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)

Abstract

Temperature-dependent NMR spectra indicate that the a -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 a -chamigren-3-01s (+)-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 H,,-C(7) make the population of the conformer 15b with Me-C(5) faced to H,,-C(10) so low that it escapes direct '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(lO), 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 oxidation of (-)-2 to 17, where the CH,=C(5) bond is maintained, and acid dehydration-isomerization of the a -chamigrene (+)-21 to the P-chamigrene (+)-24, reflect the special stability of b-chamigrenes, providing a reason for their frequent occurrence in nature.

The Laurencia Paradox: An Endless Source of Chemodiversity

“…Moreover, less frequently halogenation on a double bond or a conjugated bromodiene system is incorporated into the second six-membered ring. In the 1990s, Guella and co-workers undertook thorough conformational studies of α-and β-chamigrenes using temperaturedependent NMR spectra [164,472,473]. Majusculone (130) and mailione (131) are the only two known nor-chamigranes isolated from a number of different Laurencia species [118,122,128,134,142,143,147].…”

“…Laurencia okamurai collected near Nanji Island in the East China Sea afforded seco-laurokamurone (469) featuring a monocarbocyclic framework, possibly formed by oxidative cleavage of the rearranged laurane skeleton [275]. L. nipponica from Hokkaido, Japan, yielded the seco-chamigranes lauracetals A (470), B (471), C (474), D (472), and E (473), possessing a carbocycle that could arise from a 10-bromo-chamigrene derivative by oxidative cleavage of the bond between C-1 and C-2, followed by acetal formation [75,199,[438][439][440]. Dactylol (478) and poitediol (479) were reported as constituents of Aplysia dactylomela [442] and L. poitei [441], respectively, almost simultaneously in 1978 by two different research groups.…”

Progress in the Chemistry of Organic Natural Products 102

“…Aplysistatin L. filiformis [341], L. flexilis [331], L. implicata [91,114], L. karlae [333], L. luzonensis [334], L. saitoi [335], L. similis [325], L. snackeyi [118,150,[328][329][330]339] Palisadin A L. flexilis [331], L. implicata [91,114], L. karlae [333], L. luzonensis [323,334], L. saitoi [335], L. similis [325], L. snackeyi [118,150,328 -L. mariannensis [198] (continued) seco-chamigrane (470)(471)(472)(473)(474) not designated (486) not designated (488)(489) not designated (484) pacifigorgiane (485) guaiane (480) poitane (precapnellane) (478)(479) oppositane (481)(482)(483) seco-laurokamurane (469) germacrane (466) humulane (467)…”

“…Among these, derivatives of known skeletons, such as the germacrane 466, the humulanes 467 and 468, the cadinanes 475 and 476, the oplopane 477, the guaiane 480, the triquinanes 490-492, the cedrane 494, the africanane 495, and the aromadendranes 496-498, have been isolated, and are all non-halogenated with the exception of 494. L. nipponica from Hokkaido, Japan, yielded the seco-chamigranes lauracetals A (470), B (471), C (474), D (472), and E (473), possessing a carbocycle that could arise from a 10-bromo-chamigrene derivative by oxidative cleavage of the bond between C-1 and C-2, followed by acetal formation [75,199,[438][439][440]. Laurencia okamurai collected near Nanji Island in the East China Sea afforded seco-laurokamurone (469) featuring a monocarbocyclic framework, possibly formed by oxidative cleavage of the rearranged laurane skeleton [275].…”