Studies in Natural Product Chemistry

[摘要] Section 1. The correlation between the infra-red carbonyl frequency and conformation of delta-lactones was based on the study of a limited number of examples. To substantiate the correlation, a pair of epimeric delta-lactones were synthesised by degradation of the sesquiterpene drimenol. One of the pair was forced to adopt a half-chair conformation and had an infrared carbonyl frequency of 1740 cm -1. The other delta-lactone was forced to adopt the half-boat conformation and had an infra-red carbonyl frequency of 1755 cmThe preferred conformation of simple unstrained delta-lactones has not previously been determined. The n. m. r., infra-red and circular dichroism spectra of the lactone of 5-hydroxy-5-t-butyl-pentanoic acid could be best interpreted in terms of it existing as an equilibrium mixture of half-boat and half-chair conformers with the latter predominating. The circular dichroism and optical rotatory dispersion curves of a number of delta-lactones were examined and the signs of their Cotton effects found to be best interpreted in terms of the rule which correlates sign with the chirality of the lactone ring. Investigation of the n. m. r. spectra of columbin, isocolumbin and decarboxy columbin confirmed that the conformation of the first two delta-lactones was the same in solution as in the solid state as determined by X-ray crystallography. Section 2. The conformation of medium ring lactones have not previously been examined. The n. m. r. spectrum of cycloheptanolide at 173 K was consistent with it adopting a boat-chair conformationwith the lactone group occupying one side of the molecule. The n. m. r. spectra of cyclononanolide and derivatives at different temperatures and in different solvents were consistent with cyclononanolide adopting a boat-chair-boat conformation similar to that preferred by cyclodecane. In addition the conformational inversion processes occuring in these two molecules are discussed. The free energy of activationof cycloheptanolide was found to be 8.9+/-2.0 K cal mole -1and for cyclononanolide 9.2+/-1.0 K cal mole -1.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry