Synthesis of 1-O-methylchlorogenic acid: Reassignment of structure for MCGA3 isolated from bamboo (Phyllostachys edulis) leaves

Article abstract of DOI:10.1021/jf4042112

The first synthesis of 1-O-methylchlorogenic acid is described. The short and efficient synthesis of this compound provides laboratory-scale quantities of the material to investigate its biological properties. The synthesis involves C-1 alkylation of the

Full text of DOI:10.1021/jf4042112

Article
pubs.acs.org/JAFC
Synthesis of 1‑O‑Methylchlorogenic Acid: Reassignment of Structure
for MCGA3 Isolated from Bamboo (Phyllostachys edulis) Leaves
Wayne E. Zeller*
US Dairy Forage Research Center, Agricultural Research Service, USDA, 1925 Linden Drive, Madison, Wisconsin 53706, United
States
S
* Supporting Information
ABSTRACT: The first synthesis of 1-O-methylchlorogenic acid is described. The short and efficient synthesis of this compound
provides laboratory-scale quantities of the material to investigate its biological properties. The synthesis involves C-1 alkylation of
the known (−)-4,5-cyclohexylidenequinic acid lactone followed by methoxide opening to the hydroxyl ester. Acylation of the C-5
hydroxyl group followed by sequential removal of protecting groups afforded 1-O-methylchlorogenic acid. The NMR
spectroscopic characteristics of this compound do not coincide with those reported for the original isolation from bamboo
(Phyllostachys edulis) leaves of the compound designated MCGA3. Comparison of the published spectroscopic data reported for
MCGA3, with both reported literature values and spectroscopic data obtained from an authentic sample, leads to the conclusion
that the compound isolated from bamboo (Phyllostachys edulis) leaves is instead methyl chlorogenate.
KEYWORDS: 1-O-methylchlorogenic acid, MCGA3, Phyllostachys edulis, methyl chlorogenate
chlorogenic acid, methyl chlorogenate, 14, has shown to be a
5-lipooxygenase inhibitor30 and has exhibited activity as a HIV
protease inhibitor with an IC50 measurement of 40 μg/mL.31
Thus, discovery of any new entity belonging to the class of
caffeoyl quinates attracts attention for its potential biological
properties and activities and potential use as a therapeutic or as
a lead compound in drug discovery efforts.
INTRODUCTION
■
Phenolic plant secondary metabolites composed of caffeoyl
esters of small molecular weight hydroxy acids and amides have
displayed significant biological activities against cancer,1
inflammation,2 and cardiovascular disease3 in addition to their
well-described behavior as antioxidants.4,5 Specific examples
include compounds such as rosmarinic acid (antiviral,
antibacterial, anti-inflammatory),6,7 clovamide (anti-inflamma-
tory),7 cafftaric acid (antimutagenic),8 and chicoric acid (HIV-1
integrase inhibitor),9−12 and subulatin whose antioxidant
activity parallels that of α-tocopherol.13
In 2001, Kweon et al.32 reported the isolation of two new
chlorogenic acid derivatives, together with the known
compound 5-O-feruoylquinic acid, 5 (Figure 1), from an
aqueous ethanol extract of bamboo (Phyllostachys edilis) leaves
and identified them as the methyl ether derivatives of the C-1
As a subset of this class of secondary metabolites, caffeoyl
derivatives of quinic acid have also enjoyed detailed
examination toward their significant biological activities.
Chlorogenic acid, 1, is the most common and most widely
distributed plant phenolic of this subclass whose antioxidant
activity has been well documented.5,14,15 The radical scaveng-
ing/antioxidant activity of chlorogenic acid16−20 has led to
investigation of its inhibitory effects on lipid peroxidation,17,18
inhibition of hemolysis of mouse erythrocytes,18 action in acute
reduction of blood pressure in healthy humans,19 and
promising antitumor activity.20−22 The antibacterial activity of
chlorogenic acid has been attributed to disruption of outer and
plasma cell membranes and discharging membrane ion
gradients and leading to leakage of cell contents.23 Tomato
plants have been engineered for higher production of
chlorogenic acid,24 projected to ultimately provide even more
beneficial agricultural foodstuffs.
Chlorogenic acid and its isomers, neochlorogenic acid, 3, and
cryptochlorogenic acid, 4, have demonstrated similar antiox-
idant activity in vitro,25−28 possess tyrosinase inhibitory
activities, and moderate activity against several cancer cell
lines,26 including suppression of colon cancer,21 exhibits
potential neuroprotective activity,29 and serves as an 8-
hydroxydeoxyguanosine inhibitor.22 The methyl ester of
Figure 1. The monocaffeoylquinic acids, 1−4, and the three
compounds isolated from aqueous ethanol extraction of bamboo
(Phyllostachys edulis) leaves, 5−7.
Received: September 19, 2013
Revised: January 24, 2014
Accepted: January 26, 2014
Published: January 26, 2014
© 2014 American Chemical Society
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Figure 2. Synthesis of 1-O-methylchlorogenic acid, 7.
and C-4 hydroxyl groups of chlorogenic acid, 7 and 6,
respectively. Evaluation of these compounds revealed strong
antioxidant properties in standard tests with compound 7
showing activity superior to that of chlorogenic acid and
compound 6 in both the DPPH scavenging assay and the iron-
induced rat microsomal lipid peroxidation system.32 Com-
pound 7 was reported to possess unique antioxidant properties,
including induction of heme oxygenase-1 mRNA and protein
and providing protection of bovine aortic pulmonary
endothelial cells against ROS-mediated necrosis when
challenged with the oxidant tert-butyl hydroperoxide.33 Further
studies have shown that this compound also induces additional
selected phase II genes including ferretin, γ-glutamylcysteine
lygase, glutathione reductase, and glutathione transferase at the
micromolar level.34 Lastly, the compound assigned structure 7
was shown to enhance ultraviolet A-mediated apoptosis via
activation of p21 in HaCaT keratinocytes, potentially leading to
prevention of sunlight-induced photocarcinogenesis.35
Our interest in this class of compounds is coupled to recent
reports showing that the presence of compounds containing the
o-diphenol functionality during ensilage of dairy cow forages,
containing either naturally occurring or plants transfected with
polyphenoloxidase (PPO),36 results in enhanced protein
preservation of the stored forage. Thus, our interest lies in
securing sufficient quantities of naturally occurring o-diphenols
to evaluate them as PPO substrates and allow investigation of
the mechanism by which these compounds alter forage protein
composition toward rumen digestion. We have initiated a
program to produce naturally occurring caffeoyl-containing o-
diphenols37 and examine their ability to serve as substrates for
PPO and protein preservations.38 The extraordinary activities
reported for 7, abbreviated as MCGA3 in reports of subsequent
studies, prompted us to develop a laboratory synthesis of this
compound.
MATERIALS AND METHODS
■
1
General Experimental. H and 13C NMR spectra were obtained
on a Bruker Avance 360 instrument operating at 360 MHz (1H) and
90 MHz (13C), respectively. Spectra (presented in Supporting
Information) obtained were referenced to the residual signals: for
acetone-d6 at 2.04 and 29.8 ppm; for methanol-d4 at 3.30 and 49.0
1
ppm; for H and 13C, respectively. Mass spectra were obtained on a
Waters LCT instrument. Melting points were obtained on a Fisher-
Johns melting point apparatus and are uncorrected. IR spectra were
obtained on a Shimadzu IRPrestige-21 spectrometer operating in the
ATR mode. Elemental analyses were performed by Midwest
Microlabs, LLC, Indianapolis, IN. Optical rotations were determined
on a Rudolph Research (Flanders, NJ) Autopol III Automatic
Polarimeter. Thin-layer chromatography was performed on precoated
DC-Fertigplatten SIL G-25 UV254 plates (Alltech, Deerfield, IL).
Flash chromatography was conducted on silica gel, Merck, grade 9385,
́
́
230−400 mesh, 60 Å. Pyridine was dried over activated 4 Å sieves. For
clarity in interpretation of NMR assignments, all assignments are made
according to numbered structure 14 in Figure 2. Mention of trade
names or commercial products in this article is solely for the purpose
of providing specific information and does not imply recommendation
or endorsement by the U.S. Department of Agriculture.
Synthesis of 1-O-Methyl-3,4-cyclohexylidenequinic Acid
Lactone, 9. To a 50 mL, two-necked, round-bottomed flask equipped
with a magnetic stir bar, an equalizing addition funnel, and a septum
were added sodium hydride (193 mg, 8.02 mmol, 60% in mineral oil)
and anhydrous DMF (10 mL), and the flask was placed under an
argon atmosphere using a gas inlet adapter. The stirring suspension
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was cooled in an ice bath, and a mixture of (−)-4,5-cyclo-
hexylidenequinic acid lactone, 8 (1.36 g, 5.35 mmol), and iodo-
methane (1.14 g, 8.02 mmol) in anhydrous DMF (15 mL) was added
dropwise via the addition funnel over a 30 min period. The mixture
was allowed to slowly warm to room temperature over 2 h. The
mixture was cautiously quenched by adding it to a stirred solution of
half-saturated aqueous ammonium chloride and ethyl acetate (50 mL
each). The layers were separated, the aqueous layer was extracted with
ethyl acetate (2 × 50 mL), and the combined organic extracts were
washed with water (3 × 50 mL) and brine (20 mL), dried (Na2SO4),
filtered, and concentrated to give an off-white solid. This material was
recrystallized from hexanes (10 mL/g of residue) to give 1.30 g (90%)
of an off-white solid. Rf = 0.45 (7:3 hexanes/EtOAc), [α]D25 = +1.4 (c
(ddd, 1, J = 11.5, 7.5, 4.2 Hz, H5), 4.48 (ddd, 1, J = 5.3, 5.3, 2.9 Hz,
H3), 4.17 (dd, 1, J = 7.3, 5.8 Hz, H4), 3.70 (s, 3, CO2CH3), 3.27 (s, 3,
OCH3), 2.43 (br d, 1, J = 15.8 Hz, H2a), 2.27−2.17 (m, 8, H2b, H6a,
O2CCH3, O2CCH3), 1.82 (dd, 1, J = 13.2, 11.3 Hz, H6b), 1.71−1.65
(m, 2), 1.60−1.50 (m, 6), 1.40−1.25 (m, 2) ppm. 13C NMR (90 MHz,
acetone-d6) δ 173.6 (COOH), 168.6 (O2CCH3), 168.5 (O2CCH3),
166.2 (C9′), 145.0 (C3′ or C4′), 143.9 (C3′ or C4′), 134.0 (C1′),
127.3 (C6′), 125.0 (C5′), 123.9 (C2′), 120.0 (C8′), 110.1 (acetal C),
80.0 (C1), 77.2 (C4), 73.7 (C3), 72.2 (C5), 52.6 (OCH3), 52.5
(CO2CH3), 38.8, 36.0, 35.4 (C2), 31.1 (C6), 25.7, 24.7, 24.5, 20.5
(O2CCH3), 20.4 (O2CCH3) ppm. IR (ATR) νmax 2930, 2853, 1770,
1710, 1635, 1505, 1368, 1199, 1166, 1111, 1070, 1008, 903 cm−1.
HRMS: Calcd for C28H34O11 [M + NH4]+: 564.2440 m/z. Found:
564.2445 m/z.
1
= 1.02, CHCl3), mp 95.5−97 °C (hexanes), H NMR (360 MHz,
Synthesis of 1-O-Methyl-3,4-cyclohexylidene-5-O-caffeoyl-
quinic Acid, 13. To an 8 mL vial equipped with a magnetic stir bar
and a screw cap were added compound 12 (200 mg, 0.366 mmol) and
anhydrous pyridine (5.0 mL). To this solution was added LiI (740 mg
5.53 mmol) in one portion, and the mixture was degassed by bubbling
Ar gas through the solution for 2 min by which time the LiI had
completely dissolved. The vial was sealed and heated in an oil bath at
120 °C for 52 h. The cooled reaction mixture was diluted with EtOAc
(70 mL) and washed with 1 N aqueous HCl (30 mL). The aqueous
layer was extracted with EtOAc (30 mL), and the combined EtOAc
extracts were washed with 0.5 M aqueous sodium thiosulfate (2 × 30
mL), water (30 mL), brine (30 mL) and dried (MgSO4). The solution
was filtered, concentrated, and dried to give a light tan foam. This
residue was purified via column chromatography on flash silica gel (30
g) using 92:8:1 CH2Cl2/CH3OH/AcOH as eluent to give 131 mg
(79%) of an off-white foam. Rf = 0.25 (92:8:1 CH2Cl2/CH3OH/
AcOH); [α]D = −37.2 (c = 1.04, MeOH), mp 118−120 °C. H
NMR (360 MHz, acetone-d6) δ 7.55 (d, 1, J = 15.9 Hz, H7′), 7.16 (d,
1, J = 2.0 Hz, H2′), 7.04 (dd, 1, J = 8.2, 2.0 Hz, H6′), 6.86 (d, 1, J = 8.2
Hz, H5′), 6.27 (d, 1, J = 15.9 Hz, H8′), 5.34 (ddd, 1, J = 11.5, 7.5, 4.2
Hz, H5), 4.49 (m, 1, H3), 4.17 (dd, 1, J = 7.4, 5.7 Hz, H4), 3.32 (s, 3,
OCH3), 2.41 (m, 1, H2a), 2.29−2.22 (m, 2, H2b, H6a), 1.83 (dd, 1, J
= 13.2, 11.3 Hz, H6b), 1.70−1.60 (m, 2), 1.60−1.50 (m, 6), 1.45−1.30
(m, 2). 13C NMR (90 MHz, acetone-d6) δ 174.2 (CO2H), 166.8
(C9′), 148.7 (C4′), 146.3 (C3′), 145.9 (C7′), 127.6 (C1′), 122.6
(C6′), 116.3 (C5′), 115.7 (C8′), 115.2 (C2′), 110.0 (acetal C), 79.9
(C1), 77.3 (C4), 73.8 (C3), 71.7 (C5), 52.6 (OCH3), 38.8, 36.0, 35.5
(C6), 31.2 (C2), 25.7, 24.7, 24.5. IR (ATR) νmax 3255, 2878, 1770,
1684, 1596, 1514, 1269, 1154, 810 cm−1. HRMS: Calcd for C23H28O9
[M + H]+: 449.1807 m/z. Found: 449.1799 m/z.
acetone-d6) δ 4.68 (dd, 1, J = 6.3, 2.5 Hz, H5), 4.56 (m, 1, H3), 4.30
(ddd, 1, J = 6.4, 2.4, 1.2 Hz, H4), 3.30 (s, 3, OCH3), 2.57 (dddd, 1, J =
11.7, 6.2, 2.3, 1.3 Hz, H6a), 2.33 (d, 1, J = 11.7, H6b), 2.30 (ddd, 1, J =
14.6, 7.7, 2.4 Hz, H2a), 2.04 (dd, 1, J = 14.6, 3.0 Hz, H2b), 1.66−1.72
(m, 2), 1.56−1.66 (m, 4), 1.48−1.56 (m, 2), 1.35−1.42 (m, 2). 13C
NMR (90 MHz, acetone-d6) δ 175.8 (CO), 110.6 (acetal C), 77.6
(C1), 75.6 (C5), 72.9 (C4), 71.8 (C3), 52.0 (OCH3), 37.6, 36.8 (C2),
34.3, 29.9 (C6), 25.7, 24.7, 24.2 ppm. IR (ATR) νmax 2926, 2842,
1777, 1653, 1452, 1163, 1085, 983, 913 cm−1. Anal. Calcd for
C14H20O5: C: 62.67; H: 7.51. Found: C; 62.99, H; 7.58.
Synthesis of Methyl 1-O-Methyl-3,4-cyclohexylidenequi-
nate, 10. To a 100 mL, pear-shaped, one-necked flask equipped
with a magnetic stir bar and a ground glass stopper were added
compound 9 (1.26 g, 4.69 mmol) and anhydrous methanol (28 mL).
To this stirring solution was added freshly prepared 1.0 M sodium
methoxide in methanol (14.0 mL, 14.0 mmol, 2.98 equiv) dropwise
over ∼2 min at room temperature. After 21 h of being stirred at room
temperature, the reaction was quenched through the addition of glacial
acetic acid (1.70 mL, 29.6 mmol), diluted with EtOAc (150 mL), and
washed with water (50 mL). The layers were separated, the aqueous
layer extracted with EtOAc (50 mL), and the combined EtOAc
extracts were washed with water (50 mL), saturated NaHCO3 (50
mL), and brine (25 mL) and dried (Na2SO4). The mixture was
filtered, concentrated, and dried to give a white solid residue. The
residue was purified via flash chromatography on silica gel (80 g) using
9:1 CHCl3/ acetone as eluent to give 1.08 g (76%) of a white solid. Rf
25
1
25
1
= 0.23 (9:1 CHCl3/acetone); [α]D = −16.7 (c = 0.86, CHCl3), H
NMR (360 MHz, acetone-d6) δ 4.37 (dd, 1, J = 10.6, 5.2 Hz, H3), 4.08
(d, 1, J = 4.5 Hz, C3-OH), 3.99 (m, 1, H5), 3.88 (t, 1, J = 6.2 Hz, H4),
3.69 (s, 3, CO2CH3), 3.19 (s, 3, OCH3), 2.22 (dd, 1, J = 15.1, 5.2 Hz,
H6a), 2.14 (ddd, 1, J = 15.1, 4.7, 1.6 Hz, H6b), 1.99 (ddd, 1, J = 13.5,
4.0, 1.6 Hz, H2a), 1.70 (dd, 1, J = 13.5, 10.5 Hz, H2b), 1.53−1.66 (m,
4), 1.49−1.53 (m, 4), 1.30−1.40 (m, 2H). 13C NMR (90 MHz,
acetone-d6) δ 174.1 (C7, CO), 109.5 (acetal C), 80.8 (C4), 79.9
(C1), 73.5 (C5), 68.4 (C3), 52.3 (OCH3), 52.2 (CO2CH3), 39.1 (C2),
39.0, 35.9, 31.2 (C6), 25.8, 24.8, 24.5. IR (ATR) νmax 3203, 2938,
2851, 1721, 1450, 1436, 1160, 1078, 1066 cm−1. Anal. Calcd for
C15H24O6: C: 59.98; H: 8.05. Found: C; 60.04, H; 7.97.
Synthesis of 1-O-Methyl-5-O-caffeoylquinic Acid (1-O-Meth-
ylchlorogenic Acid), 7. To an 8 mL vial containing acetal 13 (20.8
mg, 0.0463 mmol) was added tetrahydrofuran (5 mL) followed by 0.2
M aqueous HCl (600 μL). The resulting solution was degassed by
bubbling Ar gas through the solution for 1 min, and the vial was sealed
with a screw cap and placed in a 55 °C oil bath for 24 h. The resulting
mixture was cooled to room temperature, diluted with ethyl acetate
(40 mL), washed with water (5 mL) and 80% brine (2 × 5 mL), and
dried (MgSO4). The solution was filtered and concentrated to give a
light brown residue. The residue was diluted with water (1 mL) to
provide a suspension which was frozen in liquid nitrogen and freeze-
dried on a vacuum line to give 12.0 mg (70%) of a fluffy white solid.
Attempted melting point determination of this hygroscopic material
Synthesis of Methyl 1-O-Methyl-3,4-cyclohexylidene-5-O-
bis(3′,4′-O-acetylcaffeoyl)quinate, 12. To a pear-shaped, one-
necked flask equipped with a magnetic stir bar and a septum were
added ester alcohol 10 (855 mg, 2.85 mmol) and pyridine (12 mL).
To this stirred solution was added caffeic acid anhydride tetraacetate
(11, 2.17 g, 4.27 mmol, 1.5 equiv) in one portion, and the resulting
mixture was capped with a septum and stirred at room temperature for
72 h. The mixture was diluted with EtOAc (350 mL) and transferred
to a 500 mL separatory funnel. The mixture was washed with 2 N
aqueous HCl (2 × 75 mL, second washing acidic w/litmus), saturated
aqueous NaHCO3 (4 × 60 mL), brine (50 mL) and dried (MgSO4).
The solution was filtered, concentrated, and dried to give an off-white
foam (2.0 g). The residue was purified via chromatography on flash
silica gel (200 g) using 96:4 CHCl3/acetone (1.5 L) followed by 9:1
CHCl3/acetone (500 mL) as eluent to afford 1.10 g (71%) of an off-
25
resulted in formation of a glass below 140 °C. [α]D = −16.1 (c =
1
0.55, MeOH), H NMR (360 MHz, CD3OD) δ 7.52 (d, 1, J = 15.9
Hz, H7′), 7.04 (d, 1, J = 1.9 Hz, H2′), 6.94 (dd, 1, J = 8.2, 1.9 Hz,
H6′), 6.76 (d, 1, J = 8.0 Hz, H5′), 6.21 (d, 1, J = 15.9 Hz, H8′), 5.13,
(m, 1, H5), 4.11 (m, 1, H3), 3.75 (m, 1, H4), 3.31 (s, 3, OCH3),
2.32−2.24 (m, 2, H2a, H6a), 2.11 (m, 1, H6b), 2.01 (m, H2b) ppm.
13C NMR (90 MHz, CD3OD) δ 175.0 (COOH), 168.4 (C9′), 149.6
(C4′), 147.2 (C7′), 146.8 (C3′), 127.8 (C1′), 123.0 (C6′), 116.4
(C5′), 115.2 (C2′), 115.1 (C8′), 81.2 (C1), 72.2 (C4), 72.0 (C5),
69.7 (C3), 52.2 (OCH3), 35.9 (C2), 34.2 (C6) ppm. IR (ATR) νmax
2951, 2906 1700, 1595, 1516, 1436, 1254, 1151, 1111, 976, 808 cm−1.
HRMS: Calcd for C17H20O\9 + Na [M + Na]+: 391.1000 m/z. Found:
391.1013 m/z.
25
1
white foam. [α]D = −28.3 (c = 1.05, CHCl3), H NMR (360 MHz,
acetone-d6) δ 7.67 (d, 1, J = 16.0 Hz, H7′), 7.64−7.60 (m, 2, H2′,
H6′), 7.31 (d, 1, J = 7.9 Hz, H5′), 6.54 (d, 1, J = 16.0 Hz, H8′), 5.34
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Table 1. Comparison of 1H NMR Data for MCGA3,32 for Methyl Chlorogenate from the Current Study and from Kancheva et
al.,43 and for Compound 7
MCGA3 (Kweon et al.)32
methyl chlorogenate (current study, 14) methyl chlorogenate (Kancheva et al)43
compound 7
7.51 (d, 1, J = 15.9 Hz, H7′)
7.04(d, 1, J = 2.0 Hz, H2′)
6.94 (dd, 1, J = 8.2, 2.0 Hz, H6′)
6.77 (d, 1, J = 8.4 Hz, H5′)
6.20 (d, 1, J = 15.9 Hz, H8′)
5.26−5.30 (m, 1, H5)
7.52 (d, 1, J = 15.9 Hz, H7′)
7.04 (d, 1, J = 1.9 Hz, H2′)
6.94 (dd, 1, J = 8.2, 1.9 Hz, H6′)
6.78 (d, 1, J = 8.2 Hz, H5′)
6.21 (d, 1, J = 15.9 Hz, H8′)
5.27 (m, 1, H5)
7.52 (d, 1, J = 15.9 Hz)
7.04 (d, 1, J = 2.0 Hz)
6.95 (dd, 1, J = 8.2, 2.0 Hz)
6.78 (d, 1, J = 8.2 Hz)
6.21 (d, 1, J = 15.9 Hz)
5.27 (m, 1)
7.52 (d, 1, J = 15.9 Hz, H7′)
7.04 (d, 1, J = 1.9 Hz, H2′)
6.94 (dd, 1, J = 8.0, 1.2 Hz, H6′)
6.76 (d, 1, J = 8.0 Hz, H5′)
6.21 (d, 1, J = 15.9 Hz, H8′)
5.13 (m, 1, H5)
4.12−4.15 (m, 1, H3)
3.71 (dd, 1, J = 7.5, 3.1 Hz, H4)
4.13, (dt, 1, J = 6.6, 3.3 Hz, H3)
3.73 (dd, 1, J = 7.5, 3.1 Hz, H4)
3.69 (s, 3, OCH3)
4.13 (m, 1)
4.11 (m, 1, H3)
3.73 (m, 1)
3.75 (m, 1, H4)
3.70 (s, 3, OCH3)
3.69 (s, 3, OCH3)
2.23−2.11 (m, 3)
2.03−1.99 (m, 1)
3.31 (s, 3, OCH3)
1.93−2.23 (m, 4, H2, H6)
2.23−2.19 (m, 3)
2.00 (dd, 1, J = 13.4, 6.8 Hz, H2a)
2.32−2.24 (m, 2, H2a, H6a)
2.11 (m, 1, H6b)
2.01 (m, 1, H2b)
author of another laboratory which utilized MCGA3 in
additional studies34,35 indicated that they are not in possession
of any authentic sample of the isolated MCGA3 and have no
spectroscopic data for the compound. After closer examination
of the reported spectroscopic data, we surmised that the CH3
RESULTS AND DISCUSSION
■
The approach taken for the production of MCGA3 was similar
in design to a previous report outlining a synthesis of a closely
related derivative.39 However, detailed synthetic procedures
were not presented in this communication. Previous reports
have indicated that alkylation of the C-1 hydroxyl of the 4,5-
ketal-protected quinic acid lactone can be accomplished with
iodomethane.39 Reaction of lactone 8 with methyl iodide and
sodium hydride in DMF afforded methyl ether 9 in 90% yield
after recrystallization from hexanes (Figure 2). Hydroxide
opening of lactone 9 afforded the corresponding hydroxyl
carboxylate. In our hands, acylation of this carboxylate with
acetate-protected caffeoyl chloride and imidazole derivatives39
resulted in poor isolated yields from complex mixtures. We
opted to examine the feasibility of unmasking the lactone
functionality to the hydroxyl methyl ester 10. Reaction of
lactone 9 with freshly prepared sodium methoxide (Na,
CH3OH)40 afforded hydroxy ester 10 in 74% yield after
chromatographic purification. The caffeoyl moiety was
introduced through acylation of the secondary alcohol using
caffeic acid anhydride tetraacetate, 11,37 a reagent developed
specifically for construction of caffeoyl-containing compounds,
and afforded the fully protected target compound 12. The
methyl ester functionality was cleaved with lithium iodide in
pyridine (120 °C, 52 h)41 which also resulted in concomitant
loss of the caffeoyl acetate protecting groups to give the
intermediate acetal 13 in 79% yield after chromatographic
purification. Completion of the first synthesis of 1-O-methyl
chlorogenic acid, 7, was accomplished after treatment of acetal
13 with aqueous HCl in THF (70%).
1
signal at 3.7 ppm in the reported H NMR data for MCGA3
was most likely arising from a methyl ester and not an aliphatic
methyl ether (typically absorbing at 3.2−3.3 ppm) and that the
1H NMR data as a whole was strikingly similar to that reported
for methyl chlorogenate. Literature NMR spectroscopic data
appearing both before42 and after43 the initial structure
assignment disclosure32 supports these observations. To verify
this conjecture, we converted commercially available chloro-
genic acid to methyl chlorogenate, 14, in methanolic HCl11 at
1
room temperature. Comparison of reported H and 13C NMR
absorption data for MCGA3 with the data obtained for 14 and
7 in CD3OD are given in Tables 1 and 2. Both the 1H and 13
C
NMR data of methyl chlorogenate from the published literature
Table 2. Comparison of 13C NMR Data for MCGA3,32 for
Methyl Chlorogenate from the Current Study and from
Deyama et al.,42 and for Compound 7
methyl
methyl
MCGA3,
chlorogenate, 14,
current study
(assignment)
chlorogenate,
Deyama et al.42
(assignment)
compound 7,
current study
(assignment)
Kweon et al.32
(assignment)
175.4
(COOR)
175.6 (COOR)
175.3 (COOR)
175.0 (COOR)
168.3 (C-9′)
149.7 (C-3′)
147.2 (C-4′)
168.5 (C-9′)
149.8 (C-4′)
147.4 (C-7′)
168.3 (C-9′)
149.4 (C-4′)
168.4 (C-9′)
149.6 (C-4′)
147.2 (C-7′)
Extensive NMR examination of 7 (1H, 13C, HSQC, and
HMBC) confirmed the structure as assigned. Conclusive
evidence for the assigned structure comes from diagnostic
absorption in the HMBC spectrum of compound 7 where, as
predicted, the methyl ether protons show only a single cross-
peak interaction with the assigned C-1 carbon atom. The C-5
hydrogen atom shows three-bond correlations with C-1, C-3,
and the carbonyl carbon of the caffeoyl group, as the expected
arrangement for the C-5 attachment of the caffeoyl group to the
quinic acid unit.
However, these spectroscopic absorptions differ significantly
from those reported for MCGA3.32 Unfortunately, an authentic
sample or spectroscopic data of MCGA3 is not available for
comparison purposes. No response was received through
attempted contact (e-mail) with the corresponding author of
the original isolation/activity paper.32 The corresponding
147.0 (C-3′ or C-
7′)
146.8 (C-7′)
147.0 (C-3′)
146.6 (C-3′ or
C7′)
146.8 (C-3′)
127.7 (C-6′)
123.0 (C-1′)
116.5 (C-5′)
115.1 (C-2′)
115.1 (C-8′)
75.8 (C-4)
72.6 (C-1)
72.1 (C-5)
70.3 (C-3)
53.0 (OCH3)
38.1 (C-2)
37.8 (C-6)
127.8 (C-1′)
123.2 (C-6′)
116.7 (C-5′)
115.3 (C-2′)
115.1(C-8′)
75.8 (C-1)
72.6 (C-4)
72.1 (C-5)
70.3 (C-3)
53.0 (OCH3)
38.1 (C-2)
37.8 (C-6)
127.7 (C-1′)
122.9 (C-6′)
116.5 (C-5′)
115.2 (C2′ or C8′) 115.2 (C-2′)
115.2 (C2′ or C8′) 115.1 (C-8′)
127.8 (C-1′)
123.0 (C6′)
116.4 (C-5′)
76.0 (C-1)
81.2 (C-1)
73.0 (C-5 or C-4) 72.2 (C-4)
72.0 (C-5 or C-4) 72.0 (C-5)
70.7 (C-3)
69.7 (C-3)
53.0 (OCH3)
52.2 (OCH3)
38.2 (C-2 or C-6) 35.9 (C-2)
38.2 (C-2 or C-6) 34.2 (C-6)
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Journal of Agricultural and Food Chemistry
Article
identification of caftaric acid as an antimutagenic component from
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14. In addition, subsequent reports33−35 detailing the biological
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correct structural formula.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra for compounds 9, 10, 12, and 13,
along with 1H, 13C, 1H−13C HSQC, and 1H−13C HMBC NMR
spectra for compounds 7 and 14. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: 608-890-0071. Fax: 608-890-0076. E-mail: Wayne.
Zeller@ars.usda.gov.
■
Funding
Purchase of the Waters LCT instrument in 2000 was partially
funded by NSF award no. CHE9974839 to the Department of
Chemistry, University of WisconsinMadison.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The author thanks Dr. Paul Schatz for helpful discussions, for
preparation of the Supporting Information, and for the
preparation of lactone 8. The author also thanks Professor
Steve Burke and Alex Clemens of the Department of Chemistry
at the University of WisconsinMadison for assistance in
obtaining optical rotations, and Professor Mukhtar of the
Department of Dermatology at the University of Wisconsin
Madison for his assistance in our attempt to locate an authentic
sample and spectroscopic data for MCGA3.
(18) Ohnishi, M.; Morishita, H.; Iwahashi, H.; Toda, S.; Shirataki, Y.;
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acid peroxidation and haemolysis. Phytochemistry 1994, 36, 579−583.
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Rich, L.; Mas, E.; Croft, K. D.; Hodgson, J. M. Acute effects of
chlorogenic acid on nitric oxide status, endothelial function, and blood
pressure in healthy volunteers: A randomized trial. J. Agric. Food Chem.
2012, 60, 9130−9136.
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