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Conformational Studies of Marine Polyhalogenated α‐Chamigrenes Using Temperature‐Dependent NMR Spectra. Cyclohexene‐ring flipping and rigid‐chair cyclohexane ring in the presence of equatorial halogen atoms at C(8) and C(9)
Author(s) -
Guella Graziano,
Chiasera Giuseppe,
Pietra Francesco
Publication year - 1992
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19920750626
Subject(s) - chemistry , conformational isomerism , cyclohexane , ring flip , ring (chemistry) , nmr spectra database , cyclohexane conformation , crystallography , cyclohexene , halogen , isomerization , stereochemistry , population , spectral line , hydrogen bond , molecule , alkyl , catalysis , organic chemistry , physics , demography , astronomy , sociology
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 BrC(2) and H eq −C(7) make the population of the conformer 15b with Me—C(5) faced to H ax −C(10) so low that it escapes direct 1 H‐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 CH 2 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.