Chemoenzymatic Asymmetric Synthesis of 1,4-Benzoxazine Derivatives: Application in the Synthesis of a Levofloxacin Precursor

Article abstract of DOI:10.1021/acs.joc.5b00056

A versatile and general route has been developed for the asymmetric synthesis of a wide family of 3-methyl-3,4-dihydro-2H-benzo[b][1,4]oxazines bearing different pattern substitutions in the aromatic ring. Whereas hydrolases were not suitable for resoluti

Full text of DOI:10.1021/acs.joc.5b00056

Article
pubs.acs.org/joc
Chemoenzymatic Asymmetric Synthesis of 1,4-Benzoxazine
Derivatives: Application in the Synthesis of a Levofloxacin Precursor
María Lopez-Iglesias,† Eduardo Busto,‡ Vicente Gotor,† and Vicente Gotor-Fernandez*,†
́
́
†Organic and Inorganic Chemistry Department, Biotechnology Institute of Asturias (IUBA), University of Oviedo, Avenida Julian
́
Clavería s/n, 33006 Oviedo, Spain
‡Department of Chemistry, Organic and Bioorganic Chemistry, University of Graz, Heinrichstrasse 28, 8010 Graz, Austria
S
* Supporting Information
ABSTRACT: A versatile and general route has been developed for the
asymmetric synthesis of a wide family of 3-methyl-3,4-dihydro-2H-
benzo[b][1,4]oxazines bearing different pattern substitutions in the
aromatic ring. Whereas hydrolases were not suitable for resolution of
these racemic cyclic nitrogenated amines, alternative chemoenzymatic
strategies were designed through independent pathways leading to both
amine antipodes. On one hand, bioreduction of 1-(2-nitrophenoxy)-
propan-2-ones allowed the recovery of the enantiopure (S)-alcohols in
high yields using the alcohol dehydrogenase from Rhodococcus ruber (ADH-A), whereas evo-1.1.200 ADH led to their
counterpart (R)-enantiomers also with complete selectivity and quantitative conversion. Alternatively, lipase-catalyzed acetylation
of these racemic alcohols, and the complementary hydrolysis of the acetate analogues, gave access to the corresponding optically
enriched products with high stereodiscrimination. Particularly attractive was the design of a chemoenzymatic strategy in six steps
for the production of (S)-(−)-7,8-difluoro-3-methyl-3,4-dihydro-2H-benzo-[b][1,4]oxazine, which is a key precursor of the
antimicrobial agent Levofloxacin.
different human diseases, including pneumonia, acute bacterial
sinusitis, urinary tract infections, and acute pyelonephritis.10
Asymmetric chemical strategies have been successfully carried
out for the synthesis of this drug and other related
nonfluorinated analogues.5a,11 The main efforts focused on
the production of (S)-(−)-7,8-difluoro-3-methyl-3,4-dihydro-
2H-benzo[b][1,4]oxazine, which serves as an adequate
synthetic building block for the total synthesis of Levofloxacin.
Biocatalytic methods represent elegant and sustainable
strategies for the production of enantiopure compounds
under mild reaction conditions. In the last couple of decades,
many organic chemists have incorporated the use of enzymes
into their toolbox,12 with lipases and alcohol dehydrogenases
currently being the most employed catalysts for use in industrial
applications, although others, such as transaminases, are
receiving significant attention as well.13 Enzymes have been
identified as particularly useful for the design of valuable
synthetic routes toward the synthesis of enantiopure amines by
means of the use of lipases, transaminases, monoamine
oxidases, and imine reductases, among others.14 In this context,
hydrolases are valuable hydrolytic enzymes that can also
catalyze acylation reactions for the selective formation of
amines through kinetic resolution processes.15 Among the
hydrolytic enzymes, lipases have attracted significant attention
due to their selective action in the asymmetric synthesis of a
wide range of heterocyclic nitrogenated compounds.16
INTRODUCTION
■
Benzoxazines are privileged cyclic subunits found in a wide
range of biologically active molecules with antibacterial,
anticancer, antifungal, and antimicrobial properties,1 but they
also serve as synthetic building blocks for the formation of
more complex structures with relevant medical applications.2
The synthesis of achiral and racemic benzoxazines has been
extensively reported in the literature,3 particularly for those
bearing the 3,4-dihydro-2H-benzo[b][1,4]oxazine fragment
(Figure 1),4 while there are fewer examples regarding the
Figure 1. Chemical structure of the 3,4-dihydro-2H-benzo[b][1,4]-
oxazine subunit (left) and Levofloxacin (right).
development of asymmetric routes toward enantioenriched
benzoxazines. Optically active 3,4-dihydro-2H-benzo[b][1,4]-
oxazines have been mostly synthesized through asymmetric
metal-catalyzed transfer hydrogenation5 or hydrosilylation of
imines,6 organocatalytic additions,7 and broadly using chemical
kinetic resolutions of racemic benzoxazines with optically active
acyl chlorides8 or palladium-catalyzed couplings.9
Certainly, one of the most targeted benzoxazine derivatives is
Levofloxacin (Figure 1), which is a potent fluoroquinolone
antibacterial agent currently approved for the treatment of
Received: January 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b00056
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Scheme 1. Synthesis of Racemic Benzoxazine 4a for Studying its Kinetic Resolution
Surprisingly, the production of optically enriched benzoxazine
derivatives is limited to pig liver esterase-catalyzed hydrolytic
approaches, achieving moderate selectivity values.17 Herein, we
report the versatility of enzymes for the production of
benzoxazine derivatives through the development of robust
chemoenzymatic methods, lipases, and oxidoreductases being
satisfactorily used for the production of target cyclic nitro-
genated compounds with good yields and excellent enantio-
meric excess values. Special attention will be paid to the
asymmetric synthesis of a valuable precursor of Levofloxacin.
Scheme 2. Chemoenzymatic Synthesis of Nitro Ketones 3a−
d, Alcohols 5a−d, and Acetates 6a−d
RESULTS AND DISCUSSION
■
To explore new asymmetric routes for the synthesis of
benzoxazine derivatives, we selected 3-methyl-3,4-dihydro-2H-
benzo[b][1,4]oxazine (4a) as a model for enzymatic activity
screening. The synthesis of the racemate was performed by O-
alkylation of 2-nitrophenol (1a) using chloroacetone (2) in the
presence of potassium bromide, sodium hydrogen carbonate,
and tributylmethylammonium chloride, followed by a palla-
dium-catalyzed hydrogenation-cyclization sequence of nitro
ketone 3a, which allowed the isolation of ( )-4a in 75% overall
yield. Because of the good levels of activity and stereoselectivity
found for lipases in the classical kinetic resolution of secondary
cyclic amines,18 a panel of commercially available lipases
(Candida antarctica lipase, types A and B; porcine pancreas
lipase; Candida rugosa lipase; and Pseudomonas cepacia lipase)
were used for the alkoxycarbonylation of 4a. Unfortunately, no
significant activity was observed when using different allyl
carbonates in methyl tert-butyl ether (MTBE) as solvent.
Searching for an alternative strategy, we took advantage of
the previous preparation of nitro ketone 3a. Then, three
independent strategies were undertaken: (a) nonselective
reduction of ketone 3a to racemic alcohol 5a followed by
classical kinetic resolution through lipase-catalyzed acylative
processes, (b) chemical acetylation of the racemic alcohol
obtained to analyze the complementary lipase-catalyzed
hydrolytic process in depth, and (c) selective bioreduction of
prochiral ketone 3a using alcohol dehydrogenases. For these
studies, a series of benzoxazine precursors bearing different
pattern substitutions, such as a fluorine atom, a methoxy group,
or a methyl functionality, along the aromatic ring were
chemically prepared through an efficient chemical route as
depicted in Scheme 2. This includes O-alkylation of 2-
nitrophenols 1a−d, reduction of ketones 3a−d with sodium
borohydride, and later, acetylation with acetic anhydride in the
presence of DMAP and triethylamine, affording the corre-
sponding racemic acetates 6a−d in good overall yields.
with 94% ee for the (R)-acetate and 89% ee for the remaining
(S)-alcohol. Other lipases, such as Candida antarctica lipase
type A (CAL-A) and Pseudomonas cepacia (PSL-C I), displayed
poor selectivities, whereas Candida antarctica lipase type B
(CAL-B) did not show significant activity. From a set of
solvents, the best results were found with MTBE and toluene
(Table 1, entries 1 and 2); thus, the extension to alcohols 5b−d
was performed next. A similar trend was observed, achieving
the highest rates for the reactions carried out in MTBE (entries
3, 5, and 7), whereas better selectivities were attained in toluene
(entries 4, 6, and 8). MTBE was revealed to be the solvent of
choice because the conversion values were lower in toluene
(27−49%) and, furthermore, the acetate optical purity begins
to decrease at longer time periods (data not shown).
Alternatively, we decided to study lipase-catalyzed hydrolysis
of the corresponding acetates. The results are summarized in
Table 2. Because the water content is a decisive parameter for
enzymatic activity, the amount of water was studied using
substrate 6a as reference without substitutions in the aromatic
ring (entries 1−3). In all cases, excellent selectivity was
observed, obtaining the complementary alcohol (R)-5a and
acetate (S)-6a in comparison to the lipase-catalyzed acetylation
reaction. The reaction with 5 equiv of hydrolytic agent led to
48% conversion (entry 1). Notably, increasing the amount of
water led to slower kinetics but maintained excellent stereo-
selectivity (21−35%, entries 2 and 3). Similar good results were
obtained when extending the methodology to other substituted
benzoxazine derivatives using 5 equiv of water (entries 4−6).
Finally, bioreduction experiments were considered on the
basis of accessing the final product theoretically at 100% yield.
Oxidoreductases with opposite stereopreferences were em-
ployed to develop suitable routes for both alcohol antipodes.
Thus, a set composed of Prelog alcohol dehydrogenases20 from
Rhodococcus ruber (ADH-A), Candida parapsilosis (ADH-CP)
and Baker′s yeast (BY) as well as anti-Prelog enzymes,
including Lactobacillus brevis (ADH-LB), Lactobacillus kefir
(ADH-LK), and evo-1.1.200 ADH, were screened in 50 mM
The lipase-catalyzed acetylation of alcohols 5a−d was
considered first, searching for a suitable lipase that was able
to produce the corresponding alcohols and acetates in high
optical purity (Table S1 in the Supporting Information).
Alcohol 5a was selected as the model substrate, finding
Rhizomucor miehei lipase in immobilized form (RML IM) as an
ideal candidate, leading to 48% conversion in MTBE after 5 h
B
DOI: 10.1021/acs.joc.5b00056
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Table 1. Enzymatic Kinetic Resolution of Alcohols 5a−d using RML IM (1:1 w/w) and 3 Equiv of Vinyl Acetate 7 in Dry MTBE
or Toluene at 30 °C and 250 rpm
a
b
b
c
d
entry
R
solvent
t (h)
eep (%)
ees (%)
c (%)
E
1
2
3
4
5
6
7
8
5a (H)
MTBE
5
29.5
5
94 (45)
97
89 (44)
92
48
49
49
27
50
42
50
31
103
188
5a (H)
Toluene
MTBE
5b (4-F)
5b (4-F)
5c (4-OMe)
5c (4-OMe)
5d (5-Me)
95 (47)
99
91 (48)
37
117
Toluene
MTBE
20
5
>200
102
94 (47)
>99
93 (48)
73
Toluene
MTBE
20
7
>200
103
93 (46)
94 (48)
45
5d (5-Me)
Toluene
20
>99
>200
a
b
c
d
Substitution in brackets. As determined by HPLC with isolated yields in parentheses. c = ees/(ees + eep). E = ln[(1 − c)(1 − eep)]/ln[(1 − c)(1
+ eep)].19
Table 2. Enzymatic Kinetic Resolution of Acetates 6a−d Using RML IM (1:1 w/w) in the Presence of Water Using MTBE as
Solvent at 30 °C and 250 rpm after 52 h
a
b
b
c
d
entry
substrate
H2O (equiv)
eep (%)
ees (%)
c (%)
E
1
2
3
4
5
6
6a (H)
6a (H)
6a (H)
6b (4-F)
5
10
20
5
98 (44)
99
92 (41)
53
48
35
21
50
48
49
>200
>200
>200
>200
>200
>200
>99
27
96 (44)
>99 (46)
>99 (47)
97 (48)
91 (47)
94 (48)
6c (4-OMe)
5
6d (5-Me)
5
a
b
c
d
Substitution in brackets. Determined by HPLC with isolated yields in parentheses. c = ees/(ees + eep). E = ln[(1 − c)(1 − eep)]/ln[(1 − c)(1 +
eep)].19
Tris-HCl pH 7.5 buffer, using a suitable cofactor recycling
system when required (Table 3). For the Prelog enzymes, high
to excellent selectivities were found for the formation of (S)-
alcohol 5a (entries 1−3). Notably, ADH-A showed complete
conversion and selectivity after 24 h (entry 1). On the other
hand, for the synthesis of (R)-5a, ADH-LK completely reduced
the ketone, obtaining the alcohol with very high enantiomeric
excess (entry 4), and 91% conversion was achieved in the
production of the enantiopure alcohol when using ADH-LB
(entry 5). The best result for the production of (R)-5a was
observed with evo-1.1.200 ADH (entry 6), obtaining the target
alcohol in quantitative conversion.
Efficient scale-up of the optimal ADH-catalyzed processes
was successfully achieved for both a Prelog (ADH-A) and an
anti-Prelog enzyme (evo-1.1.200 ADH), leading to the desired
(S)- and (R)-alcohol in quantitative conversion and 85% and
99% isolated yields, respectively, after a simple extraction
protocol (Table 4, entries 1 and 2). This methodology was
satisfactorily extended to the bioreduction of ketones 3b−d
(entries 3−8). Both alcohol dehydrogenases led to full
conversions with ADH-A producing the enantiopure alcohols
(S)-5b−d with very high yields (88−93%, entries 3, 5, and 7),
Table 3. Bioreduction of Nitro Ketone 3a for the Production
of Optically Active Alcohol 5a in Tris-HCl pH 7.5 Buffer
after 24 h at 30 °C
a
a
entry
enzyme
external cofactor
c (%)
ee (%)
1
2
3
4
5
6
ADH-A
ADH-CP
BY
NADH
NADH
>99
8
99 (S)
99 (S)
>99
>99
91
86 (S)
ADH-LK
ADH-LB
evo-1.1.200
NADPH
NADPH
NADH
96 (R)
>99 (R)
>99 (R)
>99
a
Conversion and enantiomeric excess values calculated by 1H NMR or
HPLC measurements of the crude reaction and absolute config-
urations appear in parentheses.
C
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Once a powerful chemoenzymatic strategy was developed for
the asymmetric synthesis of a representative number of 3-
methyl-3,4-dihydro-2H-benzo[b][1,4]oxazine derivatives, ef-
forts turned to focus on the development of an efficient and
selective preparation of the Levofloxacin precursor (4e, 7,8-
difluoro-3-methyl-3,4-dihydro-2H-benzo[b][1,4]oxazine). For
this reason, a similar route was attempted starting from
commercially available 2,3-difluoro-6-nitrophenol (1e) as
depicted in Scheme 4. O-alkylation proceeded in 85% yield
for the formation of nitro ketone 3e, which was subjected to
ADH-A-catalyzed bioreduction, leading to enantiopure (S)-5e
after 24 h in 91% isolated yield using a Tris-HCl pH 7.5 buffer.
For the preparation of its (R)-5e counterpart, the use of evo-
1.1.200 was attempted, which resulted in complete selectivity,
though the structural isomer 2-(2,3-difluoro-6-nitrophenoxy)-
propan-1-ol (11) was also found as a side product. For this
reason, the bioreduction was carried out at different pHs,
minimizing the formation of 11 at lower pH values (6−6.5),
which yielded the alcohol (R)-5e in 94% isolated yield after 24
h at 30 °C in a Tris-HCl pH 6 buffer. It must be mentioned
that the (R)-configuration is required for the formation of
Levofloxacin; thus, the use of evo-1.1.200 seems to be an
excellent tool for introducing a desired chirality.
In addition, the lipase-catalyzed hydrolysis of racemic acetate
6e was attempted, which would give direct access to the desired
(R)-alcohol 5e. First, the chemical reduction of ketone 3e was
initially performed with sodium borohydride. In this case, the
unexpected formation of a 61:39 mixture of the desired alcohol
( )-5e and the structural isomer 2-(2,3-difluoro-6-
nitrophenoxy)propan-1-ol (11) was observed. The formation
of this side-product was mostly suppressed using a mild
reducing agent, such as the ammonia borane complex,22 thus
avoiding basic reaction medium as well as a basic workup in the
reaction, recovering 5e in 78% yield after 1 h at 30 °C. Then,
the alcohol was chemically acetylated in 93% yield using acetic
anhydride to later explore its RML IM-catalyzed hydrolysis.
After 53 h, total selectivity toward the formation of the (R)-
alcohol was attained, obtaining (S)-acetate 6e in 84% ee and
the desired enantiopure (R)-alcohol 5e in 45% isolated yield.
Finally, taking alcohol (R)-5e, a four step sequence was
carried out, involving reduction of the nitro functionality,
protection of the free amine, cyclization in Mitsunobu
conditions, and N-tosyl deprotection, leading to the valuable
enantiopure Levofloxacin precursor (S)-4e in good overall yield
(36%).
Table 4. Bioreduction of Nitro Ketones 3a−d in Tris-HCl
pH 7.5 Buffer after 24 h at 30 °C
a
a
entry
enzyme
3
c (%)
ee (%)
1
2
3
4
5
6
7
8
ADH-A
3a
3a
3b
3b
3c
3c
3d
3d
>99 (85)
>99 (99)
>99 (89)
>99 (87)
>99 (88)
>99 (88)
>99 (93)
>99 (78)
99 (S)
>99 (R)
>99 (S)
>99 (R)
>99 (S)
>99 (R)
>99 (S)
>99 (R)
evo-1.1.200
ADH-A
evo-1.1.200
ADH-A
evo-1.1.200
ADH-A
evo-1.1.200
a
Conversion and enantiomeric excess values calculated by 1H NMR or
HPLC measurements of the reaction crude with absolute config-
urations and isolated yields in parentheses.
and evo-1.1.200 ADH led to the enantiopure (R)-alcohols in
78−88% yield (entries 4, 6, and 8). In this manner, the isolated
yields were improved in comparison with the lipase-catalyzed
transformations that are limited to a theoretical 50% yield due
to the inherent limitations of kinetic resolution procedures.
A four-step sequence was designed for the production of
racemic and enantiopure benzoxazines 10a−d, which occur
without any racemization of the intermediates or final products
(Scheme 3). Starting from (S)-alcohols 5a−d, the proposed
Scheme 3. Chemical Synthesis of Protected Enantiopure
Benzoxazines 10a−d
CONCLUSIONS
■
Two different classes of enzymes have efficiently served in the
development of asymmetric synthesis of both 3-methyl-3,4-
dihydro-2H-benzo[b][1,4]oxazine enantiomers. Alcohol dehy-
drogenases and lipases have been identified as good catalysts for
the synthesis of valuable optically active precursors as key
independent features of the synthetic route. The alcohol
dehydrogenase from Rhodococcus ruber has allowed the selective
bioreduction of 1-(2-nitrophenoxy)propan-2-ones with com-
plete selectivity toward quantitative conversion to the (S)-
alcohols, whereas evo-1.1.200 led to the corresponding
enantiopure (R)-enantiomers. On the other hand, it was
found that upon carrying out a lipase screen, Rhizomucor miehei
lipase seems to be a versatile hydrolase for the development of
classical kinetic resolutions through complementary acylative
and hydrolytic processes. The chemoenzymatic route has also
served to synthesize a valuable Levofloxacin precursor, which
synthesis began with platinum catalyzed hydrogenation of the
nitro functionality, forming the corresponding amino alcohols
(S)-8a−d, which were activated prior to cyclization under
Mitsunobu reaction conditions to avoid the mixture of products
as is seen using free amine or when additional catalysts were
employed, such as ZnCl2 with related amino alcohols, for
example 8e.21 This process occurred with inversion of the
absolute configuration, yielding tosylated benzoxazine deriva-
tives (R)-10a−d. As an example, the final deprotection of
activated amine 10a with the tosyl group using magnesium in
refluxing methanol allowed the recovery of (R)-3-methyl-3,4-
dihydro-2H-benzo[b][1,4]oxazine (4a) in 80% isolated yield
after 2 h.
D
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Scheme 4. Chemoenzymatic Synthetic Alternatives for the Production of the Enantiopure Alcohol (R)-5e and the
Corresponding Levofoxacin Precursor (S)-4e
2
3
Hz, CH), 121.4 (d, JFC = 23.0 Hz, CH), 139.9 (d, JFC = 6.8 Hz, C),
147.8 (C), 156.1 (d, 1JFC = 245.6 Hz, C), 204.0 (C). HRMS (ESI+, m/
z): calcd for (C9H8FNNaO4)+ [M + Na]+, 236.0330; found, 236.0335.
1-(4-Methoxy-2-nitrophenoxy)propan-2-one (3c). Yellow solid
(578 mg, 85% yield). Rf (40% EtOAc/hexane): 0.38. Mp: 80−81
°C. IR (KBr): 3055, 2987, 2348, 1739, 1718, 1534, 1499, 1430, 1360,
has been isolated in the enantiopure form after a six-step
sequence in good overall yield.
EXPERIMENTAL SECTION
■
General Procedure for the Synthesis of Ketones 3a−e. To a
solution of the corresponding nitrophenol 1a−e (3.02 mmol) in
toluene (1 mL) were added successively chloroacetone (481 μL, 6.04
mmol), potassium bromide (43 mg, 0.36 mmol), sodium hydrogen
carbonate (279 mg, 3.32 mmol), and a tributylmethylammonium
chloride solution (75% weight in water, 16 μL, 0.065 mmol). The
mixture was stirred and heated at 65 °C for 6 h, and then additional
chloroacetone (120 μL, 1.51 mmol) was added. The reaction was
further heated at 65 °C for 18 h, and after this time, water (1 mL) was
added. The pH of the mixture was adjusted to 6.5−7 at 55−60 °C by
the addition of 1N HCl (between 7 and 15 drops). The layers were
separated in a separatory funnel, and the aqueous phase was discarded.
Then, an aqueous 5% NaCl solution (2 mL) was added to the organic
phase and transferred to a round-bottom flask. The resulting mixture
was vigorously stirred at 55−60 °C for 10 min. The layers were again
separated in a separatory funnel, the organic layer was collected, dried
over Na2SO4, and filtered and the solvent was removed by distillation
under reduced pressure. The resulting crude product was washed with
toluene to ensure complete chloroacetone removal, affording the
corresponding pure ketones 3a−e (84−93%).
1224, 1052, 1035, 896, 811 cm−1. H NMR (300.13 MHz, CDCl3): δ
2.31 (s, 3H), 3.80 (s, 3H), 4.57 (s, 2H), 6.92 (d, JHH = 9.2 Hz, 1H),
7.07 (dd, JHH = 9.1 Hz, JHH = 3.1 Hz 1H), 7.39 (d, JHH = 3.1 Hz,
1H). 13C NMR (75.5 MHz, CDCl3): δ 27.0 (CH3), 56.3 (CH3), 75.1
(CH2), 110.4 (CH), 117.2 (CH), 121.0 (CH), 140.4 (C), 145.5 (C),
154.1 (C), 204.9 (C). HRMS (ESI+, m/z): calcd for (C10H11NNaO5)+
[M + Na]+, 248.0529; found, 248.0540.
1
3
3
4
4
1-(5-Methyl-2-nitrophenoxy)propan-2-one (3d). Light yellow
solid (531 mg, 84% yield). Rf (40% EtOAc/hexane): 0.41. Mp: 98−
99 °C. IR (KBr): 3056, 1724, 1723, 1608, 1521, 1419, 1348, 1179,
1097, 1051, 896, 820 cm−1. H NMR (300.13 MHz, CDCl3): δ 2.37
1
3
(s, 3H), 2.41 (s, 3H), 4.60 (s, 2H), 6.73 (s, 1H), 6.90 (d, JHH = 8.3
Hz, 1H), 7.85 (d, 3JHH = 8.3 Hz, 1H). 13C NMR (75.5 MHz, CDCl3):
δ 21.8 (CH3), 26.8 (CH3), 73.7 (CH2), 115.2 (CH), 122.2 (CH),
126.1 (CH), 137.4 (C), 146.3 (C), 151.2 (C), 204.4 (C). HRMS
(ESI+, m/z): calcd for (C10H11NNaO4)+ [M + Na]+, 232.0580; found,
232.0581.
1-(2,3-Difluoro-6-nitrophenoxy)propan-2-one (3e). Light yellow
solid (593 mg, 85% yield). Rf (40% EtOAc/hexane): 0.69. Mp: 43−45
°C. IR (KBr): 3059, 2987, 2924, 1741, 1627, 1597, 1541, 1496, 1358,
1-(2-Nitrophenoxy)propan-2-one (3a). White solid (548 mg, 93%
yield). Rf (40% EtOAc/hexane): 0.31. Mp: 68−70 °C. IR (KBr): 3055,
1217, 1070, 813 cm−1. H NMR (300.13 MHz, CDCl3): δ 2.32 (s,
1
1
2987, 2306, 1739, 1724, 1608, 1528, 1357, 1166, 1052, 860 cm−1. H
3H), 4.80 (d, 5JFH = 1.3 Hz, 2H), 7.05 (ddd, 3JHH = 9.3 Hz, 3JFH = 8.8
Hz, 4JFH = 7.2 Hz, 1H), 7.72 (ddd, 3JHH = 9.4 Hz, 4JFH = 5.2, 5JFH = 2.4
NMR (300.13 MHz, CDCl3): δ 2.35 (s, 3H), 4.62 (s, 2H), 6.94 (dd,
3JHH = 8.4 Hz, 4JHH = 0.9 Hz, 1H), 7.03−7.10 (m, 1H), 7.53 (ddd, 3JHH
= 8.6, 7.5 Hz, 4JHH = 1.7 Hz, 1H), 7.87 (dd, 3JHH = 8.1 Hz, 4JHH = 1.6
Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 27.0 (CH3), 73.8 (CH2),
114.7 (CH), 121.7 (CH), 126.1 (CH), 134.4 (CH), 140.1 (C), 151.1
(C), 204.4 (C). HRMS (ESI+, m/z): calcd for (C9H9NNaO4)+ [M +
Na]+, 218.0424; found, 218.0402.
Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 26.4 (CH3), 78.1 (d, 4JFC
=
5.3 Hz, CH2), 111.7 (d, 2JFC = 19.3 Hz, CH), 120.8 (dd, 3JFC = 9.0 Hz,
4JFC = 4.0 Hz, CH), 140.0 (d, 3JFC = 3.7 Hz, C), 142.6 (dd, 2JFC = 10.9
Hz, 3JFC = 2.9 Hz, C), 144.7 (dd, 1JFC = 252.9 Hz, 2JFC = 14.7 Hz, C),
1
2
154.1 (dd, JFC = 259.6 Hz, JFC = 11.6 Hz, C), 203.0 (C). HRMS
(ESI+, m/z): calcd for (C9H7F2NNaO4)+ [M + Na]+, 254.0235; found,
254.0249.
Synthesis of Racemic 3-Methyl-3,4-dihydro-2H-benzo[b]-
[1,4]oxazine (4a). Pd/C (10% weight loading, 100 mg) was added
to a solution of ketone 3a (2.05 mmol, 400 mg) in methanol (0.02 M,
102.5 mL) placed in the reaction vessel of a Parr hydrogenator. The air
was evacuated and hydrogen was introduced into the system until
reaching 4 atm of pressure. The suspension was stirred for 6 h at room
1-(4-Fluoro-2-nitrophenoxy)propan-2-one (3b). Light yellow solid
(592 mg, 92% yield). Rf (40% EtOAc/hexane): 0.21. Mp: 82−83 °C.
IR (KBr): 3055, 2987, 2343, 1740, 1723, 1538, 1498, 1420, 1360,
1
1204, 1049, 815 cm−1. H NMR (400.13 MHz, CDCl3): δ 2.33 (s,
3
4
3H), 4.62 (s, 2H), 6.96 (dd, JHH4 = 9.2 Hz, JFH = 4.2 Hz, 1H), 7.28
3
3
3
(ddd, JHH = 9.3 Hz, JFH = 7.4, JHH = 3.1 Hz, 1H), 7.63 (dd, JFH
=
7.7 Hz, JHH = 3.1 Hz, 1H). 13C NMR (100.6 MHz, CDCl3): δ 26.9
(CH3), 74.6 (CH2), 113.4 (d, 2JFC = 27.5 Hz, CH), 116.7 (d, 3JFC = 7.8
4
E
DOI: 10.1021/acs.joc.5b00056
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
temperature, and afterward, the solvent was evaporated under reduced
pressure. The residue was dissolved in Et2O (20 mL), and the metal
catalyst was filtered through a diatomaceous earth plug. The reaction
crude was obtained after solvent evaporation and purified by column
chromatography on silica gel (20% Et2O/hexane), affording racemic
benzoxazine 4a (245 mg, 80% yield). The spectroscopic data are in
agreement with those previously reported in the literature using a
different procedure.23
reaction was stopped by careful addition at 0 °C of an aqueous 2 M
HCl solution until an acidic pH (pH < 3) was achieved. Then, the
mixture was extracted with CH2Cl2 (3 × 10 mL). The organic layers
were combined, dried, and filtered, and the solvent was removed by
distillation under reduced pressure. The crude was purified by column
chromatography on silica gel (10% EtOAc/hexane), affording nitro
alcohol 5e (275 mg, 78% yield). Yellow oil. Rf (40% EtOAc/hexane):
0.51. IR (NaCl): 3569, 3439, 3054, 2987, 2360, 2307, 1653, 1539,
1
1355, 1163, 1022, 852, 665 cm−1. H NMR (300.13 MHz, CDCl3): δ
General Procedure for the Synthesis of Racemic Nitro
Alcohols 5a−d. Sodium borohydride (19 mg, 0.50 mmol) was added
to a solution of the corresponding ketones 3a−d (1.00 mmol) in dry
MeOH (3.8 mL) at 0 °C. The solution was stirred at room
temperature for 45 min, quenching the reaction by the addition of
water (10 mL). MeOH was removed by distillation under reduced
pressure, and the aqueous residue was extracted with CH2Cl2 (3 × 10
mL). The organic layers were combined, dried, and the solvent
removed by distillation under reduced pressure, affording the
corresponding nitro alcohols 5a−d (87−94%).
3
3
1.25 (d, JHH = 6.4 Hz, 3H), 2.83 (d, JHH = 3.7 Hz, 1H), 4.10 (ddd,
2JHH = 9.5 Hz, 3JHH = 7.7 Hz, 5JFH = 1.9 Hz, 1H), 4.15−4.28 (m, 1H),
4.38 (dt, 2JHH = 9.4 Hz, 3JHH = 2.4 Hz, 5JFH = 2.4 Hz, 1H), 7.01 (ddd,
3JHH = 9.4 Hz, 3JFH = 8.8 Hz, 4JFH = 7.2 Hz, 1H), 7.72 (ddd, 3JHH = 9.4
4
5
Hz, JFH = 5.3, JFH = 2.4 Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ
18.2 (CH3), 66.5 (CH), 81.2 (d, 4JFC = 5.7 Hz, CH2), 111.1 (d, 2JFC
=
19.3 Hz, CH), 120.9 (dd, 3JFC = 9.2 Hz, 4JFC = 4.0 Hz, CH), 139.5 (d,
3JFC = 1.9 Hz, C), 143.8 (dd, 2JFC = 10.9 Hz, 3JFC = 2.8 Hz, C), 144.7
1
2
1
(dd, JFC = 253.0 Hz, JFC = 14.3 Hz, C), 154.4 (dd, JFC = 259.6 Hz,
1-(2-Nitrophenoxy)propan-2-ol (5a). Yellow oil (172 mg, 87%
yield). Rf (40% EtOAc/hexane): 0.19. IR (NaCl): 3586, 3440, 3055,
2JFC = 11.6 Hz, C). HRMS (ESI+, m/z): calcd for (C9H9F2NNaO4)+
1
[M + Na]+, 256.0392; found, 256.0390.
2985, 2937, 2307, 1609, 1526, 1354, 1166, 1020, 860 cm−1. H NMR
(300.13 MHz, CDCl3): δ 1.25 (d, 3JHH = 6.4 Hz, 3H), 3.03 (br s, 1H),
3.88 (dd, 2JHH = 8.9 Hz, 3JHH = 7.5 Hz, 1H), 4.07 (dd, 2JHH = 9.0 Hz,
3JHH = 3.1 Hz, 1H), 4.12−4.24 (m, 1H), 6.98−7.15 (m, 2H), 7.46−
7.61 (m, 1H), 7.80 (dd, 3JHH = 8.0 Hz, 4JHH = 1.6 Hz, 1H). 13C NMR
(75.5 MHz, CDCl3): δ 18.5 (CH3), 65.8 (CH), 75.0 (CH2), 115.0
(CH), 120.8 (CH), 125.8 (CH), 134.5 (CH), 139.6 (C), 152.2 (C).
HRMS (ESI+, m/z): calcd for (C9H11NNaO4)+ [M + Na]+, 220.0580;
found, 220.0609.
General Procedure for the Synthesis of Racemic Acetates
6a−e. 4-Dimethylaminopyridine (12 mg, 0.1 mmol), triethylamine
(198 μL, 1.42 mmol), and acetic anhydride (90 μL, 0.95 mmol) were
added successively to a solution of the corresponding alcohols 5a−e
(0.47 mmol) in dry CH2Cl2 (3.2 mL). The reaction was stirred at
room temperature for 30 min, and after this time, the solvent was
removed by distillation under reduced pressure. The crude product
was purified by column chromatography on silica gel (EtOAc/hexane
mixtures), affording the corresponding acetates 6a−e (88−94%).
1-(2-Nitrophenoxy)propan-2-yl acetate (6a). Intense yellow oil
(106 mg, 94% yield). Rf (40% EtOAc/hexane): 0.41. IR (NaCl): 3055,
2986, 2940, 2343, 1734, 1609, 1528, 1355, 1239, 1091, 1035, 992, 860
cm−1. 1H NMR (300.13 MHz, CDCl3): δ 1.38 (d, 3JHH = 6.5 Hz, 3H),
1-(4-Fluoro-2-nitrophenoxy)propan-2-ol (5b). Yellow solid (202
mg, 94% yield). Rf (40% EtOAc/hexane): 0.26. Mp: 74−76 °C. IR
(KBr): 3586, 3441, 3055, 2984, 2935, 2340, 1534, 1499, 1354, 1203,
1
1141, 1022, 815, 786 cm−1. H NMR (300.13 MHz, CDCl3): δ 1.28
(d, 3JHH = 6.4 Hz, 3H), 2.98 (br s, 1H), 3.90 (dd, 2JHH = 9.0 Hz, 3JHH
=
3
2.06 (s, 3H), 4.12 (d, JHH = 5.1 Hz, 2H), 5.14−5.53 (m, 1H), 6.96−
2
3
7.4 Hz, 1H), 4.10 (dd, JHH = 9.0 Hz, JHH = 3.2 Hz, 1H), 4.15−4.26
7.17 (m, 2H), 7.51 (ddd, 3JHH = 9.0, 7.5 Hz, 4JHH = 1.7 Hz, 1H), 7.82
(m, 1H), 7.08 (dd, 3JHH = 9.2 Hz, 4JFH = 4.3 Hz, 1H), 7.28 (ddd, 3JHH
3
4
(dd, JHH = 8.1 Hz, JHH = 1.6 Hz, 1H). 13C NMR (75.5 MHz,
CDCl3): δ 16.6 (CH3), 21.2 (CH3), 68.3 (CH), 71.4 (CH2), 114.9
(CH), 121.0 (CH), 125.8 (CH), 134.2 (CH), 140.2 (C), 151.9 (C),
170.7 (C). HRMS (ESI+, m/z): calcd for (C11H13NNaO5)+ [M +
Na]+, 262.0686; found, 262.0708.
3
4
3
= 9.4 Hz, JFH = 7.3 Hz, JHH = 3.1 Hz, 1H), 7.60 (dd, JFH = 7.8 Hz,
4JHH = 3.1 Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 18.5 (CH3),
65.9 (CH), 75.8 (CH2), 113.0 (d, 2JFH = 27.5 Hz, CH), 116.6 (d, 3JFH
= 7.7 Hz, CH), 121.5 (d, 2JFH = 22.8 Hz, CH), 139.3 (d, 3JFC = 8.3 Hz,
C), 148.9 (d, 4JFC = 3.0 Hz, C), 155.5 (d, 1JFC = 244.3 Hz, C). HRMS
(ESI+, m/z): calcd for (C9H10FNNaO4)+ [M + Na]+, 238.0486; found,
238.0500.
1-(4-Fluoro-2-nitrophenoxy)propan-2-yl acetate (6b). White solid
(106 mg, 88% yield). Rf (40% EtOAc/hexane): 0.67. Mp: 63−64 °C.
IR (KBr): 3055, 2987, 2307, 1738, 1537, 1499, 1373, 1357, 1241,
1-(4-Methoxy-2-nitrophenoxy)propan-2-ol (5c). Light orange
solid (211 mg, 93% yield). Rf (40% EtOAc/hexane): 0.18. Mp: 75−
77 °C. IR (KBr): 3576, 3431, 3058, 2964, 2922, 2840, 2343, 1527,
1496, 1346, 1216, 1040, 817 cm−1. 1H NMR (300.13 MHz, CDCl3): δ
1.25 (d, 3JHH = 6.4 Hz, 3H), 2.76 (br s, 1H), 3.80 (s, 3H), 3.81−3.91
(m, 1H), 4.07 (dd, 2JHH = 9.1 Hz, 3JHH = 3.1 Hz, 1H), 4.12−4.27 (m,
1
1203, 1142, 1083, 1034, 943, 814, 788 cm−1. H NMR (300.13 MHz,
3
3
CDCl3): δ 1.37 (d, JHH = 6.5 Hz, 3H), 2.05 (s, 3H), 4.11 (d, JHH
=
3
4
5.0 Hz, 2H), 5.18−5.30 (m, 1H), 7.06 (dd, JHH = 9.2 Hz, JFH = 4.3
Hz, 1H), 7.26 (ddd, 3JHH = 9.3 Hz, 3JFH = 7.4, 4JHH = 3.1 Hz, 1H), 7.57
(dd, JFH = 7.7 Hz, JHH = 3.1 Hz, 1H). 13C NMR (75.5 MHz,
3
4
CDCl3): δ 16.6 (CH3), 21.2 (CH3), 68.2 (CH), 72.3 (CH2), 113.0 (d,
3
1H), 7.01 (d, 3JHH = 9.1 Hz, 1H), 7.09 (dd, JHH = 9.1 Hz, 4JHH = 3.0
3
2
2JFH = 27.4 Hz, CH), 116.7 (d, JFH = 7.9 Hz, CH), 121.1 (d, JFH
=
Hz, 1H), 7.37 (d, 4JHH = 3.0 Hz, 1H). 13C NMR (75.5 MHz, CDCl3):
δ 18.5 (CH3), 56.1 (CH3), 66.0 (CH), 76.1 (CH2), 110.0 (CH), 117.1
(CH), 121.3 (CH), 139.8 (C), 146.6 (C), 153.4 (C). HRMS (ESI+,
m/z): calcd for (C10H13NNaO5)+ [M + Na]+, 250.0686; found,
250.0711.
22.9 Hz, CH), 139.3 (d, 3JFC = 9.6 Hz, C), 148.5 (d, 4JFC = 2.6 Hz, C),
155.7 (d, JFC = 244.6 Hz, C), 170.6 (C). HRMS (ESI+, m/z): calcd
1
for (C11H12FNNaO5)+ [M + Na]+, 280.0592; found, 280.0613.
1-(4-Methoxy-2-nitrophenoxy)propan-2-yl acetate (6c). Yellow
solid (116 mg, 92% yield). Rf (40% EtOAc/hexane): 0.46. Mp: 55−56
°C. IR (KBr): 3055, 2986, 2307, 1734, 1533, 1499, 1373, 1354, 1243,
1-(5-Methyl-2-nitrophenoxy)propan-2-ol (5d). Light orange solid
(186 mg, 88% yield). Rf (40% EtOAc/hexane): 0.18. Mp: 53−54 °C.
IR (KBr): 3583, 3435, 3055, 2985, 2935, 1609, 1592, 1517, 1347,
1041, 812 cm−1. H NMR (300.13 MHz, CDCl3): δ 1.35 (d, JHH
=
1
3
3
1
1182, 1093, 1031, 841 cm−1. H NMR (300.13 MHz, CDCl3): δ 1.26
6.5 Hz, 3H), 2.06 (s, 3H), 3.80 (s, 3H), 4.07 (d, JHH = 4.8 Hz, 2H),
5.16−5.28 (m, 1H), 7.01 (d, 3JHH = 9.1 Hz, 1H), 7.07 (dd, 3JHH = 9.1
Hz, 4JHH = 3.0 Hz, 1H), 7.34 (d, 4JHH = 2.9 Hz, 1H). 13C NMR (75.5
MHz, CDCl3): δ 16.6 (CH3), 21.2 (CH3), 56.2 (CH3), 68.5 (CH),
72.6 (CH2), 109.9 (CH), 117.4 (CH), 120.8 (CH), 140.5 (C), 146.2
(C), 153.6 (C), 170.7 (C). HRMS (ESI+, m/z): calcd for
(C12H15NNaO6)+ [M + Na]+, 292.0792; found, 292.0796.
(d, 3JHH = 6.4 Hz, 3H), 2.37 (s, 3H), 3.02 (br s, 1H), 3.86 (dd, 2JHH
=
3
2
3
9.0 Hz, JHH = 7.6 Hz, 1H), 4.08 (dd, JHH = 9.0 Hz, JHH = 3.2 Hz,
3
1H), 4.12−4.25 (m, 1H), 6.79 (d, JHH = 8.3 Hz, 1H), 6.84 (s, 1H),
3
7.76 (d, JHH = 8.3 Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 18.5
(CH3), 22.0 (CH3), 66.0 (CH), 75.2 (CH2), 115.7 (CH), 121.7 (CH),
126.2 (CH), 137.4 (C), 146.4 (C), 152.6 (C). HRMS (ESI+, m/z):
calcd for (C10H13NNaO4)+ [M + Na]+, 234.0737; found, 234.0727.
Synthesis of Racemic 1-(2,3-difluoro-6-nitrophenoxy)-
propan-2-ol (5e). Ammonia borane complex (24 mg, 0.76 mmol)
was added to a solution of ketone 3e (1.51 mmol) in dry THF (4.6
mL), and the mixture was stirred at 30 °C for 1 h. After this time, the
1-(5-Methyl-2-nitrophenoxy)propan-2-yl acetate (6d). White
solid (107 mg, 90% yield). Rf (40% EtOAc/hexane): 0.63. Mp: 76−
77 °C. IR (KBr): 3054, 2987, 2306, 1734, 1609, 1521, 1423, 1093,
1040 cm−1. 1H NMR (300.13 MHz, CDCl3): δ 1.39 (d, 3JHH = 6.5 Hz,
3
3H), 2.06 (s, 3H), 2.40 (s, 3H), 4.11 (d, JHH = 5.1 Hz, 2H), 5.22−
F
DOI: 10.1021/acs.joc.5b00056
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The Journal of Organic Chemistry
Article
5.31 (m, 1H), 6.83 (dd, JHH = 8.3 Hz, JHH = 0.7 Hz, 1H), 6.86 (s,
198.1125; found, 198.1135. [α]2D0 + 23.2 (c 0.5, EtOH) [for (S)-8c in
>99% ee].
3
4
1H), 7.77 (d, JHH = 8.2 Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ
3
1-(2-Amino-5-methylphenoxy)propan-2-ol (8d). Light pink solid
(421 mg, 93% yield). Rf (40% EtOAc/hexane): 0.15. Mp: 76−78 °C.
IR (KBr): 3402, 3054, 2985, 2929, 2523, 2307, 1592, 1520, 1420,
1152, 1132, 1041, 812 cm−1. 1H NMR (400.13 MHz, CD3OD): δ 1.26
16.7 (CH3), 21.2 (CH3), 22.0 (CH3), 68.4 (CH), 71.4 (CH2), 115.5
(CH), 121.7 (CH), 126.0 (CH), 137.9 (C), 145.9 (C), 152.2 (C),
170.7 (C). HRMS (ESI+, m/z): calcd for (C12H15NNaO5)+ [M +
Na]+, 276.0842; found, 276.0856.
3
2
3
(d, JHH = 6.5 Hz, 3H), 2.21 (s, 3H), 3.78 (dd, JHH = 9.6 Hz, JHH
=
1-(2,3-Difluoro-6-nitrophenoxy)propan-2-yl acetate (6e). Intense
yellow oil (120 mg, 93% yield). Rf (40% EtOAc/hexane): 0.68. IR
(NaCl): 3447, 3059, 2988, 2942, 2886, 2343, 1739, 1627, 1541, 1495,
2
3
6.9 Hz, 1H), 3.88 (dd, JHH = 9.7 Hz, JHH = 3.7 Hz, 1H), 4.11
3
3
(dquint, JHH = 6.5, 3.8 Hz, 1H), 4.86 (br s, 3H), 6.56 (d, JHH = 7.8
Hz, 1H), 6.64 (s, 1H), 6.66 (d, JHH = 7.8 Hz, 1H). 13C NMR (100.6
3
1
1357, 1237, 1020, 813 cm−1. H NMR (300.13 MHz, CDCl3): δ 1.33
(d, 3JHH = 6.6 Hz, 3H), 2.03 (s, 3H), 4.23 (ddd, 2JHH = 10.3 Hz, 3JHH
=
MHz, CD3OD): δ 19.5 (CH3), 21.1 (CH3), 67.1 (CH), 74.7 (CH2),
113.8 (CH), 117.2 (CH), 122.5 (CH), 129.6 (C), 134.8 (C), 148.4
(C). HRMS (ESI+, m/z): calcd for (C10H16NO2)+ [M + H]+,
182.1176; found, 182.1173. [α]2D0 + 12.5 (c 0.6, EtOH) [for (S)-8d in
>99% ee].
1-(6-Amino-2,3-difluorophenoxy)propan-2-ol (8e). Yellow solid
(330 mg, 65% yield). Rf (40% EtOAc/hexane): 0.25. Mp: 56−58 °C.
IR (KBr): 3398, 3054, 2986, 1653, 1559, 1507, 1490, 1419, 1165,
5.9 Hz, 5JFH = 0.9 Hz, 1H), 4.33 (ddd, 2JHH = 10.3 Hz, 3JHH = 3.4 Hz,
5JFH = 1.0 Hz, 1H), 5.16−5.28 (m, 1H), 7.01 (td, 3JHH = 9.1 Hz, 3JFH
=
4
3
4
9.1 Hz, JFH = 7.2 Hz, 1H), 7.66 (ddd, JHH = 9.4 Hz, JFH = 5.2 Hz,
5JFH = 2.4 Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 16.1 (CH3), 21.0
(CH3), 68.8 (CH), 77.2 (d, 4JFC = 5.2 Hz, CH2), 111.4 (d, 2JFC = 19.4
Hz, CH), 120.4 (dd, 3JFC = 9.1 Hz, 4JFC = 4.1 Hz, CH), 140.3 (d, 3JFC
2
3
= 2.2 Hz, C), 143.4 (dd, JFC = 9.7 Hz, JFC = 4.4 Hz, C), 144.9 (dd,
1
3
1047, 896 cm−1. H NMR (400.13 MHz, CDCl3): δ 1.19 (d, JHH
=
1JFC = 253.6 Hz, 2JFC = 14.2 Hz, C), 154.0 (dd, 1JFC = 259.0 Hz, 2JFC
=
6.4 Hz, 3H), 3.61 (br s, 3H), 3.83 (dd, 2JHH = 10.8 Hz, 3JHH = 8.8 Hz,
11.3 Hz, C), 170.5 (C). HRMS (ESI+, m/z): calcd for
(C11H11F2NNaO5)+ [M + Na]+, 298.0497; found, 298.0528.
General Procedure for the Synthesis of Racemic and
Optically Active Amino Alcohols 8a−e. A hydrogen atmosphere
was generated using a hydrogen balloon connected to a round-bottom
flask, which contained a suspension of the corresponding nitro
alcohols 5a−e (2.50 mmol) and PtO2 (150 mg, 0.66 mmol) in dry
MeOH (14 mL). The resulting suspension was stirred at room
temperature overnight, and then the reaction was stopped by filtering
the mixture through a diatomaceous earth plug. The solvent was
removed by distillation under reduced pressure, and the crude was
purified by column chromatography on silica gel (EtOAc/hexane
mixtures), affording the corresponding amino alcohols 8a−e (65−
98%).
1-(2-Aminophenoxy)propan-2-ol (8a). White solid (397 mg, 95%
yield). Rf (40% EtOAc/hexane): 0.16. Mp: 66−67 °C. IR (KBr): 3391,
3054, 2986, 2924, 2340, 1653, 1558, 1506, 1219, 1154 cm−1. 1H NMR
(300.13 MHz, CD3OD): δ 1.30 (d, 3JHH = 6.4 Hz, 3H), 3.83 (dd, 2JHH
= 9.7 Hz, 3JHH = 6.9 Hz, 1H), 3.94 (dd, 2JHH = 9.7 Hz, 3JHH = 3.7 Hz,
1H), 4.16 (dquint, 3JHH = 6.5, 3.7 Hz, 1H), 4.91 (br s, 3H), 6.66−6.75
(m, 1H), 6.75−6.90 (m, 3H). 13C NMR (75.5 MHz, CD3OD): δ 19.5
(CH3), 67.1 (CH), 74.7 (CH2), 112.9 (CH), 117.0 (CH), 119.8
(CH), 122.3 (CH), 137.8 (C), 148.4 (C). HRMS (ESI+, m/z): calcd
for (C9H14NO2)+ [M + H]+, 168.1019; found, 168.1020. [α]2D0 + 37.0
(c 0.3, EtOH) [for (S)-8a in >99% ee].
1H), 4.03−4.16 (m, 2H), 6.39 (ddd, JHH = 9.0 Hz, 4JFH4 = 4.8, JFH
=
3
5
3
3
2.3 Hz, 1H), 6.70 (ddd, JFH = 9.9 Hz, JHH = 9.0 Hz, JFH = 8.1 Hz,
1H). 13C NMR (100.6 MHz, CDCl3): δ 18.5 (CH3), 66.6 (CH), 79.3
(d, 4JFC = 3.3 Hz, CH2), 109.6 (dd, 3JFC = 6.9 Hz, 4JFC = 3.2 Hz, CH),
2
2
3
111.4 (d, JFC = 17.8 Hz, CH), 135.5 (dd, JFC = 10.4 Hz, JFC = 1.3
Hz, C), 136.8 (dd, JFC = 2.7 Hz, 4JFC = 1.2 Hz, C), 144.7 (dd, JFC
=
3
1
2
1
2
239.0 Hz, JFC = 11.3 Hz, C), 144.9 (dd, JFC = 247.1 Hz, JFC = 14.6
Hz, C). HRMS (ESI+, m/z): calcd for (C9H12F2NO2)+ [M + H]+,
204.0831; found, 204.0857. [α]2D0 − 22.4 (c 0.7, EtOH) [for (R)-8e in
>99% ee].
General Procedure for the Synthesis of Racemic and
Optically Active Sulfonamides 9a−e. Pyridine (56 μL, 0.69
mmol) and p-toluensulfonyl chloride (134 mg, 0.70 mmol) were
added to a solution of the corresponding amino alcohols 8a−e (0.54
mmol) in dry CH2Cl2 (13.5 mL). The solution was stirred at room
temperature for 12 h until complete disappearance of the starting
material was observed by TLC analysis. Almost all the solvent was
removed by distillation under reduced pressure, and the resulting
residue was dissolved in EtOAc (20 mL) and washed with an aqueous
saturated NH4Cl solution (2 × 20 mL), an aqueous HCl 1 M solution
(2 × 20 mL), and finally an aqueous saturated NaCl solution (2 × 20
mL). The organic phase was dried over Na2SO4, filtered, and
concentrated by distillation under reduced pressure. The crude
product was purified by column chromatography on silica gel
(EtOAc/hexane mixtures), affording the corresponding sulfonamides
9a−e (71−80%).
1-(2-Amino-4-fluorophenoxy)propan-2-ol (8b). Brown solid (444
mg, 96% yield). Rf (40% EtOAc/hexane): 0.16. Mp: 119−121 °C. IR
(KBr): 3391, 3054, 2985, 2933, 2341, 1623, 1513, 1218, 1160, 1035,
970, 842 cm−1. 1H NMR (400.13 MHz, CDCl3): δ 1.25 (d, 3JHH = 6.4
Hz, 3H), 3.77 (br s, 3H), 3.78 (dd, 2JHH = 9.6 Hz, 3JHH = 7.8 Hz, 1H),
3.90 (dd, 2JHH = 9.7 Hz, 3JHH = 3.0 Hz, 1H), 4.06−4.25 (m, 1H), 6.36
(ddd, 3JHH = 8.6 Hz, 3JFH = 8.6 Hz, 4JHH = 2.9 Hz, 1H), 6.43 (dd, 3JFH
= 9.8 Hz, 4JHH = 2.9 Hz, 1H), 6.68 (dd, 3JHH = 8.8 Hz, 4JFH = 5.1 Hz,
1H). 13C NMR (100.6 MHz, CDCl3): δ 19.0 (CH3), 66.4 (CH), 75.1
(CH2), 102.6 (d, 2JFH = 26.7 Hz, CH), 103.9 (d, 2JFH = 23.0 Hz, CH),
N-(2-(2-Hydroxypropoxy)phenyl)-4-methylbenzenesulfonamide
(9a). White solid (135 mg, 78% yield). Rf (40% EtOAc/hexane): 0.25.
Mp: 166−167 °C. IR (KBr): 3545, 3297, 3054, 2985, 2920, 2343,
1
1596, 1501, 1404, 1340, 1156, 1114, 1088, 934, 819 cm−1. H NMR
3
(300.13 MHz, (CD3)2CO): δ 1.13 (d, JHH = 6.4 Hz, 3H), 2.34 (s,
3H), 3.61 (dd, 2JHH = 9.4 Hz, 3JHH = 7.6 Hz 1H), 3.71 (dd, 2JHH = 9.4
3
Hz, JHH = 3.2 Hz, 1H), 3.90−4.03 (m, 1H), 4.50 (br s, 1H), 6.84−
6.97 (m, 2H), 7.05 (td, 3JHH = 7.8 Hz, 4JHH = 1.7 Hz, 1H), 7.23−7.34
(m, 2H), 7.51 (dd, 3JHH = 7.8 Hz, 4JHH = 1.6 Hz, 1H), 7.58−7.69 (m,
2H), 8.46 (br s, 1H). 13C NMR (75.5 MHz, (CD3)2CO): δ 19.2
(CH3), 21.3 (CH3), 66.0 (CH), 75.4 (CH2), 113.4 (CH), 121.7 (CH),
123.7 (CH), 126.6 (CH), 127.4 (C), 127.9 (2xCH), 130.1 (2xCH),
138.2 (C), 144.2 (C), 151.0 (C). HRMS (ESI+, m/z): calcd for
3
3
113.6 (d, JFH = 10.0 Hz, CH), 138.0 (d, JFC = 11.0 Hz, C), 142.5
(C), 158.2 (d, JFC = 237.3 Hz, C). HRMS (ESI+, m/z): calcd for
1
(C9H13FNO2)+ [M + H]+, 186.0925; found, 186.0941. [α]2D0 + 21.6 (c
0.7, EtOH) [for (S)-8b in >99% ee].
(C16H19NNaO4S)+ [M + Na]+, 344.0927; found, 344.0941. [α]2D0
15.1 (c 1.0, CHCl3) [for (S)-9a in >99% ee].
+
1-(2-Amino-4-methoxyphenoxy)propan-2-ol (8c). Light yellow
solid (483 mg, 98% yield). Rf (40% EtOAc/hexane): 0.10. Mp: 75−76
°C. IR (KBr): 3583, 3391, 3054, 2986, 2340, 1623, 1516, 1419, 1220,
N-(5-Fluoro-2-(2-hydroxypropoxy)phenyl)-4-methylbenzenesul-
fonamide (9b). Colorless viscous oil (147 mg, 80% yield). Rf (60%
Et2O/hexane): 0.14. IR (NaCl): 3342, 3054, 2986, 2934, 2362, 1616,
1506, 1419, 1339, 1170, 1153, 1091, 1034, 812 cm−1. 1H NMR
1
3
1168, 962 cm−1. H NMR (400.13 MHz, CDCl3): δ 1.22 (d, JHH
=
2
6.5 Hz, 3H), 3.68 (br s, 3H), 3.70 (s, 3H), 3.75 (dd, JHH = 9.6 Hz,
3JHH = 7.8 Hz, 1H), 3.87 (dd, JHH = 9.7 Hz, JHH = 3.1 Hz, 1H),
4.07−4.22 (m, 1H), 6.22 (dd, 3JHH = 8.7 Hz, 4JHH = 2.9 Hz, 1H), 6.30
(d, 4JHH = 2.9 Hz, 1H), 6.69 (d, 3JHH = 8.7 Hz, 1H). 13C NMR (100.6
MHz, CDCl3): δ 18.9 (CH3), 55.5 (CH3), 66.3 (CH), 74.4 (CH2),
102.5 (CH), 102.6 (CH), 114.2 (CH), 137.8 (C), 140.9 (C), 155.0
(C). HRMS (ESI+, m/z): calcd for (C10H16NO3)+ [M + H]+,
2
3
3
(300.13 MHz, (CD3)2CO): δ 1.15 (d, JHH = 6.5 Hz, 3H), 2.34 (s,
3H), 3.61 (dd, 2JHH = 9.5 Hz, 3JHH = 7.6 Hz, 1H), 3.77 (dd, 2JHH = 9.5
3
Hz, JHH = 3.0 Hz, 1H), 3.94−4.07 (m, 1H), 4.69 (br s, 1H), 6.78
(ddd, 3JHH = 9.0 Hz, 3JFH = 8.3 Hz, 4JHH = 3.1 Hz, 1H), 6.91 (dd, 3JHH
= 9.0 Hz, 4JFH = 5.1 Hz, 1H), 7.19−7.39 (m, 3H), 7.66−7.74 (m, 2H),
G
DOI: 10.1021/acs.joc.5b00056
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
8.74 (br s, 1H). 13C NMR (75.5 MHz, (CD3)2CO): δ 19.1 (CH3),
(dd, 2JHH = 11.1 Hz, 3JHH = 1.3 Hz, 1H), 4.27−4.63 (m, 1H), 6.80 (d,
3JHH = 8.0 Hz, 1H), 6.95 (t, 3JHH = 7.7 Hz, 1H), 7.08 (t, 3JHH = 7.6 Hz,
2
21.3 (CH3), 66.0 (CH), 76.3 (CH2), 109.5 (d, JFC = 27.8 Hz, CH),
2
3
1H), 7.21 (dd, JHH = 7.9 Hz, 4JHH = 0.5 Hz, 2H), 7.46 (d, JHH = 7.9
Hz, 2H), 7.89 (d, 3JHH = 8.0 Hz, 1H). 13C NMR (75.5 MHz, CDCl3):
δ 17.1 (CH3), 21.7 (CH3), 48.7 (CH), 66.1 (CH2), 117.2 (CH), 121.3
(CH), 122.0 (C), 126.1 (CH), 126.3 (CH), 127.3 (2xCH), 130.0
(2xCH), 135.5 (C), 144.3 (C), 146.1 (C). HRMS (ESI+, m/z): calcd
3
3
111.6 (d, JFC = 23.0 Hz, CH), 114.9 (d, JFC = 9.4 Hz, CH), 127.9
(2xCH), 129.0 (d, 3JFC = 11.0 Hz, C), 130.3 (2xCH), 137.8 (C), 144.6
(C), 146.8 (d, 4JFC = 2.2 Hz, C), 157.6 (d, 1JFC = 237.0 Hz, C). HRMS
(ESI+, m/z): calcd for (C16H18FNNaO4S)+ [M + Na]+, 362.0833;
found, 362.0847. [α]2D0 + 16.0 (c 1.0, CHCl3) [for (S)-9b in >99% ee].
N-(2-(2-Hydroxypropoxy)-5-methoxyphenyl)-4-methylbenzene-
sulfonamide (9c). White solid (135 mg, 71% yield). Rf (40% EtOAc/
hexane): 0.18. Mp: 132−133 °C. IR (KBr): 3339, 3054, 2935, 2356,
1504, 1420, 1340, 1159, 1088, 956, 816 cm−1. 1H NMR (300.13 MHz,
for (C16H17NNaO3S)+ [M + Na]+, 326.0821; found, 326.0814. [α]2D0
164.8 (c 1.0, EtOH) [for (R)-10a in >99% ee].
+
6-Fluoro-3-methyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]oxazine
(10b). White solid (85 mg, 94% yield). Rf (60% EtOAc/hexane): 0.64.
Mp: 81−83 °C. IR (KBr): 3054, 2987, 2929, 2341, 1616, 1597, 1499,
3
(CD3)2CO): δ 1.14 (d, JHH = 6.4 Hz, 3H), 2.35 (s, 3H), 3.54 (dd,
3
2JHH = 9.6 Hz, JHH = 7.7 Hz, 1H), 3.69−3.75 (m, 1H), 3.71 (s, 3H),
1
1419, 1353, 1212, 1169, 970, 936, 814 cm−1. H NMR (300.13 MHz,
3
3.92−4.07 (m, 1H), 4.53 (d, JHH = 4.1 Hz, 1H), 6.58 (dd, 3JHH = 8.9
CDCl3): δ 1.22 (d, 3JHH = 6.9 Hz, 3H), 2.39 (s, 3H), 3.16 (dd, 2JHH
=
4
3
4
11.1 Hz, 3JHH = 2.6 Hz, 1H), 3.80 (dd, 2JHH = 11.1 Hz, 3JHH = 1.5 Hz,
1H), 4.37−4.47 (m, 1H), 6.73−6.88 (m, 2H), 7.24 (d, 3JHH = 8.4 Hz,
2H), 7.51 (d, 3JHH = 8.3 Hz, 2H), 7.69 (dd, 3JFH = 10.5 Hz, 4JHH = 2.7
Hz, 1H). 13C NMR (75.5 MHz, CDCl3): δ 17.1 (CH3), 21.7 (CH3),
48.8 (CH), 66.1 (CH2), 112.1 (d, 2JFC = 27.7 Hz, CH), 113.0 (d, 2JFC
Hz, JHH = 3.0 Hz, 1H), 6.83 (d, JHH = 8.9 Hz, 1H), 7.13 (d, JHH
=
3.0 Hz, 1H), 7.23−7.32 (m, 2H), 7.67−7.72 (m, 2H), 8.58 (br s, 1H).
13C NMR (75.5 MHz, (CD3)2CO): δ 19.3 (CH3), 21.3 (CH3), 55.8
(CH3), 66.1 (CH), 76.7 (CH2), 108.9 (CH), 110.2 (CH), 115.4
(CH), 128.1 (2xCH), 128.9 (C), 130.2 (2xCH), 138.1 (C), 144.4 (C),
144.6 (C), 155.1 (C). HRMS (ESI+, m/z): calcd for
3
3
= 23.5 Hz, CH), 117.8 (d, JFC = 9.0 Hz, CH), 122.5 (d, JFC = 10.9
Hz, C), 127.3 (2xCH), 130.1 (2xCH), 135.3 (C), 142.1 (d, 4JFC = 2.5
Hz, C), 144.5 (C), 156.8 (d, 1JFC = 238.2 Hz, C). HRMS (ESI+, m/z):
calcd for (C16H16FNNaO3S)+ [M + Na]+, 344.0727; found, 344.0742.
[α]2D0 + 171.7 (c 1.0, EtOH) [for (R)-10b in >99% ee].
(C17H21NNaO5S)+ [M + Na]+, 374.1033; found, 374.1050. [α]2D0
24.2 (c 1.0, CHCl3) [for (S)-9c in >99% ee].
+
N-(2-(2-Hydroxypropoxy)-4-methylphenyl)-4-methylbenzenesul-
fonamide (9d). Light pink solid (136 mg, 75% yield). Rf (40%
EtOAc/hexane): 0.27. Mp: 139−141 °C. IR (KBr): 3337, 3054, 2986,
6-Methoxy-3-methyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]-
oxazine (10c). White solid (88 mg, 94% yield). Rf (40% EtOAc/
hexane): 0.66. Mp: 150−152 °C. IR (KBr): 3734, 3054, 2986, 2360,
1
2928, 2307, 1596, 1507, 1419, 1339, 1164, 1123, 1092, 815 cm−1. H
NMR (400.13 MHz, (CD3)2CO): δ 1.14 (d, 3JHH = 6.5 Hz, 3H), 2.22
2
3
1
2342, 1761, 1646, 1559, 1506, 1420, 1363, 1168, 933, 814 cm−1. H
(s, 3H), 2.31 (s, 3H), 3.60 (dd, JHH = 9.4 Hz, JHH = 7.6 Hz, 1H),
3.67 (dd, 2JHH = 9.4 Hz, 3JHH = 3.2 Hz, 1H), 3.90−4.01 (m, 1H), 4.54
3
NMR (300.13 MHz, CDCl3): δ 1.22 (d, JHH = 6.9 Hz, 3H), 2.37 (s,
3
3
2
3
2
(d, JHH = 4.2 Hz, 1H), 6.69−6.74 (m, 2H), 7.23 (d, JHH = 8.1 Hz,
2H), 7.38 (d, 3JHH = 7.8 Hz, 1H), 7.61 (d, 3JHH = 8.3 Hz, 2H), 8.37 (br
s, 1H). 13C NMR (100.6 MHz, (CD3)2CO): δ 19.2 (CH3), 21.2
(CH3), 21.3 (CH3), 66.1 (CH), 75.1 (CH2), 114.0 (CH), 122.1 (CH),
124.1 (CH), 124.6 (C), 127.9 (2xCH), 130.0 (2xCH), 136.6 (C),
138.2 (C), 144.0 (C), 151.0 (C). HRMS (ESI+, m/z): calcd for
(C17H21NNaO4S)+ [M + Na]+, 358.1083; found, 358.1096. [α]2D0 + 9.2
(c 1.0, CHCl3) [for (S)-9d in >99% ee].
3H), 3.12 (dd, JHH = 11.0 Hz, JHH = 2.5 Hz, 1H), 3.74 (dd, JHH
=
3
11.0 Hz, JHH = 1.5 Hz, 1H), 3.80 (s, 3H), 4.34−4.50 (m, 1H), 6.66
3
4
3
(dd, JHH = 9.0 Hz, JHH = 2.8 Hz, 1H), 6.72 (d, JHH = 8.9 Hz, 1H),
7.21 (d, JHH = 8.1 Hz, 2H), 7.44−7.54 (m, 3H). 13C NMR (75.5
3
MHz, CDCl3): δ 17.1 (CH3), 21.7 (CH3), 48.9 (CH), 55.9 (CH3),
65.9 (CH2), 109.8 (CH), 110.1 (CH), 117.6 (CH), 122.2 (C), 127.3
(2xCH), 130.0 (2xCH), 135.4 (C), 140.1 (C), 144.3 (C), 153.7 (C).
HRMS (ESI+, m/z): calcd for (C17H19NNaO4S)+ [M + Na]+,
356.0927; found, 356.0944. [α]2D0 + 333.9 (c 1.0, EtOH) [for (R)-
10c in >99% ee].
N-(3,4-Difluoro-2-(2-hydroxypropoxy)phenyl)-4-methylbenzene-
sulfonamide (9e). Colorless viscous oil (154 mg, 80% yield). Rf (40%
EtOAc/hexane): 0.41. IR (NaCl): 3349, 3054, 2986, 2359, 1653, 1559,
3,7-Dimethyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]oxazine (10d).
Light brown viscous oil (88 mg, 99% yield). Rf (40% EtOAc/hexane):
0.57. IR (NaCl): 3032, 2981, 2934, 2892, 2340, 1918, 1598, 1577,
1500, 1349, 1321, 1219, 1167, 1069, 917, 813 cm−1. 1H NMR (300.13
1
1507, 1490, 1419, 1265, 1165, 1047, 896, 738, 705 cm−1. H NMR
3
(300.13 MHz, CDCl3): δ 1.20 (d, JHH = 6.4 Hz, 3H), 2.36 (s, 3H),
2
3
3.30 (br s, 1H), 3.59 (dd, JHH = 10.3 Hz, JHH = 8.1 Hz, 1H), 3.93
(dd, 2JHH = 10.4 Hz, 3JHH = 2.3 Hz, 1H), 4.04−4.18 (m, 1H), 6.83 (td,
3
MHz, CDCl3): δ 1.20 (d, JHH = 6.9 Hz, 3H), 2.28 (s, 3H), 2.37 (s,
3JHH = 9.4 Hz, JFH = 9.4, JFH = 8.1 Hz, 1H), 7.22 (d, JHH = 8.2 Hz,
2H), 7.30 (ddd, 3JHH = 9.4 Hz, 4JFH = 5.2, 5JFH = 2.4 Hz, 1H), 7.69 (d,
3JHH = 8.3 Hz, 2H), 8.69 (br s, 1H). 13C NMR (75.5 MHz, CDCl3): δ
3
4
3
2
3
2
3H), 3.16 (dd, JHH = 11.0 Hz, JHH = 2.5 Hz, 1H), 3.75 (dd, JHH
=
3
4
11.1 Hz, JHH = 1.4 Hz, 1H), 4.34−4.46 (m, 1H), 6.61 (d, JHH = 1.2
Hz, 1H), 6.76 (dd, 3JHH = 8.4 Hz, 4JHH = 1.4 Hz, 1H), 7.21 (d, 3JHH
=
4
19.0 (CH3), 21.7 (CH3), 66.7 (CH), 79.8 (d, JFC = 3.6 Hz, CH2),
111.8 (d, JFC = 18.1 Hz, CH), 116.5 (dd, JFC = 11.4 Hz, JFC = 3.8
8.0 Hz, 2H), 7.46 (d, 3JHH = 8.3 Hz, 2H), 7.75 (d, 3JHH = 8.4 Hz, 1H).
13C NMR (75.5 MHz, CDCl3): δ 17.1 (CH3), 21.0 (CH3), 21.7
(CH3), 48.7 (CH), 66.0 (CH2), 117.4 (CH), 119.3 (C), 122.2 (CH),
125.9 (CH), 127.3 (2xCH), 129.9 (2xCH), 135.5 (C), 136.4 (C),
144.1 (C), 145.9 (C). HRMS (ESI+, m/z): calcd for
2
3
4
3
4
Hz, CH), 127.5 (2xCH), 127.9 (dd, JFC = 3.2 Hz, JFC = 1.9 Hz, C),
129.8 (2xCH), 136.3 (C), 139.7 (dd, 2JFC = 10.6 Hz, 3JFC = 2.0 Hz, C),
144.2 (C), 144.6 (dd, 1JFC = 248.7 Hz, 2JFC = 14.3 Hz, C), 148.4 (dd,
1JFC = 247.0 Hz, JFC = 11.1 Hz, C). HRMS (ESI+, m/z): calcd for
2
(C17H19NNaO3S)+ [M + Na]+, 340.0978; found, 340.0990. [α]2D0
210.1 (c 1.0, EtOH) [for (R)-10d in >99% ee].
+
(C16H17F2NNaO4S)+ [M + Na]+, 380.0739; found, 380.0736. [α]2D0
21.5 (c 0.6, EtOH) [for (R)-9e in >99% ee].
−
7,8-Difluoro-3-methyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]-
oxazine (10e). White solid (95 mg, >99% yield). Rf (30% EtOAc/
hexane): 0.42. Mp: 85−87 °C. IR (KBr): 3054, 2987, 2343, 1598,
General Procedure for the Synthesis of Racemic and
Optically Active Tosylated Benzoxazines 10a−e. Triphenylphos-
phine (89 mg, 0.34 mmol) was added to a solution of the
corresponding sulfonamide 9a−e (0.28 mmol) in dry THF (3.1
mL). Next, diethyl azadicarboxylate (53 μL, 0.34 mmol) was added
dropwise and stirred at room temperature for 2 h. After this time, the
solvent was removed by distillation under reduced pressure, and the
crude product was purified by column chromatography on silica gel
(EtOAc/hexane mixtures), affording the corresponding tosylated
benzoxazines 10a−e (94−100%).
3-Methyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]oxazine (10a).
Colorless oil (84 mg, 99% yield). Rf (40% EtOAc/hexane): 0.72. IR
(NaCl): 3054, 2986, 2340, 1599, 1559, 1490, 1350, 1170, 1072, 1015,
815 cm−1. 1H NMR (300.13 MHz, CDCl3): δ 1.22 (d, 3JHH = 6.9 Hz,
3H), 2.38 (s, 3H), 3.20 (dd, 2JHH = 11.1 Hz, 3JHH = 2.2 Hz, 1H), 3.79
1
1508, 1484, 1361, 1183, 1166, 1038, 815 cm−1. H NMR (400.13
MHz, CDCl3): δ 1.21 (d, 3JHH = 7.0 Hz, 3H), 2.39 (s, 3H), 3.17 (dd,
3
2
2JHH = 11.1 Hz, JHH = 2.6 Hz, 1H), 3.92 (dd, JHH = 11.1 Hz, 3JHH
=
3
3
0.9 Hz, 1H), 4.43−4.50 (m, 1H), 6.78 (td, JHH = 9.6 Hz, JFH = 9.6,
4JFH = 8.2, 1H), 7.24 (d, JHH = 8.1 Hz, 2H), 7.44 (d, JHH = 8.3 Hz,
3
3
2H), 7.64 (ddd, JHH = 9.5 Hz, JFH = 5.2, JFH = 2.5 Hz, 1H). 13C
3
4
5
NMR (100.6 MHz, CDCl3): δ 17.0 (CH3), 21.7 (CH3), 48.3 (CH),
2
3
66.4 (CH2), 108.3 (d, JFC = 18.4 Hz, CH), 119.4 (d, JFC = 3.0 Hz,
C), 120.4 (dd, 3JFC = 7.8 Hz, 4JFC = 4.2 Hz, CH), 127.3 (2xCH), 130.2
(2xCH), 135.0 (C), 136.7 (dd, 2JFC = 10.1 Hz, 3JFC = 3.4 Hz, C), 140.0
(dd, 1JFC = 247.4 Hz, 2JFC = 15.6 Hz, C), 144.8 (C), 148.6 (dd, 1JFC
=
2
245.7 Hz, JFC = 10.1 Hz, C). HRMS (ESI+, m/z): calcd for
H
DOI: 10.1021/acs.joc.5b00056
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(C16H15F2NNaO3S)+ [M + Na]+, 362.0633; found, 362.0628. [α]2D0
183.2 (c 0.5, EtOH) [for (S)-10e in >99% ee].
−
reaction crude products were analyzed by NMR (conversion) and
HPLC (enantiomeric excess).
General Procedure for the Bioreduction of Ketones 3a−e
with Evo-1.1.200. In an eppendorf tube containing the correspond-
ing ketones 3a−e (0.015 mmol) and 2-propanol (25 μL) in 50 mM
Tris-HCl pH 7.5 buffer (400 μL), a 10 mM MgCl2 solution in 50 mM
Tris-HCl pH 7.5 buffer (60 μL), a 10 mM NADH solution in 50 mM
Tris-HCl pH 7.5 buffer (60 μL) and evo-1.1.200 (60 μL of a solution
composed of 1 mg of pure evo-1.1.200 in 760 μL of 50 mM Tris-HCl
pH 7.5 buffer and 240 μL of 10 mM MgCl2 solution) were added
successively. The reaction was shaken at 30 °C and 250 rpm for 24 h.
Then, the mixture was extracted with EtOAc (2 × 500 μL), and the
organic layers were combined and dried over Na2SO4, analyzing the
reaction crude products by GC (conversion) and HPLC (enantio-
meric excess). For ketone 3e, better results were found at slightly
acidic pHs (6−6.5), suppressing the appearance of side-product 11 at
an optimal pH value of 6.
General Procedure for the Synthesis of Racemic and
Optically Active Benzoxazines 4a and 4e. Magnesium turnings
(22 mg, 0.88 mmol) were added to a solution of the corresponding
tosylated benzoxazines 10a and 10e (0.18 mmol) in dry MeOH (0.9
mL). The mixtures were stirred under reflux for 2 h until complete
deprotection of the tosyl group occurred. Then, the solvents were
removed by distillation under reduced pressure, and the crude
products were purified by column chromatography on silica gel (10%
EtOAc/hexane), affording the corresponding benzoxazines 4a and 4e
(79−86%).
3-Methyl-3,4-dihydro-2H-benzo[b][1,4]oxazine (4a). Twenty-one
milligrams, 79% yield. Rf (10% EtOAc/hexane): 0.24. The
spectroscopical data are listed above in the Experimental Section.
[α]2D0 − 17.6 (c 0.5, CHCl3) [for (R)-4a in 99% ee; lit. [α]2D0 − 19 (c
1.3, CHCl3) for (S)-4a in 99% ee].8a
General Procedure for the Scale up of Bioreduction of
Ketones 3a−e with ADH-A. E. coli/ADH-A cells (20:1 weight ratio
of ketone/crude enzyme) were rehydrated in 50 mM Tris-HCl pH 7.5
buffer (22 mL) by shaking the mixture at 30 °C and 250 rpm for 5
min. The corresponding ketones 3a−e (0.94 mmol), 2-propanol (1.5
mL), and NADH (10 mg) were added successively. The suspension
was shaken at 30 °C and 250 rpm until no starting material was
detected by TLC analysis (24 h). Then, the mixture was extracted with
EtOAc (3 × 20 mL). The organic layers were combined and dried
over Na2SO4 and filtered, and the solvent was removed under reduced
pressure, affording the corresponding alcohols (S)-5a−e (85−93%,
≥99% ee).
General Procedure for the Scale up of Bioreduction of
Ketones 3a−e with Evo-1.1.200. To a suspension of ketones 3a−e
(0.15 mmol) in a mixture of 2-propanol (250 μL) and 50 mM Tris-
HCl pH 7.5 buffer (4 mL), a 10 mM MgCl2 solution in 50 mM Tris-
HCl pH 7.5 buffer (600 μL), a 10 mM NADH solution in 50 mM
Tris-HCl pH 7.5 buffer (600 μL), and evo-1.1.200 (600 μL of a
solution composed of 1 mg of pure evo-1.1.200 in 760 μL of 50 mM
Tris-HCl pH 7.5 buffer and 240 μL of 10 mM MgCl2 solution) were
added successively. The reactions were shaken at 30 °C and 250 rpm
for 24 h. The mixtures were extracted with EtOAc (3 × 10 mL). The
organic layers were combined and dried over Na2SO4 and filtered, and
the solvent was removed under reduced pressure, affording the
corresponding alcohols (R)-5a−e (78−99% yield, >99% ee). As
mentioned before, for ketone 3e, better results were found at slightly
acidic pHs (6−6.5), suppressing the appearance of side-product 11 at
an optimal pH value of 6.
General Procedure for the Enzymatic Kinetic Resolution by
Acylation of Racemic Alcohols 5a−e. Vinyl acetate (7, 140 μL,
1.52 mmol) and RML IM (1:1 weight ratio of alcohol/enzyme) were
added to a suspension containing the corresponding racemic alcohols
5a−e (0.51 mmol) in dry MTBE (5.1 mL) under nitrogen
atmosphere. The reactions were shaken at 30 °C and 250 rpm for
the necessary time to achieve good kinetic resolution (see Table 1 and
Table S1 in the Supporting Information). The reaction was followed
by HPLC analysis until ∼50% conversion was reached. The enzyme
was filtered and washed with CH2Cl2 (3 × 5 mL), and the solvent was
evaporated under reduced pressure. The crude reaction was purified by
column chromatography on silica gel (eluent gradient 20−40%
EtOAc/hexane), affording the corresponding optically active acetates
(R)-6a−e (45−47% yield, 93−95% ee) and alcohols (S)-5a−e (44−
48% yield, 89−94% ee).
General Procedure for the Enzymatic Kinetic Resolution by
Hydrolysis of Racemic Acetates 6a−e. Water (39 μL, 2.16 mmol)
was added to a suspension containing the corresponding racemic
acetates 6a−e (0.43 mmol) and RML IM (1:1 weight ratio of acetate/
enzyme) in MTBE (4.3 mL). The reaction was shaken at 30 °C and
250 rpm for the necessary time to achieve good kinetic resolution (see
Table 2). The reaction was followed by HPLC analysis until ∼50%
conversion was reached. The enzyme was filtered and washed with
CH2Cl2 (3 × 5 mL), and the solvent was evaporated under reduced
pressure. The crude reaction products were purified by column
7,8-Difluoro-3-methyl-3,4-dihydro-2H-benzo[b][1,4]oxazine (4e).
Twenty-nine milligrams, 86% yield. Rf (40% EtOAc/hexane): 0.43.
[α]2D0 − 8.6 (c 1.3, CHCl3) [for (S)-4e in 99% ee; lit. [α]2D0 − 9.1 (c
1.3, CHCl3) for (S)-4e in 99% ee].11c
Bioreduction of 1-(2-Nitrophenoxy)propan-2-one (3a) with
ADH-LK. In an eppendorf tube containing ketone 3a (2.3 mg, 0.012
mmol) in 50 mM Tris-HCl pH 7.5 buffer (450 μL), glucose-6-
phosphate (40 μL), glucose-6-phosphate dehydrogenase (3 U, 10 μL),
10 mM NADPH solution in 50 mM Tris-HCl pH 7.5 buffer (50 μL),
and ADH-LK (3 U, 2 mg) were added successively. The reaction was
shaken at 250 rpm at 30 °C for 24 h. Then, the mixture was extracted
with EtOAc (2 × 500 μL) and dried over Na2SO4, and the reaction
crude products were analyzed by NMR (conversion) and HPLC
(enantiomeric excess).
Bioreduction of 1-(2-Nitrophenoxy)propan-2-one (3a) with
ADH-CP. In an eppendorf tube containing ketone 3a (2.3 mg, 0.012
mmol) in 50 mM Tris-HCl pH 7.5 buffer (450 μL), 2-propanol (25
μL), a 10 mM NADH solution in 50 mM Tris-HCl pH 7.5 buffer (50
μL), and ADH-CP (3 U, 7.5 μL) were added successively. The
reaction was shaken at 250 rpm at 30 °C for 24 h. Then, the mixture
was extracted with EtOAc (2 × 500 μL) and dried over Na2SO4, and
the reaction crude products were analyzed by NMR (conversion) and
HPLC (enantiomeric excess).
Bioreduction of 1-(2-Nitrophenoxy)propan-2-one (3a) with
Baker′s Yeast. Baker′s yeast (1.3 g) was added to a solution of
glucose (165 mg) in H2O (11 mL), and the resulting suspension was
stirred for 15 min at 30 °C and 250 rpm. After this time, ketone 3a (33
mg, 0.17 mmol) was added, and the suspension was shaken at 30 °C
and 250 rpm for 24 h. Then, the reaction was centrifuged, and the
supernatant was extracted with EtOAc (3 × 20 mL). Organic layers
were combined, dried over Na2SO4, filtered, and the solvent was
evaporated under reduced pressure. The reaction crude products were
analyzed by NMR (conversion) and HPLC (enantiomeric excess).
General Procedure for the Bioreduction of Ketones 3a−e
with ADH-LB. In an eppendorf tube containing the corresponding
ketones 3a−e (0.018 mmol) and 2-propanol (38 μL) in 50 mM Tris-
HCl pH 7.5 buffer (555 μL), a 10 mM MgCl2 solution in 50 mM Tris-
HCl pH 7.5 buffer (75 μL), a NADPH 10 mM solution in 50 mM
Tris-HCl pH 7.5 buffer (75 μL), and ADH-LB (4.5 U, 15 μL) were
added successively. The reaction was shaken at 30 °C and 250 rpm for
24 h and then extracted with EtOAc (2 × 500 μL). Organic layers
were combined and dried over Na2SO4, and the reaction crude
products were analyzed by NMR (conversion) and HPLC
(enantiomeric excess).
General Procedure for the Bioreduction of Ketones 3a−e
with ADH-A. In an eppendorf tube containing the corresponding
ketones 3a−e (0.012 mmol) and 2-propanol (25 μL) in 50 mM Tris-
HCl pH 7.5 buffer (425 μL), a NADH 10 mM solution in 50 mM
Tris-HCl pH 7.5 buffer (50 μL) and E. coli/ADH-A cells (15 mg) were
added successively. The reaction was shaken at 30 °C and 250 rpm for
24 h. After this time, the mixture was extracted with EtOAc (2 × 500
μL), the organic layers combined and dried over Na2SO4, and the
I
DOI: 10.1021/acs.joc.5b00056
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
chromatography on silica gel (eluent gradient 20−40% EtOAc/
hexane), affording the corresponding optically active alcohols (R)-5a−
e (44−47% yield, 96−99% ee) and acetates (S)-6a−e (41−48% yield,
91−97% ee).
The following optical rotation values of alcohols 5a−e and acetates
6a−e were found after selected biocatalyzed transformations. (S)-5a:
[α]2D0 + 6.0 (c 0.6, EtOH) (>99% ee). (S)-5b: [α]2D0 + 7.0 (c 0.5,
EtOH) (>99% ee). (S)-5c: [α]D20 + 4.0 (c 0.75, EtOH) (>99% ee). (S)-
5d: [α]2D0 + 3.2(c 0.65, EtOH) (>99% ee). (R)-5e: [α]2D0 − 4.8 (c 0.4,
EtOH) (>99% ee). (S)-6a: [α]2D0 − 74.3 (c 1.0, CHCl3) (93% ee). (R)-
6b: [α]2D0 − 58.8 (c 0.75, CHCl3) (91% ee). (S)-6c: [α]2D0 − 49.7 (c
0.7, CHCl3) (80% ee). (S)-6d: [α]2D0 − 63.8 (c 0.8, CHCl3) (85% ee).
(S)-6e: [α]2D0 − 18.5 (c 0.6, CHCl3) (63% ee).
(b) Krasnov, V. P.; Levit, G. L.; Korolyova, M. A.; Bukrina, I. M.;
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ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of HPLC chiral analyses and 1H, 13C, and DEPT NMR
spectra describing the organic compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vicgotfer@uniovi.es. Phone: +34 98 5103454. Fax:
+34 98 5103456.
■
(12) (a) Hudlicky, T.; Reed, J. W. Chem. Soc. Rev. 2009, 38, 3117−
3132. (b) Clouthier, C. M.; Pelletier, J. N. Chem. Soc. Rev. 2012, 41,
1585−1605.
Notes
(13) (a) Sanchez, S.; Demain, A. L. Org. Process Res. Dev. 2011, 15,
224−230. (b) Groger, H.; Asano, Y.; Bornscheuer, U. T.; Ogawa, J.
̈
The authors declare no competing financial interest.
Chem.Asian J. 2012, 7, 1138−1153.
(14) (a) Hohne, M.; Bornscheuer, U. T. ChemCatChem. 2009, 1,
̈
ACKNOWLEDGMENTS
Financial support of this work by the Spanish Ministerio de
■
42−51. (b) Kroutil, W.; Fischereder, E.-M.; Fuchs, C. S.; Lechner, H.;
Mutti, F. G.; Pressnitz, D.; Rajagopalan, A.; Sattler, J. H.; Simon, R. C.;
Siirola, E. Org. Process Res. Dev. 2013, 17, 751−759. (c) Ghislieri, D.;
Turner, N. J. Top. Catal. 2014, 57, 284−300. (d) Simon, R. C.;
Richter, N.; Busto, E.; Kroutil, W. ACS Catal. 2014, 4, 129−143.
(15) van Rantwijk, F.; Sheldon, R. A. Tetrahedron 2004, 60, 501−
519.
Ciencia e Innovacion
24237 and CTQ-2013-44153-P projects is gratefully acknowl-
edged. M.L.-I. thanks FICYT for a predoctoral fellowship.
́
(MICINN) through the CTQ-2011-
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