ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
α-Chamigren-3-one (+)-8 bearing an axial CI-atom at C(8) exists as a largely dominant conformer with Me - C(5) at the envelope-shaped enone ring pointing away from CIax-C(8) at the cyclohexane ring (= B) in the ‘normal’ chair conformation, as shown by 1H-NMR. In contrast, the α-chamigren-3-ols (+)-9 and (+)-10, obtained from hydride reduction of (+)-8, show a temperature-dependent equilibrium of conformers where the major conformers have ring B in the inverted-chair (and twist-boat for (+)-9) conformation to avoid repulsions between Me-C(5) and CIax-C(8) (Scheme 1). This is in agreement with the conformation of the epoxidation product (+)-12 of (+)-9 where Me-C(5) is pushed away from CIax-C(8) in a ring-B chair similar to that of (+)-8 (Scheme 2). Introduction of a pseudoequatorial Br-atom at C(2) of (+)-8, as in enone (+)-15 (Scheme 3), does not affect the conformation; but a pseudoaxial Br—C(2) experiences repulsive interactions with Heq-C(7), as shown by the 1H-NMR data of the isomeric enone (+)-16 where the ‘normal’-chair conformer Cβ-16 is in an equilibrium with the inverted chair conformer ICβ-16 (Scheme 3). These results and the accompanying paper allow a unifying view on the conformational behavior of marine polyhalogenated α-chamigrenes. This view is supported by the acid-induced isomerization of α-chamigrene (+)-9 (inverted chair) to β-chamigrene (+)-17 (‘normal’ chair; Scheme 4), the driving force being the lesser space requirement of CH2=C(5) than of Me-C(5). This explains why β-chamigrenes are so common in nature.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19920750627