Spirans. Part 13. The synthesis and orientation of spirans related to Abel's ketone (naphtho[2,1-b]furan-2(1H)-spiro-1′(2′H)-naphthalen-2′one)

Abstract

The aryl substituent in stereoisomers (2) and (3) of 1-phenylnaphtho[2,1-b]furan-2(1H)-spiro-1′(2′H)naphthalen-2′-one are subject to hindered rotation by an energy barrier of ca. 15 kcal mol^–1. If the phenyl substituent carries an ortho substituent, however, rotation is prevented at temperatures up to 60 °C (the highest examined) : only one rotamer is obtainable (the other is too crowded) and it has the ortho-substituent cisoid with respect to the triarylmethine proton.

The temperature-variable broadening of the signals from the protons of the aryl substituent serves to identify these in the ¹H n.m.r, spectra. Some other resonances can be regularly identified by combinations of coupling constants, double-irradiation experiments, and shifts induced by using benzene instead of trichloromethane as solvent, but the only one of value for determining the configuration at the triarylmethine centre is that already known, i.e. the upfield shift in the resonance of the vinylic (3′) proton caused by the aryl substituent when this lies immediately below it. A less precise but more general test of configuration is afforded by the mutual shielding of the aryl substituent and ring E when these are in adjacent planes.

Methods of preparation are given for several related spirans, especially those containing the spiro-1′(4′H)naphthalen-4′-one nucleus, and n.m.r. methods used to establish the configurations.