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-benzodiazaborin-3-one (3b), and 1-hydroxy-2-phenyl-1H-2,4,1-benzodiazaborin-3-one (3c) were synthesized, and their structures were established on the basis of 1H, 13C, and 11B NMR spectroscopies, mass spectrometry, and microelemental analyses. The structures of compounds 3b and 3c were unambiguously confirmed by X-ray crystallographic analyses. 11B NMR spectral analyses of the methanolic solutions of benzoborauracils 3a−c confirmed the formation of the corresponding bis-methanol adducts 13a−c. The structure of the N-Ph bis-methanol adduct 13c was confirmed by X-ray crystallography. The stabilities of these bis-methanol adducts depend on the substituent at the N2 position of 3a−c. The bis-methanol adducts are readily reconverted to the corresponding benzoborauracils upon removal of methanol. The first stable benzoborauracil nucleoside, 4-[5-O-(tert-butyldimethylsilyl)-2,3-O-disopropylidene-α-d-ribofuranosyl]-1-hydroxy-2-methyl-1H-2,4,1-benzodiazaborin-3-one (25) was prepared in two steps by treatment of 2-aminophenylboronic acid with 5-O-(tert-butyldimethylsilyl)-2,3-O-diisopropylidene-d-ribofuranose followed by its reaction with methylisocyanate.