One-Step Synthesis of Racemic 4-Methyloctanoic Acid, a Component of the Aggregation Pheromone of Oryctes rhinoceros

Full text of DOI:10.1007/s10600-013-0489-0

Chemistry of Natural Compounds, Vol. 48, No. 6, January, 2013 [Russian original No. 6, November–December, 2012]
ONE-STEP SYNTHESIS OF RACEMIC 4-METHYLOCTANOIC ACID,
A COMPONENT OF THE AGGREGATION PHEROMONE
OF Oryctes rhinoceros
*
R. M. Sultanov, R. R. Ismagilov, and U. M. Dzhemilev
UDC 547.3+632.78
The principal constituents of the pheromone of male rhinoceros beetles of the genus Oryctes are 4-methyloctanoic
acid (1) and its ethyl ester [1].
Known methods for synthesizing racemic 1 are typically multi-step and in most instances use difficultly accessible
compounds as starting materials [2, 3–6].
We developed a convenient and efficient synthetic pathway to 1 that was based on the catalytic carbomagnesiation of
1-alkenes using EtMgCl (Et Mg) and TaCl catalyst that was recently discovered by us (Scheme 1) [7].
2
5
TaCl
+
EtMgX
+
R
5
R
[Ta] = TaL
n
[Ta]
2
EtMgX
MgX
[Ta]
R
R
[Ta]
3
4
R
R
H
H
MgX
R
R
+
XMg
MgX
A
B
R = n-C H , n-C H , n-C H etc.
4
9
5
11
6 13
X = Cl, Et
Scheme 1
Reaction of a 1-alkene with EtMgCl or Et Mg in the presence of TaCl forms organomagnesium compounds (OMC)
2
5
of type A and B in equimolar amounts in overall yield 98–99%.
Additional studies are needed in order to explain the detailed mechanism of the carbomagnesiation reaction of
1-alkenes using ethylmagnesium derivatives in the presence of TaCl . However, experimental results obtained by us and
5
published in the literature on the synthesis and transformations of tantalocyclopentane derivatives [8–14] enabled us to propose
catalytic cycles of forming OMC A and B (Scheme 1) [7].
Scheme 1 shows that the reaction of ꢀ-substituted tantalocyclopentane 2, which was formed from TaCl , a 1-alkene,
5
and EtMgX with an excess of EtMgX occurs along two practically equivalent parallel pathways. The first pathway assumes
the formation of a labile bimetallic complex 3 via transmetallation of 2 by EtMgX. Then, 3 is transformed into OMC A as a
result of a ꢀ-hydride transfer. The second pathway occurs stepwise via transmetallation of intermediate 2 using EtMgX. This
leads to bimetallic complex 4, which also is transformed into final OMC B as a result of a ꢀ-hydride transfer.
Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, Ufa, 450075, Prosp. Oktyabrya, 141, Russia,
fax: (3472) 84 27 50, e-mail: ink@anrb.ru. Translated from Khimiya Prirodnykh Soedinenii, No. 6, November–December,
2012, pp. 991–992. Original article submitted March 11, 2012.
©
1122
0009-3130/13/4806-1122 2013 Springer Science+Business Media New York

We found during a search for selective methods for preparing only one of the OMC (A or B) that TaCl activated by
5
the electron-donating ligand Ph P is capable of selective formation of OMC B.
3
In order to demonstrate the synthetic potential of the developed method, we showed that OMC synthesized in this
manner can be used in the synthesis of useful compounds, in particular, 1.
Thus, the reaction of hex-1-ene with Et Mg in the presence of TaCl -Ph P catalyst (Et Mg–hex-1-ene–TaCl -Ph P =
2
5
3
2
5
3
110:100:5; TaCl -Ph P = 1:1, concentration of Et Mg 0.65 mmol/mL THF, 20°C, 3 h) in THF at room temperature formed
5
3
2
OMC 5 in 98% yield. Then, 5 was reacted with CO . Finally, hydrolysis of the reaction mixture by aqueous HCl (5%) gave
2
target compound 1 in 70% yield (Scheme 2).
a
b
EtMgCl
ClMg
HO C
2
+
5
1
+
a. TaCl –Ph P, THF, 3 h; b. 1. CO , 2. H O
5
3
2
3
Scheme 2
Compound 1 was isolated pure by chromatographic separation over a column of SiO using petroleum ether:t-BuOMe
2
13
(5:1). The structure of 1 was proved using IR, PMR, and C NMR spectral methods and GC-MS in addition to comparing the
resulting pheromone 1 with an authentic sample [2, 3–6].
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