STEREOELECTRONIC STABILIZATION IN NON-CHAIR CONFORMATIONS OF SUBSTITUTED-2,3-DIHYDRO-1,4-BENZODIOXEPINS

Article abstract of DOI:10.1016/S0040-4020(01)86008-7

2,3-Dihydro-1,4-benzodioxepin (5) and its 3-substituted derivatives 6-9 have been studied by (13)C and (1)H dynamic NMR in two solvent systems of different polarity.The aromatic signals were found to be quite sensitive to the nature of the seven-membered cyclic geometries and were used as "conformational probes".The results show that the chair (C) conformation is the only form detected at -120 deg C for the parent compound 5 and its 3-methyl derivative 6 whereas the twist-boat (TB) comformation becomes predominant for three derivatives, 3,3-dimethyl 7, 3-metoxy 8 and 3,3-methylmethoxy 9, as a consequence of both steric and stereoelectronic interactions.Furthermore, in the case of 8, the detailed geometry of the TB forms detected reflects on the relative importance of competing stereoelectronic (anomeric and gauche) effects.