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功能基团起着重要的作用。α-氨基酸类似物，其中羧酸部分被其它酸性基团取代，例如，已知膦酸、磺酸、硼酸、四唑是α-氨基酸重要替代物。方酸是一种含氧碳化合物，具有强酸性、芳香共轭体系、环张力、缺电子和金属螯合能力等独特的物理化学性质。因此，4-羟基-2，3-二氧代环丁烯基（Sq）作为羧酸的电子等排体在药物化学中引起了广泛的关注。本项目合成了一种新型的氨基酸电子等排体α-氨基方酸（α-Amino Squaric Acid，α-ASqA），并将其掺入到肽中。我们首先研究了简单实用的碳-碳键形成反应，得到1 ′-氨基-1-方芳乙酸酯，然后将所得的烷基化加合物转化为α-ASqA。提出了两种合成α-ASqA的新方法，即引入sq-group的氨基丙二酸酯等价物法（方法A）和二价阴离子烯醇化物加成法（方法B）。其中，N-Boc-氨基酸酯与方酸二异丙酯的加成反应是关键。使用各种碱通过氘掺入实验评估从N-Boc-Phe-Ot-Bu酯生成烯醇化物的效率。结果，发现使用2当量的sec-BuLi是最有效的碱。这一结果提示了两种新的方法A和B，以获得具有天然和非天然类型的α-氨基酸侧链的各种α-ASqA。将α-ASqA掺入Leu-enkephalin中，得到三种不同类型的含α-ASqA的Leu-enkephalin。这些生物活性通过阿片受体的受体结合测定来表征。

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.