Chiral 3,6-Dihydro-2H-1,4-oxazin-2-ones as Alanine Equivalents for the Asymmetric Synthesis of α-Methyl α-Amino Acids (AMAAs) under Mild Reaction Conditions
作者:Rafael Chinchilla、Nuria Galindo、Carmen Nájera
DOI:10.1055/s-1999-3456
日期:1999.4
3,6-Dihydro-2H-1,4-oxazin-2-ones 1 act as very reactive chiral cyclic alanine equivalents and can be diastereoselectively alkylated or allylated using mild reaction conditions: potassium carbonate under phase-transfer catalysis (PTC) conditions when using activated alkyl halides, organic bases such as tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) when using unactivated alkyl halides, and neutral Pd(0)-catalysis when allylic carbonates are used. In most cases, the diastereoselectivity under all these different reaction conditions is excellent although the reactions are always carried out at room temperature. Hydrolysis of the obtained alkylated or allylated oxazinones allows the preparation of enantiomerically enriched (S)-α-methyl α-amino acids (S)-AMAAs. The PTC and organic base methodologies have also been applied to the synthesis of (R)-α-methyl α-amino acids starting from (R)-alanine. When dihalides are used as electrophiles under PTC or BEMP conditions, a spontaneous N-alkylation also takes place giving bicyclic oxazinones, which can be hydrolyzed to enantiomerically pure cyclic (S)-AMAAs.
3,6-二氢-2H-1,4-噁嗪-2-酮1作为非常活泼的手性环状丙氨酸等价物,可以在温和反应条件下进行立体选择性烷基化或烯丙基化:使用活化烷基卤化物时,在相转移催化(PTC)条件下使用碳酸钾;使用非活化烷基卤化物时,使用如叔丁基亚氨基-2-二乙氨基-1,3-二甲基过氢-1,3,2-二氮磷烷(BEMP)或1,8-二氮杂双环[5.4.0]十一碳-7-烯(DBU)等有机碱;使用烯丙基碳酸盐时,使用中性零价钯催化。在大多数情况下,尽管反应总是在室温下进行,但在所有这些不同反应条件下,立体选择性都非常优异。所得的烷基化或烯丙基化噁嗪酮的水解可以制备具有光学活性的(S)-α-甲基α-氨基酸(S)-AMAA。PTC和有机碱方法也已应用于从(R)-丙氨酸开始合成(R)-α-甲基α-氨基酸。当使用二卤化物作为亲电试剂在PTC或BEMP条件下时,也会自发发生N-烷基化,生成双环噁嗪酮,其可以水解为具有高光学纯度的环状(S)-AMAA。