General Method for the Asymmetric Synthesis of N-H Sulfoximines via C-S Bond Formation

Article abstract of DOI:10.1021/acs.orglett.0c00761

A versatile method for the synthesis of enantioenriched N-H sulfoximines is reported. The approach stems from the organomagnesium-mediated ring opening of novel cyclic sulfonimidate templates. The reactions proceed in high yield and with excellent stereofidelity with alkyl, aryl, and heteroaryl Grignard reagents. The chiral auxiliary is readily removed from the resultant sulfoximines via an unusual oxidative debenzylation protocol that utilizes molecular oxygen as the terminal oxidant. This provides a general strategy for the synthesis of highly enantioenriched N-H sulfoximines.

Full text of DOI:10.1021/acs.orglett.0c00761

pubs.acs.org/OrgLett
Letter
General Method for the Asymmetric Synthesis of N−H Sulfoximines
via C−S Bond Formation
Priscilla Mendonca̧ Matos, William Lewis, Stephen P. Argent, Jonathan C. Moore,*
and Robert A. Stockman*
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ABSTRACT: A versatile method for the synthesis of enantioen-
riched N−H sulfoximines is reported. The approach stems from
the organomagnesium-mediated ring opening of novel cyclic
sulfonimidate templates. The reactions proceed in high yield and
with excellent stereofidelity with alkyl, aryl, and heteroaryl
Grignard reagents. The chiral auxiliary is readily removed from
the resultant sulfoximines via an unusual oxidative debenzylation protocol that utilizes molecular oxygen as the terminal oxidant.
This provides a general strategy for the synthesis of highly enantioenriched N−H sulfoximines.
n the past few years, chiral sulfur(VI) derivatives have
received a considerable influx of attention in the medicinal
year that proceeds via the nucleophilic, S-functionalization of
sterically encumbered sulfinamides.31 A complementary strategy
utilizes the electrophilic nature of certain sulfur(VI) derivatives.
In 2018, as part of our own studies into the synthesis of
sulfur(VI) compounds,32−39 we reported an approach to
sulfoximine synthesis that capitalizes on the reaction of
sulfonimidates with organomagnesium reagents (Scheme 1).39
In the same year, Sharpless and co-workers reported that
sulfonimidoyl fluorides can also serve as useful synthetic
intermediates in this context.40 While these are each effective
methods, they are limited to the generation of racemic
sulfoximines. To address this challenge, we sought to develop
a procedure where the same mode of reactivity could be utilized
in a chiral templating strategy comparable to that used for
sulfoxide synthesis. To this end, we were inspired by pioneering
reports from Reggelin and co-workers concerning the cyclic
sulfonimidate 1, which upon reaction with a small range of
organometallic reagents was shown to yield nonracemic
sulfoximines bearing an N-valinyl moiety (Figure 1).41 The
main limitation to this approach is that removal of the N-valinyl
moiety requires an inconvenient three-step deprotection
sequence,42 namely, O-mesylation, bromination, and finally
C−N bond cleavage with zinc and molecular iodine.
Furthermore, this deprotection sequence was only demon-
strated on a single example. Consequently, the method has not
seen widespread adoption as a general method for the synthesis
of sulfoximines. We reasoned, however, that if this limitation
I
and synthetic chemistry communities.1−4 This renaissance has
largely been driven by observations that the once neglected
sulfoximine functionality can imbue favorable properties to a
candidate drug substance. Examples include the ATR inhibitor
AZD 6738,5,6 the antiasthmatic Sudexanox,7 the cyclin-depend-
ent kinase inhibitor BAY 1000394,8 and the COX inhibitor 3.9
When compared to their more commonly employed achiral
analogues (e.g., sulfones and sulfonamides), sulfoximines can
offer improved solubility and polarity while providing an
additional chiral point of diversity. Unsurprisingly, the stereo-
chemistry at the sulfur atom has been shown to be an important
factor in the bioactivity of many chiral sulfoximine com-
pounds.10 For example, considerable differences are observed
between the sulfur epimers of each of the aforementioned
medicinally relevant sulfoximines (Figure 1).
While metal- and organo-catalyzed methods for the kinetic
resolution of racemic sulfoximines have recently been
developed,11−13 there is still a scarcity of synthetic methods
that provide direct access to enantioenriched sulfoximines. This
is due, in part, to the fact that the majority of methods for
sulfoximine synthesis14−16 stem from the oxidation or imination
of lower oxidation state sulfur derivatives17−27 and only a few of
these methods are enantioselective. Consequently, many
methodologies require enantiopure sulfur(IV) derivatives as
substrates, such as sulfoxides. These are most commonly
obtained in their nonracemic form using a chiral templating
strategy28 such as the classical procedure developed by
Anderson that utilizes menthol-derived sulfinic esters.29,30
For the above reasons, alternative methods for sulfoximine
synthesis are in high demand. In recent years, methodologies
centered on C−S bond-forming reactions have been gaining
traction as an alternative strategy. For example, an unusual
asymmetric approach to sulfoximine synthesis was reported this
Received: February 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00761
© XXXX American Chemical Society
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Our synthetic route to the cyclic sulfonimidates 7a−9a and
7b−9b centers on the intramolecular oxidative alkoxylation of
the corresponding sulfinamides 4a−6a and 4b−6b, which are
themselves readily available from the coupling of sulfinyl
chlorides and phenyl glycinol (see the Supporting Information).
In our racemic study,39 the hypervalent iodine derived oxidant
PhIO was employed for sulfonimidate synthesis.55−57 While this
is an efficient procedure, it has previously been reported that the
sulfur stereochemistry is scrambled under these conditions.55
This was corroborated in our own studies; when the sulfinamide
4a was employed as a single diastereoisomer, oxidation with
PhIO yielded the cyclic sulfonimidates 7a and 7b as a 1:1
mixture of diastereomers (Table 1, entry 1).
Table 1. Optimization of Conditions for the Synthesis of the
a
Cyclic Sulfonimidates 7a−9a and 7b−9b
entry
R =
substrate
[O]
result (yield, %)
b
1
Ph
Ph
Ph
Ph
Me
tBu
Ph
Me
tBu
4a
4a
4a
PhIO
NCS
tBuOCl
7a/7b = 1:1
7a
7a
2
3
4
5
6
7
8
9
4a + 4b (1:1)
5a + 5b (1:1)
6a + 6b (1:1)
4a + 4b (1:1)
5a + 5b (1:1)
6a + 6b (1:1)
NCS
NCS
NCS
tBuOCl
tBuOCl
tBuOCl
7a/7b = 1:1 (86%)
8a/8b = 1:1 (79%)
9a/9b = 1:1 (60%)
7a/7b = 1:1 (92%)
8a/8b = 1:1 (94%)
9a/9b = 1:1 (80%)
a
All reactions were performed at −78 °C in THF unless otherwise
stated. Yields are for isolated material, obtained following column
chromatography. This reaction was performed at 25 °C in
acetonitrile.
b
This obstacle was overcome by using N-chlorosuccinamide
(NCS) or tert-butyl hypochlorite (tBuOCl) as oxidant. The
sulfur stereochemistry was preserved when these reagents were
employed, and the sulfonimidate 7a was obtained as a single
diastereomer in each case (Table 1, entries 2 and 3). For the
purpose of this study, we targeted both sulfonimidate
diastereomers. Accordingly, we adopted a procedure where
the sulfinamides were employed as a (1:1) mixture of
diastereomers (Table 1, entries 4−9). In the case of the phenyl,
methyl, and tert-butyl derivatives, the highest yields were
Figure 1. From top to bottom: our previous study on the racemic
synthesis of sulfoximines via C−S bond formation; seminal work by
Reggelin and co-workers; this study; examples of medicinally relevant
sulfoximines.
could be overcome, a general method could be developed for the
synthesis of enantioenriched N−H sulfoximines. Since a number
of methods exist for the functionalization of these derivatives via
alkylation, arylation, acylation and sulfonylation,43−54 N−H
sulfoximines are a versatile and sought-after synthetic inter-
mediate for the synthesis of numerous sulfoximine derivatives.
We report here the synthesis of new chiral cyclic
sulfonimidate scaffolds that are readily prepared from (R)-
phenyl glycinol. These species serve as excellent chiral templates
for the synthesis of enantioenriched sulfoximines via their
reaction with organomagnesium reagents. A salient feature of
this approach is that the chiral auxiliary can be readily removed
from the resultant sulfoximines in a single step via an unusual
oxidative debenzylation strategy that preserves the chirality on
the sulfur atom. This approach provides access to N−H
sulfoximines in excellent yield and enantiomeric excess.
t
obtained using BuOCl, and the sulfonimidate diastereomers
were found to be readily separable by standard column
chromatography in each case (Table 1, entries 7−9). With six
cyclic sulfonimidates templates in hand (7a−9a and 7b−9b), we
proceeded to investigate the proposed organomagnesium-
mediated ring opening to enantioenriched sulfoximines.
The S-tert-butyl sulfonimidates 9a and 9b were found to be
unproductive as intermediates for sulfoximine synthesis
(Scheme 1). In the case of each diastereomer, only recovered
starting material was obtained upon treatment with phenyl or
methyl magnesium bromide. It should be noted that this is
consistent with our previous report that acyclic S-tert-butyl
sulfonimidates are unreactive toward Grignard reagents.39 In
contrast, the S-methyl derivatives 8a and 8b each reacted
B
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Scheme 1. Evaluation of S−Me and S−tBu Cyclic
Sulfonimidates As Precursors for Sulfoximine Synthesis
(Top). Plausible Mechanism for the Observed
Stereodegradation in the Synthesis of the S-Me Sulfoximines
(Bottom)
(Scheme 2). As expected, the reaction proceeds with inversion
of the sulfur stereochemistry, as confirmed by X-ray
Scheme 2. Substrate Scope for the Formation of the
Enantioenriched Sulfoximines from the Sulfonimidates 7a
and 7b
crystallography (see the Supporting Information). Initially,
alkyl-substituted Grignard reagents were investigated. Pleas-
ingly, in the case of each diastereomer (7a and 7b), the reaction
proceeded well with methyl, cyclopropyl, and isopropyl
magnesium bromide, yielding the targeted sulfoximines as a
single diastereomer in each case. A limit to the steric tolerance
was found when tert-butyl magnesium bromide was employed;
in these cases, the reaction failed to proceed and only recovered
starting material was obtained. When allyl magnesium bromide
was used, the sulfoximines 21a and 21b were obtained, albeit in
moderate yields (17% and 19%, respectively). Aryl organo-
magnesium reagents were particularly well tolerated and a
number of S-phenyl sulfoximines were produced containing a
range of electron-donating and -withdrawing substituents (10
examples). The reactions proceeded well with each diaster-
eomer (7a and 7b), yielding the targeted sulfoximines 22−26a
and 22−26b in 80−98% yield. The unusual fluorene derivatives
27a and 27b could also be accessed with this method. Next,
smoothly with both electron-rich and -poor aryl magnesium
bromides. Each of the targeted sulfoximines was obtained with
this approach, but we were disappointed to observe that the
products were isolated as a mixture of diastereomers in each case
(Scheme 1). It was postulated that the observed stereo-
degradation at the sulfur center could be due to a competitive
base-mediated ring opening, followed by nucleophilic addition
to the resultant methylene derivative 15 (Scheme 1). The
observed formation of the deuterated derivative 16 upon
quenching of the reaction mixture with CD3OD is consistent
with this hypothesis. Moreover, when the S-phenyl sulfonimi-
dates 7a and 7b were employed, which lack an acidic proton
alpha to the sulfur, we were delighted to observe that the
products were obtained as a single diastereomer in each case
C
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heteroaryl organometallics were trailled. Pleasingly, thienyl (28a
and 28b), 3-pyridyl (29a and 29b), and 2-pyridyl (30a and 30b)
sulfoximines could each be obtained with this approach, again
forming as a single diastereomer in each case. Interestingly,
when ethynyl magnesium bromide was employed, the targeted
sulfoximines 31a and 31b were not observed. Instead, the
aziridine 32 was obtained, presumably via bromide-mediated
ring-opening followed by intramolecular displacement.
employs molecular oxygen as oxidant under basic conditions in
an ethereal solvent (MTBE). We were delighted to observe that
under these conditions the N−H sulfoximines were obtained in
good to excellent yields (Scheme 3). Of the 26 sulfoximines that
were trialed, the only examples where the deprotection reaction
failed to occur were the S-allyl sulfoximines 21a and 21b (which
yielded a complex mixture of degradation products) and the 2-
pyridyl derivatives 30a and 30b (from which only recovered
starting material could be obtained). In all other cases, the
targeted N−H sulfoximines were obtained in good to excellent
yield with this method. Crucially, the sulfur chirality is retained
under these conditions, and in each case, the products were
obtained in excellent enantiomeric excess (93−99% ee).
This work constitutes a general and versatile method for the
synthesis of enantioenriched N−H sulfoximines via C−S bond
formation, allowing a modular approach to chiral sulfoximine
synthesis. The strategy utilizes cyclic sulfonimidates as chiral
templates, which are readily prepared in two steps from the
simple and cheap amino acid derivative (R)-phenyl glycinol.
The method provides access to either enantiomer of the desired
sulfoximines in excellent yields and stereoselectivities through
reaction of the appropriate template with a wide range of
Grignard reagents. Removal of the auxiliary was shown to be
facile in a single step using cheap and environmentally benign
reagents.
Finally, we investigated conditions for removal of the chiral
auxiliary to yield enantioenriched N−H sulfoximines (Scheme
3). In Reggelin’s report, the analogous N-deprotection required
Scheme 3. Oxidative Debenzylation to Yield Enantioenriched
N−H Sulfoximines
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00761.
Synthetic procedures and compound characterization
(PDF)
Accession Codes
CCDC 1965332−1965341 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Jonathan C. Moore − Department of Chemistry, University of
Nottingham, Nottingham NG7 2RD, U.K.; orcid.org/0000-
0002-2990-5642; Email: j.moore@nottingham.ac.uk
Robert A. Stockman − Department of Chemistry, University of
Nottingham, Nottingham NG7 2RD, U.K.; orcid.org/0000-
0002-7915-340X; Email: robert.stockman@
nottingham.ac.uk
Authors
a rather low-yielding three-step procedure (Figure 1).42
Improving upon this would be essential for the development
of a general and synthetically tractable procedure. We were
somewhat surprised to observe that the N-benzylic functionality
appeared to be completely robust under a range of hydro-
genative debenzylation conditions (see the Supporting
Information). To combat this issue, we were drawn to a report
from Escolano and co-workers where a rather unusual oxidative
debenzylation procedure was utilized for the removal of the
same functionality from lactam derivatives.58 This method
Priscilla Mendonca̧ Matos − Department of Chemistry,
University of Nottingham, Nottingham NG7 2RD, U.K.; CAPES
Foundation, Ministry of Education of Brazil, DF 70040-020,
Brazil
William Lewis − Department of Chemistry, University of
Nottingham, Nottingham NG7 2RD, U.K.; orcid.org/0000-
0001-7103-6981
Stephen P. Argent − Department of Chemistry, University of
Nottingham, Nottingham NG7 2RD, U.K.
Complete contact information is available at:
D
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(28) Balasubramaniam, S.; Sankaran, G. S.; Arumugan, S. MOJ. Biorg
Org. Chem. 2018, 2, 93−101.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CAPES foundation (P.M.M.) and the University
of Nottingham for funding. We also thank Professor Hon Lam,
UoN, for use of HPLC.
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