was achieved by heating with aq.HOAc/ NH_4OAc to give 16, 20-α-ethyl-19,20-dihydroepi-pleiocarpamine mp 169-171°, (C_<20>H_<24>O_2N_2) and an indoline derivative 17. The compound 16 was reduced to the corresponding alcohol 18 (20-α-ethyl-19,20-dihydronormavacurine) with LiAlH_4. The mass spectrum of this alcohol 18 showed the same fragmentation peaks and the similar relative intensities as recorded for 20-β-ethyl-19,20-dihydronormavacurine derived from the natural alkaloid.
1974-10-01