K. Nabeta et al., Biosynthesis of kelsoene and prespatane in cultured cells of the liverwortPtychanthus striatus, J CHEM S P1, (16), 2000, pp. 2703-2708
Biosynthesis of the unusual sesquiterpenes, (+)-kelsoene and (-)-prespatane
, that accumulate in the cultured cells of the liverwort Ptychanthus striat
us was investigated. A mixture of (+)- and (-)-alloaromadendrene was isolat
ed from a variety of unidentified sesquiterpenes from cultured cells. Howev
er, the (-)-isomer, which is thought to be an immediate precursor to kelsoe
ne, was present only as a minor component [a ratio of (-)-isomer to (+)-iso
mer of 1:25]. When the cells were incubated with a higher dosage (1.0 mmol)
of mevalonate (MVA, 3,5-dihydroxy-3-methylvalerate) under heterotrophic co
nditions or with a normal dosage (0.5 mmol) under photoautotrophic conditio
ns, the H-2- and C-13-labels from H-2- and C-13-MVA were incorporated into
the sesquiterpenes at an extremely high level (30 to 40 and 46 atom% excess
, respectively). Labelling patterns of the biosynthetically H-2- or C-13-la
belled kelsoene and prespatane were determined by GLC-MS and H-2- and C-13-
NMR analyses. The labelling pattern of the kelsoene demonstrates the random
ization of the H-2- and C-13-labels between the isopropenylmethyl and the m
ethylene groups, with loss of one H-1 proton of farnesyl diphosphate (FPP,
farnesyl=3,7,11-trimethyldodeca- 2,6,10-trienyl), suggesting that kelsoene
is biosynthesized from a germacradienyl cation with 7R configuration by mea
ns of a (-)-alloaromadendranyl cation. Randomization of the labels between
the isopropenylmethyl and the methylene and loss of one H-1 proton of FPP w
ere also observed in H-2- and C-13-labelled prespatane. Thus, prespatane is
postulated to be biosynthesized from a (7S)-germacradienyl cation by means
of a guaianyl cation. During the third cyclization of the guaianyl cation,
one proton at the C-1 position of FPP was lost.