H. Schwab et al.
Diels–Alder reaction (b.p. 73–768C). Spectroscopic data correspond with
hyde in tert-butyl methyl ether (tBME). The solution was stirred at room
temperature and 100 mL samples were taken at 15 and 30 min, 1, 2, 3, 4,
5, and 24 h, filtered over a Celite 545 Na2SO4 pad, diluted with tBME
(1 mL), and analyzed with GC-MS: 608C, 2 min, 108Cminꢀ1, 1608C,
5 min, 1 bar He.
those reported.[22]
2,2-Dimethoxybut-3-ene: Spectroscopic data correspond to those in refer-
[23]
ence
.
4-Methoxycyclohex-3-ene carbaldehyde (2a): As reported in the litera-
ture,[17] a mixture of 2-methoxybutadiene (10 g, 119 mmol, 1 equiv), fresh-
ly distilled acrolein (12 mL, 179 mmol, 1.5 equiv), and hydroquinone
(0.5 g) in benzene (30 mL) were heated at 1608C for 30 min in a stainless
steel bomb. After cooling to room temperature, the solution was trans-
ferred and the excessive acrolein and benzene were removed by distilla-
tion at atmospheric pressure. Distillation of the crude product afforded
2a as a colorless oil (7.5 g; 45%; b.p. 80–858C (7 mbar)). 1H NMR spec-
tra correspond to those in reference [24]. 13C NMR (CDCl3): d=204.59
(CHO), 155.41 (C4), 91.09 (C3), 54.24 (OCH3), 46.30 (C1), 26.30 (C2),
23.13 (C5), 22.47 ppm (C6); MS: 141 ([M]+), 125 ([MꢀCH3]), 81
([Mꢀ(CHO + OCH3)]); Chiral GC: 1208C, 2 min, 58Cminꢀ1, 1608C,
10 min; 4.67, 4.76 min, 0.45 bar H2.
Synthesis and safe handling of anhydrous HCN—CAUTION: All reac-
tions in which HCN or cyanides were involved, were performed in a
well-ventilated hood. An electrochemical sensor for HCN detection was
used for continuous warning. The required amount of HCN was freshly
prepared by adding a saturated NaCN solution dropwise to aqueous sul-
furic acid (60%) at 808C. HCN was transferred in a nitrogen stream
through a drying column and collected in a cooling trap at ꢀ128C. Waste
solutions containing cyanides were treated with aqueous sodium hypo-
chlorite (10%). Subsequently, the pH was adjusted to 7.0 with aqueous
sulphuric acid.
General procedure for the chemical synthesis of racemic cyanohydrins
(blank): An aqueous buffer solution (30 mm, K2HPO4/citrate, 1/1 v/v) was
added to a solution of aldehyde in tBME. The resulting mixture was
stirred at 08C until an emulsion was formed. After the addition of freshly
prepared prussic acid (3.6 equiv), the mixture was stirred at 08C until
quantitative conversion. The emulsion was broken with Celite 545, fil-
tered, and dried over Na2SO4. Evaporation of the solvent under reduced
pressure yielded the crude cyanohydrins as light yellow liquids. For deter-
mination of the product distribution, a small amount was acetylated with
acetic anhydride and pyridine in dichloromethane by using standard
methods. The results are shown in Table 1.
2-Hydroxy-2-(4-oxocyclohexyl)acetonitrile (4): 1H NMR (CDCl3): d=
4.57 (d, J=5.41 Hz, 1H, H2); 2.51–1.36 ppm (m, 9H, H1’, 2ꢂH2’, 2ꢂH3’,
2ꢂH5’, 2ꢂH6’); 13C NMR (CDCl3): d=210.9 (C4O), 121.1 (CN), 64.1
(C2), 40.5 (C1’), 40.2 (C3’, C5’), 27.7 ppm (C2’, C6’).
2-Hydroxy-2-(4’-methoxycyclohex-3’-enyl)acetonitrile (5a): 1H NMR
(CDCl3): d=4.62–4.57 (m, 1H, H3’), 4.38 (dd, J=10.2, 5.9 Hz, 1H, H2),
3.51 (s, 3H, OCH3), 2.37–1.95 (m, 6H, H1’, 2ꢂH2’, 2ꢂH5’, H6’), 1.66–
1.50 ppm (m, 1H, H6’’); 13C NMR (CDCl3): d=155.3, 155.2 (C4’), 119.5,
119.4 (CN), 91.1, 91.0 (C3’), 65.4 (C2), 54.4, 54.3 (OCH3), 38.9, 38.8
(C1’), 26.9, 26.8 (C6’), 25.4, 25.3 (C2’), 24.4, 24.3 ppm (C5’).
2-Trimethylsilyloxy-1,3-butadiene (1b): As reported in the literature,[25]
a
solution of triethylamine (55 mL, 395 mmol, 1.2 equiv) in DMF (200 mL)
were heated to 70–808C under an argon atmosphere. Solutions of methyl
vinyl ketone (30 mL, 328 mmol, 1 equiv) in DMF (25 mL) and trimethyl-
silyl chloride (50 mL, 394 mmol, 1.2 equiv) in DMF (25 mL) were added
simultaneously over 30 min. The solution darkened from colorless to
brown and a precipitate of triethylamine hydrochloride was formed. The
reaction was run overnight at 80–908C. After cooling the reaction mix-
ture to room temperature, it was filtered and transferred to a separating
funnel containing pentane (300 mL). The solution was extracted with ice-
cooled, saturated NaHCO3 (1 L). The organic layer was separated and
the aqueous phase was washed with pentane (2ꢂ300 mL). The combined
pentane layers were extracted with water and dried over Na2SO4. The
pentane and other volatile compounds were removed by distillation at at-
mospheric pressure. After distillation, 1b was obtained as a colorless oil
(20 g; 40%; b.p. (70 mbar) 42–448C). The spectroscopic data are slightly
different from those in the literature.[26] 1H NMR (CDCl3): d=6.13 (q,
J=10.3, 17.1 Hz, 1H, H3), 5.40 (dd, J=2.0, 17.1 Hz, 1H, H4), 5.02 (dt,
J=2.0, 10.7 Hz, 1H, H4’), 4.28 (s, 1H, H1), 4.29 (s, 1H, H1’), 0.18 ppm (s,
9H, SiACHTUNGTRENNUNG
(CH3)3); 13C NMR (CDCl3): d=154.89 (C2), 134.64 (C3), 114.55
(C4), 96.47 (C1), ꢀ0.004 ppm (Si
ACHTUNGNERT(UNNG CH3)3); MS: 171 ([M + C2H5]), 143
2-Hydroxy-2-(4’-trimethylsilyloxycyclohex-3’-enyl)acetonitrile (5b): Un-
stable.
([M]+), 127 ([MꢀCH3]), 73 (TMS).
4-Trimethylsilyloxycyclohex-3-ene carbaldehyde (2b): As reported in the
literature,[18] freshly distilled acrolein (2.0 equiv), hydroquinone
(0.02 equiv), and 2-trimethylsilyloxy-1,3-butadiene were dissolved in tolu-
ene and heated to reflux for 24 h. The solvent and unreacted starting ma-
terial were removed under reduced pressure. Distillation gave 2b as a
colorless liquid (15 g; 66%; b.p. (4 mbar) 89–928C). The spectroscopic
data were slightly different from those reported in the literature.[18]
1H NMR (CDCl3): d=9.53 (s, 1H, CHO), 4.71 (m, 1H, H3), 2.30–2.20
(m, 1H, H1), 2.12–2.08 (m, 2H, H2, H2’), 1.94–1.88 (m, 2H, H5, H5’),
1.84–1.78 (m, 1H, H6), 1.58–1.65 (m, 1H, H6’), 0.01 ppm (s, 9H, TMS);
13C NMR (CDCl3): d=204.07 (CO), 150.17 (C4), 101.49 (C3), 45.34 (C1),
27.87 (C2), 23.02 (C5), 22.43 (C6); MS: 198 ([M]+), 170 ([MꢀCO]), 155
General procedure for the enzymatic synthesis of (S)-cyanohydrins: An
aqueous solution (1/1 v/v) that contained HbHNL (ca. 250–
5000 Ummolꢀ1 aldehyde) in buffer solution (30 mm, K2HPO4/citrate,
pH 5.0) was added to a solution of aldehyde in tBME and the resulting
mixture was stirred at 08C until an emulsion was formed. After addition
of freshly prepared prussic acid (1.8 to 3.6 equiv), the mixture was stirred
at 08C until quantitative conversion. The emulsion was broken with
Celite 545, filtered, and dried over Na2SO4. Evaporation of the solvent
under reduced pressure yielded the crude cyanohydrin as a light yellow
liquid. For the determination of the distribution of the stereoisomers, a
small amount was acetylated (for results see Table 1).
General procedure for the synthesis of (R)-cyanohydrins: An aqueous
solution (1:1 v/v) containing PaHNL (ca. 600–700 Ummolꢀ1 aldehyde) in
a buffer solution (30 mm, K2HPO4/citrate, pH 5.0) was added to a solu-
tion of aldehyde in tBME and the resulting mixture was stirred at 08C
until an emulsion was formed. Freshly prepared prussic acid (1.8–
3.5 equiv) was added and the mixture was stirred at 08C until quantita-
tive conversion occured. The emulsion was broken with Celite 545, fil-
tered, and dried over Na2SO4. Solvent removal in vacuo gave the crude
cyanohydrin as a light yellow liquid. For the determination of the distri-
bution of the stereoisomers, a small amount was acetylated by using stan-
dard procedures. Results are shown in Table 1.
([M
+
C2H5 ꢀTMS]), 127 ([MꢀTMS]), 73 ppm (TMS); chiral GC:
1008C, 2 min, 108Cminꢀ1, 1608C, 10 min, 0.45 bar H2, 7.10 min.
4-Oxocyclohexane carbaldehyde (3): A solution of 4-trimethylsilyloxycy-
clohex-3-ene carbaldehyde (2b) in aqueous HCl (2m) was stirred for 2 h
at room temperature and then extracted twice with dichloromethane.
The organic layer was separated, washed with saturated NaHCO3 and
water, and dried over Na2SO4. Solvent removal under reduced pressure
gave aldehyde 2c in 55% yield. 1H NMR (CDCl3): d=9.78 (s, 1H,
CHO), 2.70–2.64 (m, J=4.39 Hz, 1H, H1), 2.48–2.34 (m, J=6.35,
4.39 Hz, 4H, H3, H3’, H5, H5’), 2.26–2.19 (m, J=6.35, 4.39 Hz, 2H, H2,
H6), 2.02–1.94 ppm (m, J=4.88 Hz, 2H, H2’, H6’); 13C NMR (CDCl3):
d=209.74 (CO), 202.42 (CHO), 47.27 (C1), 39.45 (C3, C5), 25.42 ppm
(C2, C6); MS: 155 ([M + C2H5]), 127 ([M]+), 109 ([MꢀOH]), 81 ([Mꢀ
(CHO+O)]).
Treatement of 5a and 5b with HCl: A solution of 5a or 5b in aqueous
HCl (2m) was stirred for 4 h at room temperature and then extracted
twice with dichloromethane. The organic layer was separated, washed
with water, and dried over Na2SO4. Solvent removal under reduced pres-
sure gave aldehyde 4 in 80–90% yield. Spectroscopic data are given
above.
pH Stability of the enol ethers 2a and 2b: A buffer solution (30 mm,
K2HPO4/citrate, pH 5.5) containing HbHNL (2000 Ummolꢀ1 aldehyde)
was adjusted to the desired pH (4.5–7.0) and added to a solution of alde-
11420
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11415 – 11422