Carnosol. Crystal structure,absolute configuration,and spectroscopic properties of a diterpene

The structure and absolute configuration of naturally occurring Carnosol have been investigated by X-ray analyses. Crystal data are: orthorhombic P2₁2₁2;a=15.762(1),b=13.755(1),c=7.7747(7) Å,Z=4,V=1688.2 ų, andR=0.031 (2569 reflections). The absolute configuration is established at a significance level better than 0.995 according to HamiltonsR-factor test and is in accordance with that predicted experimentally by chemically means. Correlation of the solid-state IR pattern of the regions sensitive to hydrogen bonding (CO and OH stretching bands) with the X-ray crystal structure is excellent and shows, that even weak inter- and intramolecular hydrogen bonds may be unambiguously identified by IR spectroscopy. The variable-temperature nuclear magnetic resonance studies (NMR) show that the twinning of the signals from the isopropyl methyl protons cannot be due to hindered rotation but can be ascribed magnetic nonequivalence induced by the chiral centers.