Boron-containing heterocycles: Syntheses, structures, and properties of benzoborauracils and a benzoborauracil nucleoside

Benzo-fused boron-containing heterocycles (benzoborauracils), 1-hydroxy-1H- 2,4,1-benzodiazaborin-3-one (3a), 1-hydroxy-2-methyl-1H-2,4,1-benzodiazabor in-3-one (3b), and 1-hydroxyy-phenyl-1H-2,4,1-benzodiazaborin-3-one (3c) we re synthesized, and their structures were established on the basis of H-1, C-13, and B-11 NMR spectroscopies, mass spectrometry, and microelemental an alyses. The structures of compounds 3b and 3e were unambiguously confirmed by X-ray crystallographic analyses. B-11 NMR spectral analyses of the metha nolic solutions of benzoborauracils 3a-c confirmed the formation of the cor responding bis-methanol adducts 13a-c. The structure of the N-Ph bis-methan ol adduct 13c was confirmed by X-ray crystallography. The stabilities of th ese bis-methanol adducts depend on the substituent at the N2 position of 3a -c. The bis-methanol adducts are readily reconverted to the corresponding b enzoborauracils upon removal of methanol. The first stable benzoborauracil nucleoside, 4-[5-O-(tert-butyldimethylsilyl)-2,3-O-disopropylidene-alpha-D- ribofuranosyl]-1-hydroxy-2-methyl-1H-2,4,1-benzodiazaborin-3-one (25) was p repared in two steps by treatment of 2-aminophenylboronic acid with 5-O-(te rt-butyldimethylsilyl)-2,3-O-diisopropylidene-D-ribofuranose followed by it s reaction with methylisocyanate.