Synthesis of chiral nonracemic tertiary α-thio and α-sulfonyl acetic esters via SN2 reactions of tertiary mesylates

Article abstract of DOI:10.1055/s-0029-1219186

Syntheses of enantioenriched sulfides and sulfones via substitution of tertiary mesylate with thiolate nucleophile were achieved with modest to excellent success. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0029-1219186

470
LETTER
Synthesis of Chiral Nonracemic Tertiary a-Thio and a-Sulfonyl Acetic Esters
via SN2 Reactions of Tertiary Mesylates
C
hira
l
N
on
i
racem
m
ic Tertiary
a
-Thio a
m
nd
a
-
Sulfonyl Aceti
i
c
E
ster
e
s
D. Weaver, David K. Morris, Jon A. Tunge*
Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045-7582, USA
Fax +1(785)8645396; E-mail: tunge@ku.edu
Received 8 September 2009
quires repeated recrystallizations, resulting in very low
Abstract: Syntheses of enantioenriched sulfides and sulfones via
substitution of tertiary mesylate with thiolate nucleophile were
achieved with modest to excellent success.
overall yields (Scheme 2). Furthermore the optimal amine
is substrate dependent; consequently selecting the appro-
priate amine requires a separate search and optimization
for each substrate.
Key words: sulfide, sulfone, SN2 reaction
O
O
Recently, we have shown that the a-sulfonyl allyl esters
cleanly undergo decarboxylative coupling to afford tertia-
ry sulfones, making a-sulfonyl allyl esters a powerful pre-
cursor to tertiary sulfones that are not easily synthesized
by other methods (Scheme 1).1 In the course of this re-
search we became interested in the synthesis of chiral
nonracemic allyl a-sulfonylacetic esters.
Me
EtOH–H2O (3:1)
PhS
PhS
OH
+
OH
reflux
recryst.
5x
Ph Me
H2N
Ph
Ph Me
0.43 g, 97% ee
6% overall yield
7.04 g
Scheme 2
We felt that it would be possible to approach the synthesis
of optically active a-thio and a-sulfonyl acids by a direct
SN2 displacement of an enantioenriched tertiary a-hy-
droxy ester derivative. Specifically, it was expected that
the electron-withdrawing ester group would deactivate
the requisite substrates toward SN1 reactions. It was also
expected that the planar nature of the ester group would
better accommodate the sterically demanding transition
state for SN2 displacement. Thus, we posited that combin-
ing these two features with the use of highly nucleophilic
thiolates would allow SN2 displacement at tertiary carbon
centers (Scheme 3).5,7,8
O
PhO2S
PhO2S
Pd(PPh3)4
O
toluene, r.t.
96%
Scheme 1
To our knowledge, a general, simple, enantioselective
method to fully substituted a-sulfonyl acid derivatives
does not exist. Typically syntheses of optically active
compounds have been racemic and rely on preparatory
chiral HPLC to obtain individual stereoisomers.2 Alterna-
tively, modest success at the asymmetric synthesis has
been achieved with the use of a chiral oxazolidine auxilia-
ry to both quaternerize the a-center and control the abso-
lute stereochemistry of the sulfenylation.3 However, this
methodology requires harsh and toxic reagents and is not
general.
planar
sterically
small
OR3
O
O
O
PhS
PhS
R2 R1
MsO
R1 R2
PhS
OMs
OR3
OR3
R1 R2
deactivates
SN1
A scan of the literature revealed several less direct strate-
gies for the synthesis of a-sulfonyl acid derivatives in-
cluding chiral resolution of a-thio acids4 as well as several
procedures for the activation and substitution of a-hy-
droxy esters.5,6 More specifically, several early reports
obtained enantiomerically enriched a-thio and a-sulfonyl
acids via resolution of the racemic a-thio acids with enan-
tiopure amines via crystallization of diastereomeric salts.4
While this is a reliable method, it is not general and it re-
quires stoichiometric amounts of chiral amine and re-
Scheme 3
Searching the literature revealed two examples of SN2 dis-
placements of tertiary a-hydroxy acid derivatives with
thiolates. Mukaiyama reported SN2 displacement of
diphenyl phosphinite derived from a-hydroxy acids in the
presence of the appropriate quinone and benzothiazole9
and Corey reported the displacement of a mesylate de-
rived from the a-hydroxy atrolactic acid.6 With the goal of
verifying these procedures with our substrates, we at-
tempted to repeat the known Mitsunobu variant that has
been used for the synthesis similar a-thio esters.9 Unfortu-
nately, this method did not work well in our hands
(Scheme 4).
SYNLETT 2010, No. 3, pp 0470–0474
1
2.
0
2.
2
0
1
0
Advanced online publication: 11.01.2010
DOI: 10.1055/s-0029-1219186; Art ID: S09809ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Chiral Nonracemic Tertiary a-Thio and a-Sulfonyl Acetic Esters
471
Table 1 Synthesis of Compounds 3 and 4
S
N
O
SH
Ph2PO
OH
O
OH
OH
O
Pt/C, NaHCO3
air, H2O, 70 °C
AD-mix
recovered
starting material
t-BuOH–H2O
0 °C
Ar
Ar
Ar
Me
OH
2,6-di(tert-butyl)quinone
CDCl3
Me
Me
2
3
4
Scheme 4
Ar
AD-mix Product
Yield of Product Yield of
3 (%)
4 (%)
<10
81
Consequently, it was decided to investigate the simpler,
albeit less direct, two-step approach involving sequential
activation and substitution of an a-hydroxy ester. Previ-
ously, Corey converted (+)-atrolactic acid into the corre-
sponding sulfone by this protocol, although the
experimental details were not provided and no reaction
scope was demonstrated.6 Herein we provide a detailed
examination of the methodology, provide expanded scope
of the reaction, and outline the limits of the methodology.
4-MeO2CC6H4
4-BrC6H4
4-ClC6H4
a
b
a
a
b
(S)-3a
(R)-3b
(S)-3c
(S)-3d
(R)-3e
79
(S)-4a
(R)-4b
(S)-4c
(S)-4d
(R)-4e
78
99
85
4-MeC6H4
4-FC6H4
99
64
98
82
Table 2 Conditions for Esterification of Hydroxy Acids
While Corey was successful in his substitution of atrolac-
tic acid, our attempts to displace the mesylate from the
more sterically demanding butanoate led solely to elimi-
nation (Scheme 5). Consequently, further investigation
was limited to propanoic acid derivatives.
O
O
HO
K2CO3
HO
R2
O
OH
allyl bromide
acetone, r.t.
5f–m
4b–e
R1
R1
O
O
HO
PhS
Entry Acid
R
Allyl bromide
Yield (%) Product
OH
O
OMe
Corey
Br
1
2
3
4e
4d
4b
F
93
92
90
5f
Me
Br
5g
5h
O
Br
Br
O
HO
Ph
O
4
4b
Br
97
5i
Br
Br
5
6
7
4c
4e
4c
Cl
F
99
99
99
5j
5k
5l
Scheme 5
Br
Ph
In order to further explore this methodology we synthe-
sized several nonracemic hydroxy esters as outlined in
Table 1. a-Methyl styrenes 2a–e were subjected to Sharp-
less asymmetric dihydroxylation, which typically afford
highly enantioenriched derivatives (90–94% ee).10 Next,
the resulting diols underwent Pt/C-catalyzed oxidation
under mild conditions utilizing air as the terminal oxi-
dant.11 Using this protocol, the ester-substituted arene de-
rivative 3a, failed to give significant quantities of the
desired product 4a. However, the remaining substrates
3b–e produced the corresponding a-hydroxy acids in
good yields using this practical and simple synthesis.12,13
Cl
Br
Br
Ph
Ph
8
4d
Me
97
5m
synthesis of a-thio esters.6 The esters, 5f–m were subject-
ed to mesylation followed by careful workup, then imme-
diately subjected to the conditions for substitution with
sodium phenyl thiolate (Scheme 6).
R2
R2
R1
R1
O
O
R2
Next, the hydroxy acids 4b–e were subjected to typical re-
action conditions for esterification. Unfortunately, the ter-
tiary benzylic hydroxy acids are not well-suited for
common Fisher or DCC/DMAP-catalyzed esterifications.
However, it was gratifying to find that esterifications via
an SN2 reaction of the potassium carboxylates and allyl
bromides yielded the desired hydroxy esters in high yields
and purities (Table 2).14
PhS
Ar
PhO2S
R1
O
O
O
c
8f–m
Ar
6f–m
R2
a,b
R2
HO
Ar
+
O
R1
R1
O
O
5f–m
Ar
Ar
O
O
SPh 7'f–m
Scheme 6 Reagents and conditions: a) MsCl, DMAP, pyridine,
–5 °C to 0 °C; b) NaSPh, EtOH, 0 °C to r.t.; (c) MCPBA, CH2Cl2, r.t.
With the requisite a-hydroxy esters in hand, we turned to
evaluating the mesylation–substitution procedure for the
Synlett 2010, No. 3, 470–474 © Thieme Stuttgart · New York

472
J. D. Weaver et al.
LETTER
Table 3 Synthesis of Compounds 6–8
O
O
O
PhO2S
Entry
Substrate Yield of
Ratio
Yield of
8 (%)
ee (%) of
OEt
Ph
OEt
OEt
Bn
a,b,c
d–f
6 (%)
of 6/7a
8d
OH
Ph
9c
9a
9b
1
2
3
4
5
6
7
8
5f
29
10
41
39c
23
38
50
15
2.5:1
1:1
77
–
46
–
6 steps 45%
97% eeg
5g
5h
4i
19:1
19:1
n.d.b
8:1
38
48
38
59
72
–
97
80c
89
73
61
–
Scheme 7 Reagents and conditions: a) AD-mix-a, t-BuOH–H2O,
0 °C, 61%; b) TFAA; c) H2, Pd/C, EtOH, r.t., 99% over 2 steps, >94%
ee; d) MsCl, DMAP, pyridine ,r.t., 78%; e) NaSPh, EtOH, 96%; f)
MCPBA, CH2Cl2, r.t., 99%; g) ee determined by chiral HPLC.
5j
5k
5l
Similarly, enantioenriched dialkyl a-sulfonyl esters may
also be synthesized from 1,1-disubstituted olefins such as
10a (Scheme 8) via a straightforward dihydroxylation–
oxidation sequence. Specifically, the PMP-protected alco-
hol, 10a underwent asymmetric dihydroxylation and Pt-
catalyzed oxidation to provide the crystalline a-hydroxy
acid 10b in excellent yield.16 Esterification of the acid
provided the requisite a-hydroxy ester for mesylation.
Treatment with MsCl provided the mesylate, in 81%
yield. Once again, this a,a-dialkyl mesylate was much
more stable and less prone to undergo racemization, re-
maining unchanged after months at room temperature.
Subsequent substitution of the mesylate cleanly afforded
the sulfide in 96% yield. Oxidation to the sulfone provid-
ed 10c, with excellent enantioenrichment (94% ee), indi-
cating that the substitution of the tertiary alcohol
derivative proceeded cleanly via an SN2 mechanism. The
overall yield of sulfone 10c from 10a was a satisfactory
54%.
5:1
5m
1:1
a Determined by 1H NMR.
b Ratio not determined before oxidation.
c Mesylation inadvertently performed at r.t.
d Determined by chiral HPLC.
As can be seen from the data in Table 3, the yields of a-
thio esters 6 using this protocol are moderate, at best. In
particular, the more electron-rich p-methyl arene sub-
strates (entries 2 and 8, Table 2) formed a-thio esters in
dismal yields. The low yields in these cases are attributed
to the formation of significant quantities of b-sulfides 7
that likely arise via elimination followed by conjugate ad-
dition of the thiolate to the acrylate derivative (Scheme 6).
The mixture of 6 and 7 was subjected to oxidation with
MCPBA.1,15 Upon oxidation the a-thio esters 6 readily
formed the corresponding sulfones, while the b-thio esters
7 underwent elimination to form acrylates. The stereo-
chemical fidelity of the mesylation–substitution was good
in general, although several substrates gave substantial ra-
cemization (entries 1, 6, and 7). The delicate nature of the
mesylates is illustrated by the much lower ee of 8i (entry
4) when the mesylation was inadvertently performed at
room temperature; mesylation of an analogous substrate
at –5 °C, provided the sulfone in excellent ee (entry 3).
Thus, we hypothesized that the observed racemization
was due to ionization of the intermediate mesylate and
that the methodology might be better suited for a,a-di-
alkyl sulfonyl esters, whose requisite mesylates would
ionize less readily.
O
a,b
HO
OPMP
HO
10b
OPMP
10a
O
c–f
PhO2S
10b
O
6 steps 54%
94% eeg
10c
OPMP
Scheme 8 Reagents and conditions: a) AD-mix-a, t-BuOH–H2O,
0 °C, 90%; b) Pt/C, NaHCO3, air, H2O, 70 °C, 93%; c) K2CO3, allyl-
bromide, acetone, r.t., 99%; d) MsCl, DMAP, pyridine, r.t.,81%; e)
NaSPh, EtOH, 96%; f) MCPBA, CH2Cl2, r.t., 84%, 94% ee; g) ee de-
termined by chiral HPLC. PMP = p-methoxyphenyl.
To test this hypothesis, Sharpless dihydroxylation was
used to provide chiral nonracemic diol which was selec-
tively deoxygenated to give hydroxy ester 9b in 61% yield
from 9a (Scheme 7).7 As expected the mesylation of 9b
results in a stable mesylate that could be purified by col-
umn chromatography and stored at low temperatures
without any observed racemization. As posited, exposure
of the mesylate to sodium thiolate led to clean substitution
to provide the a-thio ester which was oxidized to form the
highly enantioenriched sulfone 9c. Here, we assume that
the reaction occurred with inversion of configuration in
analogy to the observations of Corey.6
In conclusion we have demonstrated the limits of the two-
step activation–substitution methodology on several ben-
zylic hydroxy esters. Problematic ionization of the tertiary
a-mesyl-a-aryl esters indicates that the best substituents
are those that electronically destabilize carbocation for-
mation. Additionally, we have shown that the two-step
mesylation and substitution protocol works satisfactorily
for a,a-dialkyl-a-hydroxy esters to provide chiral nonra-
cemic tertiary a-thio and a-sulfonyl esters in high yields
and ee.
Synlett 2010, No. 3, 470–474 © Thieme Stuttgart · New York

LETTER
Chiral Nonracemic Tertiary a-Thio and a-Sulfonyl Acetic Esters
473
chromatography, hexanes–EtOAc (8:2)] The substitution reaction
was carried out at r.t. to afford sulfide ester in 96%. The oxidation
was also carried out with MCPBA using standard conditions de-
scribed for synthesis of sulfone 8h. The enantioenrichment was de-
termined to be 97% ee via chiral HPLC [Chiracel OD-H, hexanes–
i-PrOH (90:1), 1 mL/min, 210 nm, tR (major) = 8.2 min, tR (minor) =
15.2 min]. 1H NMR (400 MHz, CDCl3): d = 7.89 (d, J = 7.4 Hz, 2
H, o-ArCHSO2R), 7.68 (t, J = 7.5 Hz, 1 H, 4-ArCHSO2R), 7.56 (t,
J = 7.7 Hz, 2 H, 3-ArCHSO2R), 7.24–7.18 (m, J = 7.1 Hz, 3 H,
ArCHCH2R), 7.07 (d, J = 7.6 Hz, 2 H, ArCHCH2R), 4.09 (q, J = 7.1
Hz, 2 H, ROCH2R), 3.63 (d, J = 12.9 Hz, 1 H, diastereotopic RCH-
HR), 3.05 (d, J = 13.0 Hz, 1 H, RCHHR), 1.47 (s, 3 H, qCCH3),
1.15 (t, J = 7.1 Hz, 3 H, RCH2CH3). 13C NMR (101 MHz, CDCl3):
d = 167.93 (RCO2R), 136.00 (ArC), 134.48 (ArC), 134.42 (ArC),
130.83 (ArC), 130.53 (ArC), 128.96 (ArC), 128.70 (ArC), 127.59
(ArC), 73.97 (qC), 62.55 (ROCH2R), 38.82 (PhCH2R), 16.29
(qCCH3), 14.02 (RCH2CH3).
All reactions were run in flame-dried glassware under an Ar atmo-
sphere unless otherwise noted. Commercially available reagents
were used without additional purification unless otherwise stated.
Compound purification was effected by flash chromatography us-
ing 230 × 400 mesh, 60 Å porosity, silica obtained from Sorbent
Technologies. 1H NMR and 13C NMR spectra were obtained on ei-
ther a Bruker Avance DRX 500 spectrometer equipped with a
broadband observe probe or a Bruker Avance AVIII 500 spectrom-
eter equipped with a 13C/1H cryoprobe and referenced to residual
protio solvent signals. Structural assignments are based on 1H, 13C,
DEPT-135, COSY, HSQC, and IR spectroscopy. All ee were deter-
mined using chiral HPLC with columns purchased from Daicel
(Chiracel AD, OD-H, or AS) using 99:1 to 85:15 mixtures of hex-
anes–i-PrOH.
Synthesis of Hydroxy Ester 5h
Hydroxy acid 4b was synthesized via an adaptation of a known
method.13 Compound 4b (200 mg, 0.82 mmol) and dry K2CO3 (566
mg, 4.1 mmol) were added to a flamed-dried flask followed by the
addition of allylbromide (296 mg, 2.4 mmol). Next, acetone (2 mL,
distilled from MgSO4) was added and the mixture stirred vigorously
for 5 h. The mixture was then extracted with EtOAc (20 mL),
washed with H2O (2 × 5 mL), dried over MgSO4, and concentrated
in vacuo. Azeotropic removal of excess allyl bromide and allyl al-
cohol provided pure 5h (210 mg, 0.74 mmol).14 1H NMR (500
MHz, CDCl3): d = 7.48–7.41 (m, 4 H, ArCH), 5.90–5.78 (m,
J = 16.4, 11.3, 5.7 Hz, 1 H, CH2CHCH2), 5.24 (dd, J = 7.5, 1.3 Hz,
1 H, CH2CHCHH), 5.21 (t, J = 1.3 Hz, 1 H, CH2CHCHH), 4.70–
4.57 (m, J = 13.2, 11.8, 5.7 Hz, 2 H, ROCH2R), 3.76 (s, 1 H, ROH),
1.75 (s, 3 H, quat. CCH3). 13C NMR (126 MHz, CDCl3): d = 175.13
(RCO2R), 141.92 (qArC), 131.61 (ArC), 131.24 (RCHCH2), 127.40
(ArC), 122.21 (qArC), 119.34 (RCHCH2), 75.62 (qC), 67.12
(ROCH2R), 27.02 (RCH3).
Synthesis of Sulfone 10c
The precursor 1,2-diol was synthesized according to a literature
prep.16 The diol was then oxidized to the hydroxy acid in the same
manner as outlined in reference.13 The resulting hydroxy acid was
esterified was outlined for 5h. Mesylation, substitution, and oxida-
tion of the resulting hydroxy ester followed the same procedure as
that outlined for sulfone 9c. 1H NMR (500 MHz, CDCl3): d = 7.84
(dd, J = 8.3, 1.1 Hz, 2 H, 2-ArCHSO2R), 7.66 (t, J = 7.5 Hz, 1 H, 4-
ArCHSO2R), 7.53 (t, J = 7.9 Hz, 2 H, 3-ArCHSO2R), 6.80–6.67 (m,
4 H, ROArCHOR), 5.79 (ddd, J = 16.3, 11.0, 5.8 Hz, 1 H,
RCHCH2), 5.29 (dd, J = 17.2, 1.4 Hz, 1 H, RCHCHH), 5.20 (d,
J = 10.5 Hz, 1 H, RCHCHH), 4.61–4.50 (m, 2 H, ROCH2CHCH2),
4.07–3.91 (m, 2 H, ArOCH2R), 3.72 (s, 3 H, ArOCH3), 2.79 (ddd,
J = 14.5, 8.8, 6.1 Hz, 1 H, diastereotopic ArOCH2CH2qC), 2.30 (dt,
J = 14.0, 4.9 Hz, 1 H, diastereotopic ArOCH2CH2qC), 1.67 (s, 3 H,
q CCH3). 13C NMR (126 MHz, CDCl3): d = 167.91 (RCO2R),
154.34 (ROqArCOR), 152.56 (ROqArCOR), 135.70 (ArC), 134.42
(ArC), 131.24 (RCHCH2), 130.81 (ArCH), 128.95 (ArCH), 119.35
(RCHCH2), 115.58 (ROqArCHOR), 114.84 (ROqArCHOR), 71.81
(qCCH3), 66.97 (ROCH2CHR), 64.34 (ROCH2CH2R), 55.89
(ROCH3), 32.58 (qCCH2CH2R), 16.33 (qCCH3).
Synthesis of Sulfone 8h
To a flame-dried flask with septum was added 5h (205 mg, 0.72
mmol), methane sulfonyl chloride (0.27 mL, 3.45 mmol), DMAP
(13 mg, 0.11 mmol), and pyridine (0.72 mL), and the reaction was
placed in a chiller where the temperature was maintained at –5 °C
to 0 °C for 18 h. After 12 h, additional methanesulfonyl chloride
(0.20 mL) was added. After 6 additional hours, the reaction mixture
was extracted with EtOAc (20 mL) and washed with an ice–H2O–
HCl mixture (1:1 ice–1 M HCl), dried over MgSO4 and concentrat-
ed in vacuo with minimum heat. The crude mesylate was then added
to a Schlenk flask and chilled to 0 °C. To the mesylate was added a
prechilled solution of sodium phenyl thiolate and phenyl thiol in
EtOH [NaH (248 mg, 6.5 mmol) and PhSH (0.73 mL, 7.2 mmol) in
EtOH (7.2 mL)] and allowed to slowly warm to r.t. and stirred for
12 h. The reaction mixture was extracted with CH2Cl2 (40 mL) and
washed with K2CO3 solution (2 × 10 mL). The organic layer was
dried over MgSO4, concentrated in vacuo, and purified via flash col-
umn chromatography [hexanes–CH2Cl2 (95:5 to 50:50) to afford 6h
(110 mg, 41%)]. Compound 6h was oxidized to the sulfone using
MCPBA to afford sulfone 8h (41 mg, 38%).1,15 1H NMR (500 MHz,
CDCl3): d = 7.55 (t, J = 7.4 Hz, 1 H), 7.52–7.49 (m, 2 H), 7.43–7.40
(m, J = 5.8 Hz, 2 H), 7.38–7.33 (m, 2 H), 7.23–7.19 (m, 2 H), 5.84
(ddt, J = 16.3, 10.5, 5.9 Hz, 1 H), 5.29 (dq, J = 17.1, 1.4 Hz, 1 H),
5.24 (ddd, J = 10.4, 2.3, 1.2 Hz, 1 H), 4.67 (tt, J = 4.5, 1.3 Hz, 2 H),
2.09 (s, 3 H).
Acknowledgment
We thank the National Institute of General Medical Sciences
(1R01GM079644) for support of this work.
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