Enzymatic Total Synthesis of a Banana Volatile
FID detector (J&W GC column: 30 mϫ0.25 mmϫ0.25 μm chiral
J = 6.3 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 177.5,
67.7, 41.0, 38.6, 18.6, 13.9 ppm.
1
capillary column, Cyclodextrin-B, Part No. 112-2532). H and 13C
NMR spectra were recorded with 300 and 500 MHz Bruker spec-
trometers in CDCl3 solutions using Me4Si as the internal standard.
Chemical shifts are reported in ppm downfield from Me4Si. MS
were recorded with a Shimadzu GC–MS-QP5050 instrument
equipped with an SPB-5 column and CI mass detector. Ketoreduct-
ases, glucose dehydrogenase and NADPH were a generous do-
nation from Biocatalytics. Immobilized lipase CAL-B (Novozym
435) was purchased from Novozymes.
Enzymatic Esterification of (R)-3-Hydroxyhexanoic Acid (4) with
(S)-2-Pentanol (3a): Methyl (R)-3-hydroxyhexanoic acid (4; 88 mm,
0.88 mmol, 116 mg), (S)-2-pentanol (3a; 80 mm, 0.8 mmol, 70 mg),
CAL-B (70 mg) and molecular sieves (4 Å, 500 mg) were added to
dry hexane (10 mL) and the mixture was stirred at 45 °C. After
5 d the enzyme was removed by filtration and the organic solvent
evaporated to dryness to afford 137 mg of the natural product
(85%). Analytical data of the natural product are presented on the
following experimental section.
Methyl (R)-3-Hydroxyhexanoate (2a): Methyl 3-oxohexanone (2;
150 mm, 2 mmol, 288 mg), Kred-A1C (18 mg), glucose (250 mm,
Enzymatic Transesterification of Methyl 3-Oxohexanoate (2) with
585 mg), NADPH (5 mg) and glucose dehydrogenase (10 mg) were (S)-2-Pentanol (3a)
added to a phosphate buffer solution (13 mL, 200 mm, pH 6.9) and
(S)-2-Pentyl 3-Oxohexanoate (5): Methyl 3-oxohexanoate (2;
the mixture was stirred at 37 °C. Periodically the pH was readjusted
to 6.9 with NaOH (2 m). After completion of the reaction, the
product was isolated by extracting the crude reaction mixture with
EtOAc (15 mL ϫ3). The combined organic layers were then dried
with MgSO4 and the solvents evaporated to dryness to give the
corresponding hydroxy ester 2a. Isolated yield: 90%, 263 mg. 1H
NMR (CDCl3, 300 MHz): δ = 3.94–4.07 (m, 1 H), 3.70 (s, 3 H),
2.91 (br. s, 1 H), 2.49 (dd, J = 3.6 Hz, 1 H), 2.39 (dd, J = 9 Hz, 1
H), 1.31–1.54 (m, 4 H), 0.92 (t, J = 6.9 Hz, 3 H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 173.5, 67.7, 51.7, 41.1, 38.6, 18.7, 13.9 ppm.
GC data (chiral capillary column, cyclodextrin-B, 90 °C isothermic,
carrier gas: N2, pressure: 12 psi): tR = 27.3 min.
88 mm, 0.88 mmol, 127 mg), (S)-2-pentanol (3a; 40 mm, 0.4 mmol,
35 mg) and CAL-B (70 mg) were added to dry hexane (10 mL) and
the mixture was stirred at 45 °C. After 24 h the enzyme was re-
moved by filtration and the organic solvent evaporated to dryness.
The product was purified by flash column chromatography (silica
gel, hexane/EtOAc, v/v, 20:1), to afford 72 mg (90%) of keto ester
5. Note, 66 mg of methyl 3-oxohexanoate (2) was recovered. 1H
NMR (CDCl3, 300 MHz): δ = 4.91–5.05 (m, 1 H), 3.40 (s, 2 H),
2.51 (t, J = 7.2 Hz, 3 H), 1.52–1.69 (m, 4 H), 1.28–1.52 (m, 4 H),
1.23 (d, J = 6.3 Hz, 3 H), 0.92 (t, J = 7.5 Hz, 3 H), 0.91 (t, J =
7.2 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 202.9, 166.6,
72.1, 49.7, 44.9, 37.9, 19.9, 18.6, 16.9, 13.8, 13.5 ppm.
(S)-2-Pentanol (3a): 2-Pentanone (3; 175 mm, 3.5 mmol, 300 mg),
Kred-108 (40 mg), glucose (278 mm, 1 g), NADPH (10 mg) and
glucose dehydrogenase (10 mg) were added to a phosphate buffer
solution (20 mL, 200 mm, pH 6.9) and the mixture was stirred at
37 °C. Periodically the pH was readjusted to 6.9 with NaOH (2 m).
After completion of the reaction, the product was isolated by ex-
tracting the crude reaction mixture with Et2O (25 mL ϫ3). The
combined organic layers were then dried with MgSO4 and the
product was isolated by distillation of the organic solvent to give
the corresponding alcohol 3a. Isolated yield: 92%, 284 mg. 1H
NMR (CDCl3, 300 MHz): δ = 3.76–3.87 (m, 1 H), 1.36–1.51 (m, 4
H), 1.18 (d, J = 6.0 Hz, 3 H), 0.93 (t, J = 6.9 Hz, 3 H) ppm. 13C
NMR (CDCl3. 75 MHz): δ = 67.9, 41.5, 23.5, 18.9, 14.1 ppm. GC
data (chiral capillary column, cyclodextrin-B, 50 °C isothermic,
carrier gas: N2, pressure: 12 psi): tR = 12.4 min.
Enzymatic Reduction of (S)-2-Pentyl 3-Oxohexanoate (5)
(S)-2-Pentyl (R)-3-Hydroxyhexanoate (1): (S)-2-Pentyl 3-oxohex-
anoate (5; 100 mm, 0.5 mmol, 100 mg), Kred-A1C (35 mg), glucose
(200 mm, 180 mg), NADPH (5 mg) and glucose dehydrogenase
(10 mg) were added to a phosphate buffer solution (5 mL, 200 mm,
pH 6.9) and the mixture was stirred at 37 °C. Periodically the pH
was readjusted to 6.9 with NaOH (2 m). After completion of the
reaction (24 h), the product was isolated by extracting the crude
reaction mixture with EtOAc (8 mL ϫ3). The combined organic
layers were then dried with MgSO4 and the solvents evaporated to
dryness to give the natural product (S)-2-pentyl (R)-3-hydroxyhex-
anoate (1). Isolated yield: 95%, 96 mg. 1H NMR (CDCl3,
300 MHz): δ = 4.94–5.01 (m, 1 H), 3.97–4.03 (m, 1 H), 2.84 (d, J
= 4 Hz, 1 H), 2.48 (dd, J = 3 Hz, 1 H), 2.37 (dd, J = 9 Hz, 1 H),
1.28–1.64 (m, 8 H), 1.22 (d, J = 6.5 Hz, 3 H), 0.93 (t, J = 7 Hz, 3
H), 0.92 (t, J = 7.5 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ
= 172.8, 71.3, 67.8 41.5, 38.6, 38.0, 20.0, 18.7, 18.6, 14.0, 13.9 ppm.
GC data (chiral capillary column, cyclodextrin-B, 100 °C for 3 min,
rate: 10 °C/min, final temp.: 180 °C, carrier gas: N2, pressure:
12 psi): tR = 13.2 min; [M + H]+; found 203.2.
Enzymatic Transesterification of Methyl (R)-3-Hydroxyhexanoate
(2a) with (S)-2-Pentanol (3a) in Hexane: Methyl (R)-3-hydroxyhex-
anoate (2a; 40 mm, 0.4 mmol, 58 mg), (S)-2-pentanol (3a; 80 mm,
0.8 mmol, 70 mg) and CAL-B (50 mg) were added to dry hexane
(10 mL) and the mixture was stirred at 45 °C. After 7 d the enzyme
was removed by filtration and the organic solvent evaporated to
dryness. The product was purified by flash column chromatography
(silica gel, hexane/EtOAc, v/v, 5:1) to afford 40 mg (50%) of the
product. Analytical data of the natural product are presented in
the following experimental section.
Preparation of MPA Esters. General Method for the Synthesis of
MPA Esters of Secondary Alcohols: DCC (0.11 mmol, 1.1 equiv.)
and the corresponding (R)- or (S)-MPA (0.11 mmol, 1.1 equiv.)
were added to a solution of the corresponding hydroxy ester
(0.1 mmol) in dry CH2Cl2 and the reaction mixture was stirred at
0 °C for 4–6 h. After completion of the reaction the urea produced
was filtered and the filtrate was evaporated and then purified by
chromatography with 5:1 hexane/EtOAc. The corresponding MPA
ester was isolated in 90% isolated yield.
(R)-3-Hydroxyhexanoic Acid (4): Methyl (R)-3-hydroxyhexanoate
(2a; 1.2 mmol, 176 mg) and K2CO3 (0.84 mmol, 116 mg) were
added to a MeOH/H2O solution (1:1, v/v, 10 mL) and the mixture
was stirred at room temp. for 24 h. After completion of the reac-
tion, the mixture was concentrated, diluted with H2O (8 mL) and
extracted with Et2O (8 mL). The aqueous phase was acidified to
pH 3 and extracted with EtOAc (3ϫ10 mL). The organic layer was
dried with MgSO4 and concentrated to give 146 mg (92%) of 4. 1H
1
(R)-MPA Ester of (S)-2-Pentanol: H NMR (CDCl3, 300 MHz): δ
= 7.31–7.46 (m, 5 H), 4.92–5.03 (m, 1 H), 4.73 (s, 1 H), 3.41 (s, 3
H), 1.53–1.65 (m, 1 H), 1.39–1.50 (m, 1 H), 1.21–1.36 (m, 2 H),
1.08 (d, J = 6.3 Hz, 3 H), 0.88 (t, J = 7.2 Hz, 3 H) ppm.
1
NMR (CDCl3, 300 MHz): δ = 4.01–4.11 (m, 1 H), 2.57 (dd, J = (S)-MPA Ester of (S)-2-Pentanol: H NMR (CDCl3, 300 MHz): δ
3 Hz, 1 H), 2.47 (dd, J = 8.7 Hz, 1 H), 1.34–1.61 (m, 4 H), 0.94 (t,
= 7.31–7.46 (m, 5 H), 4.89–4.99 (m, 1 H), 4.72 (s, 1 H), 3.41 (s, 3
Eur. J. Org. Chem. 2011, 3946–3950
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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