ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Temperature-dependent NMR spectra indicate that the α-chamigren-3-ones (-)-11, (+)-12, (+)-14 (-)-15, (+)-16, 18, and 19 bearing equatorial halogen atoms at C(8) and C(9) undergo slow conformational flipping of the envelope-shaped enone ring, while the cyclohexane ring is maintained in the chair conformation. The α-chamigren-3-ols (+)-20 and (+)-21, obtained by hydride reduction of (+)-12, behave similarly, with slow half-chair inversion of the cyclohexenol ring. In each case, both conformers are about equally populated and detectable by NMR, except in the case of (+)-15, where repulsive interactions between Br—C(2) and Heq-C(7) make the population of the conformer 15b with Me - C(5) faced to Hax-C(10) so low that it escapes direct 1H-NMR detection. The energy barriers to these conformational motions are viewed to arise mainly from repulsive interactions between Me - C(5) and the axial H-atoms at C(8) and C(10), while, contrary to previous beliefs, no twist-boat conformations of the cyclohexane ring intervene. Similar conclusions hold for the 4,5-epoxides of both (-)-6 and (+)-7. Clean Jones oxidatio of (-)-2 to 17, where the CH2=C(5) bond is maintained, and acid dehydration-isomerization of the α-chamigrene (+)-21 to the β-chamigrene (+)-24, reflect the special stability of β-chamigrenes, providing a reason for their frequent occurrence in nature.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19920750626