New Asymmetric Synthesis of α-Aminoboronic Acids Containing Functionalized Side Chains

Synthesis of α-aminoboronic acids

This review describes available methods for the preparation of α-aminoboronic acids in their racemic or in their enantiopure form. Both, highly stereoselective syntheses and asymmetric procedures leading to the stereocontrolled generation of α-aminoboronic acid derivatives are included. The preparation of acyclic, carbocyclic and azacyclic α-aminoboronic acid derivatives is covered. Within each section, the different synthetic approaches have been classified according to the key bond which is formed to complete the α-aminoboronic acid skeleton.

Synthesis and properties of 1-(3'-dihydroxyboryl-2',3'-dideoxyribosyl)pyrimidines

Nucleoside analogues having a boronic acid in place of the 3-hydroxyl group of deoxyribose have been synthesized. The synthesis of 3'-dihydroxyboryl-2',3'-dideoxyribose was based on asymmetric homologation of boronic esters with (dihalomethyl)lithium, beginning from a (silyloxymethyl)boronic ester. A change of chiral director is required before introduction of the second stereocenter, and the direct displacement of (S,S)-1,2-dicyclohexyl-1,2-ethanediol by (1S,2S,3R,5S)-pinanediol was used for this purpose. Coupling of the pinanediol ester of the 1-acetoxy-3-dioxyboryl-5-tert-butylsilyloxy deoxyribose analogue with silylated pyrimidine bases was accomplished with trimethylsilyl bromide. The boronic acid nucleoside analogues were not cytotoxic toward Hep G2 (human hepatocarcinoma) cells. Decomposition occurred over a period of several hours at 37 °C, pH 7.4, with liberation of free pyrimidine base.

Chemical and biological evaluation of dipeptidyl boronic acid proteasome inhibitors for use in prodrugs and pro-soft drugs targeting solid tumors

Bortezomib, a dipeptidyl boronic acid and potent inhibitor of the 26S proteasome, is remarkably effective against multiple myeloma (MM) but not against solid tumors. Dose-limiting adverse effects from "on target" inhibition of the proteasome in normal cells and tissues appear to be a key obstacle. Achieving efficacy against solid tumors therefore is likely to require making the inhibitor more selective for tumor tissue over normal tissues. The simplest strategy that might provide such tissue specificity would be to employ a tumor specific protease to release an inhibitor from a larger, noninhibitory structure. However, such release would necessarily generate an inhibitor with a free N-terminal amino group, raising a key question: Can short peptide boronic acids with N-terminal amino groups have the requisite properties to serve as warheads in prodrugs? Here we show that dipeptides of boroLeu, the smallest plausible candidates for the task, can indeed be sufficiently potent, cell-penetrating, cytotoxic, and stable to degradation by cellular peptidases to serve in this capacity.

Oxidation and Decomposition Kinetics of Thiourea Oxides

In this report the decomposition and oxidation kinetics of thiourea dioxide and thiourea trioxide are investigated with a reversed-phase ion-pair high-performance liquid chromatography (HPLC) method. The HPLC method allows us to simultaneously determine and quantify several sulfur-containing reagents such as (NH2)2CSO2, (NH2)2CSO3, (NH2)2CO, and SO3(2-) (HSO3-). Experiments illustrate that the decomposition of thiourea oxides is a first-order reaction, in which the rate constants increase with the pH of the solution. Oxidations of thiourea oxides by hydrogen peroxide are first order with respect to both reagents within the studied pH range between 4.0 and 8.0. Oxidation rate constants are measured under different pH conditions, which show that increasing the pH of the reaction solution significantly accelerates the oxidation process.

Synthesis of boronic acid analogues of alpha-amino acids by introducing side chains as electrophiles

A synthetic route has been developed which has allowed us to prepare novel alpha-aminoboronic acids as inhibitors of serine proteases. These compounds were prepared to study the roles of proteases in biological systems. This methodology affords alpha-aminoboronic acids with the general formula R'-NHCH(R)BO(2)-pinanediol, where R = -CH(2)CHF(2), -CH(2)CO(2)tBu, and -(CH(2))(2)CO(2)Me and R' = either H or C(O)R". The latter two compounds are the boronic acid analogues of the natural amino acids aspartic acid and glutamic acid with the side chain carboxylate protected as a tert-butyl or a methyl ester, respectively. Following acylation of the amino group, the side chain tert-butyl ester of boroaspartic acid was removed by treatment with TFA. Boroglutamic acid was obtained as the free boronic acid by hydrolysis with HCl. Prior syntheses of alpha-aminoboronic acids involve the initial addition of an organometallic reagent to a trialkyl borate ester. These conditions do not allow the preparation of compounds with functionalities that are not stable to the strongly basic reaction conditions. The methodology described here allows the preparation of alpha-aminoboronic acids by introducing side chains as electrophiles. This is particularly advantageous for side chains which are prone to elimination or unwanted enolate formation. Specifically, BrCH(2)CHF(2), BrCH(2)COO(t)Bu, and CH(2)=CHCOOMe were allowed to react with the stabilized anion of (phenylthio)methane boronate, PhSCH(2)BO(2)C(6)H(12), to give the substituted boronate. The substituted (phenylthio)methane boronate was converted to the corresponding sulfonium ion by treatment with methyl iodide and subsequently displaced with iodide. The alpha-iodo derivative was converted to the amine by conventional methods.