A practical synthesis of (2R)-3,5-di-O-benzoyl-2-fluoro-2-C-methyl-d-ribono-γ-lactone

Article abstract of DOI:10.1016/j.tetasy.2009.02.006

The title compound was synthesized in 23% overall yield using only one purification in four chemical steps. The key features of this practical synthesis include an asymmetric aldol condensation and an enzymatic hydrolysis to remove the major undesired iso

Full text of DOI:10.1016/j.tetasy.2009.02.006

Tetrahedron: Asymmetry 20 (2009) 305–312
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Tetrahedron: Asymmetry
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A practical synthesis of (2R)-3,5-di-O-benzoyl-2-fluoro-
2-C-methyl-D-ribono-
c
-lactone
b
a
b
Pingsheng Zhang a, , Hans Iding , Miall Cedilote , Stephan Brunner ,
*
Thomas Williamson a, Thomas P. Cleary a
a Pharmaceutical Technical Development Actives, Roche Carolina Inc. 6173, East Old Marion Highway, Florence, SC 29506, USA
b Hoffmann-La Roche Ltd, Pharma Research Basel, Biotechnology PRBT-B, 4070 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The title compound was synthesized in 23% overall yield using only one purification in four chemical
steps. The key features of this practical synthesis include an asymmetric aldol condensation and an enzy-
matic hydrolysis to remove the major undesired isomer.
Received 26 November 2008
Revised 20 January 2009
Accepted 10 February 2009
Available online 11 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
isomer could be isolated at some point in the process, a practical
synthesis for 2 would be achievable.
1-(2-Deoxy-2-fluoro-2-C-methyl-b-
D
-ribofuranosyl)-cytosine 1
is a potent and selective anti-HCV agent1, and is currently in clin-
2. Results and discussion
ical trials for the treatment of hepatitis C. 3,5-Di-O-benzoyl-2-flu-
oro-2-C-methyl-D-ribono-c-lactone 2 is a key intermediate for
The initial aldol condensation reaction between 4 and 5 was
carried out by first treating 5 with LDA at 0 °C in THF for half an
the synthesis of 1.2 A number of synthetic routes for preparing 2
have been disclosed.3 However, none of these syntheses are
deemed suitable for scaling up to commercial scale manufacturing
due to the shortcomings of very low overall yield, lack of robust-
ness, the use of expensive and toxic reagents, chromatographic iso-
lation, etc. Although in the latest version of the synthesis4 most of
the scale-up issues have been resolved, the manufacturing cost is
still a concern, and continued efforts to pursue a more cost effec-
tive and scalable alternative synthesis are warranted. Herein, we
report a practical synthesis of 2 using a combination of an asym-
metric aldol condensation and an enzymatic resolution.5
NH2
O
O
O
N
Ph
O
N
O
O
HO
O
F
O
Ph
HO
F
1
2
Our synthetic strategy is outlined in Figure 1. The target lactone
could be prepared from intermediate 3, which, in turn, could be
formed via an asymmetric aldol condensation reaction between
O
2,3-O-isopropylidene-
onate 5. This approach has been used many times in the literature
for the preparation of
-lactones.6 For example, Heathcock et al.7
D-glyceraldehyde 4 and ethyl 2-fluoropropi-
O
O
O
Ph
O
O
O
c
F
reported that the reaction of 4 with the enolate of methyl propio-
nate 6 led to the formation of a 3:2 mixture of 7 and 8 (Scheme 1).
Treatment of the mixture with 60% acetic acid at room tempera-
ture overnight resulted in a 3:2 mixture of lactones 9 and 10. We
reasoned that similar results should be observed for ester 5, and
if so, the desired adduct 3 should be the major product. If the major
O
F
O
OH
O
3
Ph
F
O
O
O
H
O
O
5
4
* Corresponding author. Tel.: +1 843 629 4241; fax: +1 843 629 4128.
E-mail address: pingsheng.zhang@roche.com (P. Zhang).
Figure 1.
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.02.006

306
P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
aldol mixture. The two products were isolated, and the major iso-
O
mer was confirmed to be 2 by NMR studies and comparison with
an authentic sample. The minor isomer was confirmed to be 14
by extensive NMR studies. One NMR feature clearly distinguishing
the two compounds was a clear NOE detected between the methyl
group and the C-4 proton in 14, whereas no such NOE was detected
in 2. This work confirmed that the two major compounds present
in the crude aldol condensation product (Scheme 2) were 3 and 11.
The third component in the GC chromatogram of the aldol mix-
ture must be 12 and/or 13. At this point, a 3:2 mixture of this com-
ponent and compound 3 was obtained through flash column
chromatography. Oxidation of this mixture with Dess–Martin re-
agent produced a mixture of two isomers in the same 3:2 ratio.
This result clearly indicated that C-2 configuration in compound
3 and the third component was different, and thus, the third com-
ponent must be 13 (Scheme 4).
O
O
7
O
OH
+
O
1) LDA, 0 °C, THF
O
2) 4, -70 °C
6
O
O
O
8
OH
O
O
O
60% AcOH
O
O
HO
HO
+
HO
HO
9
10
Scheme 1. A literature example.
O
O
F
5
O
O
O
O
O
O
2) 4, -78 °C
1) LDA, 0 °C, THF
F
F
O
O
O
OH
15 (42%)
3 (40%)
Dess-Martin
O
O
O
O
O
O
O
O
O
F
O
F
O
O
O
O
O
O
OH
OH
F
F
O
3
11
O
O
OH
13
16 (58%)
(60%)
O
O
O
O
O
Scheme 4. Confirmation of compound 13.
F
F
O
OH
OH
13
12
The fourth possible isomer, 12, was prepared by reduction of 15
with NaBH4 in ethanol at 0 °C. The reaction was surprisingly ster-
eoselective, as indicated in Scheme 5. Unfortunately, 12 and 13
co-eluted in our GC method, and thus, it was not conclusive regard-
ing the ratio of these two isomers in the mixture of the aldol
product.
Scheme 2. The condensation of 5 with 4.
hour, and then cooling the mixture to around ꢀ78 °C, followed by
the addition of 4 as a THF solution. GC analysis revealed that the
crude product was a mixture of three components, 3 (51%), 11
(38%), 12, and/or 13 (11%) (Scheme 2), a result similar to that ob-
served in Scheme 1. The initial assignment of the structures was
conducted via a tedious process. The attempts to isolate individual
isomers using column chromatography were unsuccessful. Treat-
ment of the crude mixture with 60% acetic acid at 90 °C for 2 h re-
sulted in complete conversion to a mixture of un-protected
lactones, which was reacted with an excess amount of benzoyl
chloride in pyridine (Scheme 3). Two major products with a ratio
of approximately 60:40 were detected in the reaction mixture,
which corresponded to the ratio of the two major products in the
O
O
O
F
O
OH
O
NaBH4
O
(<2%)
O
3
F
EtOH, 0 °C
+
O
O
O
15
O
O
F
O
OH
11
(38%)
12/13
(11%)
3
(51%)
+
+
12 (>98%)
Scheme 5. Preparation of compound 13.
2) PhCOCl, Pyridine
3) isolation
1) 60% AcOH
90 °C, 2 h
O
After the establishment of the stereochemistry the reaction was
optimized, so as to achieve better stereoselectivity and chemical
yield. Our studies have indicated that the solvent affected the ste-
reoselectivity, but the concentration did not. Better selectivities
were observed in TBME or toluene (typically, 56:38:6) than in
THF and 2-MeTHF (typically, 50:39:11). Two bases, LDA and
LiHMDS, were evaluated for the reaction. The batches using
LiHMDS demonstrated better selectivities (e.g., 64:32:4), but of-
fered lower conversions and crude yields. In one experiment, ester
O
O
H
O
H
O
O
Ph
O
Ph
O
F
CH3
H
H
+
O
CH3
O
F
O
O
Ph
Ph
2 (60%)
Scheme 3. Conversion of aldol adducts to lactones.
14
(40%)

P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
307
5 was added to a THF solution of LiHMDS at ꢀ78 °C, and analysis of
the mixture after 10 min at the temperature revealed that more
than 60% of the ester had been converted to a condensed product
via Claisen condensation. Our explanation is that LiHMDS is a rel-
atively weak base. Therefore, in the reaction mixture there was a
significant amount of the ester (in equilibrium with its enolate),
and thus Claisen condensation had occurred. For the same reason,
the addition order is very important. For instance, addition of the
LDA to an ester solution should be avoided. The best way to con-
duct the reaction is to add a solution of both 4 and 5 in toluene
to a solution of LDA at low temperature. Similar selectivity was ob-
served when the reaction was carried out at temperatures from
ꢀ78 to ꢀ20 °C. Deterioration in both stereoselectivity and yield
was experienced at higher temperatures. With the optimized con-
ditions, a selectivity of 62:34:4 was achieved. It is also worth men-
tioning that aldehyde 4 tends to polymerize. Freshly distilled 4 is a
liquid. It can turn into a gel in just a few hours. For our reaction,
both freshly cracked liquid and polymerized gel gave similar
results.
At this point, it seemed to us that further improvement of selec-
tivity would be hard to achieve using the lithium enolate of 5.
Thus, an evaluation of other types of enolates was attempted. It
is clear that the selectivity at C-2 and C-3 is controlled by different
factors. The selectivity at C-3 is dictated by the steric center in 4
and follows the Felkin–Anh rule.8 The fact that the ratio of 3+11/
12+13 was ꢁ9:1 indicated a fairly favorable selectivity at C-3.
The control of selectivity at C-2 is more complicated. A syn addition
from a Z-enolate9 is required to form 3.10 There are many enolates
in the literature that demonstrates high syn selectivity in aldol con-
densations; examples include 17,11 18,12 19,13, and 2014 (Fig. 2).
and the corresponding products susceptible to hydrolysis, which
accounts for the low isolated yield of the reactions.
O
O
O
O
N
S
F
F
21
22
BBu2
OTi(Oi-Pr)3
SPh
O
OLi
SPh
O
F
F
F
24
25
23
Cl
Ti
Cl
O
Cl
B
O
O
O
O
N
O
N
F
F
26
27
Figure 3. Fluorine-substituted substrates and their enolates.
Table 1
Diastereoselectivity of different enolatesa
OMLn
SPh
BBu2
O
O
4
O
AcOH/water
M = B, Ti, Li
O
G
+
F
17
18
Enolate 23-27
O
OH
Cl
R
Cl
O
R
O
HO
O
Cl
O
HO
O
O
Ti
B
N
F
O
O
O
F
HO
HO
28
29
N
O
Entry
Enolate
Ratio 28/29b
19
R'
R'' 20
1
2
3
4
5
23
24
26
25
27
28/72
32/68
78/22
No reaction
No reaction
Figure 2. Enolates demonstrated syn selectivities.
Thus, the corresponding fluorine-substituted substrates, 2115 and
22,16 were prepared, and enolates 23,17 24,17 25,18 26,19 and 2720
were generated accordingly (Fig. 3). The aldol condensation of
these enolates with aldehyde 4 was then conducted. The crude
products were treated with 60% acetic acid at ꢁ90 °C for 2 h, and
the ratio of the resulting lactones 28–29 was measured as an indi-
cator of the syn/anti selectivity in the aldol reaction. Some results
are listed in Table 1. The data indicate that the selectivity is not
easily predictable. Enolates 23 and 24 offered reversed selectivity,
despite the facts that both enolates existed almost exclusively as
Z-isomers, and excellent syn selectivity was observed in their reac-
tions with certain aldehydes.15,17 No reactions were detected for
enolates 25 and 27. The best selectivity was observed with enolate
26; however, the reaction gave a very low isolated yield. It appears
to us that the effects of a fluorine atom on the enolates are pro-
found; it alters the stereoselectivity of the reactions;21 it decreases
the reactivity of the enolates, and thus, the boron enolates are no
longer reactive toward the aldehyde; and it makes the substrates
a
The enolates were generated according to literature procedures. The aldol
reactions were quenched with 6% HCl solution and extracted with dichlorometh-
ane. The organic solution was concentrated to dryness, the residue was mixed with
60% acetic acid solution, and the resulting mixture was stirred at ꢁ88 °C for 2 h.
b
GC area ratio.
One thought was that increasing the bulk of either the aldehyde
or the ester might be beneficial to the diastereoselectivity of the
reaction. Thus, aldehyde 3022 and esters 31,23 32,24 and 3325 were
prepared (Fig. 4). Unfortunately, similar selectivities were observed
for the reactions of these esters 31–33 with aldehyde 4. The larger
esters did offer one benefit—higher crude yield. However, as will be
discussed later, the products from these larger esters are not good
substrates for the enzymatic resolution. Worse selectivities
(48:41:11) were observed for the condensation of 30 with ester 5.
As expected, the isolation of the desired isomer, 3, from the
crude product using conventional methods was not successful.

308
P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
Table 2
F
Diastereoselectivity of hydrolasesa
O
O
O
H
O
31
30
O
O
O
1) hydrolase
O
O
OH
OH
F
F
11
+
3
F
+
F
2) 0.1 M HCl
O
O
O
O
OH
OH
34
35
O
O
32
33
Enzyme
Estersb
Acidsc
Figure 4. Aldehyde and esters tested for the aldol condensation.
3
11
34
35
CALB
TOY
L6
HLE
PLE
96.9
89.3
80.3
78.2
80.0
80.3
74.8
73.7
71.3
69.2
69.1
72.8
71.6
72.0
71.6
71.4
3.1
10.7
19.7
21.8
20.0
19.7
25.2
26.3
28.7
30.8
30.9
27.2
28.4
28.0
28.4
28.6
0.0
100.0
82.7
82.6
81.6
56.4
55.8
54.2
42.1
20.4
15.1
16.7
31.7
19.9
14.0
20.5
13.1
17.3
17.4
18.4
43.6
44.2
45.8
57.9
79.6
84.9
83.3
68.3
80.1
86.0
79.5
86.9
Therefore, we turned our attention to an enzymatic diastereoselec-
tive hydrolysis approach. Selective hydrolysis of one of the major
isomers to the corresponding carboxylic acid, coupled with an
extractive workup, could be a very efficient separation method.26
Thus, a mixture (ꢁ73:27) of 3 and 11 was isolated from a crude al-
dol product. Hydrolases were screened for the hydrolysis of this
mixture using a pH-indicator, well plate assay. Among the 214
commercial hydrolases tested, 17 demonstrated activity and were
analyzed for their diastereoselectivity (see Table 2). Eight of them
showed either no selectivity or a slight preference for substrate 3.
Eight of them demonstrated different degrees of selectivity for 11.
One widely used enzyme, CALB27 (Candida antarctica lipase, form
B), was outstanding. In the experiment it hydrolyzed only substrate
11. Therefore, this enzyme was selected for an in depth reaction
optimization. First, 1 M Na2SO4 solution, 0.1 M KPI (potassium
phosphate) buffer, and 10% sorbitol solution were identified as
the best media for the reaction. Then, the reaction was optimized
using a batch of crude aldol product (containing 40.8% 3, 34.6%
11, and 7.7% 12/13), and some of the results are listed in Table 3.
In the experiments, the reaction was stopped when isomer 11
was completely hydrolyzed. In an ideal situation no amount of 3
would be hydrolyzed at this point. In reality, a portion of isomer
3 was hydrolyzed in all the experiments. Therefore, the primary
objective for the reaction optimization was to determine the opti-
mal combination of minimal hydrolysis of 3 and reasonable reac-
tion time. As indicated in the table, the pH of the reaction media
had a minor effect on the reaction rate, but did affect the percent-
CLE
L1
LCV
SP524
MME
RME
SP523
RMM
F10
OF
LAO
a
The hydrolase screening was based on a color change of the pH-indicator of
bromothymol blue. The reactions were carried out using ready to use 96 well MTPs
preloaded with an enzymes solution of 20–40
isate per well. The assay was carried out by the addition of 10
substrate solution into the reaction wells prefilled with 170
g pH indicator. The acid formed in the
l
l containing roughly 1 mg lyophil-
l of a 10% ethanolic
l 7.5 mM potassium
l
l
phosphate buffer at pH ꢁ7.2 including 1.5
l
reaction decreased the pH and resulted in a color change from green to yellow,
which was monitored by an UV microplate reader. The activity hits—emerging
yellow wells—were acidified to pH < 2 with 1 ml 0.1 M HCl and extracted with
500 ll ethyl acetate. After dramatization with diazomethane solution, the ethyl
acetate solutions were analyzed on GC for b3/11 ratio and c34/35 ratio.
age of 3 hydrolysis (entries 2 and 5–7). The percentage was signif-
icantly higher at pH of 7.5. The reaction temperature clearly
affected both the reaction rate and hydrolysis of 3 (entries 7–10).
When the temperature changed from 35 °C to 50 °C, the percent-
Table 3
Optimization of CALB-catalyzed diastereoselective hydrolysisa
Entry
S/Eb
Cc (%)
Buffer
pH
T (°C)
Time (h)
A (%) 3d
% 3 Hydrolyzede
Yieldf (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
10
10
10
10
10
10
10
10
10
10
10
10
5
5
5
5
5
5
5
5
5
5
5
5
5
10
10
10
10
10
10
3 mM PKI, 1 M Na2SO4
100 mM KPI
200 mM KPI
500 mM KPI
100 mM KPI
100 mM KPI
100 mM KPI
100 mM KPI
100 mM KPI
100 mM KPI
10% Sorbitol
10% Sorbitol, 1 M Na2SO4
10% Sorbitol
10% Sorbitol, 1 M Na2SO4
1 M Na2SO4
10% Sorbitol
10% Sorbitol
10% Sorbitol
7.2
7.2
7.2
7.2
6.5
7.0
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
35
35
35
35
35
35
35
40
45
50
45
45
45
45
45
45
45
45
63
42
42
72
46
42
42
39
24
23
42
42
18
18
18
15
23
64
61.4
56.9
51.8
52.8
45.7
51.4
52.7
55.7
53.6
47.8
66.0
50.8
56.3
53.1
53.4
48.6
55.0
76.7
15.3
11.4
11.7
10.3
11.4
11.2
12.7
13.9
14.5
16.4
13.7
12.2
9.0
62.9
64.9
62.2
57.4
52.3
65.9
66.7
64.9
60.8
47.2
66.4
66.7
76.3
70.8
71.1
60.3
65.7
53.1
5
5
5
10
20
11.3
13.2
13.3
8.8
14.0
a
CALB was the technical formulation Lipozyme CALB L*K from Novozyme.
wt/wt.
as wt %.
GC area% in the isolated crude product.
Calculated based on area% of 3 and 34 [34/(34 + 3) ꢂ 100].
Estimated with the area% of 3 and the weight of isolated crude product.
b
c
d
e
f

P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
309
O
O
O
O
F
O
O
O
O
OH
Ph
O
O
O
1) CALB
1.0 M Na2SO4
pH: 7.2, 35 °C
22h
O
O
3
F
1) AcOH/water,
88 °C, 2h
O
LDA, toluene
-78 to -20 °C
O
5
OH
O
F
O
O
2
Recrystallisation
from IPA
3
O
+
F
Ph
O
2
OH
4
+
O
2. Extraction
with CH2Cl2
2)
PhCOCl
O
11
O
DMAP, Et3N
MeCN, 1h
O
O
Ph
O
F
F
O
O
O
O
OH
O
F
H
12/13
O
Ph
OH
12/13
Scheme 6. The complete process for the synthesis of 2.
age of hydrolyzed 3 increased from 12.7% to 16.4% while the reac-
tion time decreased from 42 h to 23 h. Increasing the enzyme load
helped to boost the reaction rate (comparing entries 16–18). The
concentration of the substrate also affected the reaction rate. The
hydrolysis of 3 was largely caused by its reaction with the medium.
This is consistent with the fact that the percentage of 3 hydrolysis
was correlated to reaction time, temperature, and pH of the reac-
tion media. A ‘placebo’ experiment (same conditions without en-
zyme) further confirmed this conclusion. The reaction was
further optimized. As a compromise between the reaction rate, en-
zyme load, and minimized non-enzymatic hydrolysis, our final
reaction conditions were pH 7.2, T = 35 °C, s/e = 6.6, and 5% sub-
strate concentration in 1.0 M Na2SO4 buffer. Under these condi-
tions, the reaction was usually complete within approximately
20 h.
Our overall process is illustrated in Scheme 6. Upon completion
of the enzymatic hydrolysis, the reaction mixture was extracted
with dichloromethane. The organic solution was dried over anhy-
drous MgSO4 and filtered to remove residual enzyme. Concentra-
tion of the solution afforded a crude mixture that contained 3
and 12/13. This mixture was stirred in ꢁ60% acetic acid solution
at 88 °C for 2 h, and was then concentrated to dryness. The result-
ing residue was dissolved in acetonitrile and benzoylated with
benzoyl chloride and triethylamine. The crude lactone mixture
was recrystallized from 2-propanol to afford pure 2 in ꢁ23% overall
yield.
was suspended in DMF and reacted with an excess amount of iodo-
ethane. The crude product was a mixture of 3, 11, and 12/13 in a GC
area ratio of 28:70:2. Pure 11 was isolated from this mixture
(Scheme 7). In a different experiment, the aqueous solution was
acidified with 31% HCl to pH < 1 and heated at 90 °C for 3 h. The
resulting mixture was then concentrated to dryness. The residue
was suspended in acetonitrile and reacted with an excess amount
of benzoyl chloride in the presence of triethylamine. After workup
and purification, pure 14 was isolated.
3. Conclusion
The aldol condensation of 4 and 5 afforded a mixture of at least
three isomeric products. The desired isomer 3 was the major com-
ponent (ꢁ60%). Treatment of the crude mixture with CALB led to
the hydrolysis of the major by-product 11 to its acid, which was
then removed by extraction. This approach allows the production
of 2 with only one recrystallization. The mild reaction conditions,
coupled with the availability and inexpensiveness of the enzyme,
make the process scalable and practical. Our internal cost analysis
has concluded that this process is by far the most cost effective
one.
4. Experimental
It is also worth noting that pure 11 and 14 were prepared,
respectively, starting from the aqueous phase of the enzymatic
hydrolysis that contained mainly the sodium salts of 35 and 34.
Thus, the solution was concentrated to dryness. The resulting solid
1H NMR and 13C NMR spectra were recorded on a Varian INOVA
500 MHz spectrometer. IR spectra were recorded on a Nicolet/
Magna 550 instrument. Mass spectra were recorded on a Thermo
Fisher TSQ Quantum Ultra AM mass spectrometer. Optical rota-
tions were measured at 25 °C using a JASCO P-1010 polarimeter.
Melting Point was taken using a TA DSC Q2000 at a heating rate
of 10 °C/min. 2,3-Isopropylidene-D
-glyceraldehyde 428 and ethyl
Aqueous phase of the enzymatic hydrolysis
2-fluoropropionate 529 were prepared according to literature
procedures.
O
O
O
4.1. Aldol condensation of ethyl 2-fluoropropionate 5 and 2,3-
O-isopropylidene-D-glyceraldehyde 4
ONa
O
ONa
+
F
OH
F
O
O
OH
Minor component
Major component
To a 1.6 M MeLi solution in ethyl ether (164 mL, 261 mMol)
at <ꢀ10 °C was slowly added diisopropylamine (26.4 g, 261 mMol).
After the addition the mixture was cooled to ꢀ78 °C. To the mix-
ture was slowly added a solution of 4 (20 g, 154 mMol) and 5
(28 g, 231 mMol) in toluene (100 mL), while maintaining the tem-
perature below ꢀ70 °C. After the addition, the mixture was stirred
at ꢀ78 °C for 10 min, and was slowly warmed to ambient temper-
ature in 1 h. The resulting mixture was slowly poured into a 28%
KH2PO4 solution (210 mL) while maintaining the temperature be-
1) concentrate to dryness
2) EtI, DMF, rt, overnight
3) purification
1) acidif y to pH<1
2) 90 °C, 3 h
3) concentrate to dryness
4) BzCl, TEA, MeCN, 45 °C
5) Crystallization
11
14
Scheme 7. Preparation of 11 and 14.

310
P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
low 20 °C. After stirring briefly, the mixture was transferred to a
separatory funnel. The aqueous phase was separated and extracted
twice with ethyl acetate (2 ꢂ 100 mL). The combined organic solu-
tions were dried over MgSO4, filtered, and concentrated to dryness
to give 33.5 g of crude product as a thick oil. GC chromatogram of
the product showed a mixture of 56% 3, 40% 11, and 4% 12/13.
and dried under vacuum at 40 °C overnight to give 3 (0.8 g) as a
white solid: mp 62.8–64.5 °C; ½a D25
¼ þ16:7 (c 1.0, CH2Cl2); FTIR
ꢃ
mmax (neat, cmꢀ1): 3425, 1732; 1H NMR (500 MHz, CDCl3): d
4.42–4.17 (m, 2H), 4.14 (dd, J = 6.2, 12.4 Hz, 1H), 4.11–4.06 (m,
1H), 4.02 (dd, J = 6.0, 8.7 Hz, 1H), 3.94 (dd, J = 6.4, 23.9 Hz, 1H),
1.66 (d, J = 22.3 Hz, 3H), 1.38 (s, 3H), 1.33 (s, 3H), 1.32 (t,
J = 7.5 Hz, 3H); 13C NMR (125 MHz, CDCl3): d 170.49, 170.29,
109.40, 96.09, 94.58, 74.82, 74.76, 74.73, 74.65, 66.29, 66.26,
61.85, 26.31, 25.27, 20.87, 20.68, 14.04; MS (ESI): m/z 251 ([M+1]+).
4.2. Preparation of (2R)-3,5-di-O-benzoyl-2-fluoro-2-C-methyl-
D-ribono-c-lactone 2
A 4-neck round-bottomed flask, equipped with a mechanic stir-
4.4. Preparation of ethyl (2R,4R)-4,5-isopropylidene-2-fluoro-3-
rer, a thermo couple, a pH probe, and a base dosing pump inlet, was
charged with 1.0 M sodium sulfate buffer (620 mL), and the crude
aldol product (33.0 g) was prepared in Section 4.1. The mixture
was stirred at 35 °C to become a clear solution. Next, CALB solu-
tion30 (3.3 g) was added. The mixture was stirred at the tempera-
ture, while the pH was maintained at 7.2 by the addition of
1.0 M NaOH solution via a pH pump. After 5 h, additional CALB
(1.7 g) was added, and the reaction was continued overnight. The
reaction was stopped by the addition of dichloromethane
(60 mL). The mixture was cooled to ambient temperature and
transferred to a separatory funnel. The aqueous phase was sepa-
rated and extracted with dichloromethane twice (2 ꢂ 50 mL). The
combined organic solution was dried over MgSO4 for at least 1 h,
filtered, and concentrated to give a thick oil (20.2 g). This oil
(20 g) was mixed with acetic acid (120 mL) and water (64 mL).
The mixture was stirred at 90 °C for 2 h and then concentrated to
dryness. The residue was mixed with toluene (50 mL) and concen-
trated to dryness. The residue was dissolved in acetonitrile
(100 mL). To this solution were added DMAP (0.2 g) and benzoyl
chloride (33.7 g, 240 mMol). Triethylamine (26.4 g, 264 mMol)
was slowly added while maintaining the temperature <40 °C. After
the addition, the mixture was stirred at ambient temperature for
1 h, diluted with ethyl acetate (100 mL) and water (100 mL), trans-
ferred to a separatory funnel. The aqueous phase was extracted
with ethyl acetate (2 ꢂ 50 mL). The combined organic solution
was washed with saturated NaHCO3 (50 mL), dried over MgSO4, fil-
tered, and concentrated to a thick oil. A mixture of the oil and 2-
propanol (160 mL) was heated to ꢁ70 °C, at which point a clear
solution formed. The solution was then slowly cooled to ambient
temperature overnight. The solid was filtered, washed with 2-pro-
panol, and dried under vacuum at 50 °C for at least 5 h to give
12.7 g (23% overall yield from 4) pure 2 as an off-white solid: mp
keto-2-methylvalerate 15
A mixture of Dess–Martin reagent (1.7 g, 4.0 mMol) in dichloro-
methane (50 mL) was stirred at ambient temperature. A couple of
drops of pyridine was added to help the dissolution. To the mixture
was added a solution of 3 (500 mg, 2.0 mMol) in dichloromethane
(10 mL). The resulting mixture was stirred overnight during which
time, heavy precipitation formed. The reaction was quenched by
the addition of saturated Na2S2O3 solution (20 mL) and saturated
NaHCO3 solution (20 mL). The organic phase was separated, and
the aqueous phase was extracted with dichloromethane. The com-
bined organic solution was dried over MgSO4, filtered, and concen-
trated to give the crude product, which was purified with flash
column chromatography (silica gel, eluting with dichloromethane)
to give 15 (320 mg, 64%) as a clear liquid: ½a D25
¼ þ13:3 (c 1.0,
ꢃ
CH2Cl2); FTIR mmax (neat, cmꢀ1): 1737 (br); 1H NMR (500 MHz,
CDCl3): d 4.96 (ddd, J = 2.6, 5.6, 8.0 Hz, 1H), 4.35–4.19 (m, 3H),
4.06 (ddd, J = 1.5, 5.6, 8.8 Hz, 1H), 1.73 (d, J = 22.6 Hz, 3H), 1.46
(s, 3H), 1.40 (s, 3H), 1.30 (t, J = 7.5 Hz, 3H); 13C NMR (125 MHz,
CDCl3): d 200.45, 200.22, 166.32, 166.12, 111.28, 97.84, 96.32,
77.67, 65.99, 65.95, 62.79, 25.60, 25.06, 20.27, 20.10, 13.92.
4.5. Preparation of ethyl (2R,3S,4R)-4,5-isopropylidene-2-
fluoro-3-hydroxy-2-methylvalerate 12
A solution of 15 (100 mg, 0.40 mMol) in ethanol (10 mL) was
cooled to 0 °C. To the solution was added NaBH4 (15 mg,
0.40 mMol). The mixture was stirred at 0 °C for 1 h and then con-
centrated to dryness on a rotavapor. The residue was partitioned
between water (2 mL) and TBME (50 mL). The organic solution
was washed with brine, dried over MgSO4, filtered, and concen-
trated to give the crude product (80 mg), which was a mixture of
12 and 3 in a ratio of 98:2. The crude product was purified with
flash column chromatography (silica gel, eluting with dichloro-
methane/ethyl acetate, 8:1) to give 12 (60 mg) as a clear liquid:
132.9–133.1 °C; ½a D25
¼ þ124:6 (c 1.0, CH2Cl2); FTIR mmax (neat,
ꢃ
cmꢀ1): 1790, 1731, 1711; 1H NMR (500 MHz, CDCl3): d 8.10–8.05
(m, 2H), 8.00–7.95 (m, 2H), 7.66–7.60 (m, 1H), 7.58–7.53 (m,
1H), 7.51–7.44 (m, 2H), 7.43–7.37 (m, 2H), 5.51 (dd, J = 7.3,
17.6 Hz, 1H), 4.99 (ddd, J = 3.6, 5.1, 7.3 Hz, 1H), 4.75 (dd, J = 3.6,
12.5 Hz, 1H), 4.59 (dd, J = 5.2,12.5 Hz, 1H), 1.75 (d, J = 25 Hz, 3H);
13C NMR (125 MHz, CDCl3): d 169.18, 169.00, 165.78, 165.29,
134.15, 133.53, 130.12, 129.73, 129.00, 128.69, 128.52, 128.03,
91.72, 90.23, 77.44, 72.40, 72.28, 62.29, 18.86, 18.66; MS (ESI):
m/z 373 ([M+1]+).
½
a 2D5
ꢃ
¼ ꢀ5:7 (c 1.0, CH2Cl2); FTIR mmax (neat, cmꢀ1): 3514 (br),
1761, 1735; 1H NMR (500 MHz, CDCl3): d 4.40 (tt, J = 1.9, 7.3 Hz,
1H), 4.32–4.19 (m, 2H), 4.08 (dd, J = 6.8, 8.0 Hz, 1H), 3.90 (t,
J = 7.7 Hz, 1H), 3.64–3.57 (m, 1H), 2.97 (d, J = 10.9 Hz, 1H), 1.67
(d, J = 14.6 Hz, 3H), 1.40 (s, 3H), 1.35 (s, 3H), 1.33 (t, J = 7.2 Hz,
3H); 13C NMR (125 MHz, CDCl3): d 170.83, 170.64, 109.90, 94.88,
93.37, 73.84, 73.63, 72.97, 66.36, 61.98, 26.11, 25.40, 21.87,
21.69, 14.03; MS (ESI): m/z 251 ([M+1]+).
4.3. Isolation of ethyl (2R,3R,4R)-4,5-O-isopropylidene-2-
fluoro-3-hydroxy-2-methylvalerate 3
4.6. Preparation of ethyl (2S,3R,4R)-4,5-isopropylidene-2-
fluoro-3-hydroxy-2-methylvalerate 11
A sample of crude product after the enzymatic hydrolysis (2.0 g)
was dissolved in t-butyl methyl ether (TBME, 20 mL). To the solu-
tion was slowly added hexane until the mixture was cloudy. After
the solids formed, hexane was added to maximize precipitation.
The mixture was aged at ambient temperature for 2 h. The solid
was filtered, washed with hexanes, and dried in the air to give a so-
lid. The solid was mixed with 10 mL TBME. The mixture was
heated to become a clear solution, and was then cooled to ambient
temperature overnight. The solid was filtered, washed with TBME,
Approximately, one-sixth (170 g) of the aqueous mother liquor
(pH ꢁ 7.0 by paper) from experiment 4.2 was concentrated to dry-
ness on a rotavapor. The residual solid was mixed with toluene
(250 mL), and the mixture was concentrated to dryness. The resid-
ual solid was mixed with acetonitrile (250 mL), and the mixture
was concentrated to dryness. The resulting solid was mixed with
DMF (50 mL) and iodoethane. The mixture was stirred at ambient
temperature overnight. The mixture was concentrated on a rotava-

P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
311
5. (a) Cedilote, M.; Cleary, T.; Iding, H.; Zhang, P. U.S. Pat. Appl. Publ. US
2008145901 A1, 2008.; (b) Cedilote, M.; Cleary, T.; Zhang, P. U.S. Pat. Appl. Publ.
US 2008177079 A1, 2008.
6. For example: (a) Bernrdi, A.; Beretta, M. G.; Colombo, L.; Gennari, C.; Poli, G.;
Scolastico, C. J. Org. Chem. 1985, 50, 4442–4447; (b) Kita, Y.; Yasuda, H.;
Tamura, O.; Itoh, F.; Ke, Y. Y.; Tamura, Y. Tetrahedron Lett. 1985, 26, 5777–5780;
(c) Danilova, G. A.; Melnikova, V. I.; Pivnitsky, K. K. Tetrahedron Lett. 1986, 27,
2489–2490; (d) Kita, Y.; Tamura, O.; Itoh, F.; Yasuda, H.; Kishino, H.; Ke, Y. Y.;
Tamura, Y. J. Org. Chem. 1988, 53, 554–561.
7. Heathcock, C. H.; Young, S. D.; Hagen, J. P.; Pirrung, M. C.; White, C. T.;
Vanderveer, D. J. Org. Chem. 1980, 45, 3846–3856.
8. (a) Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968, 2199–2204; (b)
Anh, N. T.; Eisentein, O. Nov. J. Chim. 1977, 1, 61.
por to almost dryness. The resulting solid was partitioned between
water (50 mL) and dichloromethane (250 mL). The organic solution
was washed with water (10 mL), dried over MgSO4, filtered, and
concentrated to give the crude product (1.4 g), which was a mix-
ture of 3 (28%), 11 (70%), and 12/13 (2%). Column chromatography
(silica gel, eluting with hexanes/MTBE, 2:1) of the crude product
afforded pure 11 (470 mg) as a colorless oil: ½a D25
¼ þ11:0 (c 1.0,
ꢃ
CH2Cl2); FTIR mmax (neat, cmꢀ1): 3462 (br), 1738; 1H NMR
(500 MHz, CDCl3): d 4.34–4.20 (m, 3H), 4.13–3.96 (m, 3H), 2.54
(d, J = 6.2 Hz, 1H), 1.62 (t, J = 15 Hz, 3H), 1.41 (s, 3H), 1.37 (s, 3H),
1.33 (t, J = 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl3): d 170.48,
170.28, 109.25, 6.94, 95.44, 74.59, 74.35, 74.18, 65.93, 65.89,
61.99, 26.42, 25.50, 19.90, 19.71, 14.08; MS (ESI): m/z 251 ([M+1]+).
9. To simplify this discussion, the term ‘Z’ or ‘E’ describes only the relationship
between the methyl group and metallated oxygen, and the influence of the
heavier fluorine and sulfur atoms on the nomenclature is ignored.
10. Evans, D. A.; Nelson, J. V.; Vogel, E.; Taber, T. R. J. Am. Chem. Soc. 1981, 103,
3099–3111.
11. For examples: (a) Nagase, R.; Matsumoto, N.; Hosomi, K.; Hidashi, T.;
Funakoshi, S.; Misaki, T.; Tanabe, Y. Org. Biomol. Chem. 2007, 5, 151–159; (b)
Tanabe, Y.; Matsumoto, N.; Funakoshi, S.; Manta, N. Synlett 2001, 1959–1961;
(c) Eames, J.; de las Heras, M. A.; Jones, R. V. H.; Warren, S. Tetrahedron Lett.
1996, 37, 4581–4584; (d) Annunziata, R.; Cinquini, M.; Cozzi, F.; Cozzi, P. G.;
Consolandi, E. Tetrahedron 1991, 47, 7897–7910; (e) Corey, E. J.; Kim, S. S. J. Am.
Chem. Soc. 1990, 112, 4976–4977; (f) Christlieb, M.; Davies, J. E.; Eames, J.;
Hooley, R.; Warren, S. J. Chem. Soc., Perkin Trans. 1 2001, 2983–2996; (g)
Danheiser, R. L.; Nowick, J. S. J. Org. Chem. 1991, 56, 1176–1185; (h) Corey, E. J.;
Imwinkelried, R.; Pikul, S.; Xiang, Y. B. J. Am. Chem. Soc. 1989, 111, 5493–5495;
(i) Patel, D. V.; VanMiddlesworth, F.; Donaubauer, J.; Gannett, P.; Sih, C. J. J. Am.
Chem. Soc. 1986, 108, 4603–4604.
12. Tnoue, T.; Liu, J. F.; Buske, D. C.; Abiko, A. J. Org. Chem. 2002, 67, 5250–5256.
13. (a) Shaw, S. J.; Ashley, G. W.; Burlingame, M. A. PCT Int. Appl. WO 070536 A2,
2007.; (b) Ashley, G. W.; Burlingame, M.; Desai, R.; Fu, H.; Leaf, T.; Licari, P. J.;
Tran, C.; Abbanat, D.; Bush, K.; Macielag, M. J. Antibiot. 2006, 59, 392–401; (c)
Burlingame, M. A.; Mendoza, E.; Ashley, G. W. Tetrahedron Lett. 2004, 45, 2961–
2964; (d) Santi, D. V.; Ashley, G.; Myles, D. C. WO 012534 A2, 2002.; (e) Ashley,
G.; Chan-Kai, I.; Burlingame, M. A. WO 044717 A2, 2000.
4.7. Preparation of (2S)-3,5-di-O-benzoyl-2-fluoro-2-C-methyl-
D
-ribono-c-lactone 14
Approximately, two-thirds (700 g) of the aqueous mother liquor
from experiment 4.2 was acidified to pH < 1 with 31% hydrochloric
acid. The mixture was stirred at 90 °C for 3 h, and was then concen-
trated to dryness on a rotavapor. The residual solid was mixed with
toluene (500 mL), and the mixture was concentrated to dryness.
This operation was repeated with toluene (300 mL) and acetoni-
trile (400 mL). The resulting solid was mixed with acetonitrile
(150 mL). To the stirring mixture were added 4-dimethylamino-
pyridine (50 mg) and benzoyl chloride (21 g). The mixture was
cooled to 10 °C, and triethylamine (21 g) was added while main-
taining the batch temperature below 40 °C. After the addition,
the mixture was stirred at 45 °C for 30 min and cooled to room
temperature. The mixture was filtered, and the wet cake was
washed with acetonitrile. The filtrate was concentrated to dryness
to give an oil. The crude oil was mixed with ethyl acetate
(ꢁ200 mL) and washed with 10% Na2CO3 solution, brine, dried over
MgSO4, filtered, and concentrated to give crude product (ꢁ15 g) as
a thick oil. The crude product was purified with flash column chro-
matography (eluting with dichloromethane), followed by a recrys-
tallization from 2-propanol (50 mL) and drying under vacuum at
45 °C overnight to give pure 14 (4.8 g) as a white solid: mp 95.2–
14. (a) Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2123–2129;
(b) Evans, D. A.; Bartoli, J. Tetrahedron Lett. 1982, 23, 807–810.
15. Ishihara, T.; Ichihara, K.; Yamanaka, H. Tetrahedron 1996, 52, 255–262.
16. Prepared following literature procedure (Andrade, C. K. Z.; Rocha, R. O.;
Vercillo, O. E.; Silva, W. A.; Matos, R. A. F. Synlett 2003, 2351–2352): 74% yield
as white solid: mp 139.5–140.1 °C; FTIR
m
max (neat, cmꢀ1): 1797, 1732; 1H NMR
(500 MHz, CDCl3): d 8.12–8.05 (m, 1H), 7.34–7.27 (m, 2H), 7.27–7.21 (m, 1H),
6.09 (dq, J = 6.6, 48.4 Hz, 1H), 1.72 (dt, J = 12.3, 24.7 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d 169.6, 169.4, 150.6, 142.8, 127.2, 125.9, 125.2, 116.0,
110.2, 86.7, 85.2, 18.1, 17.9.
17. Ishihara, T.; Ichihara, K.; Yamanaka, H. Tetrahedron Lett. 1995, 36, 8267–8270.
18. Prepared following the literature procedure in Ref. 11
19. Prepared following the literature procedure in Ref. 12c.
99.4 °C; ½a 2D5
ꢃ
¼ þ23:6 (c 1.0, CH2Cl2); FTIR mmax (neat, cmꢀ1):
20. Prepared following the literature procedure in Ref. 9
21. One possible cause for the low observed stereoselectivity in these reactions
could be the formation of higher percentage of E-enolate favored by fluorine–
metal interactions. Although fluorine-metal interactions were detected under
certain circumstances (e.g., Takemura, H.; Kon, N.; Kotoku, M.; Nakashima, S.;
Otsuka, K.; Yasutake, M.; Shimyozu, T.; Inazu, T. J. Org. Chem. 2001, 66, 2778;
Barr, D.; Hutton, K. B.; Morris, J. H.; Mulvey, R. E.; Reed, D.; Snaith, R. J. Chem.
Soc., Chem. Commun. 1986, 127; Plenio, H. ChemBioChem 2004, 5, 650; and the
references cited in these papers), no strong evidence for the existence of such
1802, 1735, 1712; 1H NMR (500 MHz, CDCl3): d 8.03 (dd, J = 1.2,
8.3 Hz, 2H), 7.67–7.61 (m, 1H), 7.59–7.53 (m, 1H), 7.51–7.45 (m,
2H), 7.45–7.40 (m, 2H), 5.89–5.79 (m, 1H), 4.79–4.69 (m, 2H),
4.69–4.59 (m, 1H), 1.77 (d, J = 20 Hz, 3H); 13C NMR (125 MHz,
CDCl3): d 170.36, 170.16, 165.85, 164.86, 134.25, 133.49, 129.90,
129.82, 129.75, 129.73, 129.04, 128.80, 128.50, 128.47, 128.05,
94.43, 92.93, 78.53, 78.49, 74.62, 74.38, 62.96, 16.93, 16.73; MS
(ESI): m/z 373 ([M+1]+).
interactions in
a-fluoroenolates were reported in the literature. Some
literature results might have indicated the lack of such interaction or the
lack of influence of such an interaction on the E/Z ratio of enolates. For instance,
19F NMR of the enolates of N,N-dimethylfluoroacetamide indicated an almost
1:1 mixture of E/Z isomers even at ꢀ85 °C (Welch, J. T.; Eswarakrishnan, S. J.
Org. Chem. 1985, 50, 5403). Another example is that enolates 23 and 24 existed
almost exclusively as Z-enolates under the reaction conditions (Refs. 15 and
17), indicating that the coordination between F–Li or F–Ti (if it exists) has little
effect on the E/Z ratio.
Acknowledgments
We thank Drs. Geng-Xian Zhao, Stephen Chan, Shan-Ming
Kuang, Ms. Joni Bishop, Ms. Eileen Zhao, Mr. James Suchy, and
Ms. Joye Collins for their analytical supports. We also thank Dr. Jo-
die Brice for her help in preparation of the manuscript.
22. Schmid, C. R.; Bradley, D. A. Synthesis 1992, 587–590.
23. Compound 31 was prepared according to the scheme below:
O
O
O
O
OH
S
S
O
O
O
O
cat.MeSO3H
References
O
O
36
37
1. (a) Clark, J. PCT Int. Appl. WO 2005003147 A2, 2005.; (b) Clark, J. L.; Hollecker,
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F
KF
O
O
HCONH2
31
To a solution of methanesulfonyl ethyl lactate 36 (purchased from Acros, 60 g,
306 mMol) in n-butanol (100 mL) was added methanesulfonic acid (1.4 g,
14.6 mMol). The mixture was gradually heated to ꢁ120 °C, and the low boiling
point components were distilled out. The distillation was continued until batch
temperature reached ꢁ136 °C. More n-butanol (30 mL) was added, and the

312
P. Zhang et al. / Tetrahedron: Asymmetry 20 (2009) 305–312
distillation was continued for ꢁ2 h. GC analysis indicated ꢁ96% conversion.
1.43–1.28 (m, 2H), 1.25 (dt, J = 4.4, 8.7 Hz, 3H), 0.92 (t, J = 7.4 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d 170.27, 170.09, 86.45, 85.00, 72.34, 37.93, 37.91, 19.92,
19.87, 18.61, 18.59, 18.47, 18.44, 18.29, 18.26, 13.83.
The mixture was cooled to ambient temperature and diluted with ethyl
acetate. The solution was washed with saturated Na2CO3 solution, brine, dried
over MgSO4, filtered, and concentrated to give crude product 37 (68.5 g).A flask
equipped with a mechanic stirrer, an addition funnel, and a distillation outlet
was charged with KF (70 g, 1200 mMol) and formamide (150 mL). The mixture
was heated to 100 °C, and the vacuum for the distillation head was set at
40 mbar. Crude 37 (68 g, 300 mMol) was slowly added from the addition
funnel for ꢁ2.5 h. The product formed in the reaction mixture was distilled out
and collected (total liquid collected: 36 g). The crude product was purified by
fractional distillation, and pure product 31 (24 g, 55% overall yield) was
collected at 52–64 °C/40 mbar, as a clear liquid (Fujisawa, H., Takeuchi, Y. J.
Fluorine Chem. 2002, 117, 173–176): 1H NMR (500 MHz, CDCl3): d 4.98 (dq,
J = 61, 8.5 Hz, 1H), 4.18 (t, J = 8.5 Hz, 2H), 1.70–1.60 (m, 2H), 1.57 (dd, J = 29.5,
8.5 Hz, 3H), 1.44–1.34 (m, 2H), 0.93 (t, J = 9 Hz, 3H).
25. Compound 33 was prepared following the same procedure as in Ref. 21, 50%
overall yield as a clear liquid: FTIR mmax (neat, cmꢀ1): 1756, 1737; 1H NMR
(500 MHz, CDCl3): d 5.00 (dq, J = 6.9, 48.9 Hz, 1H), 4.90–4.84 (m, 1H), 1.66–1.56
(m, 4H), 1.59 (dd, J = 25.0, 10.0 Hz, 3H), 0.93–0.87 (m, 6H); 13C NMR (125 MHz,
CDCl3):
9.53.
d 170.51, 170.32, 86.43, 84.99, 78.21, 26.46, 26.42, 18.58, 18.40,
26. (a) Janes, L. E.; Cimpoia, A.; Kazlauskas, R. J. J. Org. Chem. 1999, 64, 9019–9029;
(b) Aggarwal, V. K.; Astle, C. J.; Iding, H.; Wirz, B.; Rogers-Evans, M. Tetrahedron
Lett. 2005, 46, 945–947.
27. Anderson, E. M.; Larsson, K. M.; Kirk, O. Biocatal. Biotransform. 1998, 16, 181–
204.
28. Schmid, C. R.; Bryant, J. D.; Dowlatzedah, M.; Philips, J. L.; Prather, D. E.;
Schantz, R. D.; Sear, N. L.; Vianco, C. S. J. Org. Chem. 1991, 56, 4056–4058.
29. Fritz-Langhals, E.; Schultz, G. Tetrahedron Lett. 1993, 34, 293–296.
24. Compound 32 was prepared following the same procedure as in Ref. 21, 44%
overall yield as a clear liquid: IR mmax (neat, cmꢀ1): 1757, 1737; 1H NMR
(500 MHz, CDCl3): d 5.07–4.99 (m, 1.5H), 4.95–4.89 (m, 0.5H), 1.69–1.60 (m,
1H), 1.69–1.60 (m, 1H), 1.56 (ddt, J = 2.7, 5.5, 10.4 Hz, 3H), 1.55–1.46 (m, 1H),
30. CALB was
CN02100.
a technical solution from BioCatalytics, Cat# SO L-101, Lot#