SYNTHESIS OF 3-SUBSTITUTED BENZOCHROMONES FROM ALKYL-SUBSTITUTED NAPHTHODIOXABORINS

The synthesis of substituted benzochromones via a ring closure reaction of alkyl-substituted naphthodioxaborins with a Vilsmeier reagent have been investigated. 2-Naphthol 1 and 1-naphthol 2 were allowed to react with acetic acid 3 a or substituted acetic acids such as propionic acid 3 b, phenylacetic acid 3 c and 3-phenylpropionic acid 3 d in the presence of boron trifluoride-diethyl either (BF3.Et2O) at 100-degrees-C for 3 h to produce methyl-(4 a and 5 a), ethyl-(4 b and 5 b, benzyl-4 c and 5 c and (2-phenylethyl-substituted naphtho[1,3,2]dioxaborins 4 d and 5 d in good yields, respectively. Each of dioxaborins, 4 b, 4 c, 4 d, 5 b, and 5 d, reacted with an equimolar amount of the Vilsmeier reagent at 60-degrees-C for 1 h to give the corresponding 3-methy-(6 b and 7 b), 3-phenyl-(6c), and 3-benzyl-substituted chromone derivatives (6 d and 7 d) via 2-(dimethylamino) vinyl-substituted naphthodioxaborins in high yields. A similar reaction of 4 a, 5 a, and 5 c, however, afforded 2-(dimethylamino) vinyl-substituted naphthodioxaborins 8 a, 9 a, and 9 c), which were treated with an acid such as HCl or polyphosphoric acid to produce 1 H-naphtho[2, 1-b]pyran-1-one 6a, 4 H-naptho[1, 2-b]pyran-4-one 7 a, and 2-phenyl-1 H-naphtho [2, 1-b]pyran-4-one 7 c, in yields of 85%, 78%, and 86%, respectively. It is considered that the formation of 3-substituted benzochromones from alky-substituted naphthodioxaborins takes place through the ring closure of 2-(dimethylamino)vinyl compounds, which are produced by the Vilsmeier reaction of alkyl-substituted naphthodioxaborins. In the reaction of 4 a and 5 a with two equivalent of the Vilsmeier reagent, forml-substituted benzochromones (6 f and 7 f) were formed as main products without producing 6 a or 7 a. In bromination reaction 8 a and 9 a gave bromo-substituted benzochromones (6 e and 7 e).