Protomeliacins. The triterpenes containing a C8 side chain at C-17 are supposed to be
biogenetic precursors of limonoids and hence are known as protolimonoids or protomeliacins or
melianes.87 Meliantriol (25) was the first tetracyclic triterpenyl alcohol biogenetically related to 20
(S)-tirucallol, isolated from both pure neem oil and the fresh fruits of Melia azedarach and shown
to exhibit marked feeding inhibition against desert locust.83 Its structure was well established
and confirmed by its synthesis from melianone. Siddiqui and his co-workers have added other
protomeliacins (Table 2). Nimbocinone137 (26), nimolinonel40 (27), kulactonel46 (28), limocinol
(29) and limocinone (30)145 are derivatives of euphol/tirucallol while azadirachtol132 (31),
azadirachnol (32)147 and azaditoll46 (33) fall under apo-series. Interestingly, kulactone (28) was
earlier isolated from M. azedarch.151 The latest additions, viz. limocin A and B (34 and 35) and
limocinin (36) are tetranortriterpenoids having a tetrahydrofuran hemiacetal as the side chain and
are probably precursors of limonoids.145 The structures 26-36 have been elucidated by
comparing their 1H and I3C- nmr spectra with those of known model compounds and mass
spectral fragmentation data have been complemented in support of the proposed structures. The
absolute stereochemistry of nimbocinone (26) was elucidated by the application of Horeau's
method as well as from the spectral information generated through NOESY. The position of a
trisubstituted double bond in 27-30 was ascertained as 7 (and not Δ14) by its conversion to 7,9
(11)-heteroannular dienes through dehydrogenation with Hg(OAc)2137. The absolute
stereochemistry of the synthetically known nimolinone (27) was established through NOESyl40
but that of azadirachtol (31) and its dihydro derivative (32) at the side chain has not been
established. The latter compound is now given another name, naheedin without any reason! The
compounds 26, 31 and 32 are the first triterpenoids from neem, which contain hydroxy groups at
C-26 and C-II respectively. None of the compounds was reported to have any natural pest control
properties. Nimbocinone (26) has been isolated from neem leaves while most of other
constituents from fruit coats and whole fruits.
biogenetic precursors of limonoids and hence are known as protolimonoids or protomeliacins or
melianes.87 Meliantriol (25) was the first tetracyclic triterpenyl alcohol biogenetically related to 20
(S)-tirucallol, isolated from both pure neem oil and the fresh fruits of Melia azedarach and shown
to exhibit marked feeding inhibition against desert locust.83 Its structure was well established
and confirmed by its synthesis from melianone. Siddiqui and his co-workers have added other
protomeliacins (Table 2). Nimbocinone137 (26), nimolinonel40 (27), kulactonel46 (28), limocinol
(29) and limocinone (30)145 are derivatives of euphol/tirucallol while azadirachtol132 (31),
azadirachnol (32)147 and azaditoll46 (33) fall under apo-series. Interestingly, kulactone (28) was
earlier isolated from M. azedarch.151 The latest additions, viz. limocin A and B (34 and 35) and
limocinin (36) are tetranortriterpenoids having a tetrahydrofuran hemiacetal as the side chain and
are probably precursors of limonoids.145 The structures 26-36 have been elucidated by
comparing their 1H and I3C- nmr spectra with those of known model compounds and mass
spectral fragmentation data have been complemented in support of the proposed structures. The
absolute stereochemistry of nimbocinone (26) was elucidated by the application of Horeau's
method as well as from the spectral information generated through NOESY. The position of a
trisubstituted double bond in 27-30 was ascertained as 7 (and not Δ14) by its conversion to 7,9
(11)-heteroannular dienes through dehydrogenation with Hg(OAc)2137. The absolute
stereochemistry of the synthetically known nimolinone (27) was established through NOESyl40
but that of azadirachtol (31) and its dihydro derivative (32) at the side chain has not been
established. The latter compound is now given another name, naheedin without any reason! The
compounds 26, 31 and 32 are the first triterpenoids from neem, which contain hydroxy groups at
C-26 and C-II respectively. None of the compounds was reported to have any natural pest control
properties. Nimbocinone (26) has been isolated from neem leaves while most of other
constituents from fruit coats and whole fruits.
| Table 2. Protomeliacins occurring in different parts of neem |
Stru- Name Molecular m.p. Isolated form Ref.
ture Formula Weight (*C) No.
No.
25. Meliantriol C30H50O5 490 176 Seed oil 83
26. Nimbocinone C30H46O4 470 78 Fresh winter leaves 137
27. Nimolinone C30H44O3 452 190 Ripe uncrushed fruits 140
28. Kulactone C30H44O3 452 164 Fruit coats 146
29. Limocinol C30H50O3 420 - Fruit coats 145
30. Limocinone C30H48O 424 - Fruit coats 145
31. Azadirachtol C32H46O6 526 112 Ripe uncrushed fruits 132
32. Azadirachnol/Naheedin C32H48O6 528 171 Fresh whole fruit 147
33. Azaditol C32H46O6 526 112 Fruit coats 146
34. Limocin-A C29H42O5 470 - Fruit coats 145
35. Limocin-B C29H42O5 470 - Fruit coats 145
36. Limocinin C34H44O6 548 - Fruit coats 145
ture Formula Weight (*C) No.
No.
25. Meliantriol C30H50O5 490 176 Seed oil 83
26. Nimbocinone C30H46O4 470 78 Fresh winter leaves 137
27. Nimolinone C30H44O3 452 190 Ripe uncrushed fruits 140
28. Kulactone C30H44O3 452 164 Fruit coats 146
29. Limocinol C30H50O3 420 - Fruit coats 145
30. Limocinone C30H48O 424 - Fruit coats 145
31. Azadirachtol C32H46O6 526 112 Ripe uncrushed fruits 132
32. Azadirachnol/Naheedin C32H48O6 528 171 Fresh whole fruit 147
33. Azaditol C32H46O6 526 112 Fruit coats 146
34. Limocin-A C29H42O5 470 - Fruit coats 145
35. Limocin-B C29H42O5 470 - Fruit coats 145
36. Limocinin C34H44O6 548 - Fruit coats 145
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