Synthesis of dimer and oligomers from (R)-methyl hydnocarpate

Article abstract of DOI:10.1007/s11746-000-0173-7

We report synthesis and characterization of dimer and oligomer acids from chaulmoogra oil. (R)-Methyl hydnocarpate (methyl ester of the major fatty acid component of chaulmoogra oil) was brominated to give threo-2,3-dibromocyclopentane-1-methyl undecanoate. Formation of two diastereoisomers, viz., threo-2(R),3(R)-dibromocyclopentane-1(R)-methyl undecanoate and threo-2(S),3(S)-dibromocyclopentane-(R)-1-methyl undecanoate, was observed. Dehydrobromination of bromo derivatives using alcoholic KOH gave a cyclopentadiene derivative as intermediate, which underwent Diels-Alder reaction to give dimer and oligomer fatty acids. The products were characterized by ultraviolet, direct exposure probe-mass spectroscopy, ¹H nuclear magnetic resonance (NMR), and ¹³C NMR spectroscopic techniques.

Full text of DOI:10.1007/s11746-000-0173-7

Synthesis of Dimer and Oligomers from (R)-Methyl Hydnocarpate
Navdeep B. Malkara,*, Ashish A. Vaidyab, and V.G. Kumarc
aFlorida Atlantic University, Department of Chemistry and Biochemistry, Boca Raton, Florida 33431,
bLehigh University, Chemical Engineering Department, Bethlehem, Pennsylvania 18015,
and cHindustan Lever Research Centre, Andheri,
of such things as nonreactive and reactive polyamide resin
(17), corrosion inhibitors, and lubricants (18,19). Most of the
present-day processes involve use of tall oil fatty acids as a
precursor for formation of dimer and oligomer acids.
Mumbai - 400 099, India
ABSTRACT: We report synthesis and characterization of dimer
and oligomer acids from chaulmoogra oil. (R)-Methyl hydno-
carpate (methyl ester of the major fatty acid component of chaul-
moogra oil) was brominated to give threo-2,3-dibromocyclopen-
tane-1-methyl undecanoate. Formation of two diastereoisomers,
viz., threo-2(R),3(R)-dibromocyclopentane-1(R)-methyl unde-
canoate and threo-2(S),3(S)-dibromocyclopentane-(R)-1-methyl
undecanoate, was observed. Dehydrobromination of bromo de-
rivatives using alcoholic KOH gave a cyclopentadiene deriva-
tive as intermediate, which underwent Diels-Alder reaction to
give dimer and oligomer fatty acids. The products were charac-
We report a synthesis of novel endo-1,4-bis (methyl un-
decanoate)-tricyclo [5,2,1,02,6]deca-4,8-diene (methyl ester of
dimer acid) from 2-cyclopentene-1(R)-methyl undecanoate
(methyl hydnocarpate). Methyl hydnocarpate was obtained
from chaulmoogra oil. We also report a synthesis of threo-
2(R/S), 3(R/S)-dibromocyclopentane-1(R)-methyl undecano-
ate (bromo derivative of methyl hydnocarpate) from methyl
hydnocarpate. The stereoisomers of dibromo derivatives were
terized by ultraviolet, direct exposure probe–mass spectroscopy, characterized using 1H and 13C nuclear magnetic resonance
1H nuclear magnetic resonance (NMR), and 13C NMR spectro-
scopic techniques.
(NMR). The cyclopentene moiety present in methyl hydno-
carpate provided an opportunity to prepare a 1,3-cyclopenta-
diene-1-undecanoic acid (diene intermediate). The dibromo-
derivative on dehydrobromination gave an diene interme-
diate that underwent a further Diels-Alder reaction to form the
novel endo-1,4-bis (undecanoic acid)-tricyclo [5,2,1,02,6]deca-
4,8-diene (dimer) and oligomer acids. The dimer acid obtained
was structurally different from the dimer acids prepared from
tall oil, soybean oil, and rapeseed oil or dehydrated castor oil
fatty acids. Owing to these structural differences, dimer acid
prepared from chaulmoogra oil may show different properties
as compared to conventional dimer acids.
Paper no. J9477 in JAOCS 77, 1101–1105 (October 2000).
KEY WORDS: Dimer and oligomer acids, Flacourtiaceae,
Hydnocarpus wightiana, hydnocarpic acid, 13C NMR, 1H NMR.
Chaulmoogra oil obtained (1) from seeds of Hydnocarpus
wightiana and H. anthelmintica is a triglyceride of fatty acids
containing the cyclopentenyl (Cy) group. The fatty acids
obtained from this oil are a mixture of 2-cyclopentene-1-un-
decanoic acid (hydnocarpic acid, 50%), 13-(2-cyclopentene-
1-yl) 6-tridecenoic acid (gorlic acid, 10%) and 2-cyclopen-
tene-1-tridecanoic acid (chaulmoogric acid, 32%) (2,3).
Chaulmoogra oil and its derivatives show biological activity
(4,5) and have great medicinal value (6), especially in the
treatment of leprosy (7,8). Some derivatives are also used as
an intermediate in the preparation of perfumery compounds
(9,10). Our goal was to synthesize novel dimer or oligomer
acids from chaulmoogra oil.
Dimer acids obtained by dimerization (11) of various fatty
acids (conjugated and nonconjugated) are known in the liter-
ature. The fatty acid precursors used for the dimerization
(12–14) and oligomerization (15) are generally tall oil, soy-
bean oil, rapeseed oil, or dehydrated castor oil fatty acids.
These dimer acids reportedly show biological activity (16)
and are industrially important. They are used in preparation
EXPERIMENTAL PROCEDURES
Fourier transform infrared (FT-IR) spectra were recorded
using a Bomem infrared spectrophotometer (Québec City,
Canada). The 1H and 13C NMR spectra were taken in CDCl3
using tetramethylsilane as an internal standard using 200.13
MHz Bruker spectrometer (Karlsruhe, Germany). Chemical
shifts are reported in δ-units. In addition, Distortionless En-
hancement by Polarization Transfer (DEPT) pulse sequence
was used for 13C NMR experiment. Ultraviolet (UV) spec-
troscopy (20) was carried out on a Shimadzu Ultraviolet
Spectrophotometer (Kyoto, Japan). A sample aliquot (0.1 g)
taken from reaction mixture was dissolved in 50 mL methanol
in a volumetric flask. The UV spectrum of the sample solu-
tion was recorded over a range of 200 to 400 nm using
methanol as a blank. Absorbance of the sample at 239 nm was
monitored. Uerjeeta Jain’s Herbals (Mumbai, India) supplied
chaulmoogra oil. Gas–liquid chromatography (GLC) was car-
*To whom correspondence should be addressed at Florida Atlantic Univer-
sity, Department of Chemistry and Biochemistry, 777 Glades Rd., Boca
Raton, FL 33431. E-mail: nmalkar@fau.edu
Copyright © 2000 by AOCS Press
1101
JAOCS, Vol. 77, no. 10 (2000)

1102
N.B. MALKAR ET AL.
13C NMR: δ 174 (–COOCH3), 136 (–CH2–CH=CH–CH–
in cyclopentenyl ring), 130 (–CH2–CH=CH–CH– in cy-
clopentenyl ring), 52 (–COOCH3), 46 (–CH2–CH=CH–CH–
in cyclopentenyl ring), 32 (–CH2–CH=CH–CH– in cyclopen-
tenyl ring), 30 (–CH2–CH2–CH=CH–CH– in cyclopentenyl
ring), 37 (–CH–CH2–CH2–), 30 (–CH–CH2–CH2–), 28
(–CH–CH2–CH2–CH2–), 34 (–CH2–COOCH3), 25 (–CH2–
CH2–COOCH3), 28–30 (5C, –CH2–).
Synthesis of threo-2(R/S),3(R/S)-dibromo cyclopentane-
1(R)-methyl undecanoate. A solution of 2-cyclopentene-1(R)-
methyl undecanoate (methyl hydnocarpate, 50 g, 0.19 mmol)
in chloroform (200 mL) was cooled to 0–5°C. A solution of
bromine (33 g, 0.2 mmol) in chloroform (100 mL) was added
dropwise to methyl hydnocarpate solution, until bright yel-
low color of excess bromine appeared. The reaction mixture
was stirred for 3 h at 0–5°C. The excess bromine was
removed by washing the reaction mixture with water. The
chloroform layer was separated and dried over anhydrous
Na2SO4 threo-2(R/S),3(R/S)-dibromo cyclopentane-1(R)-
methyl undecanoate (76 g, 95% yield) was obtained by dis-
tilling off chloroform. The product was then characterized
using FT-IR, NMR, and GC–MS.
ried out on Shimadzu 14A and 14B gas chromatographs using
the following columns: DB-1 (J&W Scientific, Folsom, CA),
capillary column, internal diameter: 0.5 mm; length 30 m and
3% OV-17 liquid phase, adsorbent: Chromosorb WHP,
80–100 mesh (Chrompack, Middelburg, The Netherlands),
packed stainless steel column, 2 mm i.d., 1 ft length, with
flame-ionization detector. Mass spectral studies of the methyl
ester of dimer acid were carried out by Direct Exposure
Probe–Mass Spectroscopy (DEP–MS). The following pro-
gram was used: start at 25 mA current and hold for 30 s; in-
crease at a rate of 1 mA/s to 800 mA and hold for 30 s. Thin-
layer chromatography (TLC) was carried out on Whatman 60
Å Silica Gel precoated TLC plates (2.5 × 5.5 cm in size and
200 µm in thickness; Maidstone, United Kingdom). Prepara-
tive chromatography was carried out on Whatman 60 Å Sil-
ica Gel precoated TLC plates (20 × 20 cm in size and 500 µm
in thickness). Preparative TLC plates were activated before
use by heating them at 120°C for 2 h.
Synthesis of 2-cyclopentene-1(R)-methyl undecanoate
(methyl hydnocarpate). Chaulmoogra oil (200 g) and 5% al-
coholic KOH (2 L) were refluxed for 3 h. After complete hy-
drolysis, the reaction mixture was neutralized with 10% HCl,
and excess ethanol (1.8 L) was distilled off. The reaction mix-
ture was diluted with water (1 L) and then extracted with
hexane (3 × 1 L). The hexane layer was dried over anhydrous
Na2SO4. The chaulmoogra fatty acids (175 g, yield: 92%)
were obtained by distilling hexane completely.
These chaulmoogra fatty acids were then subsequently dis-
solved in methanol (1.5 L), and concentrated H2SO4 (0.1%
vol/vol) was added. The reaction mixture was refluxed for 4 h
and was monitored using TLC. After completion of reaction,
70% of the methanol was distilled off. The reaction mixture
was diluted with water, and crude methyl ester was extracted
in hexane. The hexane layer was washed with water to ensure
complete removal of acid. The organic layer was dried over
anhydrous Na2SO4. The crude methyl ester (179 g, 97%
yield) was obtained after distilling off hexane under vacuum.
The methyl ester was then analyzed by GLC. GLC analysis
showed 50.8% of 2-cyclopentene-1-methyl undecanoate
(methyl hydnocarpate), 10.2% of 13-(2-cyclopentene-1-yl)-
6-methyl tridecenoate (methyl gorlate) and 32.1% of 2-cy-
clopentene-1-methyl tridecanoate (methyl chaulmoograte).
The crude methyl ester was fraction-distilled using a 1-ft
glass column packed with dixon rings using a perkin triangle.
Methyl hydnocarpate (78.7 g, 88% yield) was collected at
144–146°C at 1-mm vacuum. The temperature of the oil bath
was maintained at 230°C. The product was characterized
using FT-IR, NMR, and gas chromatography–mass spectrom-
etry (GC–MS).
1H NMR (CDCl3): δ 3.9–4.9 (m, 2H, J = 5.2, 12 Hz,
–CH(Br)–CH(Br)– in cyclopentane ring), 3.66 (s, 3H,
–COOCH3), 2.30 (t, 2H, –CH2–COOCH3), 2.09 (bs, 1H,
–CHBr-CHBr–CH(–CH2–)–(CH2)10–COOCH3, in cyclopen-
tane ring), 2.03 (m, 4H, –CHBr–CHBr–CH2–CH2–CH–, in
cyclopentane ring), 3.66 (s, 3H, –COOCH3), 1.61 (s, 2H,
–CH2–CH2COOCH–), 1.27 (s, 16H, –CH2–).
13C NMR (CDCl3): δ 174 (–COOCH3), 50, 57, 62, and 66
(–CHBr–CHBr– in cyclopentane ring), 52 (–COOCH3), 43
(–CHBr–CHBr–CH– in cyclopentane ring), 28 (–CH2–
CHBr–CHBr– in cyclopentane ring), 27 (–CH2–CH2–
CHBr–CHBr– in cyclopentane ring), 33 (–CH–CH2–CH2–),
28 (–CH–CH2–CH2–), 27 (–CH–CH2–CH2–CH2–), 34
(–CH2–COOCH3), 25 (–CH2–CH2–COOCH3), 28–30 (5C,
–CH2–).
Synthesis of endo 1,4-bis(methyl undecanoate)-tricyclo
[5,2,1,02,6]deca-4,8-diene via diene intermediate. Threo-
2(R/S),3(R/S)-dibromo cyclopentane-1(R)-methyl undec-
anoate (50 g) was added to 10% alcoholic KOH solution (200
mL) with constant stirring at 60°C. 1,3-Cyclopentadiene-1-
undecanoic acid (diene intermediate) obtained during the re-
action underwent self Diels-Alder reaction to give dimer and
oligomers. The reaction mixture was stirred further for 11 h
and then cooled to 25°C. It was then acidified with 2 N HCl
and diluted with diethylether (200 mL). The stirring was fur-
ther continued for 20 h. The diethylether layer was separated
and further washed with water to ensure complete removal
of acid. This was then dried over anhydrous Na2SO4. The
orange-yellow product was obtained by distilling off di-
ethylether completely (43 g, 69% yield). The crude dimer and
oligomer acids were esterified using methanol in the presence
of H2SO4 and purified by preparative chromatography. A so-
lution of 100 mg of crude methyl esters of dimer and
oligomer acid dissolved in diethylether (1 mL) was loaded
1H NMR (CDCl3): δ 5.68 (m, 2H, –CH=CH– in cyclopen-
tenyl ring), 2.6 (bs, 1H, –CH=CH–CH–(–CH2–)–(CH2)10–
COOCH3, in cyclopentenyl ring), 2.30 (m, 2H,
–CH=CH–CH2–CH2, in cyclopentenyl ring), 2.02 (m, 2H,
–CH=CH–CH2–CH2–CH–, in cyclopentenyl ring), 3.66 (s,
3H, –COOCH3), 2.30 (t, 2H, –CH2–COOCH3), 1.61 (s, 2H,
C
–CH COOCH ), 1.27 (s, 16H, –C –).
H2
H2
2
3
JAOCS, Vol. 77, no. 10 (2000)

SYNTHESIS OF DIMER AND OLIGOMERS FROM (R)-METHYL HYDNOCARPATE
1103
FIG. 1. 1H Nuclear magnetic resonance spectrum of threo-2,3-dibromocyclopentane-1-methyl undecanoate.
onto preparative TLC plates by means of a 100-µL syringe. ence of 51.3% methyl hydnocarpate [Scheme 1, 2-cyclopen-
Preparative chromatography was carried out using hexane/di- tene-1(R)-methyl undecanoate; 16:1 Cy], 34.7% methyl chaul-
ethylether/acetic acid in 7.0:2.9:0.1 proportions. Plates were moograte (18:1 Cy) and 9.8% methyl gorlate (18:2 Cy). These
developed in an iodine chamber. The colored bands obtained methyl esters were separated by fractional distillation.
were separated and extracted in diethylether. Diethylether
To form a dimer acid (via Diels-Alder reaction) it is nec-
was evaporated under vacuum to yield 58 mg of pure methyl essary to have a conjugated diene moiety in the precursor.
ester of dimer acid. The endo 1,4-bis(methyl undecanoate)- Conjugation in methyl hydnocarpate was achieved by bromi-
tricyclo [5,2,1,02,6]deca-4,8-diene thus obtained was charac- nation followed by subsequent dehydrobromination. Bromi-
terized by FT-IR and NMR.
nation of methyl hydnocarpate gave a threo-2,3-dibromocy-
1H NMR (CDCl3; Fig. 1): δ 5.9 (m, 3H, –CH=CH– in clopentane-1-methyl undecanoate (dibromo methyl hydno-
ring), 3.66 (s, 6H, –COOCH3), 3.5 (m, 2H, >CH–CH=CH– in carpate), which on dehydrobromination gave 1,3-cyclopenta-
ring), 2.5–3.1 (m, 2H, –CH2–CH=CH– in ring), 2.30 (t, J = diene-1-undecanoic acid (cyclopentadiene derivative). The
7.3 Hz, 4H, –CH2–COOCH3), 2.17–2.0 (m, 3H, >CH–CH2– cyclopentadiene derivative further underwent Diels-Alder re-
and –CH–CH2–CH=CH– in ring), 1.60 (m, 8H, action to give a dimer and oligomer acids. The crude dimer
≥C–CH2–CH2– and –CH2–CH2–CH2–COOCH3), 1.27 (m, and oligomer acids were esterified and purified to give novel
28H, –CH2–).
13C NMR (CDCl3, Fig. 4): δ 147.2 (–CH=C≤ in ring), 132 4,8-diene. The reaction sequence is given in Scheme 2.
(–CH=CH– in ring), 120 (–CH=C≤ in ring), 52 (–COOCH3), Bromination of methyl hydnocarpate gave two diastereo-
endo 1,4-bis(methyl undecanoate)-tricyclo [5,2,1,02,6]deca-
47.3 (>C< in ring), 37 (≥C–CH2–CH2–), 31 and 37.2 isomers in 1:1 proportion. Methyl hydnocarpate possesses
(>CH–CH=CH– in ring), 34 (–CH2–COOCH3), 31.5 one chiral center at C1 (Scheme 1), with R-configuration.
(–CH2–CH=CH– in ring), 30 (≥C–CH2–CH2–), 28–30 Bromination generates two more chiral centers at the C2 and
(–CH2–), 28.2 (–CH–CH2–CH=CH– in ring), 28 C3 atoms. The configuration of C2, C3 were assigned to be
(≥C–CH2–CH2–CH2–), 25 (–CH2–CH2–COOCH3).
R,R and S,S, respectively. Hence the diastereoisomers of di-
bromo derivative possess R,R,R or R,S,S configuration at C1,
C2, and C3 atoms, respectively. The structures of the two di-
astereoisomers are given in Scheme 3.
RESULTS AND DISCUSSION
Alkaline hydrolysis of chaulmoogra oil gave a mixture of cy-
The 1H NMR spectra (Fig. 1) of dibromo methyl hydno-
clopentene fatty acids, which on subsequent esterification gave carpate indicates two different sets of peaks in the region of δ
methyl esters. GLC analysis of methyl esters showed the pres- 3.9 to 4.9 due to –BrCH–CHBr– protons. This was because
of two diastereoisomers (R,R,R and R,S,S) of dibromo methyl
hydnocarpate. In the case where both –BrCH–CHBr– protons
were equatorial (Scheme 3A), Ha and Hb can couple only
with one adjacent axially oriented proton (axial-equatorial
coupling, J = 5 Hz) giving a doublet for each at δ 4.79 and
4.58, respectively. When both –BrCH–CHBr– protons were
axial (Scheme 3B), Ha can couple with two axially oriented
protons (axial-axial coupling, J = 12 Hz) giving a triplet at δ
4.0, while Hb can couple with two axially oriented protons
O
C2
C5
C3
C4
(
)
9
OCH3
C1
SCHEME 1
JAOCS, Vol. 77, no. 10 (2000)

1104
N.B. MALKAR ET AL.
Br
O
H
H
H
H
OCH3
( )9
A
H
(
)
9
OCH3
H
Br
O
H
2-cyclopentene-1(R)-methyl undecanoate
threo-2(R),3(R)-dibromocyclopentane-1(R)-methylundecanoate
H
H
Br2 in CHCl3
H
Br
OCH3
( )9
Br
B
H
Br
H
O
H
O
Br
H
(
)
9
OCH3
threo-2(S),3(S)-dibromocyclopentane-1(R)-methylundecanoate
SCHEME 3
threo-2(R/S),3(R/S)-dibromo cyclopentane-1(R)-methyl undecanoate
(i) Alcoholic KOH
60°C, 11.5 h
(ii) 2 N HCl
O
O
OCH3
( )9
O
(
)
9
OH
( )9
OCH3
1,3-cyclopentadiene-1-undecanoic acid
SCHEME 4
CH3OH, 0.1% H2SO4
O
cis-double bonds. The dimer content was analyzed by GLC
and found to be 60%, along with 36% of oligomers. The
DEP-MS analysis of dimer acid methyl ester showed a mo-
lecular ion peak at m/z 528. The fragmentation pattern
showed a peak at m/z = M − 31 = 497 (loss of –OCH3 group).
The retro-Diels-Alder fragment at m/z 264 was also observed,
confirming the bicylic nature of the dimer acid. A prominent
peak due to β-cleavage (β- to the cyclopentadiene) was ob-
served at m/z = M − 185 = 79. The base peak was observed at
m/z 81, and this was attributed to C6H9 (containing cyclopen-
tene ring). On further loss of CH2 this gave m/z 67, C5H7 (cy-
clopentene ring). The peak at m/z = M − 199 = 329 was ob-
served and was explained as loss of one of the side chains,
–C10H20COOCH3. Further loss of a second side chain gave a
peak at m/z 130 confirming the structure as a tricyclic dimer
acid (Scheme 4).
OCH3
( )9
O
( )9
OCH3
endo 1,4-bis(methyl undecanoate)-tricyclo [5,2,1,02,6]deca-4,8-diene
SCHEME 2
and one equatorially oriented proton (axial-axial and axial-
equatorial coupling, J = 5 to 12 Hz) giving a quartet at δ 4.36.
In 13C NMR spectra, four peaks were observed at δ 50, 57,
62, and 66 for –BrCH–CHBr– carbons of dibromo methyl
hydnocarpate. 13C DEPT NMR experiment was carried out
to simplify the 13C NMR spectra. It showed inversion of
peaks due to –CH2– (secondary) carbon atoms while there
was no inversion of peaks due to –CH– (primary) and –CH3
(tertiary) carbon atoms at δ 50, 57, 62, 66 (of –CHBr–), δ 43
(of –CH– in cyclopentane ring) and δ 52 (of –COOCH3). The
carbons directly attached to the bromine of dibromo methyl
hydnocarpate showed two different sets of peaks in 13C NMR
and 13C DEPT NMR experiments (as in the case of 1H NMR),
thus confirming the presence of two diastereoisomers.
ACKNOWLEDGMENTS
This work was supported by research grants from Hindustan Lever
Research Foundation, Mumbai, India. We are thankful to Dr. V.K.
Gore (research scientist, Hindustan Lever Research Center) for his
valuable suggestions.
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[Received December 3, 1999; accepted July 28, 2000]
JAOCS, Vol. 77, no. 10 (2000)