Iodine(III)-initiated bromoacetoxylation of olefins

Article abstract of DOI:10.1055/s-1998-1596

A new and mild method for the bromoacetoxylation of olefins is presented, which is initiated by iodine(III). α-Acetoxy bromides 6-11 are generated from olefins 5 in the presence of tetraethylammonium bromide (1) and (diacetoxyiodo)benzene (2). The 1,2-addition to carbohydrate-derived enol ethers results in 2-deoxy-2-bromo-pyranosyl acetates which are versatile glycosyl donors for the synthesis of 2′-deoxy-2′-bromo glycosides 16.

Full text of DOI:10.1055/s-1998-1596

February 1998
SYNLETT
195
Iodine(III)-Initiated Bromoacetoxylation of Olefins
#
Md. Abul Hashem , Alexander Jung, Monika Ries, Andreas Kirschning*
Institut für Organische Chemie der Technischen Universität Clausthal, Leibnizstraße 6, D-38678 Clausthal-Zellerfeld, Germany
FAX: int. (+)-(0)5323-72 2858; e mail: andreas.kirschning@tu-clausthal.de
#
Present address: Department of Chemistry, Jahangivnagar University, Savar, Dhakar 1342, Bangladesh
Received 3 November 1997
Abstract
: A new and mild method for the bromoacetoxylation of
clear yellow solution developed within 15 minutes. It can be reasoned
3
that tetraethylammonium [di(acyloxy)bromate (I)] ( ) is formed under
4
these conditions (Scheme 1). Alternatively, acetylhypobromite ( ) may
olefins is presented, which is initiated by iodine(III). α-Acetoxy
6 11
5
bromides
-
are generated from olefins
in the presence of
1
2
5
be responsible for the yellow colour. In fact, when olefins were added
tetraethylammonium bromide ( ) and (diacetoxyiodo)benzene ( ). The
1,2-addition to carbohydrate-derived enol ethers results in 2-deoxy-2-
bromo-pyranosyl acetates which are versatile glycosyl donors for the
4
to this solution, formal addition of to the olefinic double bond took
6 11
place, affording α-acetoxy bromides
5c f
- were transformed quantitatively (>90 %);
-
in good yield (Table 1).
7
16
synthesis of 2´-deoxy-2´-bromo glycosides
.
Cyclic enol ethers
chromatographic separation of all stereoisomers led to reduction in
8
5c
isolated yields. Electron withdrawing protecting groups on glycal
The versatility of polycoordinated iodine compounds in the oxidation
state III is well recognized. A notable feature of these molecules is
8
a d
- . In
resulted in very slow conversion to 1,2-adducts
1
dichloromethane and acetonitrile the yields were further reduced due to
their ability to transfer ligands that are bound to the central iodine atom
onto appropriate electrophiles. Under these conditions iodine(III)
reagents behave like transition metals, as they undergo reductive
elimination and subsequently iodobenzene is formed as a by-product.
Recently, we initiated an investigation on ligand transfer reactions from
9
12a
12b
(23%; 2 : 1).
formation of elimination products
and
2
iodine(III) onto iodide anions. The species generated under these
3
conditions are believed to be iodate(I) salts which have found only few
4
applications in organic synthesis. It was thought, that the analogous
ligand transfer onto the bromide anion would create the corresponding
5
bromate(I) anions.
Based on this assumption, we developed a new method for the
6
1
bromoacetoxylation of olefins. When tetraethylammonium bromide ( )
2
was mixed with (diacetoxyiodo)benzene ( ) in dichloromethane at rt, a
Scheme 1

196
LETTERS
SYNLETT
In conclusion, we presented the in situ preparation of bromate(I) salts by
ligand transfer of acetoxy groups from iodine(III) onto the bromide
anion. These new reagents can efficiently be utilized for the
bromoacetoxylation of olefins under very mild conditions.
Scheme 2
Acknowledgement: We thank the Deutschen Akademischen
Austauschdienst for a fellowship for Md. A. H. Financial support by the
Deutsche Forschungsgemeinschaft (grant Ki 397/4-2) and the Fonds der
Chemischen Industrie is grateful acknowledged.
For cyclohexene 5a and indene 5b (entries 1 and 2) the 1,2-addition was
highly trans selective. In contrast, the diastereoselectivity in reactions
with glycals is dependent on the conditions employed. When the active
agent was generated in dichloromethane (entries 3, 6 and 11) or
acetonitrile (entries 4, 7, 9 and 12), up to all four possible 2-bromo-2-
deoxy-glycosyl acetates 8-11 were formed, which could be separated by
column chromatography. With the exception of 3-deoxyglycal 5f,
formation of 1,2-cis adducts was suppressed by employing 1 and 2 ( 3
equiv.) in toluene in the presence of 0.2 equivalents of TMSOTf (entries
5, 8 and 10) . The Lewis acid is partly needed for overcoming lack of
solubility of 1 and 2 in toluene, thereby accelerating formation of the
References and Notes
(1) Reviews: (a) Moriarty, R. M., Vaid, R. K., Koser, G. F., Synlett,
1990, 365−383. (b) Varvoglis, A., The Organic Chemistry of
Polycoordinated Iodine, Verlag Chemie, New York, Weinheim,
Cambridge, 1992. (c) Stang, P. J., Zhdankin, V. V., Chem. Rev.,
1996, 96, 1123−1178. (d) Varvoglis, A., Hypervalent Iodine in
Organic Synthesis, Acadamic Press, San Diego, London, 1997.
10
active agent. In contrast to iodo acetoxylation of glycals, using excess
of NIS and HOAc in propionitrile, higher yields of rare β-1-O-acetyl-2-
(2) Kirschning, A., Plumeier, C., Rose, L., Chem. Commun. 1997, in
bromo-2-deoxy pyranoses were obtained with our reagent system.
press.
Formation of 1-O-acetyl bromides is initiated by an electrophilic
bromonium species which adds both to the α- as well as the β-face of
the olefinic double bond. When the electrophilic agent is activated by
TMSOTf in toluene (Table 1), formation of the cyclic intermediate
halocarbonium ion 14a is strongly favoured. Under these conditions
only 1,2-trans products are formed after capture of 14a by nucleophiles
like acetate. Under most other conditions, oxonium ion 14b is present in
(3) de Armas, P., Concepción, J. I., Francisco, C. G., Hernández, R,
Salazar, J. A., Suárez, E. J. Chem. Soc., Perkin Trans. I, 1989,
405-411.
(4) Szántay, C., Blaskó, G., Bárczai-Beke, M., Péchy, P., Dörnei, G.,
Tetrahedron Lett., 1980, 21, 3509-3512.
(5) A similar species may be involved in the haloacetoxylation of
dimethoxynaphtalenes: (a) Evans, P. A., Brandt, T. A.,
Tetrahedron Lett., 1996, 37, 6443-6446. (b) Evans, P. A., Brandt,
T. A., J. Org. Chem., 1997, 62, 5321-5326.
11
equilibrium with 14a which also results in 1,2-syn addition products.
In acetonitrile nitrilium ion 14c has to be considered for further
stabilization, thus accounting for generation of elimination product
9
(6) For a review on cohalogenations see: Rodriguez, J., Dulcère, J.-P.,
12b.
Synthesis, 1993, 1177-1205.
(7) The corresponding tetrabutylammonium salts, generated from
Bu NBr and PhI(OAc) , gave similar diastereomeric ratios.
4
2
However, the reaction was only complete after more than one to
two days and the α-bromoacetates were isolated in slightly
reduced yields (48-54 %).
Various groups have championed 1-O-acetyl-2-deoxy-2-iodo-
(8) Typical Experimental Procedure: PhI(OAc) (0.58 g,1.8 mmol)
2
10,12
pyranoses
as glycosyl donors for the construction of 2-
was suspended in CH Cl (10 mL) under nitrogen at rt and
2
2
13
deoxygenated oligosaccharides. Therefore, we studied the ability of
bromine at C-2 to exert anchimeric assistance in the glycosidation
process (Scheme 2). Reaction of glycosyl acetate 9a with the galactosyl
acceptor 15 in CH Cl in the presence of TMSOTf afforded the
Et NBr (0.378 g, 1.8 mmol) was added in one portion. Stirring
4
was continued for 35 min at ambient temperature until a clear
yellow solution was obtained. Glycal 5d (0.25 g, 0.6 mmol) was
added and stirring was continued for 2 h at rt. For work-up, the
2
2
corresponding disaccharide 16a as a single isomer. Likewise, 9b was
transformed under similar reaction conditions into 16b, again in a
highly stereocontrolled manner.
phases were separated after addition of saturated NaHSO
3
14
solution. The aqueous phase was separated and extracted twice
with CH Cl . The combined organic layers were dried (MgSO )
2
2
4

February 1998
SYNLETT
197
and concentrated in vacuo to afford 0.48 g of crude product. Flash
chromatography using PE/EE (85:15) gave three fractions:
(14) Selected physical and spectroscopic data for disaccharides 16a
and 16b:
1st
27
1
fraction: 9b: 123 mg (37 %), colorless crystals; m.p. 82 °C;
16a: colorless oil; [α] : -26.8 (c= 1.03; CHCl ); H NMR
D 3
26
2nd
[α]
= +44 (c = 1.02, CHCl );
fraction: 9c: 77 mg (23 %),
(CDCl ): δ 7.41-7.20 (m, 15H, H arom.), 5.51 (d, J = 5.2 Hz,
D
3
3
1,2
27
3rd
colorless oil; [α]
mg (14 %), colorless oil; [α]
= +80 (c = 1.01, CHCl );. fraction: 9a: 47
1H, 1-H), 5.27 (d, J = 1.2 Hz, 1H, 1´-H), 5.00, 4.74, 4.61, 4.51,
1´,2´
4.50, 4.43 (6d, J = 12.0 Hz, 6H, 3x CH Ph), 4.58 (dd, J = 2.4
A,B 2 3,2
D
3
26
= -9.5 (c = 1.03, CHCl ).
D
3
1
13
Hz, J = 8.0 Hz, 1H, 3-H), 4.31 (m, 1H, 2´-H), 4.31 (dd, J = 2.4
(9) Selected H- and C NMR data for 2-enopyranosyl acetate 12a:
6.53 (1-H), 5.81 (2-H); J = 3.2, J = 2.4 Hz; 147.3 (C-3), 119.6
3,4
2,3
Hz, J = 5.2 Hz, 1H, 2-H), 4.17 (dd, J = 1.6 Hz, J = 8.0 Hz,
2,1
4,5
4,3
1,2
4,5
1H, 4-H), 4.10 (dd, J
= J
= 4.8 Hz, 1H, 3´-H), 3.96 (dt, J =
3´,4´ 5,4
(C-2), 89.8 (C-1) and 12b: 6.73 (1-H), 5.68 (3-H); J = 1.2, J
=
3´,2´
1,3
4,5
+
1.6 Hz, J = J = 6.0 Hz, 1H, 5-H), 3.94 (d, J = 6.8 Hz,
5´,6a´
4.8 Hz; 145.5 (C-1), 96.7 (C-2); EI-MS, m/z =352, 350 (1:1) [M ].
For additional data on unsaturated sugars like 12a refer to:
Harders, J., Garming, A., Jung, A., Kaiser, V., Monenschein, H.,
Ries, M., Rose, L., Schöning, K.-U., Weber, T., Kirschning, A.,
Liebigs Ann. Chem., 1997, 2125-2132.
5,6a
5,6b
1H, 5´-H), 3.90 (m, 1H, 4´-H), 3.80 (dd, J
= 6.8 Hz, J
=
6a´,5´
6a´,6b´
10.8 Hz, 1H, 6a´-H), 3.71 (dd, J = 6.0 Hz, J
= 9.0 Hz, 1H,
6a,5
6a,6b
6a-H), 3.69 (d, J
= 10.8 Hz, 1H, 6b´-H), 3.66 (dd, J = 6.0
6b,5
6b´,6a´
Hz, J
= 9.0 Hz, 1H, 6b-H), 1.51, 1.41, 1.32 (3s, 12H, 4x CH ).
3
6b,6a
27
1
16b: colorless oil; [α] : -17 (c= 1.01; CHCl ); H NMR
D
3
(10) Roush, W. R., Briner, K., Sebesta, D. P. Synlett, 1993, 264-266.
(CDCl ): δ 7.43-7.22 (m, 15H, H arom.), 5.52 (d, J = 4.8 Hz,
3
1,2
(11) Bellucci, G., Chiappe, C., D´Andrea, F., Lo Moro, G.
1H, 1-H), 4.85, 4.71, 4.70, 4.55, 4.45, 4.41 (6d, J = 12.0 Hz,
A,B
Tetrahedron, 1997, 53, 3417-3424.
6H, 3x CH Ph), 4.64 (d, J
= 8.4 Hz, 1H, 1´-H), 4.58 (dd, J =
3,2
2
1´,2´
(12) Lafont, D., Boullanger, P., Carvalho, F., Vottero, P., Carbohydr.
Res., 1997, 297, 117-226 and references cited therein.
2.4 Hz, J = 8.0 Hz, 1H, 3-H), 4.30 (dd, J = 2.4 Hz, J = 4.8
3,4
2,3
2,1
Hz, 1H, 2-H), 4.29 (dd, J = 1.6 Hz, J = 2.0 Hz, 1H, 4-H), 4.18
4,5
4,3
(13) For reviews on 2-deoxygenated oligosaccharides see:
(a) Kirschning, A., Bechthold, A., Rohr, J. Top. Curr. Chem.,
1997, 188, 1-88. (b) Danishefsky, S. J., Bilodeau, M. T., Angew.
Chem., 1996, 108, 1482-1522; Angew. Chem. Int. Ed. Engl., 1996,
35, 1380-1419. (c) Kennedy, J. F., White, C. A., Bioactive
Carbohydrates in Chemistry, Biochemistry, and Biology, Ellis
Horwood, Chichester, 1983.
(dd, J
= 8.4 Hz, J
= 10.8 Hz, 1H, 2´-H), 4.06 (dt, J = 1.6
2´,1´
2´,3´ 5,4
Hz, J = 4.8, J = 6.4 Hz, 1H, 5-H), 4.00 (dd, J = 4.8 Hz,
5,6a
5,6b
6a,5
J
= 11.4 Hz, 1H, 6a-H), 3.88 (d, J
= 2.8 Hz, 1H, 4´-H), 3.79
6a,6b
4´,3´
(dd, J = 6.4 Hz, J
= 11.4 Hz, 1H, 6b-H), 3.62-3.58 (m, 3H,
6b,5
6b,6a
5´-H, 6a´-H, 6b´-H), 3.55 (dd, J
3´-H), 1.54, 1.42, 1.33, 1.31 (4s, 12H, 4x CH ).
= 2.8 Hz, J
= 10.8 Hz, 1H,
3´,4´
3´,2´
3