Abstract
Bridging two molecules of an organic thiol with the 3,5-dimethylenebenzoic acid (Dmb) group produces a rigid five-carbon Dmb crosslink that is stable to disulfide reducing agents. Radical bromination of 3,5-dimethylbenzoic acid or its succinimido ester gave four α-brominated products. So NMR-pure 3,5-bis(bromomethyl)benzoic acid (Br 2Dmb) was regiospecifically synthesized in 38% overall yield by a four-step route from 1,3,5-benzenetricarboxylic acid. S-Alkylation of cysteine methyl ester, a model thiol, with Br 2Dmb provided the 2:1 adduct (Cys-OCH 3) 2 Dmb in 79% yield. Subsequent N-protection with di( tert-butyl) dicarbonate gave (Boc-Cys-OCH 3) 2Dmb in 60% overall yield from cysteine methyl ester. This protected diamino acid is suitable for assembly of two Dmb-crosslinked peptide chains.
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