Chemoenzymatic synthesis of highly enantiomerically enriched secondary alcohols with a thiazolic core

Article abstract of DOI:10.1016/j.tetasy.2012.03.014

Stereoselective preparative enzymatic acylation and hydrolysis/methanolysis of various C-substituted rac-thiazol-2-yl-methanols were achieved for the preparation of enantiopure or enantiomerically enriched, naturally occurring 2-hydroxymethylthiazoles. The absolute configurations of the resulting secondary alcohols were determined by a detailed ¹H NMR study of Mosher's derivatives.

Full text of DOI:10.1016/j.tetasy.2012.03.014

Tetrahedron: Asymmetry 23 (2012) 474–481
Contents lists available at SciVerse ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Chemoenzymatic synthesis of highly enantiomerically enriched secondary
alcohols with a thiazolic core
Laura Pop a, Pierrik Lassalas b, László Csaba Bencze a, Monica Ioana Toßsa a, Botond Nagy a,
b,
Florin Dan Irimie a, , Christophe Hoarau
⇑
⇑
a Department of Biochemistry and Biochemical Engineering, University Babeßs-Bolyai Cluj-Napoca, Ro-400028 Cluj-Napoca, Arany János 11, Romania
b Institut de Chimie Organique Fine (IRCOF) associé au CNRS (UMR 6014), INSA et Université de Rouen, BP08, 76131 Mont Saint Aignan, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 February 2012
Accepted 28 March 2012
Stereoselective preparative enzymatic acylation and hydrolysis/methanolysis of various C-substituted
rac-thiazol-2-yl-methanols were achieved for the preparation of enantiopure or enantiomerically
enriched, naturally occurring 2-hydroxymethylthiazoles. The absolute configurations of the resulting sec-
ondary alcohols were determined by a detailed 1H NMR study of Mosher’s derivatives.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
2-Hydroxymethylthiazoles are common clusters of a wide
range of biologically active azole natural products including
archazolid, tubulysin, lyngbyabellin and hectochlorin (Fig. 1).1
They are also important as chiral building blocks for the synthe-
sis of thiopeptide antibiotics.
Current synthetic strategies for the preparation of the enantio-
merically pure forms are mainly based upon the use of Ugi, Hantzsch
or biomimetic thiazole forming reactions using chiral reagents,2
tandem stereoselective Strecker/Hantzch sequences2d and stereose-
lective reduction, nucleophilic addition and hydroxylation reactions
to give 2-formyl, 2-keto, 2-alkyl and 2-vinyl-thiazoles.2d,3 The 4-car-
boxylated and 4-brominated 2-hydroxymethylthiazole series have
attracted the most attention as common natural product subunits
as well as the direct precursors of highly valuable dipeptide
thiazoles.4
With regard to this, the ready access to 2-hydroxymethylthiaz-
ole esters through non-stereoselective synthetic routes
(Scheme 1)2g,5 and on the basis of our recent successful investiga-
tions in the enzymatic deracemisation of heteroaryl secondary
alcohols,6 we herein report the enzyme mediated preparation of
enantiomerically enriched 4-carboxylated and 4-brominated
2-hydroxymethyl-thiazoles over two pathways: (1) the stereose-
lective acylation of racemic alcohols and (2) the non-selective acyl-
ation of racemic alcohols followed by stereoselective hydrolysis/
alcoholysis (Scheme 2).
2.1. Chemical synthesis
The tert-butyl 2-hydroxymethylthiazole-4-carboxylates and 4-
bromo-2-hydroxymethylthiazoles 2a–h were prepared as race-
mates by operating a magnesium-bromide exchange reaction
using Knochel’s reagent,5d from the tert-butyl 2-bromothiazole-4-
carboxylate 1a and commercially available 2,4-dibromothiazole
1b, followed by an electrophilic quench with various aldehydes
(Scheme 1, route A). Alternatively, the formylation reaction was
also achieved using N-formyl-morpholine as an electrophile to give
2-formylated tert-butylthiazole-4-carboxylate 3a and 4-bromothi-
azole 3b, which were then reacted with commercially available
Grignard reagents (Scheme 1, route B) to give the corresponding
racemic secondary alcohols rac-2i–k.
By chemical acylation of racemic secondary alcohols rac-2a–k
with an acyl chloride or anhydride in the presence of DMAP, the
racemic esters were prepared. In order to investigate the stereose-
lectivity of the enzymatic reaction and the activity of the enzymes,
the chromatographic enantiomeric separation of racemic second-
ary alcohols 2 and of their ester derivatives 4–6 were performed
successfully using the appropriate HPLC Chiralcel or Chiralpak
chiral columns (Table 2).
2.2. Enzymatic kinetic resolutions
First the selective O-acylations of racemic alcohols rac-2a-k
using a broad panel of hydrolases [lipases A and B from from Can-
dida antartica strains (CaL A and CaL B), lipases from Candida rugosa
(CRL), Candida cylindracea (CCL), Pseudomonas cepacia (LPS), Pseudo-
monas fluorescens (LAK), Rhizopus oryzae (Lipase F), lipases from
⇑
Corresponding authors. Tel.: +40 264 593 833; fax: +40 264 590 818.
E-mail address: irimie@chem.ubbcluj.ro (F.D. Irimie).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.03.014

L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
475
HO
Cl
Cl
O
HO
S
MeO
O
N
R
OAc
O
O
NH
O
O
N
O
N
S
O
O
HO
S
O
hectochlorin
archazolid A-B
(R = H, Me)
H
O
H
N
O
OAc
H
O
N
N
S
HN
N
N
N
O
O
S
NH
O
S
N
O
O
R
O
Cl
O
HO2C
Cl
tubulysin A-D
(R = alkyl
R' = H, Me, OH)
O
HO
R'
lyngbyabellin A
Figure 1. 2-Hydroalkylthiazole clusters in natural products.
porcine pancreatic lipase (PPL), pig liver esterase (PLE), aminoacy-
lase from porcine kidney (Acylase I), recombinant Penicillin G ami-
dase expressed in Escherichia coli, Proteinase 2A from Aspergillus
oryzae, papaine, protease PS from Aspergillus meleus and Baker’s
yeast] were studied. We found CaL A and CaL B to be the optimal
biocatalysts for the O-acylation with vinyl esters (Scheme 2, route
A), with significant results being obtained. PLE and CRL also proved
to be effective in the hydrolysis or methanolysis of racemic esters
(Scheme 2, route B).
and hydrolysis/methanolysis experiments. Moreover, the optimal
enzymatic hydrolysis was attained under phosphate buffer condi-
tions at pH 7.4.
Considering that the structure of the substrate and in particular
the steric hindrance has an influence on the stereoselectivity of the
enzymatic reaction, vinyl acetate and butyrate as well as racemic
acetate, propionate and butanoate esters were examined for the
acylation and hydrolysis/methanolysis enzymatic reactions. The li-
pase mediated acylation with vinyl propanoate and butanoate was
also studied and generally no considerable changes with regard to
the selectivity of the reaction were found, compared to those
Amongst the wide range of polar and apolar solvents used, DIPE
and MTBE proved to be efficient in both the enzymatic acylation
Y
Y
N
S
N
OH
R
A
Br
i.
S
t
1a Y=CO2 Bu
1b Y=Br
iv.
rac-2a-h
Y
Y
Y
N
OH
R
N
S
OCOR'
R
N
S
O
H
B
iii.
S
iv.
ii.
t
rac-4 R': Me
rac-5 R': Et
rac-6 R': nPr
3aY=CO2 Bu
3b Y=Br
rac-2i-k
i. 1. iPrMgCl·LiCl, THF,- 78°C; 2. R-CHO; ii. 1. iPrMgCl·LiCl, THF, -78°C;
2. N-formyl-morpholine; iii. RMgX, THF, -78°C; iv. R'COCl or (R'CO)2O, CH2Cl2
Scheme 1. Preparation of racemic alcohols and esters used as substrates.

476
L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
O
A
Y
Y
Y
N
OH
R
N
N
O
R
OH
R
lipase
+
R'
S
O
R'
S
S
rac-2
(S)-2
O
(R)-4-6
acylation
O
Y
Y
B
lipase
O
N
OH
N
S
O
R
Y
N
S
O
R
+
R'
R'
R
S
H2O or MeOH
(S)-4-6
(R)-2
rac-4-6
Scheme 2. Strategy for enzymatic resolution of C-substituted thiazol-2-yl-methanols (Y = CO2t Bu or Br).
obtained by using vinyl acetate. The racemic ester resolution by
hydrolysis/methanolysis was in some cases more satisfactory
(see Table 1).
entries 3–4). Similarly, enantiopure methylated 4-bromo-thiazole
(R)-2f and the corresponding enantiomer ester derivative (S)-6f
were both produced from the enzymatic methanolysis of a mixture
of racemic esters rac-6f (Table 1, entry 10). For the other examined
alcohols and esters of the thiazole series, a good enantiomeric ex-
cess of the product (eep) and the substrate (ees) could not be simul-
taneously attained using both enzymatic resolution methods, with
three sets of results occurring.
First, enzymatic acylation and hydrolysis/methanolysis gave
alternatively good results. Vinylated 2-hydroxy-methylthiazole-4-
carboxylate esters (R)-2i (98% ee) and (S)-2i (84% ee) and 4-bro-
mo-2-hydroxymethylthiazole esters (R)-2k (83% ee) and (S)-2k
(80% ee) were prepared using enzymatic acylation and hydrolysis
experiments, respectively (Table 1, entries 7–8, 16–17). In the sec-
ond case, an excellent resolution of the product was observed at low
conversion whereas a better resolution of the substrate was
attained at a higher conversion. Thus, enantiopure n-octylated 2-
hydroxymethylthiazole-4-carboxylate (R)-2d, isobutylated and iso-
propylated 4-bromo-2-hydroxy-methylthiazoles (R)-2h and (R)-2j,
which were produced through hydrolysis and acylation reactions
respectively stopped at low conversion (Table 1, entries 5, 12 and
14). The same experiments stopped at a higher conversion provided
the corresponding enantiomers (S)-4d (88% ee), (S)-5h (72% ee) and
enantiopure (S)-5j (Table 1, entries 6, 13 and 15). In the last set of
cases, highly enantiomerically enriched phenylated 2-hydroxy-
Successful analytical scale trials were immediately followed by
preparative scale enzymatic resolutions. All experimental condi-
tions (enzymes, dilution, substrate/biocatalyst ratio) were strictly
transposed from the analytical scale. The reactions were monitored
by TLC as well by HPLC and stopped by removing the enzyme by
filtration when satisfactory enantiomeric excesses were obtained.
The most significant results presented in Table 1 clearly show
that a broad panel of racemic secondary alcohols with a thiazole
core (alkylated, alkenylated and phenylated) and their ester deriv-
atives could be successfully used in both enzymatic resolution
reactions providing enantiopure, or highly enantiomerically en-
riched thiazole derivatives. Enantiopure methylated 2-hydroxym-
ethylthiazole-4-carboxylates (R)-2a and (S)-2a as well their ester
derivatives (R)-4a and (S)-4a were prepared via enzymatic acyla-
tion of racemic alcohols rac-3a and methanolysis of racemic ace-
tates rac-4a in good to fair yields (Table 1, entries 1–2). Excellent
resolutions of the product and substrate were simultaneously ob-
tained for hydrolysis and methanolysis of racemic tert-butylated
and iso-butylated thiazole-4-carboxylate esters 4b and 6c provid-
ing enantiopure alcohol (R)-2c (99% ee) and enantiomerically en-
riched alcohol (R)-2b (94% ee) as well as the enantiopure
corresponding esters (S)-4b and (S)-6c in excellent yields (Table 1,
Table 1
Preparative scale enzymatic acylation and hydrolysis/methanolysis reactionsa
Entry
Substrate
Y
R
Method
T (h)
ees (%)
eep (%)
cb
Alcohol
(%)c
Ester
(%)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
2a
4a
4b
6c
4d
4d
2i
4i
2e
6f
6g
5h
5h
5j
5j
2k
4k
CO2tBu
CO2tBu
CO2tBu
CO2tBu
CO2tBu
CO2tBu
CO2tBu
CO2tBu
CO2tBu
Br
Br
Br
Br
Br
Br
Br
Br
Me
CaL A, DIPE, VA
2
8
99
99
99
99
78
88
72
84
93
99
38
35
72
17
99
58
80
97
99
94
99
99
88
98
88
74
97
99
99
74
99
80
83
40
51
50
51
50
44
50
42
49
56
51
28
26
49
15
55
41
67
(S)-2a
(R)-2a
(R)-2b
(R)-2c
(R)-2d
(R)-2d
(S)-2i
(R)-2i
(S)-2e
(R)-2f
(R)-2g
(R)-2h
(R)-2h
(R)-2e
(R)-2e
(S)-2k
(R)-2k
43
31
48
43
13
22
46
49
43
48
26
17
19
14
36
41
45
(R)-4a
(S)-4a
(S)-4b
(S)-6c
(S)-4d
(S)-4d
(R)-4i
(S)-4i
(R)-4e
(S)-6f
(S)-6g
(S)-5h
(S)-5h
(S)-5j
(S)-5j
(R)-4k
(S)-4k
28
46
43
44
34
33
36
46
31
49
33
25
16
36
31
36
34
Me
CaL B, DIPE/MeOH.
PLE, DIPE/PB (1:1)
CaL B, DIPE/MeOH (1:1)
PLE, DIPE/PB (1:1)
PLE, DIPE/PB (1:1)
CaL B, DIPE, VA
tBu
18
48
10
22
18
2
iBu
nOctyl
nOctyl
Vinyl
Vinyl
Ph
PLE, DIPE/PB (1:1)
CaL A, DIPE, VA
48
2
Me
CaL B, DIPE/MeOH
CRL, DIPE/PB (1:2)
CRL, DIPE/PB (1:2)
CRL, DIPE/PB (1:2)
CRL, DIPE/PB (1:2)
CRL, DIPE/PB (1:2)
CaL A, DIPE, VA
tBu
57
48
72
24
48
12
2
iBu
iBu
iPr
iPr
Vinyl
Vinyl
PLE, DIPE/PB (1:1)
a
Reaction conditions: rac-alcohol (400 mg), acylating agent (5 equiv), lipase (400 mg) in solvent (10 mL) at rt for acylation experiments, or rac-ester (400 mg), lipase
(400 mg) in DIPE (10 mL) or DIPE/PB (10 mL) for solvolysis. CaL A, B = lipase A, B from Candida antartica; CRL = lipases from Candida rugosa; PLE = pig liver esterase; VA = vinyl
acetate; MTBE = tBuOMe; DIPE = (iPr)2O; PB = phosphate buffer (KH2PO4), pH 7.4, 20 mM.
b
Conversion = [ees/(ees + eep)] ꢀ 100.
c
Isolated yield.

L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
477
Table 2
Analysis of optical purity of the enzymatic resolution products
½ ꢂ
a 2D5
Compound
Enzymatic
resolution route
(Scheme 2)
Yield
(%)
HPLC condition
Retention times
(min)
ee (%)
(in CHCl3)
HPLC Column
Mobile phase (v/v)
Secondary alcohols
(R)-2a
(S)-2a
(R)-2b
(R)-2c
(R)-2d
(S)-2e
(R)-2f
(R)-2g
(R)-2h
(R)-2i
(R)-2j
B
A
B
B
B
A
B
B
B
B
B
31
43
48
43
44
43
48
26
17
49
14
IB
IB
IB
IC
IA
IB
IC
IB-IC
AS-IB
IA
n-hexane–IpOH (95:5)
n-hexane–IpOH (95:5)
n-hexane–IpOH (99:1)
n-hexane–IpOH (97:3)
n-hexane–IpOH (96:4)
n-hexane–IpOH (95:5)
n-hexane–IpOH (97:3)
n-hexane–IpOH (97:3)
n-hexane–IpOH (97:3)
n-hexane–IpOH (95:5)
n-hexane–IpOH (99:1 /19 min, 97:3/5 min),
95:5/5 min, 90:10/15 min
13.3; 14.4
13.3; 14.4
15.8; 17.3
22.9; 25.4
9.3; 10.1
13.0; 18.0
12.0; 14.6
12.9; 14.7
18.6; 19.8
8.5; 10.4
+24.3
ꢁ24.3
+17.1
+14.4
+38.1
ꢁ3.8
99
99
94
99
99
93
99
99
99
88
99
+21.9
+63.6
+35.3
+15.25
+29.5
OJ-H-AS-H
41.0; 41.6
Esters
(R)-4a
(S)-4a
(S)-4b
(S)-6c
(S)-4d
(R)-4e
(S)-6f
A
B
B
B
B
A
B
B
B
28
28
43
44
33
31
49
16
31
IB
IB
IB
IC
IA
IB
IC
AS-IB
n-hexane–IpOH (95:5)
n-hexane–IpOH (95:5)
n-hexane–IpOH (99:1)
n-hexane–IpOH (97:3)
n-hexane–IpOH (96:4)
n-hexane–IpOH (95:5)
n-hexane–IpOH (97:3)
n-hexane–IpOH (97:3)
n-hexane–IpOH (99:1 /19 min, 97:3/5 min),
95:5/5 min, 90:10/15 min
n-hexane–IpOH (95:5)
n-hexane–IpOH (97:3)
n-hexane–IpOH (97:3)
5.6; 6.0
5.6; 6.0
7.6; 9.3
13.0; 19.0
5.5; 6.6
7.5; 9.3
6.9; 10.6
8.5; 9.0
+32.0
ꢁ32.7
ꢁ17.8
ꢁ26.9
ꢁ17.2
+6.3
ꢁ32.3
ꢁ49.1
ꢁ6.1
97
99
99
99
88
74
99
72
99
(S)-5-6h
(S)-5-6j
OJ-H-AS-H
17.04; 17.96
(R)-6f
(S)-4k
(R)-4k
A
B
A
36
34
36
IA
IA
IA
7.4; 7.9
5.6; 6.4
5.6; 6.4
+80.3
ꢁ13.9
+14.4
98
80
83
O
O
O
N
O
H
N
H
S
S
(S)
(S)
H3C
H3C
H
H
O
O
Ph
H3CO
OCH3
Ph
Figure 3. Signals of 5CH protons in the 1H NMR spectra of (R)-MTPA-(S)-2a and of
(S)-MTPA-(S)-2a.
(R)
(S)
b
a
CF3
(R)-MTPA-(S)-2a
CF3
(S)-MTPA-(S)-2a
MTPA-Cl in the presence of DMAP. The resulting diastereomers
were differentiated by their 1H NMR spectra (Fig. 2).
In the case of (S)-MTPA-(ꢁ)-2a, the strong diamagnetic effect of
the phenyl group caused the proximal CH3 protons to be more
shielded (d = 1.74 ppm), when compared with their analogues in
(R)-MTPA-(ꢁ)-2a (d = 1.82 ppm), where the distance between the
phenyl and the methyl group is longer. Also, the methoxy protons
of (S)-MTPA-(ꢁ)-2a were found to be more shielded (d = 3.56 ppm),
in comparison to the methoxy protons of (R)-MTPA-(ꢁ)-2a
(d = 3.595 ppm), due to the diamagnetic effect of the thiazole ring.
The aromatic thiazolic protons of (R)-MTPA-(ꢁ)-2a were found
to be more shielded (d = 7.96 ppm) when compared to the protons
of (S)-MTPA-(ꢁ)-2a (d = 8.01 ppm) as shown in Figure 3. Based on
all of these data, the (S)-absolute configuration was assigned for
(ꢁ)-2a.
Figure 2. Diastereomeric Mosher derivatives of (S)-2a.
thiazole-4-carboxylate (S)-2e (93% ee) and enantiopure tert-butyl-
ated 4-bromothiazole (R)-2g were prepared via enzymatic acyla-
tion and hydrolysis reactions while the corresponding esters (R)-
4e and (S)-6g were produced in fair (74% ee) and poor (38% ee)
enantiomeric excesses respectively (Table 1, entry 9, 11).
2.3. The absolute configurations of the resolution products
The (S)-absolute configuration of the novel enantiopure second-
ary alcohols was confirmed by a detailed 1H NMR study of
Mosher’s derivatives of (ꢁ)-2a. This approach originated in seminal
works of Mosher concerning the structure of Mosher’s esters7 and
the first description of the NMR application of Mosher’s method
done by Kakisawa.8 For this reason the enantiomerically pure alco-
hol (ꢁ)-2a, which remained untransformed during the CaL-B-med-
iated acylation was esterified with both (S)-MTPA-Cl and (R)-
Using the same procedure, the absolute configurations of all of
the other unreacted enantiomers (ꢁ)-2 in the CaL-B catalysed acy-
lations were assigned as (S).
The same method was used to establish the absolute configura-
tion of alcohols (+)-2, obtained by enzymatic hydrolysis which was
assigned as (R).

478
L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
In conclusion, the lipase-mediated stereoselective acylation of
azole-4-carboxylate 1a (0.397 mmol), iPrMgClꢃLiCl (1.3 M,
0.52 mmol) was added dropwise at ꢁ78 °C. After 15 min the corre-
sponding aldehyde (0.79–0.99 mmol) or N-formyl-morpholine (in
the case of 2-formylthiazoles 3a-b) was added dropwise under stir-
ring and the reaction mixture was stirred at ꢁ78 °C for 10 min,
then 1.5 h at 0 °C. Saturated aqueous NH4Cl was added and the
aqueous layer was extracted with Et2O (3 ꢀ 10 mL). The combined
organic layers were washed with 5% aqueous HCl (10 mL), brine
(10 mL), dried over MgSO4 and concentrated in vacuo. The crude
product was purified by flash column chromatography on silica
gel using a mixture of EtOAc:P (petroleum ether) as eluent (see be-
low) to give tert-butyl 2-(1-hydroxymethyl)thiazole-4-carboxylate
derivatives rac-2a–h.
the racemic alcohol provided the (R)-ester and the unreacted (S)-
alcohol, while the lipase-mediated hydrolysis/methanolysis of es-
ters furnished the (R)-alcohol and the unreacted (S)-ester.
3. Conclusions
A chemoenzymatic approach for the synthesis of enantiopure
or highly enantiomerically enriched C-substituted 2-hydroxy-
methylthiazoles was developed. Both stereoselective enzymatic
acylation and hydrolysis/methanolysis of rac-2-hydroxy-methyl-
thiazoles and the ester derivatives were investigated. Prior to pre-
parative scale reactions, various processing conditions (hydrolases,
acylating agents, solvents and buffer systems) were screened. In
particular CaL A and B, PLE and CRL proved to be the best candi-
dates. This methodology could find direct applications in total
syntheses. Thus, enantiopure (S)-sec-butylated-2-hydroxythia-
zole-4-carboxylate 2c and n-butylated protected ester 6c as het-
erocycle clusters of the archazolids1a,c,g were prepared. Moreover,
the newly prepared enantiopure (R)-isopropylated-4-bromo-2-hy-
droxy-methylthiazole (R)-2j is currently used in the design of a no-
vel synthetic route towards azolylpyridine core of amythiamycin
thiopeptide antibiotic.9
4.3.1.1. tert-Butyl 2-bromothiazole-4-carboxylate 1a.
Fol-
lowing literature procedure10 starting from ethyl 2-bromothia-
zole-4-carboxylate (2.89 g, 0.012 mol). Purification by flash
chromatography (P/EtOAc 9:1, v/v) afforded pure 1 (2.06 g, 65%)
as a white solid (mp = 104 °C); 1H NMR (CDCl3, 300 MHz): d= 1.57
(s, 9H), 7.98 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 28.2, 82.8,
130.1, 136.5, 148.7, 159.3; IR (KBr):
1008, 1108, 1164, 1232, 1339, 1367, 1432, 1713, 2984,
3094 cmꢁ1 HRMS (ESI+): calcd for C8H11BrNO2S+ (M+H+)
m = 610, 776, 794, 853, 933,
;
263,9688, found 263,9684; Anal. Calcd for C8H10BrNO2S: C, 36.38;
H, 3.82; N, 5.30; S, 12.14. Found: C, 36.68; H, 3.97; N, 5.12; S, 12.23.
4. Experimental
4.3.1.2. tert-Butyl-2-formylthiazole-4-carboxylate 3a.
Fol-
4.1. Analytical methods
lowing procedure using N-formylmorpholine (17.65 mL,
A
0.175 mol, 25 equiv). Solid (mp = 141 °C). m = 940 mg, 63%.
Rf = 0.39 (P/EtOAc 8:2, v/v); 1H NMR (CDCl3, 300 MHz): d = 1.59
(s, 9H), 8.37(d, 1H, J = 1.2 Hz), 10.01 (d, 1H, J = 1.2 Hz); 13C NMR
(CDCl3, 75 MHz): d = 28.2, 83.2, 132.3, 151.0, 159.7, 165.8, 183.8;
The 1H- and 13C NMR spectra were recorded on a Bruker Advance
spectrometer operating at 300 MHz and 75 MHz, respectively.
Chemical shifts (d) are given as ppm relative to the residual solvent
peak. Preparative chromatographic separations were performed by
IR (KBr):
m 854, 954, 1110, 1244, 1331, 1371, 1394, 1446, 1469,
column chromatography on silica gel (63–200 lm). High perfor-
1697, 1721, 2873, 2995, 3083 cmꢁ1; HRMS (ESI+, ACN): calcd for
C9H11NNaO3S+ (M+Na+): 236.0352, found: 236.0355; Anal. Calcd
for C9H11NO3S: C, 50.69; H, 5.20; N, 6.57; S, 15.04. Found: C,
50.66; H, 5.19; N, 6.32; S, 14.96.
mance liquid chromatography analyses were conducted on appro-
priate Chiralcel or Chiralpak columns. The optical rotations were
determined on a Bellingham–Stanley ADP 220 polarimeter. Specific
rotations (½a 2D5
ꢂ
) are given in units of 10ꢁ1 deg cm2 gꢁ1 (Table 2).
4.3.1.3. 4-Bromothiazole-2-carbaldehyde 3b.
Following pro-
4.2. Reagents, solvents and biocatalysts
cedure using N-formylmorpholine (20.76 mL, 0.206 mol,
A
25equiv.). Standard work-up followed by flash chromatography
(EtOAc:P 1:9, v/v, Rf = 0.23) afforded 3b (1.22 g, 61%) as a solid
(mp = 160 °C); 1H NMR (CDCl3, 300 MHz): d = 7.65 (d, 1H,
J = 1.2 Hz), 9.85 (d, 1H, J = 1.2 Hz); 13C NMR (CDCl3, 75 MHz):
THF was distilled from benzophenone/Na. Melting points are
uncorrected. Column chromatography was performed using silica
gel (mesh size 60–80 lm). The tert-butyl 2-bromothiazole-4-car-
boxylate 1 was prepared starting from ethyl 2-bromothiazole-4-
carboxylate according to literature procedure.10 Commercially
available Grignard reagents were used without further purifica-
tion. All aldehydes used were purified by distillation. Lipases F
and lipases from Pseudomonas fluorescens (LAK) and Pseudomonas
cepacia (LPS) were purchased from Amano. Lipases from Candida
rugosa (CRL), lipase from hog pancreas (PPL), pig liver esterase
(PLE), proteinase 2A from Aspergillus oryzae, esterase from Rhyz-
opus oryzae and penicillin G-amidase immobilised from E. coli were
purchased from Fluka. Lipase B (CaL B, Novozyme 435) and lipase A
(CaL A) from Candida antarctica were purchased from Novozymes,
Denmark. Acylase I from porcine kidney and protease PS from
Aspergillus melleus were purchased from Sigma–Aldrich. Baker’s
yeast produced as wet cakes by Lesaffre Magyaroszág Ltd, Hungary,
was purchased from a local store.
d = 125.1, 128.4, 165.6, 182.4; IR (KBr):
1088, 1227, 1263, 1335, 1388, 1455, 1550, 1677, 1711, 2877,
3122 cmꢁ1 HRMS (ESI+): calcd for C4H3BrNOS+ (M+H+):
m 657, 712, 768, 845, 900,
;
191.9113; found 191.9119; Anal. Calcd for C4H2BrNOS: C, 25.02;
H, 1.05; N, 7.29; S, 16.70. Found: C, 25.27; H, 0.99; N, 7.38; S, 16.56.
4.3.1.4. rac-tert-Butyl 2-(1-hydroxyethyl)thiazole-4-carboxylate
rac-2a.
Following procedure A using acetaldehyde (44.90 lL,
0.794 mmol, 2 equiv). White solid (mp = 118 °C). m = 68 mg, 75%.
Rf = 0.33 (P/EtOAc 7:3, v/v). 1H NMR (CDCl3, 300 MHz): d = 1.58 (s,
9H), 1.63 (d, 3H, J = 6.3 Hz), 3.54 (s, 1H), 5.2 (q, 1H, J = 6.3 Hz),
7.95 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 24.1, 28.3, 68.2, 82.2,
126.6, 148.2, 160.6, 177.0; IR (KBr):
m 851, 955, 1159, 1236, 1269,
1348, 1393, 1456, 1476, 1493,1722, 2931, 2989, 3272 cmꢁ1; HRMS
(ESI+, ACN): calcd for C10H15NNaO3S+ (M+Na+): 252.0665; found
252.0672; Anal. Calcd for C10H15NO3S: C, 53.85; H, 7.81; N, 5.71;
S, 13.07% Found: C, 53.49; H, 7.45; N, 5.73; S, 13.32%.
4.3. Chemical synthesis
4.3.1. Procedure A for the preparation of racemic alcohols rac-
2a–h and 2-formylthiazoles 3a,b
The reaction was carried out in a sealed tube under N2 using
THF (4 mL) as the solvent. To the solution of tert-butyl 2-bromothi-
4.3.1.5. rac-tert-Butyl 2-(1-hydroxy-2,2-dimethylpropyl)thia-
zole-4-carboxylate rac-2b.
aldehyde (86.11 L, 0.794 mmol, 2 equiv). Yellowish solid
(mp = 111 °C). m = 71 mg, 66%. Rf = 0.25 (P/EtOAc 8:2, v/v). 1H
Following procedure A using piv-
l

L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
479
NMR (CDCl3, 300 MHz); d = 0.92 (s, 9H), 1.49 (s, 9H), 4.39 (s, 1H),
4.68 (s, 1H), 7.85 (s, 1H); 13C NMR (CDCl3, 75 MHz); d = 25.7,
0.824 mmol, 2 equiv). White solid (mp = 101 °C). m = 74 mg, 72%.
Rf = 0.52 (P/EtOAc 8:2, v/v). 1H NMR (CDCl3, 300 MHz): d = 1.01 (s,
9H), 2.71 (d, 1H, J = 5.1 Hz), 4.65 (d, 1H, J = 5.4 Hz), 7.20 (s, 1H);
13C NMR (CDCl3, 75 MHz): d = 25.7, 36.1, 79.5, 117.0, 123.8, 174.5;
28.1, 35.6, 79.1, 81.8, 126.3, 147.1, 160.6, 174.8; IR (KBr);
780,891, 954, 1160, 1233, 1288, 1312, 1342, 1370, 1393, 1480,
1697, 2903, 2972, 3535 cmꢁ1
HRMS (ESI+, ACN): calcd for
13H21NNaO3S+ (M+Na+) 294.1134, found 294.1136; Anal. Calcd
m
;
IR (KBr) m 7.21, 748, 835, 881, 901, 1017, 1070, 1087, 1173, 1249,
C
1289, 1366, 1479, 2873, 2975, 3107, 3123, 3308 cmꢁ1. HRMS
(ESI+, ACN): calcd for C8H13BrNOS+ (M+H+) 249.9896, found
249.9904; Anal. Calcd for C8H12BrNOS: C, 38.41; H, 4.84; N, 5.60;
S, 12.82%; Found: C, 38.42; H, 4.95; N, 5.95; S, 12.42%.
for C13H21NO3S: C, 55.54; H, 7.80; N, 5.16; S, 11.82%; Found: C,
55.93; H, 7.85; N, 5.49; S, 11.74%.
4.3.1.6. rac-tert-Butyl 2-(1-hydroxy-3-methylbutyl)thiazole-4-
car-boxylate rac-2c.
butanal (108 L,
Following procedure A using 3-methyl-
0.993 mmol, 2.5 equiv). White solid
4.3.1.11. rac-1-(4-Bromothiazol-2-yl)-3-methylbutan-1-ol rac-
l
2h.
Following procedure A using 3-methylbutanal (89.70 lL,
(mp = 112 °C). m = 96 mg, 89%. Rf = 0.33 (P/EtOAc 8:2, v/v). 1H
NMR (CDCl3, 300 MHz): d = 0.86 (d, 6H, J = 4.5 Hz), 1.48 (s, 9H),
1.66 (dddd, 2H, J = 5.2, 4.5, 3.8, 1.8 Hz), 1.78–1.87 (m, 1H), 4.18
(d, 1H, J = 3.9 Hz), 5.05 (ddd, 1H, J = 6.5, 5.4, 4.7 Hz), 7.84 (s, 1H);
13C NMR (CDCl3, 75 MHz): d = 21.5, 23.3, 24.4, 28.0, 46.9, 69.9,
0.824 mmol, 2 equiv). Colourless liquid. m = 88 mg, 85%. Rf = 0.48
(P/EtOAc 8:2, v/v). 1H NMR (CDCl3, 300 MHz): d = 0.97 (d, 3H,
J = 2.4 Hz), 0.99 (d, 3H, J = 2.4 Hz), 1.74 (dd, 2H, J = 6.6, 6.5 Hz),
1.85–1.92 (m, 1H), 2.68 (d, 1H, J = 5.3 Hz), 5.07 (dt, 1H, J = 6.5,
5.6 Hz), 7.18 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 21.7, 23.4,
81.9, 126.3, 147.6, 160.5, 177.9; IR (KBr):
m
778, 802, 853, 1039,
24.6, 47.0, 70.3, 116.8, 124.3, 177.7; IR (KBr): m 733, 888, 1082,
1095, 1110, 1166, 1234, 1309, 1345, 1371, 1385, 1396, 1478,
1498, 1717, 2935, 2965, 3098, 3110, 3306 cmꢁ1; HRMS (ESI+,
ACN): calcd for C13H22NO3S+ (M+H+): 272.1315, found: 272.1321;
Anal. Calcd for C13H21NO3S: C, 57.54; H, 7.80; N, 5.16; S, 11.82%;
Found: C, 57.23; H, 7.79; N, 5.55; S, 11.62%.
1186, 1251, 1297, 1368, 1386, 1468, 1480, 1706, 2871, 2958,
3380 cmꢁ1; HRMS (ESI+, ACN): calcd for C8H13BrNOS+ (M+H+)
249.9896, found 249.9906; Anal. Calcd for C8H12BrNOS: C, 38.41;
H, 4.84; N, 5.60; S, 12.82. Found: C, 38.81; H, 5.01; N, 5.81; S, 12.44.
4.3.2. Procedure B for preparation of racemic alcohols rac-2i–k
In a dried round bottom flask, a solution of aldehyde 3a or 3b
(100 mg, 0.469 mmol) in THF (4 mL) was cooled to ꢁ78 °C under
N2, and the Grignard reagent (0.47–1.62 mmol in THF) was added
slowly. After complete consumption of the starting material (mon-
itored by TLC), the reaction was quenched with saturated aqueous
NH4Cl (2 mL). The aqueous layer was extracted with Et2O
(3 ꢀ 5 mL) and the combined organic layers were washed with 5%
aqueous HCl (5 mL) and brine, dried over MgSO4 and concentrated
in vacuo. The residue was purified by flash column chromatography
on silica gel using a mixture of EtOAc:P as eluent to give tert-butyl-
2-(1-hydroxymethyl)thiazole-4-carboxylate derivatives rac-2i–k.
4.3.1.7. rac-tert-Butyl 2-(1-hydroxynonyl)thiazole-4-carboxyl-
ate rac-2d.
(136
Following procedure A using nonyl aldehyde
l
L, 0.794 mmol, 2 equiv). White solid (mp = 48 °C). m = 60
mg, 46%. Rf = 0.36 (P/EtOAc 7:3, v/v). 1H NMR (CDCl3, 300 MHz):
d = 0.82 (t, 3H, J = 6.6 Hz), 1.20–1.30 (m, 12H), 1.55 (s, 9H), 1.74–
1.99 (m, 2H), 3.58 (t, 1H, J = 6.75 Hz), 4.99 (dd, 1H, J= 8.2, 4.5 Hz),
7.92 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 14.0, 22.7, 25.3, 28.2,
29.3, 29.4, 29.5, 31.9, 38.1, 71.9, 82.1, 126.4, 148.0, 160.6, 177.0;
IR (KBr):
m 784, 1164, 1288, 1351, 1371, 1393, 1470, 1688, 2852,
2922, 3502 cmꢁ1
.
HRMS (ESI+, ACN): calcd for C17H30NO3S+
(M+H+) 328.1941, found 328.1947; Anal. Calcd for C17H29NO3S: C,
62.35; H, 8.93; N, 4.28; S, 9.79%; Found: C, 62.02; H, 9.05; N, 4.51;
S, 9.39%.
4.3.2.1. rac-tert-Butyl 2-(1-hydroxyallyl)thiazole-4-carboxylate
rac-2i.
Following procedure B using vinylmagnesium bro-
4.3.1.8. rac-tert-Butyl 2-(hydroxybenzyl)thiazole-4-carboxylate
mide (671 lL, 0.7 M, 0.47 mmol, 1 equiv). Oil. m = 61 mg, 54%.
rac-2e.
Following procedure A using benzaldehyde (80.16
lL,
Rf = 0.17 (P/EtOAc 8:2, v/v). 1H NMR (CDCl3, 300 MHz): d = 1.52
(s, 9H), 5.24 (dd, 1H, J = 10.5, 2.6 Hz), 5.48 (dd, 1H, J = 16.9,
2.6 Hz), 5.55 (d, 1H, J = 2.8 Hz), 6.08 (ddd, 1H, J = 16.9, 10.3,
5.5 Hz), 7.93 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 28.1, 72.3,
0.794 mmol, 2 equiv). Yellowish solid (mp = 122 °C). m = 86 mg,
74%. Rf = 0.20 (P/EtOAc 8:2, v/v). 1H NMR (CDCl3, 300 MHz):
d = 1.55 (s, 9H), 5.12 (d, 1H, J = 3.6 Hz), 6.17 (d, 1H, J = 3.3 Hz),
7.27–7.34 (m, 3H), 7.48–7.51 (m, 2H), 7.94 (s, 1H); 13C NMR13
C
82.2, 117.0, 126.9, 137.3, 148.0, 160.5, 174.8; IR (KBr):
1098, 1160, 1250, 1347, 1369, 1483, 1714, 2980, 3418 cmꢁ1; HRMS
(ESI+, ACN): calcd for
11H16NO3S+ (M+H+) 242.0845, found
m 756,
NMR (CDCl3, 75 MHz): d = 28.0, 73.3, 81.9, 126.4, 126.9, 128.0,
128.4, 141.1, 147.8, 160.4, 175.9; IR (KBr) 525, 697, 756, 843,
m
C
957, 1056, 1152, 1256, 1284, 1332, 1396, 1455, 1496, 1621,
242.0843; Anal. Calcd for C11H15NO3S: C, 54.75; H, 6.27; N, 5.80;
S, 13.29. Found: C, 54.70; H, 6.24; N, 5.40; S, 13.
1707, 1799, 2984, 3130, 3219 cmꢁ1; HRMS (ESI+, ACN): calcd for
C
15H17NNaO3S+ (M+Na+) 314.0821, found 314.0830; Anal. Calcd
for C15H17NO3S: C, 61.83; H, 5.88; N, 4.81; S, 11.01%; Found: C,
61.48; H, 5.95; N, 4.51; S, 10.91%.
4.3.2.2. rac-1-(4-Bromothiazol-2-yl)-2-methylpropan-1-ol rac-
2j.
Following procedure B using i-PrMgCl (1.31 mL, 2 M,
2.63 mmol, 2.5 equiv). Oil. m = 99 mg, 40%. Rf = 0.30 (P/EtOAc 8:2,
v/v). 1H NMR (CDCl3, 300 MHz): d = 0.9 (d, 3H, J = 6.8 Hz), 0.96 (d,
1H, J = 6.9 Hz), 2.12–2.23 (m, 1H), 3.52 (d, 1H, J = 4.14 Hz), 4.77
(dd, 1H, J = 6.9, 4.1 Hz), 7.16 (s, 1H); 13C NMR (CDCl3, 75 MHz):
4.3.1.9. rac-1-(4-Bromothiazol-2-yl)ethanol rac-2f.
Follow-
ing procedure using acetaldehyde (46.60 L, 0.824 mmol,
A
l
2 equiv). Colourless liquid, m = 56 mg, 65%. Rf = 0.33 (P/EtOAc 8:2,
v/v). 1H NMR (CDCl3, 300 MHz): d = 1.55 (d, 3H, J = 6.0 Hz), 4.46
(d, 1H, J = 4.8 Hz), 5.08 (dq, 1H, J = 6.0, 4.8 Hz), 7.13 (s, 1H); 13C
NMR (CDCl3, 75 MHz): d = 24.1, 68.2, 116.9, 124.4, 177.5; IR
d = 16.34, 18.93, 34.94, 116.81, 124.15, 154.08, 176.39; IR (KBr):
m
736, 839, 891, 1016, 1048, 1081, 1171, 1250, 1285, 1479, 2873,
2931, 2964, 3122, 3323 cmꢁ1
; HRMS (ESI+, ACN): calcd for
(KBr):
m
736, 832, 880, 1009, 1079, 1110, 1184, 1252, 1308, 1446,
C7H11BrNOS+ (M+H+) 235,9739, found 235,9747; Anal. Calcd for
C7H10BrNOS: C, 35.61; H, 4.27; N, 5.93; S,13.58. Found: C, 35.49;
H, 4.38; N, 6.02; S, 13.36.
1482, 2979, 3121, 3337 cmꢁ1
.
HRMS (ESI+, ACN): calcd for
C5H7BrNOS+ (M+H+) 207.9426, found 207.9420; Anal. Calcd for
C5H6BrNOS: C, 28.86; H, 2.91; N, 6.73; S, 15.41. Found: C, 28.48;
H, 3.06; N, 6.98; S,15.48.
4.3.2.3.
2k.
(0.7 M, 749 l
rac-1-(4-Bromothiazol-2-yl)prop-2-en-1-ol
Following procedure B using vinylmagnesium bromide
L, 0.52 mmol, 1 equiv). Colourless liquid. m = 71 mg,
62%. Rf = 0.28 (P/EtOAc 7:3, v/v). 1H NMR (CDCl3, 300 MHz):
rac-
4.3.1.10. rac-1-(4-Bromothiazol-2-yl)-2,2-dimethylpropan-1-ol
rac-2g.
Following procedure A using pivaldehyde (89.36 lL,

480
L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
d = 3.12 (d, 1H, J = 4.5 Hz), 5.33 (d, 1H, J = 10.2 Hz), 5.47 (d, 1H,
J = 6.2 Hz), 5.49 (d, 1H, J = 17.1 Hz), 6.1 (ddd, 1H, J = 17.1, 10.2,
6.2 Hz), 7.21 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 72.5, 117.5,
2.3 Hz), 8.01 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 21.0, 28.2,
73.3, 82.2, 119.2, 127.1, 133.5, 148.8, 160.3, 168.6, 169.3; IR
(KBr):
m 782, 811, 927, 1027, 1106, 1172, 1232, 1373, 1476, 1717,
118.0, 124.7, 136.8, 174.5; IR (KBr):
m
738, 837, 886, 933, 987,
1750, 2928, 3094 cmꢁ1; HRMS (ESI+, ACN): calcd for C13H18NO4S+
(M+H+) 284.0951, found 284.0960; Anal. Calcd for C13H17NO4S: C,
55.11; H, 6.05; N, 4.94; S, 11.32. Found: C, 55.45; H, 6.41; N,
4.81; S, 10.96.
1050, 1135, 1174, 1258, 1479, 3120, 3306 cmꢁ1; HRMS (ESI+,
ACN): calcd for C6H7BrNOS+ (M+H+) 219.9426, found 219.9432;
Anal. Calcd for C7H8BrNOS: C, 35.91; H, 3.44; N, 5.98; S, 13.70.
Found: C, 36.29; H, 3.49; N, 5.59; S, 13.53.
4.3.3.6. rac-1-(4-Bromothiazol-2-yl)allyl acetate 4k.
Colour-
4.3.3. Preparation of racemic acetates rac-4
less liquid. m = 117 mg, 99%. 1H NMR (CDCl3, 300 MHz): d = 2.17 (s,
3H), 5.37 (dd, 2H, J = 17.1, 10.4 Hz), 6.08 (ddd, 1H, J = 17.1, 10.4,
Acetyl chloride (1.5 mmol, 108
lL) and a catalytic amount of 4-
N,N-dimethylaminopyridine in pyridine (100
lL, 1% solution) were
6.2 Hz), 6.47 (dd, 1H, J = 6.2, 2.4 Hz), 7.23 (s, 1H); 13C NMR13
C
added to a solution of alcohol 4 (1 mmol) in CH2Cl2 (10 mL). After
30 min of stirring at room temperature, the solvent was removed
in vacuo and the crude product purified by column chromatogra-
phy using n-hexane/EtOAc or CH2Cl2 as eluent.
NMR (CDCl3, 75 MHz): d = 21.0, 73.0, 117.8, 119.8, 125.4, 133.1,
169.1, 169.4; IR (KBr):
m 839, 888, 937, 983, 1024, 1081, 1221,
1371, 1481, 1748, 2853, 2925 cmꢁ1; HRMS (ESI+, ACN): calcd for
C8H8BrNNaO2S+ (M+Na+) 283.9351, found 283.9357. Anal. Calcd
for C8H8BrNO2S: C, 36.66; H, 3.08; N, 5.34; S, 12.23. Found: C,
36.89; H, 3.26; N, 5.54; S, 12.09.
4.3.3.1. rac-tert-Butyl 2-(1-acetoxyethyl)thiazole-4-carboxylate
4a.
Oil. m = 66 mg, 80%. 1H NMR (CDCl3, 300 MHz): d = 1.58 (s,
9H), 1.69 (d, 3H, J = 6.5 Hz), 2.13 (s, 3H), 6.15 (q, 1H, J = 6.5 Hz),
4.3.4. Preparation of racemic propionates and butyrates rac-5–6
7.99 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 20.2, 21.0, 28.3, 70.0,
Propionic/butyric anhydride (2 mmol) and a catalytic amount of
82.2, 126.7, 148.6, 169.8, 170.8, 171.2; IR (KBr):
m
830, 1012,
4-N,N-dimethylamino-pyridine in pyridine (100 lL, 1% solution)
1079, 1160, 1230, 1369, 1453, 1483, 1722, 1750, 2926 cmꢁ1; HRMS
(ESI+, ACN): calcd for C12H17NNaO4S+ (M+Na+) 294.0770, found
294.0765; Anal. Calcd for C12H17NO4S: C, 53.12; H, 6.32; N, 5.16;
S, 11.82. Found: C, 53.50; H, 6.37; N, 5.20; S, 11.96.
were added to a solution of one of the alcohols rac-2 (1 mmol) in
CH2Cl2 (10 mL). After 30 min of stirring at room temperature, the
solvent was removed in vacuo and the crude product purified by
column chromatography using CH2Cl2/EtOAc or CH2Cl2 as eluent.
4.3.3.2. rac-tert-Butyl 2-(1-acetoxy-2,2-dimethylpropyl)thia-
4.3.4.1. rac-tert-Butyl 2-(1-(butyryloxy)-3-methylbutyl)thia-
zole-4-carboxylate 6c.
300 MHz): d = 0.92–0.97 (m, 9H), 1.56 (s, 9H), 1.63–1.71 (m, 4H),
1.88–1.97 (m, 1H), 2.33–2.43 (m, 2H), 6.19 (dd, 1H, J = 8.6,
4.9 Hz), 7.96 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 13.6, 18.3,
21.7, 23.1, 24.6, 28.0, 36.1, 44.0, 71.4, 82.0, 126.3, 148.4, 160.3,
zole-4-carboxylate
4b.
White
solid
(mp = 122 °C).
Oil. m = 64 mg, 99%. 1H NMR (CDCl3,
m = 59 mg, 85%. 1H NMR (CDCl3, 300 MHz): d = 1.02 (s, 9H), 1.57
(s, 9H), 2.14 (s, 3H), 5.89 (s, 1H), 7.96 (s, 1H); 13C NMR (CDCl3,
75 MHz): d = 20.9, 26.0, 28.3, 35.5, 79.9, 81.9, 126.2, 148.4, 160.4,
168.9, 169.7; IR (KBr):
m 810, 1025, 1048, 1098, 1166, 1231,
1371, 1476, 1717, 1747, 2975 cmꢁ1; HRMS (ESI+, MeOH): calcd
for C15H24NO4S+ (M+H+) 314.1421, found 314.1430; Anal. Calcd
for C15H23NO4S (313.41): C, 57.48; H, 7.40; N, 4.47; S, 10.23. Found:
C, 57.49; H, 7.65; N, 4.67; S, 9.98.
171.3, 172.5; IR: m 755, 1051, 1094, 1157, 1221, 1368, 1459,
1709, 1731, 2873, 2962 cmꢁ1; HRMS (ESI+): calcd for C17H27NO4S+
(M+) 341.1661, found 341.1656. Anal. Calcd for C17H27NO4S: C,
59.80; H, 7.97; N, 4.10; S, 9.39; Found: C, 59.90; H, 8.13; N, 4.25;
S, 9.49.
4.3.3.3. rac-tert-Butyl 2-(1-acetoxynonyl)thiazole-4-carboxylate
4d.
White solid (mp = 100 °C). m = 117 mg, 99%. 1H NMR
4.3.4.2. rac-1-(4-Bromothiazol-2-yl)-3-methylbutyl propionate
15h.
300 MHz): d = 0.93 (d, 6H, J = 6.8 Hz), 1.15 (t, 3H, J = 7.6 Hz),
1.63–1.74 (m, 1H), 1.87 (ddd, 2H, J = 16.0, 9.0, 5.3 Hz), 2.38 (q,
2H, J = 7.6 Hz), 6.12(dd, 1H, J = 9.3, 5.3 Hz), 7.16 (s, 1H); 13C NMR
(CDCl3, 75 MHz): d = 9.1, 21.9, 23.1, 24.6, 27.6, 44.0, 71.2, 117.0,
(CDCl3, 300 MHz): d = 0.83 (t, 3H, J = 6.7 Hz), 1.23–1.35 (m, 12H),
1.57 (s, 9H), 1.917–2.08 (m, 2H), 2.13(s, 3H), 6.07 (dd, 1H, J = 7.8,
5.1 Hz), 7.96 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 14.2, 21.0
22.7, 25.1, 28.3, 29.3, 29.3, 29.4, 31.9, 35.2, 73.3, 82.1, 126.4,
Colourless liquid. m = 61 mg, 99%. 1H NMR (CDCl3,
148.6, 160.4, 169.9, 170.8; IR (KBr):
m 811, 1039, 1069, 1107,
1174, 1229, 1372, 1480, 1715, 1745, 2950, 2927, 2955,
3090 cmꢁ1; HRMS (ESI+, MeOH): calcd for C19H32NO4S+ (M+H+)
370.2047, found 370.2067; Anal. Calcd for C19H31NO4S: C, 61.76;
H, 8.46; N, 3.79; S, 8.68. Found: C, 61.67; H, 8.85; N, 3.39; S, 8.28.
124.9, 171.7, 173.4; IR:
2880, 2959 cmꢁ1; HRMS (ESI+): calcd for C11H16BrNO2S+ (M+)
305.0085, found 305.0079; Anal. Calcd for 11H16BrNO2S: C,
43.14; H, 5.27; N, 4.57; S, 10.47; Found: C, 43.47; H,5.52; N,
4.68; S, 10.57.
m 732, 1080, 1117, 1163, 1450, 1745,
C
4.3.3.4. rac-tert-Butyl 2-(acetoxy(phenyl)methyl)thiazole-4-
carbo-xylate 4e.
White solid (mp = 113 °C). m = 5 mg, 90%.
4.3.4.3. rac-1-(4-Bromothiazol-2-yl)-2-methylpropyl propionate
5j.
300 MHz): d = 0.93 (d, 6H, J = 6.9 Hz), 1.17 (t, 3H, J = 7.5 Hz),
2.31–2.38 (m, 1H), 2.43 (q, 2H, J = 7.5 Hz), 5.879 (d, 1H,
J = 5.7 Hz), 7.17 (s, 1H); 13C NMR (CDCl3, 75 MHz): d = 9.2, 17.3,
1H NMR (CDCl3, 300 MHz): d = 1.57 (s, 9H), 2.18 (s, 3H), 7.12 (s,
1H), 7.32–7.38 (m, 3H), 7.45–7.49 (m, 2H), 8.00 (s, 1H); 13C NMR
(CDCl3, 75 MHz): d = 21.1, 28.2, 74.6, 82.2, 127.0, 127.2, 128.9,
Colourless liquid. m = 61 mg, 96%. 1H NMR (CDCl3,
129.0, 137.6, 148.9, 160.2, 169.2, 169.8; IR (KBr):
813, 913, 1034, 1111, 1166, 1212, 1238, 1371, 1458, 1478, 1716,
1760, 2930, 2986, 3099 cmꢁ1
HRMS (ESI+, ACN): calcd for
17H20NO4S+ (M+H+) 334.1108, found 334.1107; Anal. Calcd for
m 618, 722, 772,
18.7, 27.6, 33.3, 116.8, 124.9, 170.8, 173.3; IR:
1376, 1462, 1755, 2852, 2923, 2957 cmꢁ1; HRMS (ESI+): calcd for
10H14BrNO2S+ (M+) 290.9923, found 290.9917; Anal. Calcd for
m 892, 1016, 1166,
;
C
C
C17H19NO4S: C, 61.24; H, 5.74; N, 4.20; S, 9.62. Found: C, 61.33;
H, 5.88; N, 4.58; S, 9.39.
C10H14BrNO2S: C, 41.11; H, 4.83; N, 4.79; S, 10.97%; Found: C,
41.28; H, 5.01; N, 4.91; S, 10.67%.
4.3.3.5. rac-tert-Butyl 2-(1-acetoxyallyl)thiazole-4-carboxylate
4i.
300 MHz): d = 1.57 (s, 9H), 2.16 (s, 3H), 5.33 (dd, 2H, J = 17.0,
10.4 Hz), 6.1 (ddd, 1H, J = 17.0, 10.4, 6.1 Hz), 6.54 (dd, 1H, J = 6.1,
4.3.4.4. rac-1-(4-Bromothiazol-2-yl)ethyl butyrate 6f. Colour-
less liquid. m = 66 mg, 99%. 1H NMR (CDCl3, 300 MHz): d = 0.95
(t, 3H, J = 7.6 Hz), 1.64 (d, 3H, J = 6.8 Hz), 1.69–1.74 (m, 2H), 2.33
(t, 2H, J = 7.6 Hz), 6.09 (q, 1H, J = 6.8 Hz), 7.18 (s, 1H); 13C NMR
White solid (mp = 88 °C). m = 70 mg, 99%. 1H NMR (CDCl3,

L. Pop et al. / Tetrahedron: Asymmetry 23 (2012) 474–481
481
(CDCl3, 75 MHz): d = 13.7, 18.4, 20.8, 36.2, 69.4, 117.2, 124.9, 171.7,
removed in vacuo and the product was purified by column
chromatography.
172.4; IR:
m 832, 889, 1008, 1078, 1165, 1252, 1375, 1489, 1708,
1742, 2875, 2965 cmꢁ1: HRMS (ESI+): calcd for C9H12BrNO2S+
(M+) 276.9772, found 276.9785; Anal. Calcd for C9H12BrNO2S: C,
38.86; H, 4.35; N, 5.04; S, 11.53%; Found: C, 38.55; H, 4.46; N,
4.97; S, 11.47%.
Acknowledgment
Financial support to FDI from the Romanian Ministry of Educa-
tion and Research (UEFISCDI No. 205/05.10.2011) is gratefully
acknowledged.
4.3.4.5. rac-1-(4-Bromothiazol-2-yl)-2,2-dimethylpropyl buty-
rate 6g.
Colourless liquid. m = 61 mg, 96%. 1H NMR (CDCl3,
References
300 MHz): d = 0.92 (t, 3H, J = 7.6 Hz), 1.00 (s, 9H), 1.61–1.73 (m,
2H), 2.33 (t, 2H, J = 7.6 Hz), 5.79 (s, 1H), 7.16 (s, 1H); 13C NMR
(CDCl3, 75 MHz): d = 13.8, 18.4, 25.9, 35.3, 36.2, 79.5, 116.8,
1. (a) Horstmann, N.; Essig, S.; Bockelmann, S.; Wieczorek, H.; Huss, M.; Sasse, F.;
Menche, D. J. Nat. Prod. 2011, 74, 1100–1105; (b) Chai, Y.; Pistorious, D.; Ullrich,
A.; Weissman, K.; Kazmaier, U.; Muller, R. Chem. Biol. 2010, 17, 296–309; (c)
Minche, D.; Hassfeld, J.; Steinmetz, H.; Huss, M.; Wieczorek, H.; Sasse, F. J.
Antibiot. 2007, 60, 328–331; (d) Han, B.; McPhail, K. L.; Gross, H.; Goeger, D. E.;
Mooberry, S. L.; Gerwick, W. H. Tetrahedron 2005, 61, 11723–11729; (e)
Vinivious, M.; De Souza, N. J. Sulf. Chem. 2005, 26, 4–5; (f) Steinmetz, H.; Glazer,
N.; Herdtweck, E.; Sasse, F.; Reichenbach, H.; Hofle, G. Angew. Chem., Int. Ed.
2004, 43, 4888–4892; (g) Sasse, F.; Steinmetz, H.; Hofle, G.; Reichenbach, H. J.
Antibiot. 2003, 56, 520–525; (h) Marquez, B. L.; Watts, K. S.; Yokochi, A.;
Roberts, M. A.; Verdier-Pinard, P.; Jimenez, J. I.; Hamel, E.; Scheuer, P. J.;
Gerwick, W. H. J. Nat. Prod. 2002, 65, 866–871; (i) Cetusic, J. R. P.; Green, F. R.,
III; Graupner, P. R. Paige Olivier, M Org. Lett. 2002, 4, 1307–1310; (j) Luesch, H.;
Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Mooberry, S. L. J. Nat. Prod. 2000, 63,
611–615.
124.5, 169.7, 172.3; IR:
m
750, 835, 1030, 1080, 1248, 1355, 1481,
HRMS (ESI+): calcd for
12H18BrNO2S+ (M+) 319.0242, found 319.0248; Anal. Calcd for
12H18BrNO2S (319.02): C, 45.01; H, 5.67; N, 4.37; S, 10.01%;
1709, 1748, 2887, 2964 cmꢁ1
;
C
C
Found: C, 45.33; H, 5.50; N, 4.36; S, 10.37%.
4.4. Enzymatic reactions
4.4.1. Preparative scale enzymatic acylation
A mixture of rac-alcohol (400 mg), vinyl acetate (5 equiv) and
CaL A or CaL B (400 mg) in DIPE or MTBE (10 mL) was stirred at
room temperature. The enantiomeric composition of the reaction
counterparts was monitored by HPLC. Samples from the reaction
2. (a) Shibue, T.; Hirai, T.; Okamoto, I.; Morita, N.; Masu, H.; Azumaya, I.; Tamura,
O. Tetrahedron Lett. 2009, 50, 3845–3848; (b) Menche, D.; Hassfeld, J.; Li, J.;
Mayer, K.; Rudolph, S. J. Org. Chem. 2009, 74, 7220–7229; (c) Chandrasekhar, S.;
Mahipal, B.; Kavitha, M. J. Org. Chem. 2009, 74, 9531–9534; (d) Bull, J. A.;
Balskus, E. P.; Horan, R. A.; Langner, M.; Ley, S. V. Chem. Eur. J. 2007, 13, 5515–
5538; (e) Menche, D.; Hassfield, J.; Li, J.; Rudolph, S. J. Am. Chem. Soc. 2007, 129,
6100–6101; (f) Bull, J. A.; Balskus, E. P.; Horan, R. A. J.; Langner, M.; Ley, S. V.
Angew. Chem., Int. Ed. 2006, 45, 6714–6718; (g) Domling, A.; Beck, B.;
Eichelberger, U.; Sakamuri, S.; Menon, S.; Chen, Q.-Z.; Lu, Y.; Wessjohann, L.
A. Angew. Chem., Int. Ed. 2006, 45, 7235–7239; (h) Pang, H.; Xu, Z.; Chen, Z.; Ye,
T. Lett. Org. Chem. 2005, 2, 699–702; (i) Wipf, P.; Takada, T.; Rishel, M. Org. Lett.
2004, 6, 4057–4060; (j) Yokokawa, F.; Sameshima, H.; Katagiri, D.; Aoyama, T.;
Shioiri, T. Tetrahedron 2002, 58, 9445–9458.
3. (a) Chandrasekhar, S.; Sudhakar, A. Org. Lett. 2010, 12, 236–238; (b) Patterson,
A. W.; Peltier, H. M.; Ellman, J. J. Org. Chem. 2008, 73, 4362–4369; (c) Sani, M.;
Fossati, G.; Huguenot, F.; Zanda, M. Angew. Chem., Int. Ed. 2007, 46, 3526–3529;
(d) Peltier, H. M.; McMahon, J. P.; Patterson, A. W.; Ellman, J. A. J. Am. Chem. Soc.
2006, 128, 16018–16019; (e) Delgado, O.; Heckmann, G.; Müller, H. M.; Bach, T.
J. Org. Chem. 2006, 71, 4599–4906; (f) Spieb, A.; Heckmann, G.; Bach, T. Synlett
2004, 131–133; (g) Cetusic, J. R. P.; Green, F. R., III; Graupner, P. R.; Olivier, M. P.
Org. Lett. 2002, 4, 1307–1310; (h) Mulder, R. J.; Shafer, C. M.; Molinski, T. F. J.
Org. Chem. 1999, 64, 4995–4998.
4. Deng, S.; Tauton, J. Org. Lett. 2005, 7, 299–301. and references cited therein.
5. (a) Wipf, P.; Wang, Z. Org. Lett. 2007, 9, 1605–1607; (b) Boros, E. E.; Johns, B. A.;
Garvey, E. P.; Loble, C. S.; Miller, W. H. Bioorg. Med. Chem. Lett. 2006, 16, 5668–
5672; (c) Abrabi, M.; Thibonnet, J.; Berillon, L.; Dehmel, F.; Rottlander, M.;
Knochel, P. J. Org. Chem 2000, 65, 4618–4634; (d) Krasovskiy, A.; Knochel, P.
Angew. Chem., Int. Ed. 2004, 43, 3333–3336; (e) Henkel, B.; Beck, B.; Westner, B.;
Mejat, B.; Dömling, A. Tetrahedron Lett. 2003, 44, 8947–8950; (f) Yoo, S.-K.
Tetrahedron Lett. 1992, 33, 2159–2162; (g) Crosby, J.; Stone, R.; Lienhard, G. E. J.
Am. Chem. Soc. 1970, 92, 2891–2900.
6. (a) Bencze, L. C.; Paizs, C.; Tosa, M. I.; Trif, M.; Irimie, F. D. Tetrahedron:
Asymmetry 2010, 21, 1999–2004; (b) Bencze, L. C.; Paizs, C.; Tosa, M. I.; Vass, E.;
Irimie, F. D. Tetrahedron: Asymmetry 2010, 21, 443–450; (c) Podea, P. V.; Paizs,
C.; Tosa, M.; Irimie, F. D. Tetrahedron: Asymmetry 2008, 19, 1959–1964.
7. (a) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512–519; (b) Sullivan, G.
R.; Dale, J. A.; Mosher, H. S. J. Org. Chem. 1973, 38, 2143–2147.
mixture (10 lL) were diluted with n-hexane (400 lL). After filter-
ing the enzyme, the solvents were removed in vacuo and the prod-
ucts [(S)-alcohols and (R)-ester derivatives] were isolated by
column chromatography using n-hexane-EtOAc (9:1, v/v) as elu-
ent. Spectroscopic data of the products were identical to those ob-
tained for the racemic compounds. The enantiomeric excess of the
resolution products was determined by HPLC (Table 2).
4.4.2. Preparative scale enzymatic hydrolysis/methanolysis
A mixture of rac-esters (400 mg) and CaL B, PLE or CRL (400 mg)
in DIPE:phosphate buffer or DIPE:MeOH was stirred at room tem-
perature. The enantiomeric composition of the reaction counter-
parts was monitored by HPLC as previously described. After
stopping the hydrolysis reaction, the reaction mixture was ex-
tracted with dichloromethane, the organic layer was dried over
Na2SO4 and the solvent was evaporated in vacuo. The products
[(R)-alcohols and (S)-esters] were isolated by column chromatog-
raphy using n-hexane-EtOAc (9:1, v/v) as eluent. The spectroscopic
data of the products were identical to those obtained for the race-
mic compounds. The enantiomeric excesses of the resolution prod-
ucts were determined by HPLC (Table 2).
4.5. The absolute configuration of the resolution products
8. Ohtani, I.; Kusumi, Y.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092–4096.
Into a solution of enantiomerically enriched alcohol (0.071
9. (a) Martin, T.; Laguerre, C.; Hoarau, C.; Marsais, F. Org Lett. 2009, 11, 3690–
3693; (b) Bagley, M. C.; Dale, J. W.; Jenkins, R. L.; Bower, J. Chem. Commun. 2004,
102–103.
10. Menche, D.; Hassfeld, J.; Li, J.; Rudolph, S. J. Am. Chem. Soc. 2007, 129, 6100–
6101.
mmol) in CH2Cl2 (2 mL), 4-N,N-dimethylamino-pyridine in pyri-
dine (100 lL, 1% solution) was added, followed by the addition of
(R)- or (S)-MTPA-Cl (35 mg, 0.14 mmol). The reaction mixture
was stirred for 24 h at room temperature. The solvent was further