Stereoselective synthesis of carbohydrate-derived N-Sulfonyl aziridines

Article abstract of DOI:10.1055/s-0032-1317954

A novel reaction of iodomethyllithium with a variety of sugar-derived N-sulfinyl imines, under very mild conditions, is described. N-Sulfonyl aziridines were easily obtained in high yields and in moderate to good stereoselectivities through an addition/oxidation protocol. A mechanism is proposed that is supported by the results of X-ray diffraction studies. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317954

LETTER
▌181
lSettteer reoselective Synthesis of Carbohydrate-Derived N-Sulfonyl Aziridines
Carbohydrate-Derived N-Sulfonyl Aziridines
Humberto Rodríguez-Solla,*a Carmen Concellón,a Noemí Alvaredo,a Ricardo Llavona,a Santiago García-Granda,a
M. Rosario Díaz,a Raquel G. Soengasb
a
Departamento de Química Orgánica e Inorgánica and Química Física y Analítica, Facultad de Química, Universidad de Oviedo,
Julián Clavería 8, 33071 Oviedo, Spain
Fax +34(985)102971; E-mail: hrsolla@uniovi.es
b
Departamento de Química & QOPNA, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Received: 07.11.2012; Accepted after revision: 05.12.2012
usually highly selective, giving just one of the possible
Abstract: A novel reaction of iodomethyllithium with a variety of
isomers.11
sugar-derived N-sulfinyl imines, under very mild conditions, is
described. N-Sulfonyl aziridines were easily obtained in high yields
and in moderate to good stereoselectivities through an
addition/oxidation protocol. A mechanism is proposed that is
supported by the results of X-ray diffraction studies.
Although synthetic routes toward aziridines have includ-
ed many procedures such as the addition of nitrenes to al-
kenes,12 nucleophilic substitutions of derivatives of
epoxides13 or β-amino alcohols14 and methods based on
the use of imines as starting materials,15 almost all the re-
ported syntheses of carbohydrate aziridines are based on
SN2 intramolecular nucleophilic substitution.16 Generally,
these methods are multistep procedures, requiring long re-
action times and taking place in only moderate overall
yields.
Key words: aziridines, carbohydrates, imines, iodomethyllithium,
sulfonamides
Chiral aziridines are important building blocks in organic
synthesis.1 Due to their highly strained nature, aziridines
readily open with excellent stereo- and regiocontrol to re-
lieve ring strain, affording a wide variety of chiral
amines,2 and can be employed as starting materials for the
synthesis of important biomolecules such as amino acids
and alkaloids.3 In addition, aziridines can function as
sources of chirality in stereocontrolled reactions and have
found use both as ligands and auxiliaries in asymmetric
synthesis.1b,c
In this context, the development of a novel and efficient
procedure to obtain aziridines derived from sugars in good
yields and stereoselectivities would be desirable. We have
previously described an efficient, simple, and rapid azirid-
ination process by reaction of imines derived from p-tolu-
enesulfonamide with iodomethyllithium generated in
situ.17 Taking into account the inherent usefulness of this
procedure for the rapid preparation of sugar aziridines, we
decided to investigate the application of this methodology
for the aziridination of carbohydrate-derived imines.
Moreover, the aziridine ring is present in a number of
molecules that show biological activity, which is intimate-
ly associated with the reactivity of the strained heterocy-
cle. Naturally occurring compounds containing the
aziridine moiety exhibit antitumor and/or antibiotic activ-
ity, due to their ability to cross-link DNA,4 or inhibit the
bacterial enzyme diaminopimelic acid epimerase.5 Other
molecules related to mitosanes and mytomycins and bear-
ing the aziridine ring have been synthesized and shown to
possess activity against a variety of cancers.6
First attempts focused on the synthesis of carbohydrate-
based imines derived from either p-toluenesulfonamide or
tert-butylsulfonamide. In this sense, methods based on the
synthesis of these substrates according to different proce-
dures previously reported in the literature18 failed, and no
p-toluene or tert-butylsulfonyl imines were isolated. For
this reason, we also explored the use of other protecting
groups on the imine group such as p-methoxyphenyl and
tert-butylsulfinyl. In both cases, the corresponding imines
1a and 2a were successfully prepared according to litera-
ture procedures (Figure 1).19,20
Despite the importance of chiral aziridines in biological
systems and as intermediates in the synthesis of biologi-
cally important compounds, sugar aziridines have been
much less exploited.7 This is somewhat surprising given
the interest in using aziridines for the preparation of ami-
no-substituted sugar derivatives of great biological value8
such as azasugars9 and sugar amino acids.10 As the nu-
cleophilic opening of the aziridine ring of sugar aziridines
is strongly influenced by the configuration of the starting
sugar and by its conformational flexibility, this process is
O
PMP
H
S
N
O
N
O
t-Bu
H
H
O
O
O
O
O
O
H
O
O
1a
2a
SYNLETT 2013, 24, 0181–0184
Advanced online publication: 21.12.2012
Figure 1 Galactose-derived imines 1a and 2a
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317954; Art ID: ST-2012-D0961-L
© Georg Thieme Verlag Stuttgart · New York

182
H. Rodríguez-Solla et al.
LETTER
Initial attempts to prepare aziridines were performed start-
ing from imine 1a. Iodomethyllithium was generated in
situ by treatment of diiodomethane with methyllithium in
the presence of imine 1a at –78 °C using tetrahydrofuran
(THF) as solvent. Unfortunately, no addition of iodo-
methyllithium to imine 1a took place under a range of re-
Table 1 Synthesis of Carbohydrate-Derived N-Sulfonyl Aziridines 3
O
S
SO2t-Bu
1) CH2I2 / MeLi, –78 °C
2) MCPBA, r.t.
N
R1
N
t-Bu
R1
2
3
action conditions. A similar behavior was observed in the Entrya 3b
addition of iodomethyllithium to N-PMP imines derived
from octanal or benzaldehyde.17b
R1
dr
Yield (%)c
O
O
When the same reaction was performed on a stereoiso-
1
3a
>98:2
86
meric mixture of sulfinyl imine 2a,21 a complex mixture
of stereoisomers was isolated; for this reason, oxidation of
N-sulfinilaziridines was carried out in the presence of
MCPBA, with the galactose-derived N-sulfonyl aziridine
being isolated as a single stereoisomer in 86% yield (Ta-
ble 1, entry 1).22 It is important to remark that, when the
reaction was carried out on either (S)- or (R)-tert-butylsul-
finylimine, similar results were observed in terms of yield
and stereoselectivity. Thus, in the case of (S)-tert-butyl-
sulfinylimine or (R)-tert-butylsulfinylimine derived from
galactose, single stereoisomers of aziridine 3a were isolat-
ed in 83 and 82% yield, respectively, after application of
the oxidation protocol. This result supported the idea that
the tert-butylsulfinyl group does not act in the reaction as
a chiral auxiliary. As a consequence, all the aziridination
reactions were finally performed on stereoisomeric mix-
tures of sulfinyl imines 2.
O
O
O
O
O
2
3
3b
3c
80:20
87:13
81
79
O
OBn
O
TBSO
O
O
O
BnO
4
5
3d
3e
80:20
72:28
95
80
O
O
Aziridination reactions on other sugar-based imines are
summarized in Table 1. From a general point of view, re-
actions took place in good yields (>70%) and in stereo-
isomeric ratios ranging from 72:28 to >98:2 (Table 1).
O
O
O
O
O
The diastereoisomeric ratio (dr) of sugar-based N-sulfo-
nyl aziridines 3 was determined on the basis of 1H NMR
spectroscopic analysis (300 MHz) of the crude reaction
products.
TBDPSO
6
7
3f
80:20
79:21
70
85
O
It is worth mentioning that this process tolerates a broad
scope of sugar-derived imines with different protecting
moieties on the hydroxyl groups. Furthermore, all the re-
actions took place with good stereoselectivities and in
high yields with no epimerization of any stereogenic cen-
ter. In general terms, it is noteworthy that this approach to
the synthesis of aziridines 3 is experimentally simple and
the reaction times are short.
3g
O
O
a All reactions were carried out on mixtures of imine stereoisomers
derived from (±)-tert-butylsulfinylimine.
b Diastereoisomeric ratio (dr) was determined on the basis of 1H NMR
(300 MHz) analysis of the crude products 3.
The structure of sugar-based aziridines 3 (as depicted in
Table 1) were established on the basis of 1H and 13C NMR
spectroscopic analysis, and the absolute configuration of
the major stereoisomer was established for 3d by X-ray
diffraction (Figure 2).23 The structure and the absolute
configuration of other sulfonyl aziridines 3 were assigned
by analogy.
c Isolated yield (based on 2) of analytically pure compounds 3 after
column chromatography.
The absolute configuration obtained after the addition of
iodomethyllithium 4 to imines 2 can be easily explained
through the Felkin–Ahn model. Thus, iodomethyllithium
would attack the si face giving preferentially anti-stereo-
isomers (Scheme 1) through a model similar to that used
to explain the stereoselectivity observed in the addition of
Figure 2 ORTEP diagram for 3d
Synlett 2013, 24, 181–184
© Georg Thieme Verlag Stuttgart · New York

LETTER
Carbohydrate-Derived N-Sulfonyl Aziridines
183
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analysis of compound 3d (Figure 2). The same stereo-
chemical course was recently used to explain other
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imines.19,24
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Pg
Pg
I
Pg
N
LiN
N
O
O
O
H
H
HH
H
H
Li
I
2
4
5
3
Scheme 1 Mechanistic proposal for the addition of iodomethyl-
lithium to sugar-derived imines 2
In conclusion, we have described the reaction of iodo-
methyllithium with a variety of imines to afford, in high
yields, sugar-based N-sulfonyl aziridines. This process
took place with good to high stereoselectivities while
maintaining the stereochemical integrity of the sugar
backbone. The aziridines 3 obtained were those predicted
by the Felkin–Ahn model.
Acknowledgment
We appreciate financial support from the Spanish Ministerio de
Economía y Competitividad (CTQ2010-14959, MAT2006-01997,
MAT2010-15094 and Factoría de Cristalización Consolider Inge-
nio 2010), Fundação para a Ciência e a Tecnologia (project PEst-
C/QUI/UI0062/2011) and FEDER funding. C.C.F. and N.A. thank
MICINN for a Juan de la Cierva contract and FICYT for a
predoctoral fellowship, respectively. R.S. thanks the FCT for an
‘Investigador Auxiliar’ position.
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1519. (c) Davis, F. A.; Liu, H.; Zhou, P.; Fang, T.; Reddy, G.
V.; Zhang, Y. J. Org. Chem. 1999, 64, 7559. (d) Kasai, M.;
Kono, M. Synlett 1992, 778. (e) Sweeney, J. B. Chem. Soc.
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(b) Caldwell, J. J.; Craig, D. Angew. Chem. Int. Ed. 2007, 46,
2631. (c) Crawley, S. L.; Funk, R. L. Org. Lett. 2006, 8,
3995. (d) Banwell, M. G.; Lupton, D. W. Org. Biomol.
Chem. 2005, 3, 213. (e) Smith, A. B. III.; Kim, D. Org. Lett.
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(19) Rodríguez-Solla, H.; Concellón, C.; Alvaredo, N.; Soengas,
R. G. Tetrahedron 2012, 68, 1736.
(20) Synthesis of 6-Deoxy-1,2:3,4-di-O-isopropylidene-6[(4-
methoxyphenyl)imino]-α-D-galactopyranose (1a):19 N-p-
Methoxyphenylimine 1a was prepared in nearly quantitative
yields by stirring a solution of the corresponding aldehyde
(10 mmol, 1.0 equiv) in CH2Cl2 with p-anisidine (10 mmol,
1.0 equiv), in the presence of MgSO4 (2 g) for 16 h. Yield:
3.4 g (93%); orange oil; [α]D20 +7.2 (c 0.6 in CHCl3);
Rf = 0.62 (hexane–EtOAc, 3:1). IR (neat): 3060, 1674, 1510,
1260 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.75 (d,
J = 4.0 Hz, 1 H), 7.03 (d, J = 8.6 Hz, 2 H), 6.78 (d,
J = 8.6 Hz, 2 H), 5.56 (d, J = 4.9 Hz, 1 H), 4.58 (dd, J = 7.7,
2.3 Hz, 1 H), 4.45–4.22 (m, 2 H),4.28 (dd, J = 5.0, 2.4 Hz,
1 H), 3.69 (s, 3 H), 1.48 (s, 3 H), 1.39 (s, 3 H), 1.26 (s, 3 H),
1.24 (s, 3 H); 13C NMR (75 MHz, CDCl3): δ = 160.9 (CH),
(4) Lefemine, D. V.; Dann, M.; Barbatschi, F.; Hausmann, W.
K.; Zbinovsky, V.; Monnikendam, P.; Adam, J.; Bohnos, N.
J. Am. Chem. Soc. 1962, 34, 3184.
(5) Gerhart, F.; Higgins, W.; Tardif, C.; Ducep, J. J. Med. Chem.
1990, 33, 2157.
© Georg Thieme Verlag Stuttgart · New York
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184
H. Rodríguez-Solla et al.
LETTER
158.1 (C), 143.7 (C), 121.8 (2 × CH), 113.9 (2 × CH), 109.2
(C), 108.5 (C), 96.0 (CH), 73.1 (CH), 70.3 (CH), 70.2 (CH),
55.1 (CH3), 53.2 (CH), 25.8 (CH3), 25.7 (CH3), 24.6 (CH3),
24.0 (CH3); MS (ESI+): m/z (%) = 364 (100) [M + H]+, 338
(7), 322 (30), 306 (4); HRMS (ESI+-TOF): m/z [M + H]+
calcd for C19H26NO6: 364.1760; found: 364.1755.
CH2I2 (0.6 mmol, 1.5 equiv) in anhydrous THF (2 mL), was
added a solution of MeLi (1.5 M in Et2O, 0.48 mmol, 1.2
equiv) at –78 °C. The solution was stirred at –78 °C for 30
min, and then allowed to warm to r.t. and stirred for an
additional 30 min. The reaction was then quenched with aq.
NH4Cl and the organic layer was extracted with Et2O (3 × 10
mL). The combined extracts were dried with Na2SO4 and
concentrated under vacuum to yield crude N-sulfinyl
aziridines. Finally, to a solution of the corresponding
sulfinyl aziridine in CH2Cl2 (1 mL), anhydrous MCPBA (0.4
mmol, 1 equiv) was added at r.t. in one portion. When the
reaction was complete (less than 1 min), the reaction was
diluted with CH2Cl2 (2 mL) and washed with sat. NaHCO3
(3 × 3 mL). Organic layers were dried over anhydrous
Na2SO4 and the solvents were evaporated, giving the
corresponding N-sulfonyl aziridines 3, which were purified
by flash chromatography on silica gel (hexane–EtOAc, 3:1).
6,7-Deoxy-6,7-(N-tert-butylsulfinyl)imino-1,2:3,4-di-O-
isopropylidene-D-glycero-α-D-galacto-heptapyranose
(3a): Yield: 135.1 mg (86%); white solid; m.p. 84–85 °C
(Et2O–hexane); [α]D20 –9.2 (c 1.0, CHCl3); Rf = 0.30
(hexane–EtOAc, 3:1). IR (neat): 1640, 1458, 1308, 1067,
724 cm–1. 1H NMR (300 MHz, CDCl3): δ = 5.50 (d,
J = 4.9 Hz, 1 H), 4.61 (dd, J = 7.9, 2.4 Hz, 1 H), 4.33 (dd,
J = 7.9, 1.5 Hz, 1 H), 4.30 (dd, J = 4.9, 2.4 Hz, 1 H), 3.63
(dd, J = 5.2, 1.5 Hz, 1 H), 3.09 (dt, J = 6.9, 5.2 Hz, 1 H),
2.64 (d, J = 6.9 Hz, 1 H), 2.33 (d, J = 5.2 Hz, 1 H), 1.49 (s,
9 H), 1.47 (s, 3 H), 1.44 (s, 3 H), 1.34 (s, 3 H), 1.32 (s, 3 H);
13C NMR (75 MHz, CDCl3): δ = 109.3 (C), 108.4 (C), 96.0
(CH), 71.4 (CH), 70.3 (2 × CH), 67.0 (CH), 59.1 (C), 37.0
(CH), 31.2 (CH2), 25.8 (CH3), 25.7 (CH3), 24.6 (CH3), 24.1
(CH3), 23.9 (3 × CH3); MS (ESI+): m/z (%) = 430 (18) [M +
K]+, 416 (14) [M + 2 + Na]+, 415 (32) [M + 1 + Na]+, 414
(100) [M + Na]+; HRMS (ESI+-TOF): m/z [M + Na]+ calcd
for C17H29NO7NaS: 414.1562; found: 414.1558.
(21) Synthesis of N-tert-Butylsulfinyl Imines 2: To a solution of
the corresponding aldehyde (0.5 mmol, 1 equiv) in
anhydrous CH2Cl2 (10 mL), [Ti(OEt)4] (2 mmol, 4 equiv)
was added slowly by using a syringe. N-tert-
Butanesulfinamide (0.5 mmol, 1 equiv) was added in one
portion and the reaction mixture was stirred at r.t. for16 h.
The excess [Ti(OEt)4] was decomposed by slow addition of
the reaction mixture to a solution of brine. The resulting
suspension was filtered through a pad of Celite, washed with
ethyl acetate, the organic layer was separated, and the
aqueous phase was extracted with ethyl acetate (3 × 25 mL).
The combined organic extracts were dried over Na2SO4 and
concentrated under vacuum to yield crude N-tert-
butylsulfinyl imines 2 without further purification.
6-Deoxy-6-(N-tert-butylsulfinyl)imino-1,2:3,4-di-O-
isopropylidene-α-D-galactopyranose (2a): Yield: 155.8
mg (86%); yellow oil; Rf = 0.36, 0.33 (hexane–EtOAc, 3:1).
IR (neat): 3051, 1633, 1458, 1071, 735, 702 cm–1; 1H NMR
(300 MHz, CDCl3): δ = 8.00 (d, J = 2.7 Hz, 1 H), 5.62 (d,
J = 4.9 Hz, 1 H), 4.64 (dd, J = 7.8, 2.4 Hz, 1 H), 4.60 (app. t,
J = 2.7 Hz, 1 H), 4.49 (dd, J = 7.8, 2.4 Hz, 1 H), 4.35 (dd,
J = 4.9, 2.4 Hz, 1 H), 1.52 (s, 3 H), 1.39 (s, 3 H), 1.33 (s,
3 H), 1.28 (s, 3 H), 1.20 (s, 9 H); δ (minor isomer) = 8.00 (d,
J = 2.6 Hz, 1 H), 5.65 (d, J = 5.2 Hz, 1 H), 4.64 (dd, J = 7.7,
2.4 Hz, 1 H), 4.56 (dd, J = 7.6, 2.6 Hz, 1 H), 4.40–4.37 (m,
1 H), 4.31–4.20 (m, 1 H), 1.43 (s, 3 H), 1.32 (s, 3 H), 1.29 (s,
3 H), 1.28 (s, 3 H), 1.20 (s, 9 H); 13C NMR (75 MHz,
CDCl3): δ = 167.1 (CH), 109.6 (C), 109.4 (C), 96.1 (CH),
72.6 (CH), 70.4 (2 × CH), 70.2 (CH), 57.2 (C), 25.9 (CH3),
25.6 (CH3), 24.6 (CH3), 24.1 (CH3), 22.2 (3 × CH3); δ
(minor isomer) = 165.2 (CH), 109.8 (C), 109.7 (C), 96.1
(CH), 71.9 (CH), 70.4 (2 × CH), 70.1 (CH), 57.1 (C), 25.9
(CH3), 25.6 (CH3), 24.6 (CH3), 24.1 (CH3), 22.3 (3 × CH3);
MS (ESI+): m/z (%) = 400 (5) [M + K]+, 386 (9) [M + 2 +
Na]+, 385 (20) [M + 1 + Na]+, 384 (100) [M +Na]+; HRMS
(ESI+-TOF): m/z [M + Na]+ calcd for C16H27NO6SNa:
384.1457; found: 384.1462.
(23) CCDC 900247 contains the supplementary crystallographic
data for compound 3d. This data can be obtained free of
charge via: www.ccdc.cam.ac.uk/conts/retrieving.html [or
the Cambridge Data Center, 12 Union Road, Cambridge
CB2 1EZ, UK; fax +44(1223)336033; or
deposit@ccdc.cam.ac.uk].
(24) (a) Rodríguez-Solla, H.; Alvaredo, N.; Soengas, R. G.
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