2942
J. Agric. Food Chem. 1996, 44, 2942−2943
RAPID COMMUNICATIONS
Im p r oved P r ep a r a tion of Con ifer yl a n d Sin a p yl Alcoh ols
Keyw or d s: Coniferyl alcohol; sinapyl alcohol; lignin; lignin monomer; sodium borohydride
INTRODUCTION
Variable-scale syntheses of coniferyl and sinapyl
alcohols 2a ,b have become increasingly important in
recent years due to the usefulness of these lignin
monomers in preparing synthetic lignins and lignin
model compounds (Sarkanen and Ludwig, 1971; Adler,
1977; Landucci, 1995; Landucci et al., 1981; Helm and
Ralph, 1992; Ralph et al., 1992a,b). In the past, reduc-
tion of ethyl ferulate and ethyl sinapate with lithium
aluminum hydride (Allen and Byers, 1949; Freudenberg
F igu r e 1. NaBH4 reduction of coniferyl (1a ) and sinapyl (1b)
and Hu¨bner, 1952; Freudenberg and Swaleh, 1969),
sodium bis(2-methoxyethyl)aluminum hydride (Minami
et al., 1974; Kirk and Brunow, 1988), or diisobutyl
aluminum hydride (DIBAL-H) (Newman et al., 1986;
Quideau and Ralph, 1992) was the method of choice.
The problems associated with these methods are the
moderate yields, the necessity of handling air-sensitive
reagents under protective gases, and the varying
amounts of undesired double-bond-reduction byprod-
ucts, the so-called 1,4-reduction products 3. The toler-
ably low amount of 1,4-reduction products was the
strength of a DIBAL-based method (Quideau and Ralph,
1992). Nonreductive synthetic methods (Nakamura and
Higuchi, 1976; Steglich and Zechlin, 1978; Zanarotti,
1982; Rothen and Schlosser, 1991) leading to p-hydroxy-
cinnamyl alcohols also have the disadvantages of re-
quiring several steps, requiring nonambient conditions,
or producing poor to moderate overall yields.
Sodium borohydride is a versatile reducing agent that
is particularly attractive for general use by nonchemists
because of its ease of handling. However, reduction of
conjugated aldehydes and ketones is generally compli-
cated by competing 1,4- and 1,2-processes (Kropf, 1981).
For example, borohydride reduction of cyclopenten-2-
one produces cyclopentanol quantitatively (Chaiken and
Brown, 1949). Presumably for this reason, and because
of the previous unavailability of the aldehydes, borohy-
dride approaches have not appeared in the literature,
with one exception; sodium borohydride reduction of the
protected aldehydes using a buffer system under argon
at 0 °C has been reported but does not address the 1,4-
reduction products (Daubresse et al., 1994).
aldehydes.
variable amounts of coniferyl and sinapyl alcohols 2a ,b.
The advantages are that the yields of the desired
compounds are high, the reaction products may be used
directly in following reactions, there is no need to protect
the phenolic hydroxy group as in some of the other
preparation methods, and under 1% of the troublesome
double-bond-reduced compound 3 is produced. The
simplicity allows access to high-quality coniferyl and
sinapyl alcohols; even without purification by crystal-
lization, the purity is notably superior to that which can
be obtained commercially.
EXPERIMENTAL METHODS
Melting points are uncorrected. 1H and 13C NMR spectra
(with assignments made using the usual complement of 2D
spectra) were identical to those reported previously (Quideau
and Ralph, 1992) and reported in three solvents in the lignin
model database (compounds 2001 and 2002) (Ralph et al.,
1996). Petroleum ether was the 30-60 °C fraction, usually
distilled. Ethyl acetate was distilled. Coniferaldehyde and
sinapylaldehyde were obtained from Aldrich Chemical Co.,
Milwaukee, WI. Products 2a ,b were identical with those fully
characterized previously (Quideau and Ralph, 1992).
Con ifer yl Alcoh ol 2a . Coniferaldehyde 1a (50 mg, 0.28
mmol) was dissolved in ethyl acetate (10 mL). Sodium
borohydride (21 mg, 0.56 mmol, 2 equiv) was added and the
solution stirred for 1 h at room temperature. During that time
a yellow precipitate formed. The reaction mixture was poured
into water (50 mL), the organic layer was separated, and the
aqueous layer was extracted with ethyl acetate (2 × 50 mL).
The combined organic layers were dried over anhydrous
MgSO4 and evaporated to dryness under reduced pressure at
40 °C to give coniferyl alcohol 2a as a white/pale yellow solid,
or sometimes as a pale yellow oil (49.9 mg, 99%). 1H NMR of
the crude product 2a showed <1% of the 1,4-reduction product
3a . Recrystallization from dichloromethane/petroleum ether
gave 2a as very pale yellow plates: mp 78.2-78.6 °C [lit.
(Freudenberg and Hu¨bner, 1952) 74-76 °C; lit. (Quideau and
Ralph, 1992) 77.9-78.6 °C]. For large-scale preparations (5
g) the reaction was run for 6.5 h at room temperature using
TLC to monitor progress and worked up as above to yield crude
2a (4.66 g, 92%). Crystallization from dichloromethane/
petroleum ether afforded first-crop 2a in 76% yield.
This paper describes a quick, efficient, and clean
synthesis of milligram to decagram quantities of the
substituted cinnamyl alcohols 2a ,b using sodium boro-
hydride reduction of coniferyl and sinapyl aldehydes
1a ,b which are now commercially available.
RESULTS AND DISCUSSION
Coniferyl and sinapyl alcohols 2a ,b were prepared
cleanly from their corresponding aldehydes 1a ,b by
sodium borohydride reduction in ethyl acetate at room
temperature (Figure 1). The yields were 99% and 94%
for 2a and 2b, respectively, as described under the
Experimental Methods. Large-scale preparation of 2a
yielded 76% crystalline (first crop) material. Unlike
prior methods, the synthesis presented offers a very
efficient and easy to handle method for preparing
Sin a p yl Alcoh ol 2b. Sinapyl aldehyde 1b was reduced as
described for 1a to yield sinapyl alcohol 2b as a yellow-orange
oil in 94% yield. Crystallization was more difficult than for
2a but could be effected from methylene chloride/petroleum
ether as described previously (Quideau and Ralph, 1992).
S0021-8561(96)00490-6
This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society
Ludley, Frank H.
