122(P-60) 光学活性四級アンモニウム塩を触媒としたアルドール反応によるβ-ヒドロキシ-α-アミノ酸類の不斉合成(ポスター発表の部)
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Recent studies in this laboratory have resulted in the development of an excellent catalyst for highly enantioselective alkylation and Michael additions under phase transfer conditions. Reported herein is the extension of this system to the synthesis of chiral β-hydroxy-α-amino acids by aldol coupling of aldehydes with the trimethylsilyl enol ether derivative of tert-butylglycinatebenzophenone Schiff base (4) using the cinchonidine-derived bifluoride salt 3 as catalyst. The aldol coupling of 4 with isobutyraldehyde in the presence of catalyst 3 was optimized to synthesize α-amino-β-hydroxy ester 7a with 95% ee and 7b with 83% ee, as shown in Table 1. The aldol coupling of 4 with a number of other aldehydes was studied; the results are summarized in Table 2. Variability in the ratio of syn to and aldol products, 7a to 7b, was observed with lower ratios being associated with unbranched aldehydes of the type RCH_2CH_2CHO. On the other hand the highest syn/anti ratio, 13:1 was found for cyclohexanecarboxaldehyde. The aldol products 7, R=Cl(CH_2)_3, illustrate a broader utility of the new methodology as outlined in Scheme 2. The mixture 9a+9b was separated chromatographically and transformed into the diastereomeric 3-hydroxy-(S)-popecolic acids 10a and 10b. Similarly, after chromatographic separation, 11a and 11b were converted to the diastereomeric amino acids 12a and 12b, as shown. The syntheses of β-hydroxy-α-amino acids and derivatives described herein provide ready access to these useful substances.
- 天然有機化合物討論会の論文
- 2000-10-01
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