Facile Synthesis of 4-Hydroxycinnamyl p-Coumarates

Article abstract of DOI:10.1021/jf980440y

Coniferyl p-coumarate (4-hydroxy-3-methoxycinnamyl 4-hydroxycinnamate) and sinapyl p-coumarate (3,5-dimethoxy-4-hydroxycinnamyl 4-hydroxycinnamate) were synthesized in high overall yield. In this improved method acetate was used as the phenol protecting group instead of 2,4-dinitrophenyl ether; selective deacetylation, without cinnamate ester hydrolysis, was accomplished with neat pyrrolidine.

Full text of DOI:10.1021/jf980440y

J. Agric. Food Chem. 1998, 46, 2911−2913
2911
F a cile Syn th esis of 4-Hyd r oxycin n a m yl p-Cou m a r a tes
Fachuang Lu and J ohn Ralph*
U.S. Dairy Forage Research Center, Agricultural Research Service, U.S. Department of Agriculture, and
Department of Forestry, University of WisconsinsMadison, Madison, Wisconsin 53706
Coniferyl p-coumarate (4-hydroxy-3-methoxycinnamyl 4-hydroxycinnamate) and sinapyl p-coumarate
(3,5-dimethoxy-4-hydroxycinnamyl 4-hydroxycinnamate) were synthesized in high overall yield. In
this improved method acetate was used as the phenol protecting group instead of 2,4-dinitrophenyl
ether; selective deacetylation, without cinnamate ester hydrolysis, was accomplished with neat
pyrrolidine.
Keyw or d s: Coniferyl p-coumarate; sinapyl p-coumarate; lignin; acylation; grasses; radical coupling;
deacetylation
INTRODUCTION
(Nakamura and Higuchi, 1978a). Here we report a
facile way to prepare 4-hydroxycinnamyl p-coumarates
in 80-85% overall yields from 4-hydroxycinnamalde-
hydes. The new method protects both the alcohol and
the acid moieties using acetyl protecting groups that are
not normally considered due to the impression that they
could not be selectively removed (without cleaving the
coumarate ester). Deprotection is consequently a single
step.
Cell walls of grasses contain substantial amounts of
p-coumarates on their lignins. Recent NMR studies on
maize and other grasses have demonstrated that ex-
clusively the primary γ-positions of lignin side chains
are acylated by p-coumaric acid (Ralph et al., 1994;
Crestini and Argyropoulos, 1997). Coniferyl and sinapyl
p-coumarates 6 are logical precursors of these γ-ester
structures (Nakamura and Higuchi, 1976, 1978a,b;
Ralph et al., 1994). Although functional roles for related
ferulates have been adduced (Ralph et al., 1992, 1998),
the role of p-coumarates in cell wall structure and
development is still not well understood. An interesting
observation is that the p-coumarate moiety does not
incorporate (via the radical coupling mechanisms that
characterize lignification) into lignin and remains as a
terminal unit with a free phenolic group. However,
when synthetic lignins are made with coniferyl p-
coumarate and coniferyl alcohol, the p-coumarate moi-
eties of the esters appear to be incorporated into lignin
via radical pathways (Nakamura and Higuchi, 1978b).
The reasons for such a difference between natural lignin
and synthetic lignins are only now being revealed.
Recent studies indicated that p-coumarates may func-
tion as radical transfer carriers to help sinapyl alcohol
incorporate into lignin when the wall peroxidases have
a low reactivity with sinapyl alcohol directly (Takahama
et al., 1996; Hatfield et al., 1997).
Coniferyl and sinapyl p-coumarates are possible
precursors of grass lignins and are required for basic
studies into their role in lignification. They are tradi-
tionally synthesized by coupling of protected p-couma-
royl chloride and protected 4-hydroxycinnamyl alcohols
(Nakamura and Higuchi, 1978a; Ralph et al., 1994;
Grabber et al., 1996). Many protecting groups have
been used for protecting the phenolic hydroxyl of 4-hy-
droxycinnamyl alcohols; 2,4-dinitrophenyl ether was
considered the best one for this purpose, allowing
coniferyl p-coumarate to be prepared in moderate yield
EXPERIMENTAL PROCEDURES
Gen er a l Meth od s. Melting points were measured on an
Electrothermal (Southend, U.K.) digital mp apparatus and
were uncorrected. Evaporations were conducted under re-
duced pressures at temperatures <40 °C. Further removal of
organic solvents, as well as drying of residues, was ac-
complished under higher vacuum (100-200 mTorr) at room
temperature. Flash chromatography was performed on FLASH
40M cartridges (Biotage, Dyax Corp., Charlottesville, VA)
using a UA-6 UV-vis detector (ISCO, Lincoln, NE). Solid
phase extraction was carried out with 3 mL LC-Si tubes
(Supelco, Bellefonte, PA) using CHCl3/EtOAc (19:1) as solvent.
NMR spectra of samples in acetone-d6 were run at 300 K on a
Bruker AMX-360 MHz narrow-bore instrument (Bruker In-
struments, Billerica, MA) fitted with a 5 mm four-nucleus
(QNP) probe with normal geometry (proton coil further from
the sample). The central solvent signals were used as refer-
ence (1H, 2.04 ppm; 13C, 29.80 ppm). Fully authenticated
assignments were made by the usual complement of 1D and
2D methods, using standard Bruker pulse programs. Petro-
leum ether refers to the boiling range 40-60 °C. Coniferal-
dehyde, sinapaldehyde, and acetylation reagents were from
Aldrich (Milwaukee, WI) and used as supplied. Solvents (AR
grade) were from Mallinckrodt (Paris, KY).
P r ecu r sor s. 4-Acetoxycinnamoyl Chloride (2). Chloride 2
was prepared (white crystals in 96% yield) from p-coumaric
acid by acetylation followed by chlorination using SOCl2
according to a previous method (Helm et al., 1992).
4-Acetoxycinnamyl Alcohols 4a ,b. 4-Acetoxycinnamalde-
hydes 3a ,b were prepared in 94-96% yield by acetylation of
the corresponding 4-hydroxycinnamaldehydes with Ac2O/py-
ridine and then reduced with borane/tert-butylamine complex
to give the corresponding alcohols 4a ,b as follows. The
4-acetoxycinnamaldehyde was dissolved in methylene chloride
to which borane/tert-butylamine complex (1.5 equiv) was
added. The mixture was stirred at room temperature for 2 h,
when TLC showed that the starting material had disappeared
completely. The solvent was evaporated at 40 °C under
* Address correspondence to this author at the U.S. Dairy
Forage Research Center, USDA-ARS, 1925 Linden Dr. W.,
Madison, WI 53706-1108 [telephone (608) 264-5407; fax (608)
264-5147; e-mail jralph@facstaff.wisc.edu].
S0021-8561(98)00440-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/24/1998

2912 J. Agric. Food Chem., Vol. 46, No. 8, 1998
Lu and Ralph
Sch em e 1. Syn th etic Sch em e for Con ifer yl a n d Sin a p yl p-Cou m a r a tes
reduced pressure. The residue was hydrolyzed with 0.5 M H2-
SO4 in ethanol/water (1:1) for 1.5 h. Most of the ethanol was
removed by evaporation, and the product was extracted with
ethyl acetate. The ethyl acetate solution was washed with
saturated NH4Cl and dried over MgSO4. Evaporation of the
ethyl acetate gave the product as a light yellow syrup. 4a :
1.3 Hz, γ’s), 6.44 (1H, dt, J ) 15.9, 6.3 Hz, â), 6.57 (1H, d, J )
16.0 Hz, P8), 6.72 (1H, dt, J ) 15.9, 1.3 Hz, R), 6.86 (2H, s,
A2/6), 7.20 (2H, m, P3/5), 7.72 (1H, d, J ) 16.0 Hz, P7), 7.74
(2H, m, P2/6); 13C NMR δ 20.2 (A-OAc), 20.9 (P-OAc), 56.5
(OMe), 65.3 (γ), 104.2 (A2/6), 118.9 (P8), 123.3 (P3/5), 125.0
(â), 129.7 (A1), 130.2 (P2/6), 132.9 (P1), 134.2 (R), 135.7 (A1),
144.6 (P7), 153.4 (A3/5), 153.5 (P4), 166.7 (P9), 168.5 (A-OAc),
169.4 (P-OAc); FAB MS, found Na + M+, 463.1, NaC24H24O8
requires M, 463.1.
4-Hyd r oxycin n a m yl p-Cou m a r a tes 6a ,b. For compound
6a , 4-acetoxy-3-methoxycinnamyl 4-acetoxy-p-coumarate (5a ;
80 mg, 0.195 mmol) was dissolved in pyrrolidine (1 mL). Once
dissolution was complete, the pyrrolidine solution was diluted
with 50 mL of ethyl acetate and washed with 1 M H2SO4 (3 ×
20 mL) and saturated NH4Cl (2 × 20 mL). After drying over
MgSO4 and evaporation, the resulting syrup was submitted
to solid phase extraction [CHCl3/EtOAc (19:1)] to afford 6a (59
mg, 93%) as a pale yellow syrup. 6a : 1H NMR δ 3.86 (6H, s,
A3/5-OMe’s), 4.79 (2H, dd, J ) 6.5, 1.2 Hz, γ’s), 6.25 (1H, dt,
J ) 15.9, 6.5 Hz, â), 6.38 (1H, d, J ) 16.0 Hz, P8), 6.65 (1H,
dt, J ) 15.9, 1.2 Hz, R), 6.80 (1H, d, J ) 8.1 Hz, A5), 6.90 (2H,
m, P3/5), 6.92 (1H, dd, J ) 8.1, 1.9 Hz, A6), 7.11 (1H, d, J )
1.9 Hz, A2), 7.54 (2H, m, P3/5), 7.64 (1H, d, J ) 16.0 Hz, P7);
13C NMR δ 56.2 (A-OMe), 65.5 (γ), 110.1 (A2), 115.5 (P8), 115.8
(A5), 116.7 (P3/5), 121.2 (A6), 121.7 (â), 127.0 (P1), 129.4 (A1),
130.9 (P2/6), 134.9 (R), 145.5 (P7), 147.7 (A4), 148.7 (A3/5),
160.5 (P4), 167.4 (P9).
1
pale yellow oil (96% yield); H NMR (acetone-d6) δ 2.31 (3H,
s, OAc), 3.83 (3H, s, OAc), 3.90 (1H, t, J ) 5.5 Hz, γ-OH), 4.22
(2H, dt, J ) 5.5, 1.7 Hz, γ’s), 6.38 (1H, dt, J ) 15.9, 5.2 Hz, â),
6.58 (1H, dt, J ) 15.9, 1.7 Hz, R), 6.97 (2H, m, A5/6), 7.15 (s,
1H, A2); 13C NMR δ 20.5 (OAc), 56.2 (OMe), 63.1 (γ), 110.9
(A2), 119.5 (A6), 123.6 (A5), 129.3 (R), 131.4 (â), 137.2 (A1),
140.2 (A4), 152.3 (A3), 169.0 (OAc). 4b: (94.5% yield); crystal-
lization from methylene chloride/petroleum ether afforded
white plate crystals; mp 108.6-109.8 °C [lit. (Daubresse et al.,
1994) 85 °C]; 1H NMR δ 2.21 (3H, s, OAc), 3.80 (6H, s, OMe’s),
3.89 (1H, t, J ) 5.5 Hz, γ-OH), 4.23 (2H, td, J ) 5.5, 1.6 Hz,
γ’s), 6.40 (1H, dt, J ) 15.9, 5.0 Hz, â), 6.57 (1H, dt, J ) 15.9,
1.6 Hz, R), 6.78 (2H, s, A2/6); 13C NMR δ 20.2 (OAc), 56.4 (A3/
5-OMe’s), 63.0 (γ), 103.8 (A2/6), 129.1 (A1), 129.7 (â), 131.43
(R), 136.6 (A4), 153.3 (A3/5), 168.6 (OAc).
4-Acetoxycin n a m yl p-Cou m a r a tes 5a ,b. Coupling of
4-acetoxycinnamoyl chloride (2) with 4-acetoxycinnamyl alco-
hols 4a ,b was efficiently carried out using 4-(dimethylamino)-
pyridine (DMAP). Thus, 4a (2.3 g, 10.36 mmol) and chloride
2 (2.6 g, 11.4 mmol) were dissolved in dry CH2Cl2 (120 mL) to
which DMAP (316 mg, 0.25 equiv) and Et3N (1.24 mL, 0.85
equiv) were added. The mixture was stirred for ∼2 h, when
TLC [CHCl3/EtOAc (5:1)] showed the starting material 4a was
converted into a faster moving compound. The solution was
diluted with CH2Cl2 and washed successively with aqueous
3% HCl and saturated NH4Cl. Drying over MgSO4, evapora-
tion, and purification by flash chromatography [CHCl3/EtOAc
(19:1)] gave 5a (4.09 g, 94%) as a pale yellow oil. 5a : 1H NMR
δ 2.22 (3Η, s, A-OAc), 2.27 (3H, s, P-OAc), 3.84 (3H, s, A3-
OMe), 4.85 (2H, dd, J ) 6.3, 1.3 Hz, γ’s), 6.44 (1H, dt, J )
15.9, 6.3 Hz, â), 6.57 (1H, d, J ) 16.0 Hz, P8), 6.75 (1H, dt, J
) 15.9, 1.3 Hz, R), 7.00 (1H, d, J ) 8.2 Hz, A5), 7.05 (1H, dd,
J ) 8.2, 1.7 Hz, A6), 7.20 (2H, m, P3/5), 7.24 (1H, d, J ) 1.7
Compound 6b was prepared as a clear syrup in 94% yield
from 5b according to the procedure described for 6a . 6b: 1H
NMR δ 3.84 (6H, s, A3/5-OMe’s), 4.78 (2H, dd, J ) 6.4, 1.3
Hz, γ’s), 6.28, 1H, dt, J ) 15.8, 6.4 Hz, â), 6.38 (1H, d, J )
16.0 Hz, P8), 6.63 (1H, dt, J ) 16.0, 1.3 Hz, R), 6.80 (2H, s,
A2/6), 6.89 (2H, m, P3/5), 7.55 (2H, m, P2/6), 7.60 (1H, d, J )
16.0 Hz, P7); 13C NMR δ 56.6 (A3/5-OMe’s), 65.4 (γ), 105.1 (A2/
6), 115.5 (P8), 116.7 (P3/5), 122.1 (â), 127.0 (P1), 128.2 (A1),
130.9 (P2/6), 135.1 (R), 137.2 (A4), 145.5 (P7), 148.8 (A3/5),
160.6 (P4), 167.2 (P9).
RESULTS AND DISCUSSION
Hz, A2), 7.72 (1H, d, J ) 16.0 Hz, P7), 7.74 (2H, m, P2/6); 13
C
NMR δ 20.5 (A-OAc), 20.9 (P-OAc), 56.2 (OMe), 65.3 (γ),
111.17 (A2), 118.9 (P8), 120.0 (A6), 123.3 (P3/5), 123.8 (A5),
124.9 (R), 130.2 (P2/6), 132.9 (P1), 133.9 (â), 136.3 (A1), 140.8
(A4), 144.6 P7), 152.5 (A3), 153.5 (P4), 166.7 (P9), 168.9 (A-
OAc), 169.4 (P-OAc); FAB MS, found Na + M+, 433.0,
NaC23H22O7 requires M, 433.1.
Synthetic strategies for coniferyl p-coumarate have
been discussed (Nakamura and Higuchi, 1978a). The
ideal way to prepare the target compound would be
direct coupling of the acid and alcohol moieties without
using any protecting groups. Previous attempts using
this method failed for the obvious regioselectivity rea-
sons; to our knowledge, the use of enzymes (lipases, for
example) for this esterification has not been examined.
Therefore, the phenolic hydroxyls on the precursor acid
and alcohol have to be protected prior to esterification.
5b was prepared in 92% yield as a pale yellow oil in the
same way as described for 5a . Crystallization from acetone/
petroleum ether afforded white crystalline 5b in 80% yield.
1
5b: mp 100.8-102.0 °C; H NMR δ 2.22 (3Η, s, A-OAc), 2.27
(3H, s, P-OAc), 3.82 (6H, s, A3-OMe), 4.85 (2H, dd, J ) 6.3,

Facile Synthesis of 4-Hydroxycinnamyl p-Coumarates
J. Agric. Food Chem., Vol. 46, No. 8, 1998 2913
The benzyl ether has been avoided because its depro-
tection conditions, catalytic hydrogenolysis, concomi-
tantly saturate the double bonds on both acid and
alcohol moieties. Esters are not normally considered
as protecting groups due to the possibility of simulta-
neous hydrolysis of the desired ester linkage during
deprotection. 4-Hydroxycinnamyl p-coumarates are
sensitive to both basic and acidic conditions; even acetic
acid at room temperature caused some hydrolysis of
these esters. The most successful strategy to prepare
coniferyl p-coumarate has been to start with coniferyl
alcohol and use 2,4-dinitrophenyl ether for protection
of one or both phenols because of its relatively mild
removal by nucleophilic agents (Nakamura and Higuchi,
1978a; Grabber et al., 1996).
Pyrrolidine was found to selectively deacetylate phe-
nolic acetates over aliphatic acetates and has been used
for determination of phenolic hydroxyl groups in lignins
(Mansson, 1982, 1983). We have used pyrrolidine for
preparation of 4-hydroxy-3-methoxycinnamyl acetate in
high yield from coniferyl diacetate (Lu, 1996, unpub-
lished results). However, coniferyl alcohol cannot be
used as the starting material if acetate is chosen as the
protecting group because selective acetylation of the
phenolic hydroxyl over the aliphatic one, although
possible, is not straightforward. Fortunately, with the
recent commercial availability of 4-hydroxycinnamal-
dehydes, 4-acetoxy-3-methoxycinnamyl alcohol (the phe-
nolic acetate of coniferyl alcohol) is readily prepared.
Starting with 4-hydroxycinnamaldehydes, 4-hydroxy-
cinnamyl p-coumarates can now easily be prepared in
high overall yields (Scheme 1).
4-Acetoxycinnamaldehydes 3a ,b were obtained from
coniferaldehyde/sinapaldehyde by acetylation with ace-
tic anhydride/pyridine in 94-96% yield. Regioselective
1,2-reduction using borane/tert-butylamine complex in
methylene chloride gave rise to 4-acetoxycinnamyl al-
cohols 4a ,b in high yield (95%) without dihydro products
from 1,4-reduction. 4-Acetoxycinnamyl alcohol 4b was
crystallized from methylene chloride/petroleum ether as
white plates with a melting point of 108-109 °C, higher
than previously described (Daubresse et al., 1994).
From 4-hydroxycinnamic acid, 4-acetoxycinnamoyl chlo-
ride (2) was prepared as white crystals in 96% yield by
acetylation and chlorination (SOCl2) according to a
previously described method (Helm et al., 1992). The
coupling conditions (DMAP in Et3N) used for ester
formation were the same as used previously (Grabber
et al., 1996). Deacetylation of 5a ,b is the key to
accomplish the efficient syntheses of the target com-
pounds 6a ,b. Preliminary trials for deacetylation of 5a
using pyrrolidine with dioxane as cosolvent, similar to
conditions for determination of phenolic hydroxyls in
lignins (Mansson, 1982, 1983), gave 6a in only 50% yield
due to hydrolysis. However, deacetylation in neat
pyrrolidine produced 6a ,b in >90% yield each, after
purification through solid phase extraction (see Experi-
mental Procedures).
method are (1) the use of acetates as protecting groups
and simple selective deacetylation techniques and (2)
the use of commercial 4-hydroxycinnamaldehydes as
starting materials and their simple pathway into suit-
ably protected 4-hydroxycinnamyl alcohols.
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Received for review May 1, 1998. Revised manuscript received
May 1, 1998. Accepted J une 16, 1998. We gratefully acknowl-
edge support through USDA-NRI Competitive Grant 96-02587
(Plant Growth and Development section).
The resulting syntheses of 4-hydroxycinnamyl p-
coumarates are simpler and more efficient than previous
methods. The primary advantages of the present
J F980440Y