Synthesis of the seed germination stimulant 3-methyl-2H-furo[2,3-c]pyran-2-ones utilizing direct and regioselective Ti-crossed aldol addition

Article abstract of DOI:10.1016/j.tetlet.2008.05.052

3-Methyl-2H-furo[2,3-c]pyran-2-ones 1 and 2, a unique and remarkable seed germination stimulant, and its analogue were synthesized using direct and regioselective Ti-crossed aldol addition between dihydro-2H-pyran-3(4H)-ones and methyl pyruvate as the key step, followed by furanone formation.

Full text of DOI:10.1016/j.tetlet.2008.05.052

Tetrahedron Letters 49 (2008) 4509–4512
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Tetrahedron Letters
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Synthesis of the seed germination stimulant 3-methyl-2H-furo[2,3-c]pyran-2-
ones utilizing direct and regioselective Ti-crossed aldol addition
Ryohei Nagase a, Mayumi Katayama a, Hiroaki Mura a, Noritada Matsuo b, Yoo Tanabe a,
*
a Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
b Agricultural Chemicals Research Laboratory, Sumitomo Chemical Co. Ltd, 4-2-1 Takatsukasa, Takarazuka, Hyogo 665-0051, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Methyl-2H-furo[2,3-c]pyran-2-ones 1 and 2, a unique and remarkable seed germination stimulant, and
its analogue were synthesized using direct and regioselective Ti-crossed aldol addition between dihydro-
2H-pyran-3(4H)-ones and methyl pyruvate as the key step, followed by furanone formation.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 14 April 2008
Revised 8 May 2008
Accepted 12 May 2008
Available online 15 May 2008
Keywords:
Seed germination
Aldol addition
Titanium tetrachloride
Furanone
Pyranone
1. Introduction
ing interest in Ti-Claisen condensations,6 relevant Ti-direct aldol-
type,7 and Mannich-type additions8 led us to investigate the syn-
In 2004, Flematti’s group disclosed the unique and remarkable
seed germination stimulatory activity of 3-methyl-2H-furo[2,3-c]-
pyran-2-one 1 in a wide range of plant species.1 This lead gener-
ation finding is considered to be a promising candidate tool for
the plant production industry. Since this discovery, three syntheses
have appeared: the first synthesis by Flematti’s group themselves,2
the second by Goddard-Borger’s group,3 and the third by Xu’s
group.4 Recently, a patent by one of the author (N.M.) groups
was released.5
thesis of 1 and its dihydro analogue 2 (Fig. 1). We present herein
an efficient short synthesis of 1 and 2 utilizing a direct and
regioselective Ti-crossed aldol addition between dihydro-2H-pyr-
an-3(4H)-ones and methyl pyruvate as the key step, followed by
furanone formation.
2. Results and discussion
Scheme 1 outlines the retrosynthetic route of 1 starting from 2-
furfurylmethanol (3). The reported oxidative ring expansion of 3
using mCPBA,9 followed by treatment with AcCl–Et3N–cat. DMAP,
gave 5,6-dihydro-5-oxo-2H-pyran-2-yl acetate (4) in 82% yield
The first method, starting from pyromeconic acid, is short and
straightforward but results in low overall yield (ca. <10%). The sec-
ond method produces a significantly greater total yield to 30%, but
requires 9 steps from 1,2-O-isopropylidene-D-xylofuranose as the
starting compound. The third method is performed in 8 steps from
diethyl isopropylidenemalonate in 8% overall yield. Our longstand-
O
OH
CO2Me
O
O
O
O
O
O
O
O
O
OiPr
OiPr
O
1
7
5
O
O
O
1
2
OH
O
O
Figure 1. 3-Methyl-2H-furo[2,3-c]pyran-2-one 1 and the dihydro analogue 2.
OAc
3
4
* Corresponding author. Tel.: +81 79 565 8394; fax: +81 79 565 9077.
E-mail address: tanabe@kwansei.ac.jp (Y. Tanabe).
Scheme 1. Retrosynthesis of 3-methyl-2H-furo[2,3-c]pyran-2-one 1.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.052

4510
R. Nagase et al. / Tetrahedron Letters 49 (2008) 4509–4512
O
O
O
OH
CO Me
O
a,b
c,d
a,b
O
CO Me
2
2
OH
O
O
O
O
O
OiPr
3
i
i
OAc
5
O Pr
O Pr
7
10
4
Scheme 2. Reagents and conditions: (a) mCPBA/CH2Cl2, 0–5 °C; (b) AcCl–Et3N–cat.
DMAP/CH2Cl2, 0–5 °C; (c) iPrOH–cat. SnCl4/ClCH2CH2Cl, 20–25 °C; (d) H2–cat. Pd–C/
AcOEt, 20–25 °C.
c
O
O
1
O
R
O
R
OH
Scheme 5. Reagents and conditions: (a) (CF3CO)2O–Et3N–cat. DMAP/ CH3CN,
0–5 °C; (b) DBU/CH3CN, 0–5 °C; (c) PTSÁH2O/ toluene, reflux.
MeCOCO Me
2
4
CO Me
2
O
O
TiCl - Bu N
4
3
3. Experimental
3.1. General
R = H; 6
R = H; 8
i
i
R = O Pr; 5
R = O Pr; 7
Scheme 3. Regioselective direct Ti-aldol reaction of dihydro-2H-pyran-3(4H)-ones
5 and 7.
Melting points were determined on a hot-stage microscope
apparatus (Yanagimoto) and were uncorrected. NMR spectra were
recorded on a JEOL DELTA 300 spectrometer, operating at 300 MHz
for 1H NMR and 75 MHz for 13C NMR. Chemical shifts (d ppm) in
CDCl3 were reported downfield from TMS (=0) for 1H NMR. For
13C NMR, chemical shifts were reported in the scale relative to
CDCl3 (77.00 ppm) as an internal reference. IR Spectra were
recorded on a SHIMADZU FT/IR-8100A spectrophotometer.
i
(Scheme 2).10 An SnCl4-catalyzed acetal exchange with PrOH and
subsequent catalytic hydrogenation gave 6-isopropoxy-2H-pyran-
3(6H)-one (5) in 68% yield (2 steps).11
The initial model study of the Ti-crossed direct aldol addition
was guided using commercially available dihydro-2H-pyran-
3(4H)-one 6 and methyl pyruvate (Scheme 3). Regioselectivity of
the enolate formation of dihydro-2H-pyran-3(4H)-ones 5 and 6
was speculated to be of primary importance in this synthetic plan.
Extensively reported studies on this issue revealed that the desired
enolate formation was kinetically as well as thermodynamically
predominant.12 Indeed, Ti-direct aldol addition using TiCl4–Bu3N
reagent between 6 and methyl pyruvate proceeded smoothly at
the C4 position, presumably under kinetic conditions (À55 to
À78 °C), to give the aldol adduct 8 as a desired sole regioisomer
in 71% yield. A similar result was observed in our previous syn-
thetic study of (R)-mintlactone.7b The presence of a TMSCl co-
reagent did not affect the present reaction.7a
3.1.1. 5,6-Dihydro-5-oxo-2H-pyran-2-yl acetate (4)8,10
mCPBA (18.1 g, 105 mmol) was added to a stirred solution of
2-furfurylmethanol (3; 6.87 g, 70.0 mmol) in CH2Cl2 (210 mL) at
0–5 °C under an Ar atmosphere, and the mixture was stirred for
6 h at the same temperature. The resultant white precipitates were
removed by filtration and the filtrate was concentrated under
reduced pressure. The obtained residue was diluted with CH2Cl2
(190 mL) and cooled to 0–5 °C. Et3N (12.0 g, 119 mmol) and DMAP
(420 mg, 3.5 mmol) in CH2Cl2 (10 mL), and AcCl (9.34 g, 119 mmol)
in CH2Cl2 (10 mL) were successively added to the stirred solution
at 0–5 °C under an Ar atmosphere, and the mixture was stirred
for 2 h at the same temperature. The mixture was poured into
NaHCO3 aqueous solution and extracted twice with CH2Cl2. The
combined organic phase was washed with water, brine, dried
(Na2SO4), and concentrated. The obtained crude oil was purified
by SiO2-column chromatography (neutral) (hexane–AcOEt = 4:1)
to give the desired product 4 (8.96 g, 82%).
With this result in hand, the reaction using 5 also successfully
afforded the key aldol-adduct 7 in 47% yield. Although the yield
was decreased, probably due to the labile acetal moiety in 5, the
regiochemistry was similar to that when 6 was used.
Subsequent furanone transformation was investigated. The di-
rect transformation of 8 using p-toluenesulfonic acid (PTS)ÁH2O
catalyst gave the desired furanone 2 in 48% yield (Scheme 4). The
identical reaction using 6, however, gave disappointing results:
only a trace amount of the desired product 1 was detected due
to decomposition of the acetal moiety in 7. To overcome this prob-
lem, the stepwise dehydration and lactonization procedures were
examined: Successive treatment of 6 with (CF3CO)2O–Et3N–cat.
DMAP and DBU gave the conjugated ene-dicarbonyl intermediate
10, which was treated with a PTSÁH2O catalyst to successfully
afford the desired furanone 1 in 45% yield (Scheme 5).
In conclusion, we have achieved a new synthesis of 3-methyl-
2H-furo[2,3-c]pyran-2-ones 1 (7 steps; 12% overall yield) and 2
(2 steps; 34% overall yield) through a direct and regioselective
Ti-aldol addition. Further studies of the analogue synthesis and
the structure–activity relationship are in progress.
Yellow oil; 1H NMR (CDCl3) d 2.15 (3H, s), 4.22 (1H, d,
Jgem = 16.9 Hz), 4.52 (1H, d, Jgem = 16.9 Hz), 6.28 (1H, d, J =
10.3 Hz), 6.50 (1H, d, J = 3.9 Hz), 6.94 (1H, dd, J = 3.9, 10.3 Hz);
13C NMR (CDCl3) d 20.8, 67.3, 86.5, 128.6, 142.2, 169.4, 193.3; IR
(neat) 3485, 2988, 1751, 1373, 1221, 1006, 933 cmÀ1
.
3.1.2. Dihydro-6-isopropoxy-2H-pyran-3(4H)-one (5)
SnCl4 (1.0 M in CH2Cl2, 750 lL, 0.75 mmol) was added to a stir-
red solution of 4 (2.34 g, 15.0 mmol) and iPrOH (1.80 g, 30.0 mmol)
in CH2Cl2 (45 mL) at 20–25 °C under an Ar atmosphere, followed by
being stirred for 2 h. Satd NaHCO3 aqueous solution was added to
the mixture, which was extracted twice with AcOEt. The combined
organic phase was washed with brine, dried (Na2SO4), and concen-
trated. The obtained crude oil was purified by SiO2-column
chromatography (neutral) (hexane–AcOEt = 10:1) to give 6-
isopropoxy-2H-pyran-3(6H)-one (1.97 g, 84%).
Yellow oil; 1H NMR (CDCl3) d 1.23 (3H, d, J = 6.2 Hz), 1.27 (3H, d,
J = 6.2 Hz), 4.05 (1H, sept, J = 6.2 Hz), 4.09 (1H, d, Jgem = 16.9 Hz),
4.49 (1H, d, Jgem = 16.9 Hz), 5.31 (1H, d, J = 3.4 Hz), 6.13 (1H, d,
J = 10.3 Hz) 6.86 (1H, dd, J = 3.4, 10.3 Hz); 13C NMR (CDCl3) d
21.7, 23.2, 66.2, 71.0, 91.3, 127.6, 145.0, 195.0; IR (neat) 2975,
O
O
OH
CO Me
O
.
PTS H O
2
2
O
O
8
2
Scheme 4. Direct furanone transformation of 8–2 using PTSÁH2O.
1709, 1383, 1262, 1181, 1159, 1121, 1102, 1080, 1042 cmÀ1
.

R. Nagase et al. / Tetrahedron Letters 49 (2008) 4509–4512
4511
10% Pd–C (670 mg, 0.63 mmol) was added to a stirred solution
of 6-isopropoxy-2H-pyran-3(6H)-one (1.97 g, 12.6 mmol) in AcOEt
(25 mL), and the mixture was stirred equipped with a H2 balloon
for 1.5 h at 20–25 °C. The mixture was filtered through the Celite
using glass filter, and the filtrate was concentrated under reduced
pressure. The obtained crude oil was purified with SiO2-column
chromatography (hexane–AcOEt = 10:1) to give the desired prod-
uct 5 (1.62 g, 81%).
67.9, 68.7, 68.9, 69.2, 69.4, 72.2, 72.8, 74.8, 75.9, 93.5, 93.8,
95.1, 95.5, 175.0, 175.1, 176.8, 177.0, 206.8, 209.3, 210.7,
210.8; IR (neat) 2974, 1736, 1458, 1372, 1333, 1250, 1190,
1127, 1044, 980 cmÀ1
.
HRMS (ESI) calcd for C12H20O6 (M+)
260.1260, found 260.1258.
3.1.5. 3-Methyl-2H-furo[2,3-c]pyran-2-one (1)1
(CF3CO)2O (71 lL, 0.51 mmol) was added to a stirred solution of
7 (78 mg, 0.30 mmol), Et3N (46 mg, 0.45 mmol), and DMAP
Colorless oil; 1H NMR (CDCl3) d 1.18 (3H, d, J = 6.2 Hz), 1.23 (3H,
d, J = 6.2 Hz), 1.96 (1H, dddd, J = 4.1, 6.2, 7.2 Hz, Jgem = 14.1 Hz),
2.25 (1H, dddd, J = 4.1, 5.9, 8.3 Hz, Jgem = 14.1 Hz), 2.45 (1H, ddd,
(1.8 mg, 0.015 mmol) in CH3CN (1.0 mL) at 0–5 °C under an Ar
atmosphere, followed by being stirred for 30 min. Water was
added to the mixture, which was extracted twice with AcOEt.
The combined organic phase was washed with water, brine, dried
(Na2SO4), and concentrated to the product, to which DBU (64 mg,
0.42 mmol) in CH3CN (1.0 mL) was added at 0–5 °C under an Ar
atmosphere, followed by being stirred for 1 h. Satd NaHCO3 aque-
ous solution was added to the mixture, which was extracted twice
with AcOEt. The combined organic phase was washed with water,
brine, dried (Na2SO4), and concentrated. A solution of the obtained
crude product and PTSH2O (68 mg, 0.36 mmol) in toluene (2 mL)
was refluxed for 2 h under an Ar atmosphere, followed by being
cooled down to room temperature. Satd NaHCO3 aqueous solution
was added to the mixture, which was extracted twice with AcOEt.
The combined organic phase was washed with water, brine, and
dried (Na2SO4), and concentrated. The obtained crude oil was puri-
fied with SiO2-column chromatography (hexane–AcOEt = 4:1) to
give the desired product 1 (20 mg, 45%).
J = 6.2, 7.2 Hz,
Jgem = 16.5 Hz), 2.59 (1H, ddd, J = 5.9, 8.3 Hz,
Jgem = 16.5 Hz), 3.90 (1H, d, Jgem = 16.9 Hz), 3.98 (1H, sept, J =
6.2 Hz), 4.20 (1H, d, Jgem = 16.9 Hz), 5.10 (1H, t, J = 4.1 Hz); 13C
NMR (CDCl3) d 21.4, 23.3, 28.6, 33.8, 67.2, 68.9, 93.8, 209.1; IR
(neat) 2975, 1736, 1381, 1248, 1200, 1171, 1125, 1076, 1046,
1009 cmÀ1. HRMS (ESI) calcd for C8H14O3 (M+) 158.0943, found
158.0938.
3.1.3. Methyl 2-(tetrahydro-5-oxo-2H-pyran-4-yl)-2-
hydroxypropanoate (8)
TiCl4 (162 lL, 1.5 mmol) and a solution of Bu3N (371 mg,
2.0 mmol) in CH2Cl2 (0.5 mL) were successively added to a stirred
solution of dihydro-2H-pyran-3(4H)-one (6; 100 mg, 1.0 mmol) in
CH2Cl2 (4.0 mL) at À78 °C under an Ar atmosphere. After 15 min,
methyl pyruvate (204 mg, 2.0 mmol) in CH2Cl2 (0.5 mL) was added
to the mixture at À78 °C, followed by being stirred for 1 h. Water
was added to the mixture, which was extracted twice with AcOEt.
The combined organic phase was washed with water, brine, dried
(Na2SO4) and concentrated. The obtained crude oil was purified by
SiO2-column chromatography (neutral) (hexane–AcOEt = 10:1) to
give the desired product 8 (144 mg, 71%).
Yellow crystals; mp 118–119 °C; 1H NMR (CDCl3) d 1.94 (Me;
3H, s), 6.52 (4-H; 1H, d, J = 5.5 Hz), 7.32 (5-H; 1H, d, J = 5.5 Hz),
7.44 (7-H; 1H, s); 13C NMR (CDCl3) d 7.6, 100.2, 103.4, 126.8,
139.7, 142.2, 148.0, 171.2; IR (KBr) 1734 cmÀ1. These spectroscopic
parameters agreed with those reported.
Diastereomixtures; Colorless oil; 1H NMR (CDCl3)
d 1.33
3.1.6. 4,5-Dihydro-3-methyl-2H-furo[2,3-c]pyran-2-one (2)
A solution of 8 (61 mg, 0.3 mmol) and PTS (p-toluenesulfonic
acid)ÁH2O (114 mg, 0.6 mmol) in toluene (9 mL) was refluxed for
1 h, under an Ar atmosphere, followed by being cooled down to
room temperature. Satd NaHCO3 aqueous solution was added to
the mixture, which was extracted twice with AcOEt. The combined
organic phase was washed with water, brine, and dried (Na2SO4),
and concentrated. The obtained crude oil was purified with SiO2-
column chromatography (hexane:AcOEt = 5:1) to give the desired
product 2 (22 mg, 48%).
(3H Â 3/5, s), 1.46 (3H Â 2/5, s), 2.12–2.30 (2H, m), 2.83 (1H Â 2/5,
dd, J = 7.6, 10.7 Hz), 3.12 (1H Â 3/5, dd, J = 8.3, 10.7 Hz), 3.73–
3.88 (1H, m), 3.78 (3 H Â 2/5, s), 3.79 (3H Â 3/5, s), 3.88–4.13
(3H, m); 13C NMR (CDCl3) d 23.0, 24.9, 47.5, 53.1, 66.6, 73.0,
175.7, 176.0, 207.3; IR (neat) 3489, 2974, 1748, 1452, 1379,
1184, 1122, 1026, 979 cmÀ1. HRMS (ESI) calcd for C9H14O5 (M+)
202.0841, found 202.0939.
3.1.4. Methyl 2-(tetrahydro-2-isopropoxy-5-oxo-2H-pyran-4-
yl)-2-hydroxypropanoate (7)
Yellow crystals; mp 71–73 °C; 1H NMR (CDCl3) d 1.89 (3H, s),
2.84 (2H, t, J = 6.5 Hz), 4.19 (2H, t, J = 6.5 Hz), 6.92 (1H, s); 13C
NMR (CDCl3) d 8.3, 23.0, 66.8, 115.9, 131.6, 138.7, 142.4, 170.8;
IR (KBr) 3011, 1779, 1294, 1148, 1067, 878 cmÀ1. HRMS (ESI) calcd
for C8H8O3 (M+) 152.0473, found 152.0471.
TiCl4 (484 lL, 4.4 mmol) was added to a stirred solution of
dihydro-6-isopropoxy-2H-pyran-3(4H)-one (5; 316 mg, 2.0 mmol),
Bu3N (1.11 g, 6.0 mmol), and methyl pyruvate (408 mg, 4.0 mmol)
in CH2Cl2 (6.0 mL) at À60–À55 °C under an Ar atmosphere, and
the mixture was stirred for 1.5 h at the same temperature. A similar
work-up in the case of 8 gave the desired product 7 (245 mg, 47%).
Diastereomixtures; Yellow oil; 1H NMR (CDCl3) d 1.15–1.27
(6H, m), 1.29 (3H Â 3/10, s), 1.34 (3H Â 2/10, s), 1.42 (3H Â 4/
10, s), 1.48 (3H Â 1/10, s), 1.92 (1H Â 2/10, dt, J = 6.5 Hz,
Jgem = 14.1 Hz), 1.99 (1H Â 1/10, ddd, J = 6.2, 12.0 Hz, Jgem = 14.5
Hz), 2.11 (1 H Â 4/10, ddd, J = 2.4, 7.2 Hz, Jgem = 13.1 Hz), 2.19–
2.34 (1H Â 4/10 + 2H Â 3/10, m), 2.38 (1H Â 1/10, dt, J = 6.2 Hz,
Jgem = 14.5 Hz), 2.52 (1H Â 2/10, dt, J = 6.5 Hz, Jgem = 14.5 Hz),
2.92 (1H Â 1/10, dd, J = 5.9, 12.4 Hz), 3.09 (1H Â 4/10, dd, J =
7.2, 12.4 Hz), 3.14 (1H Â 2/10, dd, J = 6.2, 13.8 Hz), 3.38 (1H Â
3/10, dd, J = 7.2, 12.0 Hz), 3.762 (3H Â 3/10, s), 3.763 (3 H Â
2/10, s), 3.79 (3H Â 1/10, s), 3.81 (3H Â 4/10, s), 3.83–4.04
(2H, m), 4.10–4.27 (1H, m), 5.11 (1H Â 4/10, t, J = 2.4 Hz),
5.17 (1H Â 3/10, t, J = 2.4 Hz), 5.18 (1H Â 1/10, t, J = 6.2 Hz),
5.25 (1H Â 2/10, t, J = 6.5 Hz); 13C NMR (CDCl3) d 21.2, 21.3,
21.5, 23.0, 23.2, 23.3, 23.36, 23.40, 23.6, 23.9, 27.6, 29.9, 30.6,
32.3, 49.9, 50.1, 50.6, 51.2, 52.6, 52.8, 52.9, 66.3, 66.7, 67.4,
Acknowledgements
This research was partially supported by Grants-in-Aid for
Scientific Research on Basic Areas (B) ‘18350056’, Priority Areas
(A) ‘17035087’ and ‘18037068’, and Exploratory Research
‘17655045’ from MEXT. We thank Professor Kuniaki Tatsuta of
Waseda University for his helpful discussions.
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