Stereochemistry of 1,6-germacradien-5-ol, a constituent of the needles of Scots pine (Pinus sylvestris) and of the defence secretion from larvae of the pine sawfly Neodiprion sertifer

Abstract

(-)-1,6-Germacradien-5-ol [(E,E)-1,5-dimethyl-8-isopropylcyclodeca-1,6-dien-5-ol, 1] has been isolated from the defence secretion from larvae of the pine sawfly Neodiprion sertifer and from needles of Scots pine (Pinus sylvestris). It was characterised by means of spectroscopic methods and by its optical rotation. Acid-promoted transannular cyclisation isomerised 1 to the known (-)-α-cadinol (4), whereby the configuration at position 8 was established as S. The configuration at the second stereocentre, position 5, carrying the alcohol group is discussed on the basis of molecular mechanics calculations (MM2), NMR shift values, coupling constants and NOESY experiments. 1-endo-Bourbonanol (7a) and (-)-5-methoxy-1,6-germacradiene (8) were prepared from 1 and investigated by NMR spectroscopy. The evidence collected indicated that the stereoisomer isolated was (5S,8S)-1,6-germacradien-5-ol.