Transannular Interactions in Difunctional Medium Rings. 2. Molecular Structure and Conformational Properties of 1-Alkylhexahydroazocin-5-ones

Article abstract of DOI:10.1021/jo00391a035

Conformational properties of 1-alkylhexahydroazocin-5-ones 1-4 with a methyl, ethyl, isopropyl, or tert-butyl group as a substituent have been studied by He I photoelectron, 13C NMR, and IR spectroscopy as well as by MM2 and MNDO calculations.The molecular structure of the tert-butyl derivative 4 was determined by X-ray crystallography.Transannular interactions of the two functional groups in 1-4 have been determined from the first two ionization potentials with reference to the corresponding data of the respective monofunctional compounds.Since the ΔIP(n₀) values of 1-4 are nearly identical, these molecules must have rather similar conformations. δ_13C values of the carbonyl group in 1-4 show a systematic variation with substituent size, but this is probably caused by different solvation.From the IR spectra of 1 and 4 in the solid and liquid state and in solution it is concluded that phase transition is not accompanied by conformational changes.The eight-membered ring in 1-4 has a boat-chair conformation with the functional groups occupying positions with the shortest possible transannular distance (approximately 270 pm).It has been found difficult to determine the most stable conformation by theoretical methods, since transannular amide resonance is not treated correctly.For this interaction an energy of approximately 12 kJ/mol is estimated for 4 by comparison of experimental and calculated molecular structures.