?-Alkylation of Amino Acids without Racemization. Preparation of Either (S)- or (R)-?-Methyldopa from (S)-Alanine
作者:Dieter Seebach、Johannes D. Aebi、Reto Naef、Theodor Weber
DOI:10.1002/hlca.19850680118
日期:1985.2.13
(S)-methionine, and (R)-phenylglycine are deprotonated to chiral enolates (cf.3, 4, 12, 21). Diastereoselective alkylation of these enolates to 5,5-dialkyl- or 5-alkyl-5-arylimidazolidinones (5, 6, 9, 10, 13a-d, 17, 18, 22) and hydrolysis give α-alkyl-α-amino acids such as (R)- and (S)-α-methyldopa (7 and 8a, resp.), (S)-α-methylvaline (14), and (R)-α-methyl-methionine (19). The configuration of the products
对映体纯的顺式-和反式-5-烷基-1-苯甲酰基-2-(叔丁基)-3-甲基咪唑烷-4-酮(1,2,11,15,16)和反式-2-(叔丁基易于从(S)-丙氨酸,(S)-缬氨酸,(S)-蛋氨酸和(R)-苯基甘氨酸获得的)-3-甲基-5-苯基咪唑啉酮-4-酮(20)被去质子化为手性烯醇盐(参见图3,4,12,21)。这些烯醇盐的非对映选择性烷基化为5,5-二烷基或5-烷基-5-芳基咪唑烷酮(5、6、9、10、13a -d,17、18、22)和水解得到α-烷基-α-氨基酸,例如(R)-和(S)-α-甲基多巴(分别为7和8a),(S)-α-甲基缬氨酸(14)和(R) -α-甲基蛋氨酸(19)。产品的构型已通过化学相关性和NOE 1 H-NMR测量证明(请参阅23、24)。在整个过程中,简单的对映体纯的α-氨基酸可以通过新戊醛缩醛衍生物保留或反转构型进行α-烷基化。由于不需要手性助剂,因此该过程被称为“