A simple, large-scale synthesis of ammonia borane from NaBH4 and (NH4)2SO4 at 0 °C-rt in THF containing 5% NH3 is described. The presence of ammonia is critical for the reaction to proceed and allows the reaction at high concentrations and ambient temperature without the need for anhydrous solvent or inert atmosphere.
A three-component condensation involving reactive phenols, aldehydes, and active methylene substrates is described under BF3.Et2O catalysis to afford benzopyranyl products in satisfactory yields.
Synthesis of biaryl type systems, 11-aryl/heteroarylnaphtho[2,1-b]furans 8-11 has been described with a view to studying the conformational orientation of C-11 aryl/heteroaryl groups. Synthesis of 8-11 was accomplished by a two-step sequence involving O-alkylation of 2-naphthol with appropriate halo-ketones 2-4, followed by cyclization of the resulting naphthoxy-ketones 5-7 with methanesulphonic acid. The structures of 8-11 are based on detailed 2D NMR spectral analysis. The H8 in these compounds is not subject to significant anisotropic upfield shielding effects. The slightly upfield chemical shift of H8 in molecules 9-11, relative to 8 has been correlated with the electron density at C17 and C8 positions. While molecular modeling indicated dihedral angle (ϕ) between the C11 aryl/heteroaryl groups and the naphthofuran plane to be in the range of 70-60°in 8-10, a single X-ray crystal structural analysis of 11-(4'-pyridyl)naphtho[2,1-b]furan 10 indicated (ϕ) of 64.36°. In view of significant deviations from the orthogonal oreintation, the absence of any significant anisotropic shielding in 8-11 is not entirely unexpected.
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