Conformational Studies of Marine Polyhalogenated α-Chamigrenes Using Temperature-dependent NMR spectra inverted-chair and twist-boat cyclohexane moieties in the presence of an axial halogen atom at C(8)

✍ Scribed by Graziano Guella; Giuseppe Chiasera; Francesco Pietra

• Publisher: John Wiley and Sons
• Year: 1992
• Tongue: German
• Weight: 911 KB
• Volume: 75
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19920750627

✦ Synopsis

~e~~~ a-Chamigren-3-one (+)-8 bearing an axial C1-atom at C(8) exists as a largely dominant conformer with MeeC(5) at the envelope-shaped enone ring pointing away from CI,,-C(8) at the cyclohexane ring ( = B) in the 'normal' chair conformation, as shown by 'H-NMR. In contrast, the a -chamigren-3-ols (+)-9 and (+)-lo, 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 CI,,-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 CI,,-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 H,,-C(7), as shown by the 'H-NMR data of the isomeric enone (+)-16 where the 'normal'-chair conformer Ca-16 is in an equilibrium with the inverted chair conformer ICP-16 (Scheme 3 ) . These results and the accompanying paper allow a unifying view on the conformational behavior of marine polybalogenated a -chamigrenes. This view is supported by the acid-induced isomerization of c( -chamigrene (+)-9 (inverted chair) to 8-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 P-chamigrenes are so common in nature.

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Coupling patterns in C,D6 (J = 10.2 and 4.7 Hz) and in CD,OD (J = 10.4 and 4.7 Hz) indicated solvent-dependent conformational equilibria, as expected for conformers which differ largely in dipole moments.