3. VII. 92)
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( ) 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. ') ' ) 3, Presented in part by G.G. at both CISCI 90, S. Benedetto del Tronto, 30 September-5 October, 1990, and the Congress0 Nazionale di Risonanze Magnetiche, Pisa, 22-24 October, 1990. We use the chamigrene numbering for the structural formulae and spectroscopic data (see (+)-1); for IUPAC nomenclature and numbering for retrieval purposes, see Exper. Part. Many other compounds of red seaweeds are involved [3]. (although not yet proven by biosynthetic experiments) central role of chamigrene precursors in the biogenesis of many sesquiterpene skeletons found in seaweeds of the genus Laurencia [2] [5], and d ) a general interest in spirocyclic terpenes [4]. Since the bioactivity and the reactivity of chamigrenes are likely to depend largely onconformational factors, we deemed it interesting to investigate their conformational behavior.
In this paper, we study a -chamigrenes bearing equatorial halogen atoms at C(8) and C(9) and, in the accompanying paper, those having an axial substituent at C(8
Representative examples of the former were obtained by chemical transformations of the polyhalogenated P-chamigren-3-01s (+)-1 to (-)-4 [ 11 [9]. a -Chamigrenes of the first type for which the conformational behavior is already known include the a -chamigren-9-01 glanduliferol ((-)-5) and its 9-chloro analogue (-)-6 [7] (isolated from Laurencia glandulifera), besides the a -chamigren-3-01(+)-7 (isolated from Laurencia nipponica collected at Hamamasu and other locations in Japan [7]), its acetate (+)-8 [7], the a -chamigren-3one (-)-9 [7], and laurencenone D [8] (10; isolated from Laurencia obtusa collections of Jamaica). The structure of laurencenone D [8] (10) needs a revision, however [9]: probably it belongs to the above mentioned second type of a -chamigrenes [6].
' )
9,
Further support for the occurrence of conformational phenomena was provided by the I3C-NMR spectra of enone (-)-15 where several signals were broad (Table ). On elution of (+)-20/(+)-21 from a silica-gel column (HPLC), some isomerization to the corresponding P-chamigrenes 22 and 23 was observed (Scheme 4 ) .