51(P12) ニトリルオキシドの閉環反応を利用するタキサン類の合成研究(ポスター発表の部)
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Taxol^[○!R](1), a typical taxane diterpene, is one of the most attractive synthetic targets in organic chemistry because of its unique structural features, as well as its antileukemic and tumor-inhibiting activity. Recently, the total synthesis of Taxol^[○!R] has been accomplished by four groups, however, the efforts for developing an efficient synthetic method of taxanes are continuously being made. We now report a new synthetic route for the taxane skeleton using a fragmentation reaction and an intramolecular nitrile oxide cyclization reaction as key steps. To confirm the viability of nitrile oxide cyclization, the model compound 9 having a simple C ring synthon was chosen. The cyclization reaction of nitro olefin 9 promoted by p-chlorophenyl isocyanate and a catalytic amount of triethyl amine directly afforded the oxime derivative 10 as a single isomer in high yield. The expected isoxazoline derivative was not detected in reaction mixture. Next, the synthesis of a key intermediate nitro olefin 29 and its cyclization reaction were investigated. Keto mesylate 16, precursor of the fragmentation reaction, was prepared from optically active bicyclo[2.2.2]octane derivative 2 by a five-step sequence. To a solution of 16 in MeOH was added 2.0 equiv. of NaBH_4 and after 5min, DMSO and 1.5 equiv. of t-BuOK were added to furnish pentasubstituted cyclohexane 17 in 93% yield. The reaction of aldehyde 20, obtained from 17 by a six-step sequence, with vinyl lithium f provided 23 bearing A/C ring system. The key cyclization of nitro compound 29, derived from 23, with p-chlorophenyl isocyanate gave 30 having taxane skeleton.
- 天然有機化合物討論会の論文
- 1997-07-20
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