Use of α-chlorinated N-(tert-butanesulfinyl)-imines in the synthesis of chiral aziridines

Article abstract of DOI:10.1021/ol0611245

Reaction of chiral α-chloro tert-butanesulfinyl aldimines with Grignard reagents efficiently afforded β-chloro N-sulfinamides in high diastereomeric excess. The latter compounds were cyclized toward the corresponding chiral aziridines in a high-yielding o

Full text of DOI:10.1021/ol0611245

ORGANIC
LETTERS
2006
Vol. 8, No. 14
3129-3132
Use of
r-Chlorinated N-(tert-Butanesulfinyl)-
imines in the Synthesis of Chiral Aziridines
1
Bram Denolf,† Sven Mangelinckx,† Karl W. Tornroos,‡ and Norbert De Kimpe*,†
Department of Organic Chemistry, Faculty of Bioscience Engineering, Ghent
UniVersity, Coupure Links 653, B-9000 Ghent, Belgium, and Department of Chemistry,
UniVersity of Bergen, Allegaten 41, N-5007 Bergen, Norway
norbert.dekimpe@UGent.be
Received May 8, 2006
ABSTRACT
Reaction of chiral
excess. The latter compounds were cyclized toward the corresponding chiral aziridines in a high-yielding one-pot reaction or after separate
treatment with base. The diastereoselectivity obtained in the newly synthesized -chloro sulfinamides is explained via the coordinating ability
of the -chloro atom with magnesium resulting in the opposite stereochemical outcome as generally observed for nonfunctionalized N-sulfinyl
imines.
r-chloro tert-butanesulfinyl aldimines with Grignard reagents efficiently afforded â-chloro N-sulfinamides in high diastereomeric
â
r
R-Halogenated imines have received considerable attention
by Davis3 and Ellman,4 has proven to be a straightforward
method to synthesize chiral amines, amino alcohols, and
amino acids, among other interesting compounds. The use
of halogenated N-sulfinyl imines has been limited to some
fluorinated examples,5 despite the advantageous reactivity
of known alkylated and arylated halogenated imines men-
tioned. A few examples of chloro or bromo N-sulfinyl imines
are described,6 but their reactivity has not been investigated
up to now. Incorporation of a chlorine atom in N-sulfinyl
imine 2, as compared to N-sulfinyl imine 1, could lead to
chiral aziridines, which are of considerable interest in organic
chemistry,7-10 in a very straightforward way. The stereose-
lective synthesis of aziridines 4 via R-halo N-sulfinyl imines
as versatile intermediates for the synthesis of biologically
active compounds.1 The simple synthesis of R-halo imines
from the corresponding readily available R-chloro aldehydes
or R-chloro ketones and the subsequent cyclization, after a
nucleophilic addition reaction, toward azaheterocycles make
the R-halogenated imidoyl function an interesting building
block.2 Even though these R-halogenated imines have found
wide application, their use is not without problems (e.g.,
hydrolytically unstable, poor electrophiles).1d The use of a
chiral N-protective group to activate the imino function for
nucleophilic addition in a diastereofacial way and for easy
removal after reaction under mild conditions is of interest
to synthesize a variety of compounds. The diastereoselective
addition of organometallics to N-sulfinyl imines, pioneered
(3) (a) Davis, F. A.; Reddy, R. E.; Szewczyk, J. M.; Reddy, G. V.;
Portonovo, P. S.; Zhang, H.; Fanelli, D.; Reddy, R. T.; Zhou, P.; Carroll,
P. J. J. Org. Chem. 1997, 62, 2555. (b) Zhou, P.; Chen, B.-C.; Davis, F. A.
Tetrahedron 2004, 60, 8003 and references cited therein.
(4) (a) Cohan, D. A.; Lui, G.; Ellman, J. A. Tetrahedron 1999, 55, 8883.
(b) Ellman, J. A.; Owens, T. D.; Tang, T. P. Acc. Chem. Res. 2002, 35,
984. (c) Ellman, J. A. Pure Appl. Chem. 2003, 75, 39.
(5) (a) Wang, H.; Zhao, X.; Li, Y.; Lu, L. Org. Lett. 2006, 8, 1379. (b)
Kuduk, S. D.; Marco, C. N. D.; Pitzenberger, S. M.; Tsou, N. Tetrahedron
Lett. 2006, 47, 2377. (c) Crucianelli, M.; De Angelis, F.; Lazarro, F.;
Malpezzi, L.; Volonterio, A.; Zanda, M. J. Fluorine Chem. 2004, 125, 573.
(d) Asensio, A.; Bravo, P.; Crucianelli, M.; Farina, A.; Fustero, S.; Soler,
J. G.; Meille, S. V.; Panzeri, W.; Viani, F.; Volonterio, A.; Zanda, M. Eur.
J. Org. Chem. 2001, 1449. (e) Davis, F. A.; Srirajan, V.; Titus, D. D. J.
Org. Chem. 1999, 64, 6931. (f) Bravo, P.; Crucianelli, M.; Vergani, B.;
Zanda, M. Tetrahedron Lett. 1998, 38, 7771.
† Ghent University.
‡ University of Bergen.
(1) (a) Mangelinckx, S.; Giubellina, N.; De Kimpe, N. Chem. ReV. 2004,
104, 2353. (b) Giubellina, N.; Aelterman, W.; De Kimpe, N. Pure Appl.
Chem. 2003, 75, 1433. (c) Stevens, C.; De Kimpe, N. Org. Prep. Proced.
Int. 1990, 22, 589. (d) De Kimpe, N.; Verhe´, R. In The chemistry of
R-haloketones, R-haloaldehydes and R-haloimines; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1988.
(2) (a) Concellon, J. M.; Riego, E.; Rivero, I. A.; Ochoa, A. J. Org.
Chem. 2004, 69, 6244. (b) Sulmon, P.; De Kimpe, N.; Schamp, N.
Tetrahedron 1989, 45, 3907. (c) De Kimpe, N.; De Smaele, D. Tetrahedron
Lett. 1994, 35, 8023.
10.1021/ol0611245 CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/14/2006

and subsequent stirring for 12 h at ambient temperature,
3-ethylaziridine 8a was formed in modest yield (46%) and
diastereoselectivity (dr ) 70:30) (Table 1, entry a).
Scheme 1
Table 1. Addition of Grignard Reagents Across R-Chloro
N-Sulfinyl Aldimine (RS)-6a
R′ (equiv)
solvent
t (h) [°C]
product
yield %a (dr)b
a
b
c
d
e
f
g
h
i
Et (2.0)
Ph (2.0)
Et (2.0)
Ph (1.1)
Et (1.1)
Et (1.1)
Et (1.1)
Et (1.1)
Et (1.1)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
THF
4 [-78]c
1 [-78]d
4 [-78]
5 [-78]c,d
4 [-78]c
4 [-78]
4 [-78]
4 [-78]
4 [-78]
8a
9b
9a
9b
8a
9a
9a
9a
9a
46 (70:30)
43 (98:2)
2 has not been described (Scheme 1). In this report, the above
methodology with R-halo N-sulfinyl imines 2, carrying a
chiral (RS)-tert-butanesulfinyl group at nitrogen, will be
worked out toward the chiral synthesis of aziridines.
R-Chloro N-sulfinyl aldimines (RS)-6, a new class of
functionalized N-sulfinyl imines, were synthesized via
condensation of R-chloro aldehydes 5 (1.1 equiv)1d with (RS)-
tert-butanesulfinamide in the presence of 2 equiv of Ti(OEt)4
in THF under reflux (Scheme 2).
73 (88:12)
82 (99:1)
95e (96:4)
94e (95:5)
92e (72:28)
96e (78:22)
95e (96:4)
Et2O
CH2Cl2
a Determined by a mass balance after chromatography. b Determined by
NMR analysis of the reaction mixture. c Followed by stirring at room
temperature for 12 h. d Addition of the Grignard reagent at -97 °C.
e Determined by a mass balance of the reaction mixture.
The temperature effect was of great importance for both
the yield and the stereoselectivity of the addition reaction.
Reaction of aldimine (RS)-6a with 2 equiv of PhMgCl for 4
h at -78 °C and subsequent stirring at room temperature
for 12 h yielded a complex reaction mixture from which only
traces of the desired 2-phenylaziridine 8b were obtained next
to some of the corresponding arylated â-chloro N-sulfinamide
9b. At higher temperatures (-20 °C), side reactions became
even more important and it was a major drawback if PhMgCl
was used as the reagent of choice. In contrast, if the Grignard
reagent was added at -97 °C and the reaction mixture was
allowed to react further at -78 °C for 5 h, a very clean
addition reaction yielded â-chloro N-sulfinamide (RS,R)-9b.
No traces of the corresponding aziridine (RS,R)-8b were
detected under these conditions.
Scheme 2. Synthesis of R-Chloro N-Sulfinyl Aldimines (RS)-6
R-Chloro N-sulfinyl aldimines (RS)-6 were isolated in high
yields (87-95%) after distillation to separate the small excess
of aldehydes 5 used.
Upon treatment of R-chloro N-sulfinyl imine (RS)-6a with
2 equiv of EtMgBr in dichloromethane at -78 °C for 4 h
By lowering the excess of Grignard reagent, the yields of
â-chloro sulfinamide (RS,R)-9b improved. Even more, the
diastereoselectivity was also slightly improved (Table 1,
entries a-e). A lower diastereoselectivity was obtained in
ethereal solvents, such as THF or diethyl ether. The use of
noncoordinating solvents such as dichloromethane or toluene
resulted in better diastereoselectivities (Table 1, entries f-i).
The best results were obtained if 1.1 equiv of Grignard
reagent was allowed to react with aldimine (RS)-6a in
dichloromethane at -78 °C for 4-5 h. Though the addition
of PhMgCl was preferred at -97 °C, these low-temperature
conditions were not required for the addition reaction of
EtMgBr, vinylMgBr, and allylMgCl. Upon treatment of (RS)-
6a with Grignard reagent, under the former conditions,
â-chloro N-sulfinamides (RS,R)-9 were isolated in high yield
(82-99%) and diastereoselectivity (62:38-99:1) (Scheme
3). Reaction of allylMgCl with (RS)-6a afforded â-chloro
N-sulfinamide (RS,R)-9d in 99% yield but as a (62:38)
mixture of diastereomers (Scheme 3). Both diastereomers
were separated by flash chromatography after further ring
closure (vide infra) toward the aziridines (RS,R)-8d and
(RS,S)-8d in 57% and 10% yield, respectively. Notably,
addition of i-PrMgCl to (RS)-6a at -97 °C and reaction at
(6) (a) Davis, F. A.; Prasad, K. R.; Nolt, B.; Wu, Y. Org. Lett. 2003, 5,
925. (b) Schenkel, L. B.; Ellman, J. A. Org. Lett. 2003, 5, 545. (c) Davis,
F. A.; Nolt, M. B.; Wu, Y.; Prasad, K. R.; Li, D.; Yang, B.; Bowen, K.;
Lee, S. H.; Eardley, J. H. J. Org. Chem. 2005, 70, 2184. (d) Davis, F. A.;
Zhang, Y.; Andemichael, Y.; Fang, T.; Fanelli, D. L.; Zhang, H. J. Org.
Chem. 1999, 64, 1403.
(7) (a) Padwa, A.; Murphee, S. ArkiVoc 2006, 3, 6. (b) Hu, X. E.
Tetrahedron 2004, 60, 2701. (c) Sweeney, J. B. Chem. Soc. ReV. 2002, 31,
247. (d) Zwanenburg, B.; ten Holte, P. Top. Curr. Chem. 2001, 216, 93.
(e) McCoull, W.; Davis, F. A. Synthesis 2000, 1347. (f) Lindstro¨m, U. M.;
Somfai, P. Synthesis 1998, 109. (g) Padwa, A.; Woolhouse, A. D. In
ComprehensiVe Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W., Eds.;
Pergamon: Oxford, 1984; Vol. 7, pp 47-97.
(8) (a) Ferna`ndez, I.; Valdivia, V.; Gori, B.; Alcudia, F.; Alvarez, E.;
Khiar, N. Org. Lett. 2005, 7, 1307. (b) Morton, D.; Pearson, D.; Field, R.
A.; Stockman, R. A. Org. Lett. 2004, 6, 2377. (c) Yang, Y.-F.; Zhang,
M.-J.; Hou, X.-L.; Dai, L.-X. J. Org. Chem. 2002, 67, 8097. (d) Garc´ıa
Ruano, J. L.; Ferna´ndez, I.; Hamdouchi, C. Tetrahedron Lett. 1995, 36,
295. (e) Davis, F. A.; Zhou, P.; Liang, C.-H.; Reddy, R. E. Tetrahedron:
Asymmetry 1995, 6, 1511.
(9) (a) Zheng, J.-C.; Liao, W.-W.; Sun, X.-X.; Sun, X.-L.; Tang, Y.;
Dai, L. X.; Deng, J. G. Org. Lett. 2005, 7, 5789. (b) Ferreira, F.; Audouin,
M.; Chemla, F. Chem.-Eur. J. 2005, 11, 5269. (c) Chemla, F.; Ferreira, F.
Synlett 2004, 983.
(10) (a) Davis, F. A.; Wu, Y.; Yan, H.; McCoull, W.; Prasad, K. R. J.
Org. Chem. 2003, 68, 2410. (b) Davis, F. A.; Deng, J.; Zhang, Y.;
Haltiwanger, R. C. Tetrahedron 2002, 58, 7135. (c) Volonterio, A.; Bravo,
P.; Pesenti, C.; Zanda, M. Tetrahedron Lett. 2001, 42, 3985. (d) Kim, D.
Y.; Suh, K. H.; Choi, J. S.; Mang, J. Y.; Chang, S. K. Synth. Commun.
2000, 30, 87. (e) Davis, F. A.; McCoull, W.; Titus, D. D. Org. Lett. 1999,
1, 1053.
3130
Org. Lett., Vol. 8, No. 14, 2006

d). Longer reaction times or higher temperatures did not
afford the desired aziridines (RS,R)-8b in acceptable yield.
Therefore, a series of bases (BuLi, LDA, NaH, and K2CO3)
were applied in different solvents to facilitate the cyclization
reaction of â-chloro N-sulfinamide (RS,R)-9b toward aziridine
(RS,R)-8b (Supporting Information).
Scheme 3. Stereoselective Aziridination of Imine (RS)-6a
Experimentally, it was shown that KOH in H2O/THF (1:1
ratio) at 50 °C gave the best results to ring close the â-chloro
N-sulfinamide (RS,R)-9b toward the corresponding aziridine
(RS,R)-8b in high yields. Even more, these conditions were
applied to all â-chloro N-sulfinamides (RS,R)-9 affording the
corresponding aziridines (RS,R)-8 in excellent yield without
detectable loss of chirality. Thus, these aziridines 8 were
synthesized in a high-yielding one- or two-pot reaction
(Scheme 3) and were obtained as a single diastereomer after
flash chromatography.
Upon treatment of the more sterically hindered R-chloro
N-sulfinyl imine (RS)-6b (R ) Et) with EtMgBr at -78 °C
for 4 h in THF, most of the aldimine (RS)-6b was recovered.
Almost no addition reaction was observed under the latter
conditions (Table 2, entry a). Upon raising the temperature
from -78 °C to -40 °C, after the addition of the Grignard
reagent at -78 °C or -97 °C (vide supra), depending on
the reagent used, mixtures of the â-chloro N-sulfinamide
(RS,R)-11 and the corresponding aziridine (RS,R)-12 were
obtained in reasonable yield. The â-chloro N-sulfinamides
(RS,R)-11 could only be isolated in low yields after flash
chromatography of the reaction mixture. Various attempts
failed to synthesize the â-chloro N-sulfinamide (RS,R)-11a
as such, without any of the aziridine 12a present. However,
if the reaction mixture was stirred for 12 more hours at
ambient temperature, the â-chloro N-sulfinamides (RS,R)-
-78 °C resulted in the reduction of the imidoyl function
affording the reduced â-chloro sulfinamide 10a in high yield
(95%) (Scheme 3).11
Quenching of the reaction mixture of (RS)-6a upon
treatment with Grignard reagent (R′ * Ph) with aq NH4Cl
at -78 °C after 4 h afforded the alkylated â-chloro
N-sulfinamide (RS,R)-9 in high yield (92-99%). Moreover,
if the reaction mixture was allowed to stir at room temper-
ature after completion of the addition at -78 °C, the desired
aziridines 8 were isolated in excellent yield (Scheme 3).
However, if R′ ) Ph, it was noticed that the â-chloro
N-sulfinamides (RS,R)-9b were not ring closed toward the
corresponding aziridines (RS,R)-8b if the reaction mixture
was stirred at ambient temperature for 12 h (Table 1, entry
Table 2. Optimization of the Addition of Grignard Reagents Across R-Chloro N-Sulfinyl Aldimine (RS)-6b,c
entry
substrate
reagent
equiv
solvent
t (h) [°C] (rt)
product
yield %d (%)e
(dr)f
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
6b
6b
6b
6b
6b
6b
6b
6b
6b
6b
6b
6c
6c
6c
6c
6c
EtMgBr
EtMgBr
EtMgBr
EtMgBr
EtMgBr
EtMgBr
PhMgCl
EtMgBr
PhMgCl
vinylMgBr
allylMgCl
EtMgBr
EtMgBr
PhMgCl
vinylMgBr
allylMgCl
2
THF
4 [-78]
11a
12a
12a
12a
12a
12a
11b
12a
12b
12c
12d
13a
14a
14b
14c
14d
8
(-)
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
CH2Cl2
CH2Cl2
CH2Cl2
Et2O
3 [-40]a 12 (rt)
1 [-40]a 12 (rt)
6 [-40]a 12 (rt)
6 [-40]a 12 (rt)
6 [-40]a 12 (rt)
6 [-40]b 12 (rt)
6 [-40]a,c
83
(86:14)
(-)
29
82
(92:8)
(78:22)
(91:9)
(96:4)
(92:8)
(96:4)
(93:7)
(62:38)
(-)
(92:8)
(70:30)
(86:14)
(70:30)
84
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
81
77
82 (66)
77 (57)
83 (62)
90 (45)[21]
(25)
6 [-40]b,c
6 [-40]a,c
6 [-40]a,c
6 [-40]a
6 [-40]a,c
82 (63)
43 (20)
85 (56)
84 (41)
6 [-40]b,c
6 [-40]a,c
6 [-40]a,c
a Addition of the Grignard reagent at -78 °C. b Addition of the Grignard reagent at -97 °C. c Subsequent ring closure with 3 equiv of KOH in THF/H2O
(1:1) for 24 h at 50 °C. d Determined by a mass balance of the reaction mixture. e Isolated yield of the (major) [minor] diastereomer after flash chromatography.
f Determined by NMR analysis of the reaction mixture.
Org. Lett., Vol. 8, No. 14, 2006
3131

11 were ring closed toward the corresponding aziridines 12
if R′ * Ph.
If the reaction was performed for less than 6 h at -40 °C
before raising the temperature to 20 °C for 12 h, lower
diastereoselectivities were obtained, which was an indication
that the reaction was not completed at -40 °C (Table 2;
entries b-d).
Altering the solvent from CH2Cl2 to Et2O or toluene did
not improve the yield or the stereoselectivity of the reaction
(entries d-f).
Thus, addition of 1.1 equiv of Grignard reagent to imine
(RS)-6b in dichloromethane at -78 °C and subsequent stirring
for 6 h at -40 °C afforded a mixture of â-chloro N-
sulfinamide (RS,R)-11 and aziridine (RS,R)-12, after aqueous
workup. The latter mixture was treated with base (3 equiv
of KOH) at 50 °C for 24 h to obtain the ring-closed
N-sulfinyl aziridine 12 (Table 2). No racemization was
observed under these conditions. These conditions were
applied with good result (77-90% yield and dr of 62:38-
96:4) for the synthesis of a variety of new chiral sterically
hindered aziridines (RS,R)-12a-d starting from R-chloro
N-sulfinyl imine (RS)-6b (Table 2). It was found that chiral
1-azaspiro[2.5]octanes (RS,R)-14a-d could be synthesized
in moderate to good yield (43-85%) and diastereoselectivity
(70:30-92:8), starting from R-chloro N-sulfinyl imine (RS)-
6c under these optimized conditions (Table 2). All chiral
aziridines 12,14 were obtained as a single enantiomer [(RS,R)-
12,14] after flash chromatography. â-Chloro N-sulfinamide
(RS,R)-13a could be isolated as a pure product in 25% yield,
starting from R-chloro N-sulfinyl imine (RS)-6c, after the
addition of 1.1 equiv of EtMgBr at -78 °C, subsequent
stirring for 6 h at -40 °C, aqueous workup with NH4Cl at
this temperature, and subsequent flash chromatography of
the crude reaction mixture (Table 2, entry l).
of N-sulfinyl aziridine (RS,R)-8b resulted mainly in aziridine
(R)-15b, though up to 15% of â-chloro amine (R)-18b (R )
Me; R′ ) Ph) was also formed.
Without any of the â-chloro amines (R)-18,19 or aziridines
(R)-15-17 reported in the literature, the absolute configu-
ration had to be determined by X-ray diffraction analysis.
From the X-ray diffraction analysis of N-(tert-butanesulfinyl)-
2,2-dimethyl-3-phenylaziridine (RS)-8b the absolute config-
uration of the structure was undoubtedly characterized as
being the (RS,R)-aziridine 8b. This configuration is opposite
to the one that is predicted via the chelation-controlled
transition state A (Figure 1), which is the general intermediate
Figure 1. Proposed transition states.
proposed for nonfunctionalized N-sulfinyl imines.4,12 The
reversal of the stereochemical outcome of the reaction is
attributed to the R-coordinating ability of the chlorine atom
as depicted in Figure 1 (transition states B and C). The
reversal of selectivity is analogous to the results obtained
with other N-sulfinyl imines containing an R-coordinating
group, such as a nitrogen or oxygen atom.12a,13
In conclusion, a novel stereoselective synthesis of chiral
2-arylated and 2-alkylated aziridines (R)-15-17 has been
developed. Reaction of R-chloro N-sulfinyl imines (RS)-6
with Grignard reagents afforded â-chloro N-sulfinamides
(RS,R)-9 in good yields or 11 and 13 as nonisolated
intermediates. The latter compounds were ring closed toward
the corresponding N-sulfinyl aziridines (RS,R)-8,12,14 in a
high-yielding one-pot reaction or after separate treatment with
base. Chiral aziridines (R)-15-17 were synthesized by
subsequent deprotection of the N-protecting group. The
absolute configuration of the aziridines formed was proven
by X-ray analysis.
In the literature, no precedents of the synthesized amides
(RS,R)-9,13 or aziridines (RS,R)-8,12,14 have been reported.
The N-sulfinamides (RS,R)-9,13 and N-sulfinyl aziridines
(RS,R)-8,12,14 could be deprotected by simple treatment with
a saturated solution of dry HCl in dioxane. Stirring for 5
min at room temperature afforded the HCl salts of the
â-chloro amines (R)-18,19 or aziridines (R)-15-17 in high
yield (>90%) and purity (>80%) (Scheme 4). Deprotection
Acknowledgment. Research funded by a Ph.D. grant of
the Institute for the Promotion of Innovation through Science
and Technology in Flanders (IWT-Vlaanderen).
Scheme 4. N-Sulfinyl Deprotection in 1,4-Dioxane‚HCl
Supporting Information Available: Experimental details
and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Org. Lett., Vol. 8, No. 14, 2006