Phase Diagrams to Evaluate the Opportunity for Enantiomeric Enrichment of Some Nonracemic Mixtures of Amino Acid Benzyl Esters by Crystallization as p-Toluenesulfonate Salts,Organic Process Research & Development

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Phase Diagrams to Evaluate the Opportunity for Enantiomeric Enrichment of Some Nonracemic Mixtures of Amino Acid Benzyl Esters by Crystallization as p-Toluenesulfonate Salts
Organic Process Research & Development ( IF 3.1 ) Pub Date : 2017-10-17 00:00:00 , DOI: 10.1021/acs.oprd.7b00233
Cristiano Bolchi ₁ , Francesco Bavo ₁ , Marco Pallavicini ₁

Affiliation

A number of amino acids undergo moderate-to-high racemization when converted to benzyl ester under inadequate conditions. As they are usually recovered and purified at the end of the esterification by precipitating/crystallizing the p-toluenesulfonate salt, it is of most importance to be aware that such isolation procedures can improve but also lessen the enatiomeric excess of the product. To predict the chance of enantioenrichment by crystallization, it is necessary to define whether the enantiomers of these ester salts form conglomerates or racemic compounds in the solid state and, in case of racemic compounds, to know the enantiomeric composition of the eutectics and to verify that it is similar to the eutectic composition obtained from ternary solubility curves. We constructed the binary melting point phase diagrams of a number of amino acid benzyl ester p-toluenesulfonates showing that most of them form racemic compounds. Of these, we studied those of Ala, Phe, Leu, and Glu, in particular, demonstrating that when prepared in l form with moderate racemization the first three cannot be enantioenriched by crystallization because their l fraction is much higher at the ternary solubility eutectic than at the binary eutectic, whereas the last one can be isolated enantiopure by crystallization because its l fractions at both the ternary solubility and binary eutectics are accordingly low.

中文翻译：

相图来评估由结晶氨基酸的苄基酯的一些非外消旋混合物的对映体富集的机会，p甲苯磺酸盐盐

当在不适当的条件下转化为苄基酯时，许多氨基酸会经历中度到高度的外消旋作用。因为它们通常在酯化结束时通过沉淀/结晶p来回收和纯化-甲苯磺酸盐，最重要的是要意识到这样的分离程序可以改善但也减少了产物的对映体过量。为了预测通过结晶的对映体富集的机会，有必要确定这些酯盐的对映异构体是形成固态的团块还是外消旋化合物，如果是外消旋化合物，则需要了解低共熔物的对映体组成并验证它类似于从三元溶解度曲线获得的低共熔组成。我们构建了许多氨基酸苄基酯对甲苯磺酸盐的二元熔点相图，表明它们大多数形成外消旋化合物。其中，我们特别研究了Ala，Phe，Leu和Glu的那些，证明了在具有中等消旋作用的l形式，前三个不能通过结晶被对映体富集，因为它们的l分数在三元共晶时比在二元共晶时要高得多，而最后一个可以通过结晶分离出对映体，因为它的l分数在两个三元溶解度中因此，二元共晶率较低。

更新日期：2017-10-17

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