A New Route to 2,3-Disubstituted Pyrazines; Regioselective Metalation of Chloropyrazine
作者:A. Turck、L. Mojovic、G. Quéguiner
DOI:10.1055/s-1988-27736
日期:——
Chloropyrazine is regioselectively metalated by lithium amides in tetrahydrofuran to give 3-chloro-2-lithiopyrazine. The reaction of this lithio derivative with electrophiles can either be performed in situ (with the electrophile, e. g., chlorotrimethylsilane, present in the mixture already during metalation), i. e., by the "equilibrium shift" procedure, or with the solution of the preformed lithio derivative, i. e., by the "accumulation" procedure. With aldehydes and benzophenone, the "accumulation" procedure affords 3-chloro-2-(1-hydroxyalkyl)pyrazines in good yields. The secondary alcohols thus obtained can be oxidized to 2-acyl-3-chloropyrazines with manganese(IV) oxide and these ketones can be converted into 3-acyl-2-aminopyrazines by reaction with ammonia in ethanol; cyclization of 3-chloro-2-(2-methoxybenzoyl) pyrazine using pyridine hydrochloride at 210°C affords 10-oxo-10H-[1] benzopyrano[2,3-b]pyrazine (1,4-diazaxanthone) in high yield. 3-Chloro-2-formylpyrazine is obtained by reaction of the lithio derivative with ethyl formate and 3-chloropyrazine-2-carboxylic acid by reaction with carbon dioxide.
氯吡嗪在四氢呋喃中被锂胺区域选择性地金属化,生成3-氯-2-锂吡嗪。该锂衍生物与电亲体的反应可以通过“平衡移位”程序在原位进行(电亲体,如氯三甲基硅烷,在金属化过程中已存在于混合物中),也可以通过“积累”程序与预先形成的锂衍生物的溶液进行。在醛类和二苯甲酮的反应中,“积累”程序可以高产量地得到3-氯-2-(1-羟基烷基)吡嗪。由此获得的二级醇可以通过四氧化锰氧化为2-酰基-3-氯吡嗪,而这些酮可以通过与氨在乙醇中的反应转化为3-酰基-2-氨基吡嗪;在210°C下使用盐酸吡啶对3-氯-2-(2-甲氧基苯甲酰)吡嗪进行环化,可以高产量地得到10-氧代-10H-[1]苯并吡喆烷[2,3-b]吡嗪(1,4-二氮杂喹啉)。3-氯-2-甲酰吡嗪通过锂衍生物与乙酸乙酯的反应以及3-氯吡嗪-2-羧酸与二氧化碳的反应获得。