Boron-Containing Heterocycles:  Syntheses, Structures, and Properties of Benzoborauracils and a Benzoborauracil Nucleoside

Benzo-fused boron-containing heterocycles (benzoborauracils ), 1-hydroxy-1 H -2,4,1-benzodiazaborin-3-one ( 3a ), 1-hydroxy-2-methyl-1 H -2,4,1-benzodiazaborin-3-one ( 3b ), and 1-hydroxy-2-phenyl-1 H -2,4,1-benzodiazaborin-3-one ( 3c ) were synthesized, and their structures were established on the basis of 1 H, 13 C, and 11 B NMR spectroscopies, mass spectrometry, and microelemental analyses. The structures of compounds 3b and 3c were unambiguously confirmed by X-ray crystallographic analyses. 11 B 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-1 H -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.