Stereoselective olefination and regiospecific vicinal difunctionalization of imines with α-(benzothiazol-2-ylsulfonyl) carbonyl compounds

Article abstract of DOI:10.1002/ejoc.201101684

Depending on their structures, imines are able to undergo either olefination or vicinal difunctionalization with various α-(benzothiazol-2- ylsulfonyl) carbonyl compounds in the absence of external bases. The olefination reaction of aromatic imines with α-(benzothiazol-2-ylsulfonyl) carbonyl compounds proceeds smoothly in tetrahydrofuran at 70 °C to give structurally diverse α,β-unsaturated esters, amides, and ketones in good to excellent yields and with extremely high (E) selectivity. In contrast, the carbon-nitrogen double bonds of cyclic imines and the carbon-carbon double bonds of α,β-unsaturated imines are subjected to regiospecific vicinal difunctionalization with α-(benzothiazol-2-ylsulfonyl) carbonyl compounds under the same reaction conditions to give a variety of benzothiazole derivatives in good to excellent yields. It is noteworthy that the benzothiazole moiety is present in a number of antitumor agents and bioluminescent molecules. In addition, plausible reaction pathways have been proposed to account for these transformations, and these are substantially supported by ESI-MS analysis of the reaction mixtures.

Full text of DOI:10.1002/ejoc.201101684

FULL PAPER
DOI: 10.1002/ejoc.201101684
Stereoselective Olefination and Regiospecific Vicinal Difunctionalization of
Imines with α-(Benzothiazol-2-ylsulfonyl) Carbonyl Compounds
You-Dong Shao,[a] Xue-Song Wu,[a] and Shi-Kai Tian*[a,b]
Keywords: Alkenes / Reaction mechanisms / Regioselectivity / Sulfur heterocycles / Synthetic methods
Depending on their structures, imines are able to undergo
either olefination or vicinal difunctionalization with various
α-(benzothiazol-2-ylsulfonyl) carbonyl compounds in the ab-
sence of external bases. The olefination reaction of aromatic
unsaturated imines are subjected to regiospecific vicinal di-
functionalization with α-(benzothiazol-2-ylsulfonyl) carbonyl
compounds under the same reaction conditions to give a vari-
ety of benzothiazole derivatives in good to excellent yields.
imines with α-(benzothiazol-2-ylsulfonyl) carbonyl com- It is noteworthy that the benzothiazole moiety is present in a
pounds proceeds smoothly in tetrahydrofuran at 70 °C to give
structurally diverse α,β-unsaturated esters, amides, and
ketones in good to excellent yields and with extremely high
(E) selectivity. In contrast, the carbon–nitrogen double bonds
of cyclic imines and the carbon–carbon double bonds of α,β-
number of antitumor agents and bioluminescent molecules.
In addition, plausible reaction pathways have been proposed
to account for these transformations, and these are substan-
tially supported by ESI-MS analysis of the reaction mixtures.
Introduction
Benzothiazol-2-yl sulfones are readily accessible and fre-
quently employed in the modified Julia reaction for trans-
forming aldehydes into alkenes with generally high levels of
stereoselectivity.[1] In recent years, the olefination of alde-
hydes with α-(benzothiazol-2-ylsulfonyl) carbonyl com-
pounds has emerged as a powerful method for the prepara-
tion of electron-deficient alkenes,[2,3] which are not only
present in many natural products of biologically signifi-
cance,[4] but also serve as useful electrophilic species in a
broad range of chemical transformations such as Michael
addition, cycloaddition, and cross-coupling reactions.[5] Ac-
cording to the generally accepted mechanism for the modi-
fied Julia reaction, stoichiometric amounts of bases are
needed to deprotonate benzothiazol-2-yl sulfones, including
α-(benzothiazol-2-ylsulfonyl) carbonyl compounds, and the
resulting sulfonyl-stabilized carbanions undergo addition to
C=O bonds followed by the Smiles rearrangement and β-
elimination of the heterocyclic moiety to yield alkenes
through extrusion of sulfur dioxide (Scheme 1, Path a).[1]
Scheme 1. Reactions of C=X bonds with benzothiazol-2-yl sulf-
ones.
The mechanism for the modified Julia reaction allowed
us to envision three new reactions of sulfonyl-stabilized
carbanions (or equivalents) with imines rather than alde-
hydes through similar reaction pathways under appropriate
conditions: (1) olefination of imines to give alkenes
(Scheme 1, Path b);[6] (2) vicinal difunctionalization of
imines to give 2-(alkylamino)benzothiazoles (Scheme 1,
Path c); and (3) vicinal difunctionalization of the carbon–
carbon double bonds of α,β-unsaturated imines to give 2-
alkylbenzothiazoles (Scheme 1, Path d). Whereas the first
reaction was anticipated to provide an alternative stereose-
lective synthesis of electron-deficient alkenes, the latter two
[a] Joint Laboratory of Green Synthetic Chemistry, Department of
Chemistry, University of Science and Technology of China
Hefei, Anhui 230026, China
Fax: +86-551-3601592
E-mail: tiansk@ustc.edu.cn
[b] Key Laboratory of Synthetic Chemistry of Natural Substances,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences
354 Fenglin Road, Shanghai 200032, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101684.
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Selective Olefination and Vicinal Difunctionalization of Imines
could yield structurally diverse benzothiazole derivatives. It group in the carbon nucleophile. Almost no reaction was
is noteworthy that the benzothiazole moiety is present in a observed when the benzothiazol-2-ylsulfonyl group in sub-
number of antitumor agents and bioluminescent molecules strate 2a was replaced by a pyridin-2-ylsulfonyl group or by
(Figure 1).[7,8] Although a Mannich addition was recently a 1-phenyl-1H-tetrazol-5-ylsulfonyl group (Table 1, En-
reported to occur between N-Boc-protected imines and α- tries 20 and 21).[15]
sulfonyl carbonyl compounds in the presence of a thio-
urea[9] or a base,[10,11] we believed that our proposed reac-
tion pathways could be realized by tuning the electronic and
Table 1. Optimization of reaction conditions.[a]
steric properties of the substituents on the imine nitrogen
atoms. In the course of exploring the synthetic utilities of
carbon–nitrogen bond cleavage,[12] we found that a variety
of imines could react with α-(benzothiazol-2-ylsulfonyl)
carbonyl compounds through either stereoselective ole-
fination or regiospecific vicinal difunctionalization under
base-free conditions. Herein, we report our results.
Figure 1. Some biologically important benzothiazole derivatives.
Results and Discussion
We initiated our investigation by employing Schiff base
imines as carbon electrophiles to react with α-(benzothi-
azol-2-ylsulfonyl) carbonyl compounds. To our surprise, in
the absence of external bases, the model reaction of N-
benzylideneaniline (1aa) with ethyl α-(benzothiazol-2-yl-
sulfonyl)acetate (2a) proceeded smoothly in a number of
organic solvents at 70 °C (Table 1, Entries 1–11).[13,14]
Tetrahydrofuran was identified as the solvent of choice; in
this solvent the reaction gave α,β-unsaturated ester 3a in
87% yield and with a selectivity of (E)/(Z) Ͼ 99:1 (Table 1,
[a] Reagents and conditions: imine (0.30 mmol), sulfone
(0.36 mmol), solvent (1.0 mL), 70 °C (oil bath), 24 h. p-Meth-
oxyphenyl (PMP), p-tolylsulfonyl (Ts), cyclohexyl (Cy), 1,2-dichlor-
oethane (DCE), tetrahydrofuran (THF), N,N-dimethylformamide
(DMF), dimethyl sulfoxide (DMSO). [b] Isolated yield. [c] Deter-
1
mined by H NMR spectroscopic analysis.
A broad range of aromatic and heteroaromatic N-(p-
Entry 5). A slightly enhanced yield (88%) was obtained methoxyphenyl)imines smoothly underwent olefination
from the reaction with N-benzylidene-p-methoxyaniline with alkyl α-(benzothiazol-2-ylsulfonyl)acetates to give
(1ab; Table 1, Entry 12). Nevertheless, replacement of the structurally diverse α,β-unsaturated esters in good to excel-
phenyl group on the imine nitrogen atom in substrate 1aa lent yields and with excellent (E) selectivity (Table 2, En-
with either a bulky aryl group or an alkyl group led to tries 1–12). It is noteworthy that both electron-withdrawing
either a much lower yield or no alkene product at all and electron-donating groups were successfully introduced
(Table 1, Entries 13–18). Interestingly, no reaction was ob- into the alkene products by employing imines bearing such
served with N-benzylidene-p-toluenesulfonamide (1ai), groups on the aromatic rings. To our delight, the imine ole-
which is more electrophilic but less basic than Schiff base fination reaction worked well with α-(benzothiazol-2-ylsulf-
imine 1ab (Table 1, Entry 19). Moreover, the imine ole- onyl)acetamides having a wide variety of substituents on
fination was significantly affected by the heteroarylsulfonyl the amide nitrogen atoms, and a range of α,β-unsaturated
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amides were obtained in good to excellent yields and with Table 3. Vicinal difunctionalization of cyclic imines with α-(benzo-
thiazol-2-ylsulfonyl) carbonyl compounds.[a]
selectivities of (E)/(Z) Ͼ 99:1 (Table 2, Entries 13–21).
Moreover, α-(benzothiazol-2-ylsulfonyl) ketones served as
suitable substrates for the highly (E)-selective synthesis of
the corresponding α,β-unsaturated ketones (Table 2, En-
tries 22–24).[16,17]
Table 2. Olefination of aromatic imines with α-(benzothiazol-2-yl-
sulfonyl) carbonyl compounds.[a]
[a] Reagents and conditions: imine 1 (0.30 mmol), sulfone 2
(0.36 mmol), THF (1.0 mL), 70 °C (oil bath), 12–48 h. [b] Isolated
1
yield. [c] Determined by H NMR spectroscopic analysis. [d] The
reaction was conducted at 120 °C in a sealed tube. [e] The reaction
was conducted at 140 °C in a sealed tube.
[a] Reagents and conditions: imine 4 (0.30 mmol), sulfone 2
(0.36 mmol), THF (1.0 mL), 70 °C (oil bath), 9–40 h. [b] Isolated
yield. [c] A solution of imine 4e in ethanol (1.2 m) was used.
In sharp contrast, the carbon–nitrogen double bonds of
cyclic imines were subjected to vicinal difunctionalization
with α-(benzothiazol-2-ylsulfonyl) carbonyl compounds un-
der the same reaction conditions (Table 3). No alkene inter-
mediate was detected by 1H NMR spectroscopic analysis of
Treatment of α,β-unsaturated imines with sulfone 2a in
the reaction mixture of imine 4a and sulfone 2a (Table 1, tetrahydrofuran at 70 °C did not result in the formation of
Entry 1). The derivatives of 3H-indole, 2H-benzo[b][1,4]- α,β,γ,δ-unsaturated esters. Instead, the carbon–carbon
thiazine, 3,4-dihydroisoquinoline, and 2,3,4,5-tetrahydro- double bonds of α,β-unsaturated imines underwent regio-
pyridine were found to react well with α-(benzothiazol-2- specific vicinal difunctionalization to give 2-(2-aminoalk-
ylsulfonyl) carbonyl compounds to give the corresponding enyl)benzothiazoles in good yields and with exclusive (Z)
functionalized 2-(alkylamino)benzothiazoles in good to ex- selectivity (Table 4). Owing to hydrogen bonding between
cellent yields. In addition, the structure of product 5f was the enamine N–H bond and the benzothiazole nitrogen
confirmed by single-crystal X-ray analysis (Figure 2).[18]
atom, such products are sufficiently stable to enable purifi-
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Selective Olefination and Vicinal Difunctionalization of Imines
acetamides, which gave a mixture of α,β,γ,δ-unsaturated
amides and 2-(2-aminoalkenyl)benzothiazoles. For example,
the reaction of imine 6a with sulfone 2i proceeded in tetra-
hydrofuran at 70 °C to give α,β,γ,δ-unsaturated amide 3y in
63% yield [(E)/(Z) Ͼ 99:1] together with 2-(2-aminoalk-
enyl)benzothiazole 7g in 17% yield [Equation (1)]. In con-
trast, complex mixtures were obtained from the reaction of
α,β-unsaturated imines with α-(benzothiazol-2-ylsulfonyl)
ketones under the same reaction conditions.
Figure 2. X-ray crystal structure of compound 5f.[18]
cation by chromatography on silica gel. The structure of
product 7a was confirmed by single-crystal X-ray analysis
(Figure 3).[18]
Table 4. Vicinal difunctionalization of α,β-unsaturated imines with
sulfone 2a.[a]
Because Schiff base imines are generally prepared by de-
hydrative condensation of aldehydes with primary
amines,[19] we examined the reaction of sulfone 2a with an
aldehyde and p-methoxyaniline, wherein the corresponding
Schiff base imine could be generated in situ (Scheme 2). The
reaction with benzaldehyde gave α,β-unsaturated ester 3a in
52% yield, which is much lower than that for the olefination
reaction with imine 1ab (88%, Table 2, Entry 1). Mean-
while, 2-[(p-methoxyphenyl)amino]benzothiazole (8a) was
obtained as a byproduct in a similar yield (53%). The ab-
sence of 2-hydroxybenzothiazole (9a) (as a byproduct) sug-
gests that the modified Julia reaction did not occur between
benzaldehyde and sulfone 2a in the presence of p-meth-
oxyaniline. Instead, imine 1ab was generated in situ from
benzaldehyde and p-methoxyaniline and, subsequently,
underwent olefination with sulfone 2a. Similarly, cinnamal-
dehyde reacted with sulfone 2a and p-methoxyaniline to af-
ford 2-(2-aminoalkenyl)benzothiazole 7a in 49% yield,
which is significantly lower than that obtained for the vici-
nal difunctionalization of the corresponding imine (70%;
Table 4, Entry 1).
[a] Reagents and conditions: imine 6 (0.30 mmol), sulfone 2a
(0.36 mmol), THF (1.0 mL), 70 °C (oil bath), 24 h. [b] Isolated
yield.
Scheme 2. Reactions of sulfone 2a with aldehydes in the presence
of p-methoxyaniline.
According to our experimental observations and the
mechanistic studies on the modified Julia reaction,[1] we
Figure 3. X-ray crystal structure of compound 7a.[18]
A competition was observed between imine olefination propose the following reaction pathway for the olefination
and alkene vicinal difunctionalization in the reaction of of aromatic imines with α-(benzothiazol-2-ylsulfonyl) carb-
α,β-unsaturated imines with α-(benzothiazol-2-ylsulfonyl)- onyl compounds (Scheme 3). Initial Mannich addition of α-
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(benzothiazol-2-yl)sulfonyl carbonyl compound 2 (enol
form) to aromatic imine 1 gives adduct 10,[9] which un-
dergoes the Smiles rearrangement through spirocycle 11 to
give inner salt 12.[1] Proton transfer followed by extrusion
of sulfur dioxide results in the formation of β-amino carb-
onyl compound 14,[20] which prefers to eliminate the amino
group through reactive conformation 14A to give α,β-unsat-
urated carbonyl compound 3 with (E) configuration.[21,22]
ESI-MS (positive mode) analysis of the reaction mixture of
imine 1 (1ab; Ar = Ph) and sulfone 2 (2m; R = Ph) substan-
tially supports the formation of imine/sulfone adduct 10
(11, 12, and/or 13)[23] and intermediate 14 during the ole-
fination reaction.[24]
Scheme 4. Proposed reaction pathway for the vicinal difunctionali-
zation of cyclic imines with α-(benzothiazol-2-ylsulfonyl) carbonyl
compounds.
(2a; R = OEt) substantially supports the formation of
imine/sulfone adduct 19 (20, 21, and/or 22)[23] during the alk-
ene vicinal difunctionalization reaction.[25]
Scheme 5. Proposed reaction pathway for the vicinal difunctionali-
zation of α,β-unsaturated imines with α-(benzothiazol-2-ylsulfonyl)
carbonyl compounds.
Scheme 3. Proposed reaction pathway for the olefination of aro-
matic imines with α-(benzothiazol-2-ylsulfonyl) carbonyl com-
pounds.
Likewise, vicinal difunctionalization of cyclic imines with
α-(benzothiazol-2-ylsulfonyl) carbonyl compounds is pro-
posed to proceed successively through Mannich addition,
Smiles rearrangement, proton transfer, and the extrusion of
Conclusions
We have developed three new reactions of imines with
sulfur dioxide (Scheme 4). Cleavage of the carbon–nitrogen α-(benzothiazol-2-ylsulfonyl) carbonyl compounds through
bond of product 5 does not take place to yield an alkene, stereoselective olefination and regiospecific vicinal difunc-
probably due to the stable heterocycle moiety originated tionalization under base-free conditions. The olefination re-
from cyclic imine 4.
action of aromatic imines with α-(benzothiazol-2-ylsulf-
A similar reaction pathway is proposed for the vicinal onyl) carbonyl compounds proceeds smoothly in tetra-
difunctionalization of α,β-unsaturated imines with hydrofuran at 70 °C to give structurally diverse α,β-unsatu-
α-(benzothiazol-2-ylsulfonyl)
carbonyl
compounds rated esters, amides, and ketones in good to excellent yields
(Scheme 5). Initial Michael addition of sulfone 2 to α,β- and with extremely high (E) selectivity. In contrast, the car-
unsaturated imine 6 gives enamine 19, which undergoes the bon–nitrogen double bonds of cyclic imines and the car-
Smiles rearrangement through spirocycle 20 to give inner bon–carbon double bonds of α,β-unsaturated imines are
salt 21. Owing to the hydrogen bond between the iminium subjected to regiospecific vicinal difunctionalization with α-
N–H bond and the benzothiazole nitrogen atom, proton (benzothiazol-2-ylsulfonyl) carbonyl compounds under the
transfer in inner salt 21 results in the formation of interme- same reaction conditions to give a variety of benzothiazole
diate 22 with an enamine moiety of (Z) configuration. Ex- derivatives in good to excellent yields. Plausible reaction
trusion of sulfur dioxide from intermediate 22 gives product pathways have been proposed to account for these transfor-
7. Again, ESI-MS (positive mode) analysis of the reaction mations, and these are substantially supported by ESI-MS
mixture of imine 6 (6a; Ar = Ph, R1 = PMP) and sulfone 2 analysis of the reaction mixtures.
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Selective Olefination and Vicinal Difunctionalization of Imines
[6]
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Experimental Section
General Procedure for the Olefination of Aromatic Imines with α-
(Benzothiazol-2-ylsulfonyl) Carbonyl Compounds: Table 2. To a
solution of aromatic imine 1 (0.30 mmol) in dry tetrahydrofuran
(1.0 mL) under nitrogen, was added α-(benzothiazol-2-ylsulfonyl)
carbonyl compound 2 (0.36 mmol). The mixture was stirred at
70 °C for the period specified in Table 2, cooled to room tempera-
ture, and purified either by column chromatography on silica gel
or by preparative thin layer chromatography (TLC), eluting or de-
veloping with petroleum ether/ethyl acetate (10:1 to 3:1), to give
compound 3.
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General Procedure for the Vicinal Difunctionalization of Cyclic
Imines with α-(Benzothiazol-2-ylsulfonyl) Carbonyl Compounds:
Table 3. To a solution of cyclic imine 4 (0.36 mmol) in dry tetra-
hydrofuran (1.0 mL) under nitrogen, was added α-(benzothiazol-2-
yl)sulfonyl carbonyl compound 2 (0.30 mmol). The mixture was
stirred at 70 °C for the period specified in Table 3, cooled to room
temperature, and purified either by column chromatography on sil-
ica gel or by preparative TLC, eluting or developing with petroleum
ether/ethyl acetate (20:1 to 3:1), to give compound 5.
General Procedure for the Vicinal Difunctionalization of α,β-Unsatu-
rated Imines with Sulfone 2a: Table 4. To a solution of α,β-unsatu-
rated imine 6 (0.30 mmol) in dry tetrahydrofuran (1.0 mL) under
nitrogen, was added sulfone 2a (103 mg, 0.36 mmol). The mixture
was stirred at 70 °C for 24 h, cooled to room temperature, and puri-
fied by column chromatography on silica gel, eluting with petro-
leum ether/ethyl acetate (50:1 to 10:1) and triethylamine (1% v/v),
to give compound 7.
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Supporting Information (see footnote on the first page of this arti-
[9]
cle): General information, experimental procedures, characteriza-
1
tion data, copies of H and 13C NMR spectra for products.
[10]
Acknowledgments
We are grateful for the financial support from the National Natural
Science Foundation of China (20972147 and 20732006) and the
National Basic Research Program of China (973 Program
2010CB833300).
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Aidhen, Eur. J. Org. Chem. 2006, 2851–2855; c) B. Zajc, S.
Kake, Org. Lett. 2006, 8, 4457–4460; d) E. Pfund, C. Lebargy,
J. Rouden, T. Lequeux, J. Org. Chem. 2007, 72, 7871–7877; e)
D. A. Alonso, M. Fuensanta, E. Gómez-Bengoa, C. Nájera,
Adv. Synth. Catal. 2008, 350, 1823–1829; f) A. K. Ghosh, S.
Banerjee, S. Sinha, S. B. Kang, B. Zajc, J. Org. Chem. 2009, 74,
3689–3697; g) A. Kumar, S. Sharma, V. D. Tripathi, S. Srivas-
tava, Tetrahedron 2010, 66, 9445–9449; h) H. E. Giesbrecht,
B. J. Knight, N. R. Tanguileg, C. R. Emerson, P. R. Blakemore,
Synlett 2010, 374–378.
[3] For related examples, see: a) D. Mirk, J.-M. Grassot, J. Zhu,
Synlett 2006, 1255–1259; b) D. A. Alonso, M. Fuensanta, E.
Gómez-Bengoa, C. Nájera, Eur. J. Org. Chem. 2008, 2915–
2922.
[4] Comprehensive Natural Products Chemistry, vol. 1–9 (Eds.: D.
Barton, K. Nakanishi), Elsevier, New York, 1999.
[12]
[13]
[14]
The reaction did not take place at room temperature.
No olefination reaction was found to occur between benzalde-
hyde and sulfone 2a under the same conditions.
[5] F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry, Part
B: Reactions and Synthesis, 5th ed., Springer, New York, 2007.
Eur. J. Org. Chem. 2012, 1590–1596
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Y.-D. Shao, X.-S. Wu, S.-K. Tian
FULL PAPER
[15] Both pyridin-2-yl sulfones and 1-phenyl-1H-tetrazol-5-yl sulf-
ones are widely employed in the modified Julia reaction; see
ref.[1]
[16] When compared to the modified Julia reaction with α-(benzo-
thiazol-2-ylsulfonyl) carbonyl compounds,[2] the imine ole-
fination reaction not only saves stoichiometric amounts of
bases, but also significantly expands the scope for the synthesis
of electron-deficient alkenes in a highly stereoselective manner.
[17] No reaction was observed with the corresponding aliphatic
sulfones under the same conditions.
[18] CCDC-838181 (5f) and -817120 (7a) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] V. Sridharan, P. T. Perumal, C. Avendaño, J. C. Menéndez, Org.
Biomol. Chem. 2007, 5, 1351–1353 and references cited therein.
[20] W. Löwe, G. Eggersmann, A. Kennemann, Arch. Pharm. 1984,
317, 15–21.
F. van der Pijl, M. M. J. H. P. Willems, P. J. L. M. Quaedflieg,
F. L. van Delft, F. P. J. T. Rutjes, J. Org. Chem. 2009, 74, 3207–
3210; c) G. F. Busscher, L. Lefort, J. G. O. Cremers, M. Motti-
nelli, R. W. Wiertz, B. de Lange, Y. Okamura, Y. Yusa, K. Mat-
sumura, H. Shimizu, J. G. de Vries, A. H. M. de Vries, Tetrahe-
dron: Asymmetry 2010, 21, 1709–1714.
Reactive conformation 14A is energetically favored relative to
reactive conformation 14B due to minimization of allylic 1,3-
strain.
[22]
[23]
These intermediates have the same molecular formula, and at
least one was detected.
[24] HRMS (ESI) for intermediate 10 (10a; Ar = R = Ph): calcd. for
C29H25N2O4S2 [M + H]+ 529.12503; found 529.12439. HRMS
(ESI) for intermediate 14 (14a; Ar = R = Ph): calcd. for
C29H25N2O2S [M + H]+ 465.16313; found 465.16296.
[25] HRMS (ESI) for intermediate 19 (19a; Ar = Ph, R1 = PMP, R
= OEt): calcd. for C27H27N2O5S2 [M + H]+ 523.13559; found
523.13522.
[21] For examples on the elimination of the amino groups from β-
amino carbonyl compounds, see: a) R. Fan, W. Wang, D. Pu,
J. Wu, J. Org. Chem. 2007, 72, 5905–5907; b) J. M. M. Verkade,
Received: November 24, 2011
Published Online: January 26, 2012
1596
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1590–1596