Electrophilic substitution in naphthalenes: cyclisation of naphthylbutanols to tetrahydrophenanthrene

Abstract

Both 4-(1-naphthyl)butanol and 4-(2-naphthyl)butanol cyclised in refluxing boron trifluoride–ether to give 1,2,3,4-tetrahydrophenanthrene. By use of the corresponding, 1,1-dideuteriobutanol derivatives and analysis of the products by 220 MHz ¹H n.m.r. spectroscopy it has been demonstrated that the 1-naphthylbutanol cyclises by two distinct pathways, (a) by direct attack (84%) at the 2-position, and (b) by ipso-attack (16%) at the 1-position of the naphthalene nucleus, followed by rearrangement. The 2-naphthylbutanol cyclises exclusively by direct substitution at the 1-position. With 4-(4-methoxy-1-naphthyl)-1,1-dideuteriobutanol on the other hand the proportion of ipso-substitution rises to 71% as shown by the 360 MHz ¹H n.m.r. spectra of the resulting mixture of tetrahydrophenanthrenes.