2
SONI ET AL.
been applied in the enantiopure synthesis of intermedi-
ates of important beta blocker drugs like propanolol44,
atenolol2, and CNS drug intermediates like 3‐hydroxy-
methyl‐1‐tetralone tosylates6 antifungal N‐substituted
benzimidazole derivatives24 and has also been used at an
industrial scale for the synthesis of Carbovir, an antiviral
agent.26 Several additions to the biocatalytic toolbox of
PFL have been made by immobilization of PFL on various
matrices like nanoscaffolds, polymeric supports, and
hydrophobic sol‐gels.5,8
β‐Aryloxyalcohols are key intermediates in the synthe-
sis of many important drugs.30 It has been well established
that the desirable therapeutic activities of β‐aryloxyalcohols
reside mainly in the (S)‐enantiomers.27,42,43 The chemical
synthesis of enantiopure β‐aryloxyalcohols was per-
formed by the reaction of substituted phenols with
chiral epichlorhydrin or glycidol, to give epoxide inter-
mediates28,46 followed by the ring opening of these
epoxides.3,7 However, the use of chiral reagents makes
the process more cost intensive. Here we report an
extremely time‐efficient, economic, and environmen-
tally benign methodology for the synthesis of two
important pharmaceutical intermediates, which depict
the promising biocatalytic potential of PFL.
In the present study, two important racemic
β‐aryloxyalcohols, (RS)‐1‐chloro‐3‐(2,5‐dichlorophenoxy)
propan‐2‐ol [(RS)‐4], and (RS)‐3‐(4‐chlorophenoxy)pro-
pane‐1,2‐diol [(RS)‐8] were chemically synthesized and
subjected to lipase‐catalyzed kinetic resolution. Five
commercial lipase preparations were screened. The
enantiopure intermediates obtained by lipase‐catalyzed
kinetic resolution can be used for the enantiopure synthe-
sis of cloranolol (a nonselective β‐blocker)23,40 and
chlorphenesin (a muscle relaxant and antifungal).34 All
the reaction parameters were optimized using “one factor
at a time” approach. Finally, experimental observations
were validated by docking the compounds into the active
sites of the lipase.
2.1 | Analytical methods
Biocatalytic reactions were incubated in an incubator
shaker (Kuhner, Switzerland) at 200 rpm. H NMR and
1
13C NMR were obtained using Bruker DPX 400 (1H
400 MHz) in CDCl3. All the chemical shift values were
expressed in δ (ppm) units relative to tetramethylsilane
(TMS). HPLC (Shimadzu LC‐10AT pump, SPD‐10A UV‐
Vis detector) with Chiralcel® OD‐H column
(0.46 mm × 250 mm; 5 μm, Daicel Chemical Industries,
Japan) and Chiralcel® AD‐H column (0.45 mm × 250 mm;
5 μm, Daicel Chemical Industries, Japan) were used to
determine the conversion rate enantiomeric excess of
the substrates and products.
2.2 | Synthesis of (RS)‐1‐chloro‐3‐(2,5‐
dichlorophenoxy)propan‐2‐ol [(RS)‐4]
The intermediate of cloranolol was prepared via chemical
route using the following steps.
The starting racemic epoxide intermediate (RS)‐3 was
synthesized by dissolving 2,5‐dichlorophenol (1) (2.5 g,
15 mM, 1 eqv) in anhydrous acetonitrile followed by the
addition of K2CO3 (4.14 g, 15 mM, 2 eqv) at 80°C. After
stirring the reaction mixture for 30 minutes, (RS)‐
epichlorhydrin (RS)‐2 (1.8 mL, 15 mM, 1.5 eqv) was added
dropwise. The reaction mixture was extracted with ethyl
acetate after the completion of reaction in 30 hours, and
the filtrate was concentrated under vacuum. Recovered
product (RS)‐3 was subjected to the epoxide ring‐opening
reaction using acetyl chloride (1.1 mL, 10 mM, 1.5 eqv) in
dichloromethane : water (1:1) mixture. After the comple-
tion of reaction in 10 hours, the reaction mixture was
extracted with dichloromethane, dried using anhydrous
Na2SO4, and concentrated on a Rotavapor. The product
(RS)‐4 was purified by column chromatography using
hexane : ethyl acetate (94:6) as an eluent on silica gel.
(RS)‐4 was used as the substrate for lipase‐catalyzed
kinetic resolution (Scheme 1). Using similar method the
enantiopure intermediate of cloranolol, (R)‐4 was
synthesized.
(RS)‐3, light yellow liquid (96% yield), 1H NMR
(400 MHz; CdCl3) δ (ppm): 7.28 (dd, 1H, Ar–H), 7.25
(dd, 1H, Ar–H), 6.93 (m, 1H, Ar–H), 4.12 (m, 1H,
−CH2–), 3.98 (m, 1H, −CH2–), 3.40 (d, 1H, −CH–), 2.93
(dd,1H, oxirane), 2.83 (dd,1H, oxirane).13C NMR
(100 MHz, CDCl3): δ (ppm): 44.81, 53.88, 69.85, 114.67,
120.62, 122.81, 130.15, 135.21, 158.65.
2 | MATERIALS AND METHODS
2,5‐Dicholorophenol, 4‐chlorophenol, (RS)‐epichlorhydrin,
(S)‐epichlorhydrin, (RS)‐cyclopropylmethanol (glycidol),
(S)‐cyclopropylmethanol (glycidol), Candida antarctica
lipase (CALA and CALB), Candida rugosa lipase, and
P. fluorescens lipase were purchased from Sigma (St.
Louis, Missouri). Burkholderia cepacia lipase was pur-
chased from Fluka™. Anhydrous Na2SO4, K2CO3, TLC
plates, and HPLC grade solvents were purchased from
Merck (Germany). Silica gel (60‐120 mesh) for column
chromatography was obtained from SRL (India).
(RS)‐4, pale yellow liquid (95% yield), 1H NMR
(400 MHz; CdCl3) δ (ppm): 7.64 (dd, 1H, Ar–H), 7.25
(dd, 1H, Ar–H), 7.12 (dd, 1H, Ar–H), 4.11 (dd, 1H,
−CH), 4.05 (d, 2H, −CH2), 3.72 (d, 2H, −CH2). 13C
NMR (100 MHz, CDCl3): δ (ppm): 46.05, 69.67, 69.85,