P-12 酸化モリブデン触媒を用いる脱水環化反応によるオキサゾリン、チアゾリンの効率的合成(ポスター発表の部)
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Oxazoline, oxazole, thiazoline, and thiazole rings are important constituents of numerous bioactive natural products and pharmaceuticals. The biosynthesis of many naturally occurring oxazolines and thiazolines appears to involve the dehydrative cyclization of serine, threonine, and cysteine residues. Oxazoles and thiazoles are synthesized by the oxidation of oxazolines and thiazolines, respectively. We describe here a biomimetic synthesis of oxazolines and thiazolines catalyzed by molybdenum (IV or VI) oxides, their mode of cyclization is retentive at the β-position. In the course of screening various metal oxides as catalysts for the dehydrative cyclization of dipeptides 1a and 1b, we found that molybdenum oxides had good catalytic activities. Interestingly, the ammonium salts of molybdenum(VI) oxides, (NH_4)_6Mo_7O_<24>・4H_2O and (NH_4)_2MoO_4, exhibited remarkable catalytic activities and gave oxazolines 2a and 2b in a short reaction time, along with small amounts of 3a and 3b. Next, we examined the dehydrative cyclization of cysteine derivatives 4a and 4b to thiazolines 5a and 5b using molybdenum oxides as catalysts. In the case of the dehydrative cyclization of 4a, (NH_4)_6Mo_7O_<24>・4H_2O, (NH_4)_2MoO_4 and MoO_2(acac)_2 showed excellent catalytic activities. MoO_2(acac)_2 could catalyze the dehydrative cyclization of 4b, to give 5b in 70% yield along with 5c (15%). To the best of our knowledge, this is the first example of the catalytic dehydrative cyclization of dipeptide substrates that include serine, threonine, and cysteine residues. A key synthetic intermediate 11 of hennoxazole A and bis(oxazoline)s 13a and 13b were synthesized by the dehydrative cyclization using molybdenum oxide catalysts. In addition, polyaniline-supported MoO_2(acac)_2 could easily be recovered and reused.
- 2005-09-15
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- P-12 酸化モリブデン触媒を用いる脱水環化反応によるオキサゾリン、チアゾリンの効率的合成(ポスター発表の部)