Synthesis and its function of novel squaric acid containing amino acids.

α-氨基酸中的羧基作为质子供体和形成肽键的官能团起着重要的作用。α-氨基酸类似物中的羧酸部分被其他酸性基团取代，如磷酸、磺酸、硼酸、四氮唑，已知是α-氨基酸的重要替代品。方酸是一种氧碳化合物，具有强酸性、芳香族共轭体系、环张力、电子缺陷和金属螯合能力等一系列独特的物理化学性质。因此，4-羟基-2，3-二氧环丁基-1-烯基(Sq)作为羧酸的同功酶在药物化学中备受关注。在本项目中，合成了一种新型的含有Sq基团的氨基酸异构体，作为α-氨基酸中的羧酸盐等价物(α-氨基方酸(α-ASQA))，并将其掺入到多肽中。我们首先研究了1‘-氨基-1-方乙酸乙酯的简单和实用的碳-碳键形成反应，然后将生成的烷基化加合物转化为α-ASQA。发展了两种新的方法，即利用含有氨基丙二酸酯的SQ基团(方法A)和二阴离子烯醇酸的加成反应(方法B)来获得α-ASQA。N-Boc-氨基酸酯生成的双阴离子烯酸酯与方酸二异丙酯的加成反应是这些方法的关键。以N-Boc-Phe-OT-Bu酯为原料，通过不同碱基的氚掺入实验，评价了N-Boc-Phe-OT-Bu酯生成烯醇的效率。因此，使用2当量的SEC-BuLi被发现是最有效的碱。这一结果促使两种新的方法A和B获得具有自然和非自然类型的α-氨基酸侧链的各种α-ASQA。将α-ASQA掺入亮氨酸脑啡肽中，得到三种不同类型的α-ASQA亮氨酸脑啡肽。这些生物活性通过阿片受体的受体结合实验来表征。

The carboxyl group in α-amino acids plays an important role as a proton donor and a peptide-bond-forming functional group. α-Amino acid analogs in which the carboxylic acid moiety is replaced with other acidic groups, e.g., phosphonic acid, sulfonic acid, boronic acid, tetrazole are known to be important surrogates of α-amino acid. Squaric acid is an oxocarbon and exhibits several unique physicochemical and chemical properties characterized by strong acidity, aromatic conjugate system, ring strain, electron deficiency, and metal chelating ability. Therefore, 4-hydroxy-2,3-dioxocyclobut-1-enyl (Sq) group has attracted much attention as an isostere of carboxylic acid in medicinal chemistry. In this project, the synthesis of a novel amino acid isostere bearing Sq group as a carboxylate equivalent in α-amino acid (α-Amino Squaric Acid (α-ASqA)) and its incorporation into peptide was implicated. We initially investigated facile and practical carbon-carbon bond-forming reaction to 1'-amino-1-squarylacetate followed by conversion of the resulting alkylated adducts into α-ASqA. Two novel methods involving the use of sq-group incorporating amino malonate equivalent (method A) and addition reaction of dianion enolate (method B) to access α-ASqA were developed. Addition reaction of dianion enolate generated from N-Boc-amino acid ester to squaric acid diisopropyl ester was the key to these methods. Generation efficiency of enolate from N-Boc-Phe-Ot-Bu ester was assessed by deuterium incorporation experiments using various bases. As a result, the use of 2 equiv of sec-BuLi was found to be the most efficient base. This result prompted two new methods A and B to access various α-ASqA possessing natural and unnatural types of α-amino acid side chain. α-ASqA was incorporated into Leu-enkephalin to give three different type of α-ASqA-containing Leu-enkephalin. These biological activities were characterized by receptor binding assay of opioid receptors.