C. Sanfilippo et al.
MolecularCatalysis449(2018)79–84
suitable time the solids were filtered off. To an aliquot (0.5 mL) of the
solution propionic anhydride (40 μL) was added and the mixture left to
react for 24 h at room temperature. After standard workup, this mixture
was analysed by chiral HPLC to determine the enantiomeric excesses of
unreacted amine (S)-3 (as the corresponding propionic amide 4b) and
the formed (R)-4a (ees and eep, respectively) from which the substrate
conversion was calculated according the equation c = [ees/
(ees + eep)] × 100 [25]. HPLC conditions: Lux Cellulose-1 column, n-
hexane/2-PrOH 90:10 eluent, tR/min: 24.97 [(R)-4b] and 30.10 [(S)-
4b]; 36.41 [(R)-4a] and 54.00 [(S)-4a].
2.7. Synthesis of (R)-2-hydroxy-5-[[N-(1-methyl-3-phenylpropyl)-N-
(phenylmethy)amino]acetyl] benzamide, (R)-12
To a solution of (R)-2a (500 mg, 2.11 mmol, > 99% ee) in anhy-
drous methyl ethyl ketone (10 mL) 5-(2-bromoacetyl)-2-hydro-
xybenzamide 11 (272 mg, 1.06 mmol) was added and the mixture
stirred at reflux until disappearance of the ketone reagent (1 h), as
stated from TLC analysis (n-hexane/EtOAc 60:40). The solution was left
to cool at room temperature and taken to dryness in vacuo. The residue
was suspended in diethyl ether, from which the hydrobromide of un-
reacted amine was separated as insoluble salt and removed by filtration.
Excess of HCl in ethanol was added to the filtrate solution to give (R)-
2.4. Screening of solvents for the kinetic resolution of ( )-3 with Novozym
435
12·HCl (203 mg, 0.45 mmol, 43% yield) as
a white powder,
[α]D25 = +16.8 (c 0.5, EtOH), lit. [α]D25 = +18 (solvent and con-
centration not given) [19]. Structural characterization was carried out
on the free base (R)-12: 1H NMR (CDCl3) δ: 1.11 (d, J = 6.4, 3H), 1.64
(m, 1H), 1.93 (m, 1H), 2.48 (m, 1H) 2.66 (m, 1H), 2.79 (q, J = 6.4, 1H),
3.43 and 3.77 (d, AB system, J = 13.2, each 1H), 3.63 and 3.85 (d, AB
system, J = 14.4, each 1H), 6.91 (d, J = 8.4, 1H), 7.06 (d, J = 7.2, 1H),
7.18-7.33 (m, 9H), 7.92 (d, J = 7.2, 1H), 8.03 (s, 1H); 13C NMR (CDCl3)
δ: 13.72, 33.03, 35.76, 53.90, 55.31, 57.87, 112.72, 118.08, 125.72,
127.29, 128.17, 128.32, 128.40, 129.48, 135.57, 139.33, 142.21,
To a solution of ( )-3 (100 μL, 0.62 mmol) and methyl benzoate
(78 μL, 0.62 mmol) in 1 mL of the solvent of choice Novozym 435
(50 mg) and molecular sieves (40 mg) were added. The suspension was
then shaken at 300 rpm at 45 °C until a suitable conversion of substrate
was reached, as stated by TLC chromatography (n-hexane/EtOAc
70:30). The reaction mixture was then treated as above in order to
determine the substrate conversion and the enantiomeric excesses of
(R)-4a and (S)-3 by chiral HPLC analysis.
166.03, 172.06, 197.32. HRMS ESI–MS calcd for
C26H29N2O3
[M + H]+ 417.2172, found 417.1749.
2.5. Enzymatic synthesis of (R)-N-(1-methyl-3-phenylpropyl)-benzamide,
(R)-4a
2.8. Synthesis of optically active labetalol [diastereoisomeric mixture of
(R,R)-1 and (R,S)-1]
Amine ( )-3 (0.5 mL, 3.09 mmol) was dissolved in 5 mL of dry
toluene containing 500 mg of molecular sieves and 0.3 mL methyl
benzoate (0.3 mL, 2.77 mmol). The reaction was started by addition of
Novozym 435 (600 mg) and was stirred at 45 °C and 300 rpm. After
5 days (45% conversion) the enzyme was filtered off and the solution
taken to dryness under reduced pressure to partially remove the un-
reacted amine and methyl benzoate. The residue was dissolved in die-
thyl ether and the solution extracted three times with 1N HCl. The
organic phase was washed with satd. NaHCO3 and brine, dried over
Na2SO4 and the solvent evaporated to give (R)-4a as a white solid
(328 mg, 1.3 mmol, 42% yield, ee > 99%), 1H NMR (CDCl3) δ: 1.26 (d,
J = 6.8, 3H), 1.87 (m, 2H), 2.71 (t, J = 8.0, 2H), 4.26 (m, 1H), 5.91 (d,
J = 8.0, 1H), 7.18 (m, 3H), 7.27 (m, 2H), 7.40 (m, 2H), 7.48 (m, 1H),
7.66 (m, 2H); 13C NMR (CDCl3) δ: 20.99, 32.48, 38.51, 45.68, 125.87,
126.77, 128.29, 128.43, 131.21, 134.88, 141.71, 166.79.
In a 50-mL round-bottomed flask equipped with a gas inlet tube
with valve stopcock 10% Pd-C (20 mg) was added to a solution of (R)-
12·HCl (203 mg, 0.45 mmol) in absolute EtOH (6 mL). The flask was
then filled with hydrogen gas (1 atm) and the suspension magnetically
stirred at room temperature. When the complete conversion of sub-
strate was reached (4 h, TLC analysis, n-hexane/EtOAc 60:40) the cat-
alyst was filtered off over a short pad of Celite and the solution taken to
dryness at reduced pressure to afford 1:1 diastereoisomeric mixture of
(R,R)-1 and (S,R)-1 hydrochloride salts (156 mg, 0.42 mmol) in quan-
titative yield. 1H NMR (DMSO-d6) d: 1.31 (t, sum of two partially
overlapped doublets for diastereoisomeric −CH3, 2 × 3H), 1.78 (m,
2 × 1H), 2.11 (m, 2 × 1H), 2.56 (m, 2 × 1H), 2.71 (m, 2 × 1H), 3.05
(m, 2 × 2H), 3.19 (m, 2 × 1H), 4.86 (br t, 2 × 1H), 6.09 (s, 2 × 1H,
−OH), 6.89 (d, J = 8.4, 2 × 1H), 7.19-7.31 (m, 2 × 6H), 7.44 (d,
J = 8.4, 2 × 1H), 7.90 (s, 2 × 2H), 8.47 (br, 2 × 1H), 9.05 (br,
2 × 1H); 13C NMR (DMSO-d6) d: 15.55 and 16.05 (doubled signals due
to diastereoisomerism, d*), 31.32 and 31.42 (d*), 34.38, 36.25, 50.39
and 50.48 (d*), 53.69 and 53.79 (d*), 68.28 and 68.49 (d*), 114.71,
117.66, 126.39 and 126.57 (d*), 128.63 and 128.77 (d*), 132.05 and
132.39 (d*), 141.22, 160.82, 172.01. HRMS ESI–MS calcd for
2.6. Synthesis of (R)-N-benzyl-N-(1-methyl-3-phenylpropyl)-amine, (R)-
2a
To a solution of (R)-4a (300 mg, 1.2 mmol, > 99% ee) in diethyl
ether (8 mL) LiAlH4 (45 mg, 1.2 mmol) was added portionwise and the
suspension was stirred at room temperature. The reaction was mon-
itored by TLC analysis (n-hexane/EtOAc 60:40) and quenched at sub-
strate disappearance by careful dropwise addition of 15% aqueous so-
dium hydroxide. The organic phase was washed with water, then brine
and dried on Na2SO4. After the evaporation of solvent under reduced
pressure (R)-2a (243 mg, 1.03 mmol, 86% yield, > 99% ee) was iso-
C
19H23N2O3 [M + H]+ 329.1860, found 329.1842.
3. Results and discussion
At the onset of our work, the enzymatic benzoylation of some se-
lected amines was carried out in dry toluene in the presence of Candida
antarctica lipase (CAL-B) immobilized on acrylic resin (Novozym 435),
reported as the most effective catalyst in the acyl transfer reactions with
amines in non-aqueous media [11]. Methyl benzoate was used as acyl
donor and molecular sieves were also added to the reaction mixture in
order to prevent any competitive enzymatic hydrolysis of the benzoate
ester.
Achiral benzylamine 5, ( )-1-phenylethylamine 6, ( )-1-methyl-
3-phenylpropylamine 3 and ( )-2-amino-2-phenylethanol 7 were
chosen as simple models for arylalkyl amines and aminoalcohols and
the progress of reactions was monitored by 1H NMR while the optical
purity of the formed amides was determined by chiral HPLC (Table 1).
In all the cases the benzoylation reaction gave the expected amide
25
lated of a pale yellow oil. [α]D for (R)-2a·HCl = + 3.8 (c 0.5, EtOH),
lit. [α]D25 = +5.0 (solvent and concentration not given) [19]. NMR
data were in agreement with literature values [26]: 1H NMR (CDCl3) δ:
1.09 (d, J = 6.4, 3H), 1.63 (m, 1H), 1.76 (m, 1H), 2.60 (m, 2H), 2.67
(m, 1H), 3.66 (d, J = 13.2, 1H), 3.75 (d, J = 13.2, 1H), 7.19 (m, 10H);
13C NMR (CDCl3) δ: 20.05, 32.07, 38.39, 51.00, 51.74, 125.51, 126.64,
126.67, 126.95, 127.97, 128.13, 128.19, 140.41, 142.16. The en-
antiomeric purity of (R)-2a was checked by chiral HPLC analysis on Lux
Cellulose-3 column (n-hexane/2-PrOH 90:10) of the corresponding
acetamide, tR/min: 23.02 [(S)-enantiomer] and 25.24 [(R)-en-
antiomer].
81