New Melanoidin-like Maillard Polymers from 2-Deoxypentoses
J. Agric. Food Chem., Vol. 46, No. 1, 1998 105
for LC. Autoclaving was done in a stainless steel laboratory
autoclave (Roth, I series) equipped with a 100 mL duran glass
tube and heated by an electric heater with magnetic stirrer.
During autoclaving the peak temperature (120 °C) was reached
after 45 min.
Rea ction of 2-Deoxy-D-r ibose w ith Meth yla m in e-HCl.
2-Deoxy-D-ribose (1 g) was reacted with methylamine-HCl (1.5
g) in 0.1 M phosphate buffer (20 mL, pH 7) at 120 °C for 1 h.
The reaction mixture was extracted with diethyl ether (3 ×
50 mL). The combined organic phases were washed with (a)
0.025 M HCl (saturated with NaCl) (20 mL) and (b) 5%
NaHCO3 (10 mL). After drying over Na2SO4 and concentration
on a 20 cm Vigreux column to ∼0.2 mL, the extract was
directly analyzed by capillary GC/MS. N-Methyl-2-(hydroxy-
methyl)pyrrole (1), bis(N-methyl-2-pyrrolyl)methane (2) and
colorless product polymerizes in an exothermic spontaneous
reaction to form a colorless solid. Exposed to air, the polymer
became pink and finally dark red: MS (EI, 70 eV, 160 °C),
m/z 454 (2.5), 453 (7.6), 360 (8.4), 280 (5.5), 267 (23.2), 266
(8.7), 187 (9.8), 174 (43.0), 173 (51.1), 111 (53.3), 110 (1.3), 109
(20.6), 108 (21.4), 95 (11.8), 94 (100), 93 (20.8), 87 (32.5), 82
(8.3), 80 (8.8), 74 (14.1), 73 (14.1), 72 (6.8), 59, 19.3), 53 (9.4),
45 (21.3), 44 (39.9), 43 (15.8), 42 (17.4), 41 (8.1), 31 (26.8). A
red polymer generated from 1 by addition of a catalytic amount
of 2 N HCl (see below) shows essentially the same MS.
Sa m p le P r ep a r a tion . Polymerization of 1. N-Methyl-2-
formylpyrrole (40 µL) was dissolved in 9% aqueous methanol
(11 mL). After addition of NaBH4 (10 mg) and stirring (1 h,
20 °C), the mixture was extracted with diethyl ether (3 × 20
mL). The organic phase was washed with (a) 0.1 N HCl (2 ×
5 mL) and (b) saturated aqueous NaCl (2 × 5 mL) and dried
over Na2SO4. The ether (an aliquot was analyzed by GC/MS)
was evaporated on a 20 cm Vigreux column. The residue (25
mg) was redissolved in CHCl3 (10 mL). After 2 days at room
temperature (light excluded), the solvent was evaporated and
the dark colored product (10 mg) was analyzed by GC/MS and
MALDI-TOF-MS (see Figure 2): UV-vis (0.15 mg/mL in
CHCl3) λmax (E) ) 247 (1.20), 290 (0.76), broad shoulder 350-
550 nm; fluorescence (in CHCl3) λex ) 385 nm, λem ) 493 nm.
Polymerization of 2-(Hydroxymethyl)furan. Furfuryl alcohol
(1 mL) was dissolved in distilled water (40 mL) and stirred
(12 h, 20 °C) after addition of 1 N HCl (100 µL). The brown
solid was recovered by filtration, washed with water, dissolved
in diethyl ether, and analyzed by GC/MS: MALDI-TOF-MS,
m/z 257.3 (trimer), 281.3, 321.4, 337.3 (tetramer), 361.2, 379.2,
401.4, 417.2 (pentamer), 435.4, 459.3, 475.4, 497.3 (hexamer),
539.3, 577.4.
Isola tion of Oligom er s. A 10-fold amount of 1 was reacted
as described for sample preparation. The product was im-
mediately isolated after evaporation of the ether extract,
redissolved in chloroform (1 mL), and fractionated by column
LC (silica gel, 20 × 1 cm) with petroleum ether/ethyl acetate
(5:1). The dimeric bis(N-methyl-2-pyrrolyl)methane (2) (30
mg) and the trimeric 5-bis(N-methyl-2-pyrrolylmethyl)-N-
methylpyrrole (3) (15 mg) were isolated, and the pure com-
pounds were characterized by TLC (Rf ) 0.53 and 0.33,
petroleum ether/ethyl acetate 5:1), GC/MS, and NMR. 2: MS,
2,5-bis(N-methyl-2-pyrrolylmethyl)-N-methylpyrrole (3) were
identified by their MS data and by comparison with authentic
samples.
N-Meth yl-2-[13C]for m ylp yr r ole. At 4 °C 0.85 g (5.50
mmol) of POCl3 was added to 0.32 g (4.32 mmol) of N,N-
dimethyl[13C]formamide. After 15 min of stirring at 20 °C, 5
mL of 1,2-dichloroethane and 0.35 g (4.32 mmol) of N-
methylpyrrole in 1 mL of 1,2-dichloroethane were added at 4
°C. After warming up to room temperature, the mixture was
refluxed for 15 min. Five milliliters of a solution (5.5 M) of
CH3CO2Na‚3H2O was added at 20 °C. After that, the mixture
was refluxed for 15 min. The aqueous phase was extracted
with diethyl ether, and the combined organic phases were
washed with saturated Na2CO3 and dried over Na2SO4. Yield
after evaporation and vacuum distillation (11 mmHg) of the
resulting oil at 80 °C was 0.30 g (65%): 1H NMR δ 3.94 (s,
3H, NCH3), 6.19 (m, 1H, H-4), 6.86 (m, 1H, H-5), 6.90 (m, 1H,
H-3), 9.53 (d, 1H, J ) 173 Hz, CHdO); 13C NMR δ 179.6 (s,
13CdO).
1
m/z 174 (100), 173 (82), 94 (71), 93 (40), 42 (15); H NMR δ
3.52 (s, 6H, N-CH3), 3.85 (s, 2H, CH2), 5.81 (mc, 2H, H-3 of
pyrroles), 6.02 (mc, 2H, H-4 of pyrroles), 6.55 (mc, 2H, 5-H of
pyrroles). 3: MS, m/z 267 (82), 187 (18), 173 (78), 94 (100),
93 (51), 42 (20); 1H NMR δ 3.37 (s, 3H, central N-CH3), 3.515
(s, 6H, peripheral N-CH3), 3.816 (s, 4H, CH2), 5.69 (s, 2H, H-3,4
of central ring), 5.787 (mc, 2H, H-3 of peripheral rings), 6.01
(mc, 2H, H-4 of peripheral rings), 6.538 (mc, 2H, H-5); 13C
NMR δ sp3C-H nd, 33.83 (central NCH3-group), 34.53 (pe-
ripheral NCH3-groups), 106.48, 108.61 (ring CH), 121.93
(5-CH of peripheral pyrroles), 131.83 (quartet C of pyrroles).
Eth yl 2-(2-F or m yl-1-p yr r olyl)a ceta te (4). Ethyl bro-
moacetate (21.1 g, 127 mmol) was dropped into 4 g (42.1 mmol)
of 2-formylpyrrole and 25 g of K2CO3 in 90 mL of p-dioxane.
The mixture was refluxed (3 h), cooled to room temperature,
and extracted with 140 mL of toluene. The filtered dark brown
solution was evaporated after addition of 100 mL of water (45
°C, 20 mmHg): 6.63 g of 4 (86%) brown oil, which was used
without further purification; MS, m/z 181 (70.5, M+), 153
(19.7), 136 (15.5), 124 (15.2), 109 (9.0), 108 (100), 94 (46.9), 80
(24.5), 53 (16.3); 1H NMR δ 1.30 (3H, t, CH3), 4.20 (2H, q,
OCH2), 5.05 (2H, s, NCH2), 6.30 (1H, dd, H-4), 6.95 (1H, m,
H-3), 7.0 (1H, dd, H-5), 9.5 (1H, s, CHO).
(2-F or m yl-1-p yr r olyl)a cetic Acid (5). At room temper-
ature 5 g of 4 was stirred for 15 min in methanolic K2CO3 (10%
w/v, 250 mL). After addition of 15 mL of distilled water, the
mixture was refluxed for 15 min. Four hundred forty mil-
liliters of distilled water was added after the mixture had
cooled to 5 °C, and the mixture was kept at 5 °C for 12 h. The
aqueous phase was extracted with ethyl acetate (320 mL) and,
after the pH was adjusted to 6.0, extracted again with ethyl
acetate (3 × 100 mL). The combined ethyl acetate extracts
N-Meth yl-2-(h yd r oxym eth yl)p yr r ole (1). According to
the method of Ryskiewicz and Silverstein (1954) during 10 min
5.0 g (132 mmol) of NaBH4 in 50 mL of distilled water was
dropped into 5.54 g (5.80 mmol) of N-methyl-2-formylpyrrole,
suspended in 100 mL of distilled water. After 3 h of stirring
at 20 °C, 50 g of K2CO3 was added at ∼10 °C. After stirring
for 15 min, the mixture was extracted three times with diethyl
ether (each 60 mL). The combined diethyl ether extracts were
dried over Na2SO4, and 1.52 g of triethylamine was added.
Diethyl ether (100 mL) was evaporated at 20 °C and then at
0 °C by vacuum distillation. The remaining colorless oil (6.86
g) was a 3.5:1 (w/w) mixture of pyrrole 1 and triethylamine as
analyzed by 1H NMR: δ 2.9 (br s, 1H, OH), 3.60 (s, 3H, NCH3),
4.55 (s, 2H, CH2), 6.10 (m, 2H, H-3, H-4), 6.60 (m, 1H, H-5);
signals of triethylamine at 1.0 (t) and 2.55 (q); 13C NMR δ 33.41
(NCH3), 56.05 (CH2OH), 106.4 (C-3), 108.46 (C-4), 123.11 (C-
5), 131.73 (C-2); MS (EI, 70 eV), m/z 112 (4.1), 111 (55.6), 110
(10.6), 95 (7.6), 94 (100), 93 (6.7), 82 (12.9), 80 (5.7), 67 (10.4),
53 (7.9), 42 (8.5), 41 (6.0).
Several attempts to isolate the pure compound failed. After
evaporation of the diethyl ether, even at low temperature, the