The racemization rates of N-acetyl-(S)-tyrosine, N-acetyl-(S)-phenylalanine, N-acetyl-(R)-2-(4-hydroxyphenyl)glycine [(R)-AcHpg], N-acetyl-(R)-2-phenylglycine, and N-acetyl-(S)-alanine were measured by use of (RS)-α-methylbenzylamine [(RS)-MBA] as a base-catalyst. The first-order rate constant for racemization tended to increase with an increase in the polar substituent constant of the N-acetylamino acid side chain. The racemization appeared to be subject to the inductive effect by the side chain. An asymmetric transformation of (RS)-AcHpg by using (R)-MBA, based on the result of racemization, gave an optically pure salt of (R)-AcHpg with (R)-MBA by successive use of the filtrate as the solvent. Optically pure (R)-2-(4-hydroxyphenyl)glycine [(R)-Hpg] was separated from the salt in 87–90% yield based on the starting (RS)-AcHpg. In addition, the asymmetric transformation of (R)-AcHpg was achieved by using (S)-MBA to give optically pure (S)-Hpg in 80% yield after purification of the salt of (S)-AcHpg with (S)-MBA followed by hydrolysis.
以(RS)-α-
甲基苄胺[(RS)-M
BA]作为碱
催化剂,测定了
N-乙酰基-(S)-
酪氨酸、
N-乙酰基-(S)-
苯丙
氨酸、
N-乙酰基-(R)-2-(
4-羟基
苯基)甘
氨酸[(R)-AcHpg]、
N-乙酰基-(R)-2-
苯基甘
氨酸和
N-乙酰基-(S)-丙
氨酸的消旋化速率。消旋化的速率常数(一级反应速率常数)通常随
N-乙酰
氨基酸侧链极性取代常数的增加而增加。消旋化过程似乎受侧链诱导效应的影响。基于消旋化结果,利用(R)-M
BA对(RS)-AcHpg进行不对称转换,通过连续使用滤液作为溶剂,得到了光学纯的(R)-AcHpg与(R)-M
BA的盐。从盐中分离光学纯的(R)-2-(
4-羟基
苯基)甘
氨酸[(R)-Hpg],基于起始的(RS)-AcHpg,产率为87-90%。此外,使用(S)-M
BA实现了(R)-AcHpg的不对称转换,经(S)-AcHpg与(S)-M
BA盐的纯化并随后
水解,得到了产率为80%的光学纯(S)-Hpg。