Steric Requirements in the Cyclization of Naphthalene Derivatives

While polyphosphoric acid cyclization followed by dehydrogenation of γ-(2-naphthyl)butyraldehyde (Ia) yields exclusively phenanthrene, the corresponding substituted ketones Ib, Ic and Id lead predominantly to linear cyclization products of the anthracene series. These results are ascribed to steric interference of the 4 and 5 substituents in the phenanthrene series and are not operative in the next lower homolog as demonstrated by the cyclization of 5-(2′-naphthyl)pentan-3-one (VIc) to 1-ethyl-3H-benz [e]indene (VII).