Boron heterocycles bearing a peripheral resemblance to naturally-occurring purines: Design, syntheses, structures and properties

Article abstract of DOI:10.1021/ja00096a017

To test the design and initiate the development of a new class of boron-containing purine nucleoside and aglycon analogs, the benzo-fused boron heterocycles 1-hydroxy-1H-2,4,1-benzoxazaborine (2), 1,2-dihydro-1-hydroxy-2,4,1-benzodiazaborine (3), and 3-amino-1,2-dihydro-1-hydroxy-2,4,1-benzodiazaborine (4) were synthesized, and their structural properties and chemical reactivities were investigated. The heterocyclic peripheries of these targets possess heteroatom, hydrogen atom, and in-plane lone-pair electron loci specifically selected to match closely those of the pyrimidine ring portions of the naturally-occurring purines adenine, hypoxanthine, and guanine. According to ¹H and ¹¹B NMR spectral analyses, 2-4 are stable to facile hydrolysis, but not to facile hydration, which occurs in a 1,4-fashion to give zwitterionic adducts. In addition, these benzo-fused boron heterocycles form bis-methanol adducts simply upon warming in methanol solution. A single-crystal X-ray diffraction analysis of the bis-methanol adduct (14) derived from 3 confirmed it to be a zwitterion comprised of tetrahedral borate anion and formamidinium cation molecular fragments. From NMR-based solution structure determinations made of 2-4 and a series of substituted derivatives, the facile 1,4-hydration property appears to be endemic to the 2,4,1-oxaza- and diazaborine classes of boron heterocycles and is projected to imbue certain future imidazo[5,4-e]-fused members with the requisite aqueous solution structural features for 'transition-state' analog inhibition of the enzyme adenosine deaminase (EC 3.5.4.4). This work underscores the attraction of employing boron-for-carbon replacement strategies in the design of new, potentially bioactive agents.