Naphthopyrans and their C4 Alcohols by Cyclization of Substituted Naphthalenes using Potassium t-Butoxide in Dimethylformamide. Generality and Stereochemical, including Conformational, Effects

The generality of the high-yielding, stereoselective cyclization of 2-allyl-3-(1′-hydroxyethyl)-1,4-dimethoxynaphthalene 1 to afford trans-3,4-dihydro-5,10-dimethoxy-1,3-dimethylnaphtho[2,3-c]pyran 2 is investigated by replacing each of the methoxy groups in the substrate by an ethyl substituent and subjecting these to cyclization reaction conditions identical to those originally reported. For shorter reaction times with potassium tert-butoxide in dimethylformamide under nitrogen, the derived 1-ethyl-, 4-ethyl-, and 1,4-diethylnaphthalenes all cyclize to the trans-1,3-dimethyl compounds, whereas longer times yield increasing proportions of the corresponding cis-isomers. Under air, the epimeric C4 alcohols rel-(1R,3R,4S)-3,4-dihydro-5-ethyl-4-hydroxy-10-methoxy-1,3-dimethylnaphtho[2,3-c]pyran 50 and its rel-(1R,3R,4R) isomer 51 are also isolated from 2-allyl-1-ethyl-3-(1′-hydroxyethyl)-4-methoxynaphthalene 11. The half-chair conformation of pyran 50 is inverted relative to that of its C4 epimer 51.