Most of the biogenetic proposals .are tentative as they are not supported by valid biosynthetic
studies. There is only one instance of biosynthetic investigation in neem, that of nimbolide in
neem leaves42-43. Feeding experiment42 with the leaves have shown that euphol is more
efficiently incorporated into nimbolide than are its isomers 7 butyrospermol or 8 (20 α-H)
tirucallol and 7-tirucallol, thereby suggesting that the biosynthesis of C-secomeliacins may
involve the 7.9(11) diene which can easily be formed from an 8α, 9 α -epoxide. Subsequent
studies43 involving the feeding of 3H- euphol, 8 α, 9 α -epoxy 3H) euphol and the latter's 24,25-
epoxy derivative revealed that the epoxy-euphols were more efficiently utilized than euphol,
clearly confirming the intermediatory role of 7.9(11) diene. This was supported by the
biosynthesis of nimbolide from (2_14C (4R)-4.3H) mevalonic acid lactone, with the retention of
radioactivity in a 3H: 14C ratio of 3:5, a ratio theoretically expected for the above diene pathway.
On the basis of the above results, it was postulated that in the biosynthesis of C-secomeliacins,
the 7 function of the above diene would be oxidatively rearranged via its epoxide to give the 7-
0H-apo skeleton and the .9(11) bond formed would activate the C-12 position towards eventual
oxidation leading to C-ring fission. Thus, the biogenetic pathway for C-secomeliacins seems to
be different from others.
studies. There is only one instance of biosynthetic investigation in neem, that of nimbolide in
neem leaves42-43. Feeding experiment42 with the leaves have shown that euphol is more
efficiently incorporated into nimbolide than are its isomers 7 butyrospermol or 8 (20 α-H)
tirucallol and 7-tirucallol, thereby suggesting that the biosynthesis of C-secomeliacins may
involve the 7.9(11) diene which can easily be formed from an 8α, 9 α -epoxide. Subsequent
studies43 involving the feeding of 3H- euphol, 8 α, 9 α -epoxy 3H) euphol and the latter's 24,25-
epoxy derivative revealed that the epoxy-euphols were more efficiently utilized than euphol,
clearly confirming the intermediatory role of 7.9(11) diene. This was supported by the
biosynthesis of nimbolide from (2_14C (4R)-4.3H) mevalonic acid lactone, with the retention of
radioactivity in a 3H: 14C ratio of 3:5, a ratio theoretically expected for the above diene pathway.
On the basis of the above results, it was postulated that in the biosynthesis of C-secomeliacins,
the 7 function of the above diene would be oxidatively rearranged via its epoxide to give the 7-
0H-apo skeleton and the .9(11) bond formed would activate the C-12 position towards eventual
oxidation leading to C-ring fission. Thus, the biogenetic pathway for C-secomeliacins seems to
be different from others.
The biogenesis of nimbinene can be anticipated by C-28 decarboxylation of nimbin, desacetyl
nimbin etc. It is generally believed that the C-28 oxidation and decarboxylation precedes those of
C-29 in the biogenetic process because of the occurrence of several nimbin analogues
containing C-28 oxygen function.38
nimbin etc. It is generally believed that the C-28 oxidation and decarboxylation precedes those of
C-29 in the biogenetic process because of the occurrence of several nimbin analogues
containing C-28 oxygen function.38
Steroids
24-MethylenecycloartanoI40 (126), cyclo-eucalenoI40 (127), β-sitosterol and its β -D¬glucoside,
4O 4,14α-dimethyl-5 α -ergosta-8,24(28)-dien '3 β -ol and 4 α -methyl-5 α ¬ergosta-8,24(28)-
dien-3 β-01 from heartwood,18 stigmasteroll37 and cholesterol136 from flowers arid two new
glycosides of stigmasterol from seeds 144 were isolated.
24-MethylenecycloartanoI40 (126), cyclo-eucalenoI40 (127), β-sitosterol and its β -D¬glucoside,
4O 4,14α-dimethyl-5 α -ergosta-8,24(28)-dien '3 β -ol and 4 α -methyl-5 α ¬ergosta-8,24(28)-
dien-3 β-01 from heartwood,18 stigmasteroll37 and cholesterol136 from flowers arid two new
glycosides of stigmasterol from seeds 144 were isolated.
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |