Methyltrioxorhenium-catalyzed epoxidation-methanolysis of glycals under homogeneous and heterogeneous conditions

Article abstract of DOI:10.1002/adsc.200505412

The efficient and high yielding domino epoxidation-methanolysis of glycals 8-15 has been achieved by oxidation with UHP in MeOH catalyzed by MTO. The products have been conveniently isolated as 2-acetoxy derivatives 16-23a, b by direct acetylation of the crude mixtures. Homogeneous MTO-amine complexes 5-7, heterogeneous poly(4-vinyl-pyridine)/MTO compounds I-III, and microencapsulated polystyrene/MTO systems IV-VII were also tested and demonstrated their effectiveness as catalysts for the oxidation step. The facial diastereoselectivity of the oxidation ranged from satisfactory to excellent depending on the substrate and could be optimized by ample screening of catalysts. Complete conversions of substrates and nearly quantitative yields of products were obtained under environmentally friendly experimental conditions and with the use of simple work-up procedures.

Full text of DOI:10.1002/adsc.200505412

FULL PAPERS
DOI: 10.1002/adsc.200505412
Methyltrioxorhenium-Catalyzed Epoxidation-Methanolysis of
Glycals under Homogeneous and Heterogeneous Conditions
Andrea Goti,a,* Francesca Cardona,a Gianluca Soldaini,a Claudia Crestini,b
Cinzia Fiani,c Raffaele Saladinoc,*
a
`
Dipartimento di Chimica Organica “Ugo Schiff”and ICCOM-CNR, affiliated with INSTM, Universita di Firenze,
via della Lastruccia 13, 50019 Sesto Fiorentino (FI), Italy
Fax: (þ39)-055-457-3531, e-mail: andrea.goti@unifi.it
b
c
`
Dipartimento di Scienze e Tecnologie Chimiche, Universita Tor Vergata, 00185 Roma, Italy
`
Dipartimento di Agrobiologia ed Agrochimica, Universita della Tuscia, Via S. Camillo de Lellis s.n.c.,
01100 Viterbo, Italy
Fax: (þ39)-0761-357-242, e-mail: saladino@unitus.it
Received: October 19, 2005; Accepted: January 12, 2006
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de or from the author.
Abstract: The efficient and high yielding domino ep- tivity of the oxidation ranged from satisfactory to ex-
oxidation-methanolysis of glycals 8–15 has been ach- cellent depending on the substrate and could be opti-
ieved by oxidation with UHP in MeOH catalyzed by mized by ample screening of catalysts. Complete con-
MTO. The products have been conveniently isolated versions of substrates and nearly quantitative yields of
as 2-acetoxy derivatives 16–23a, b by direct acetyla- products were obtained under environmentally
tion of the crude mixtures. Homogeneous MTO- friendly experimental conditions and with the use of
amine complexes 5–7, heterogeneous poly(4-vinyl- simple work-up procedures.
pyridine)/MTO compounds I–III, and microencapsu-
lated polystyrene/MTO systems IV–VII were also Keywords: glycals; green chemistry; hydrogen perox-
tested and demonstrated their effectiveness as cata- ide; methylrhenium trioxide; oxidation; supported
lysts for the oxidation step. The facial diastereoselec- catalysts
Introduction
thyldioxirane (DMDO).[2] DMDO is unstable, has to
be freshly prepared and poses serious safety problems
The recent advances in glycobiology have established connected with its potential explosiveness. Therefore,
oligosaccharides and glycoconjugates as essential com- its replacement with safer and more stable oxidants, par-
ponents for the transfer of information in biological sys- ticularly for large-scale preparations, is a valuable task.
tems. Therefore, the synthesis of carbohydrate-based
structures has become an important field of research
and one of the biggest challenges of organic synthesis.
In this context, glycals have recently found a wide appli-
cation as synthetic building blocks in the construction of
various glycoconjugates.[1]
The most common strategy to synthesize glycoconju-
gates from glycals 1 involves an initial epoxidation of
the double bond to give the corresponding 1,2-anhydro
sugar derivatives 2, that behave as good glycosyl donors
and, in the presence of suitable nucleophiles, generate
directly the desired C-2 hydroxy glycosides
(Scheme 1).
3
The epoxidation of glycals is not a trivial task, due also
to the sensitive nature of 2, particularly in acidic media.
Scheme 1. Synthetic sequence for use of glycals as building
This transformation is usually performed using dime- blocks in the construction of glycoconjugates.
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Methyltrioxorhenium-Catalyzed Epoxidation-Methanolysis of Glycals
FULL PAPERS
Other reagentshave been proposed toperform the ep-
oxidation, namely methyl(trifluoromethyl)dioxirane,[3]
MCPBA/KF,[4] or perfluoro-cis-2,3-dialkyloxaziri-
dines,[5] always as stoichiometric oxidants, but have
never entered into practical use. No general catalytic ox-
idation procedure had been reported for this type of
transformation until our recent disclosure of a domino
oxidation-nucleophilic addition catalyzed by methyl-
trioxorhenium.[6,7]
Methyltrioxorhenium (CH3ReO3, MTO, 4),[8] in com-
bination with hydrogen peroxide (or urea hydrogen per-
oxide, UHP[9]), has become in recent years an important
catalyst for a variety of synthetic transformations, such
as oxidation of olefins,[10] alkynes,[11] aromatic deriva-
tives,[12] sulfur compounds,[13] amines and other nitro-
genated compounds,[14] phosphines,[15] Bayer–Villiger
rearrangement,[16] and oxygen insertion into C H
À
bonds.[17]
Recently, with the aim of developing clean oxidation
processes, we described the preparation of novel heter-
ogeneous rhenium compounds of general formula (poly-
mer)f/(MTO)g (the f/g quotient expresses the ratio by
weight of the two components) by heterogenization of
MTO on poly(4-vinylpyridine) and poly(4-vinylpyri-
dine N-oxide) 2% and 25% cross-linked with divinyl-
benzene (I–III) and polystyrene 2% cross-linked with
divinylbenzene (IV; Figure 1),[18] applying an extension
[19]
of the “mediator”concept
and the microencapsula-
tion technique,[20] respectively. All the new MTO com-
pounds were characterized by FT-IR, scanning electron
microscopy (SEM), and wide-angle X-ray diffraction
(WAXS).[18] To the best of our knowledge, apart from
MTO supported on silica,[21] derivatized silica,[22] or nio-
bia,[23] and a NaY zeolite/MTO supercage system,[24] no
further data are available in the literature about hetero-
genized MTO catalysts for oxidation reactions. Poly-
Figure 1. Structures of poly(4-vinylpyridine)/MTO and poly-
styrene/MTO catalysts I–IV.
mer/MTO catalysts have already been proved as effi- the oxidation processes, for example, decreasing the
cient and selective systems for the epoxidation of simple formation of diols in epoxidation reactions, especially
olefins,[18] terpenes,[25] for Baeyer–Villiger oxidation of in the case of sensitive substrates, and increasing the
flavonoids,[26] as well as for the oxidation of substituted catalytic efficiency. MTO reacts with monodentate
phenol and anisole derivatives,[27] C H bonds,[28] and hy- and bidentate nitrogen ligands to give trigonal bipyra-
À
droxylamines.[29]
midal and distorted octahedral adducts, respectively.[36]
In this paper we report full details of the domino Since a strong influence of added nitrogen ligands has
MTO-catalyzed epoxidation-methanolysis of a series been observed in a related MTO-catalyzed epoxida-
of structurally diversified glycals, which we have previ- tion-phosphorylation of glycals either in the reaction
ously reported in preliminary form only for glucal deriv- rate or the stereoselectivity,[37] a series of pre-formed
atives.[6] Although MTO/H2O2 had been used for the ep- amine-MTO compounds 5–7 has been also prepared
oxidation of a large variety of differently substituted al- and tested in this reaction [Figure 2, Eq. (A)]. More-
kenes,[10] its use with enol ethers was barely document- over, compounds 5–7 were used to prepare microen-
ed,[30] while a properly modified procedure had been re- capsulated Lewis base adducts of MTO, compounds
ported for the oxidation of silyl enol ethers.[31] Shortly V–VII [Figure 2, Eq. (B)]. Heterogeneous poly(4-vi-
after our preliminary communication, Quayle and co- nylpyridine)/MTO, polystyrene/MTO catalysts I–IV
workers reported full details of a similar procedure, em- and polystyrene/MTO.L (L¼5–7) catalysts V–VII
ploying aqueous H2O2 instead of UHP.[32]
have been employed in this oxidative addition to
Lewis base adducts of MTO with nitrogen-containing some of the glycal substrates, in order to compare the
ligands, such as pyridine,[33] pyridine derivatives,[34] pyra- results with those from the reactions under homogene-
zole[35] and others are known to influence significantly ous conditions.
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Andrea Goti et al.
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Figure 2. Lewis bases-MTO adducts 5–7 and microencapsulated PS 2%/MTO-Lewis base adducts V–VII used as catalysts.
Results and Discussion
Complexes 5 (L¼2-aminomethylpyridine),[38] 6 (L¼
trans-1,2-diaminocyclohexane),[38] and 7 (L¼1-phenyl-
ethylamine) of MTO, poly(4-vinylpyridine) 2% and
25% cross-linked (with divinylbenzene)/MTO (PVP-
2%/MTO I and PVP-25%/MTO II, respectively),
poly(4-vinylpyridine-N-oxide) 25% cross-linked/MTO
(PVPN-25%/MTO III), microencapsulated polystyrene
2% cross-linked/MTO (PS-2%/MTO IV) and microen-
capsulated polystyrene complexes/5–7 (PS-2%/5, V;[38]
PS-2%/6, VI;[38] PS-2%/7, VII), schematically represent-
Figure 3. Glycals used as substrates for the catalyzed domino
epoxidation-methanolysis.
ed in Figures 1 and 2, have been synthesized according and tribenzyl-d-glucal (9). The MTO/UHP system was
to our recently published procedures.[18,38]
able to transform the starting glucals 8 and 9 into the cor-
Acetyl- and benzyl-protected d-glucal 8 and 9, d-gal- responding b/a mixtures of methyl glycosides 16a/b[40]
actal 10 and 11, l-rhamnal 12 and 13 and d-arabinal 14 and 17a/b,[41] respectively, with complete conversion
and 15 (Figure 3) have been synthesized using literature and very high yield (entries 1 and 2, Table 1). These
procedures[39] and have been used assubstrates in the ep- products are derived from methanolytic ring opening
oxidation-methanolysis domino process.
of the intermediate epoxides. The epoxides were not
Preliminary experiments were focused to test the ef- isolable under the reaction conditions because of their
fectiveness of the system MTO/UHP in the epoxidation high reactivity: they were selectively opened by the sol-
of glucals.
vent via SN2 nucleophilic displacement at the anomeric
Standard conditions, employing catalytic amount of carbon. For practical reasons, it was more convenient
MTO (2%) and UHP (3 equivs.) in MeOH as solvent, to analyze the crude reaction mixtures after acetylation
were applied in the oxidation of triacetyl-d-glucal (8) ofthe free hydroxy groupofthe resulting products. Thus,
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Methyltrioxorhenium-Catalyzed Epoxidation-Methanolysis of Glycals
Table 1. Oxidation of triacetyl- and tribenzyl-protected d-glucal and d-galactal under homogeneous conditions.[a]
FULL PAPERS
Entry
Glycal
Time [h]
Product ratio a:b[b]
Yield [%][c]
1
2
3
4
8: R¼Ac, R1 ¼H, R2 ¼OAc
9: R¼Bn, R1 ¼H, R2 ¼OBn
10: R¼Ac, R1 ¼OAc, R2 ¼H
11: R¼Bn, R1 ¼OBn, R2 ¼H
20
15
19
3.5
16: 1.9: 1
17: 5.3: 1
18: 7.5: 1
19: 1.7: 1
92
87
97
90
[a]
Reagents and conditions: i) MTO (2%), UHP (3 equivs.), MeOH, room temperature; ii) Pyridine, Ac2O, room temperature,
15 h.
[b]
[c]
1
Calculated by integration of the H NMR spectra of the crude mixtures.
Isolated yields after purification by flash column chromatography (all conversions>98%).
Table 2. Oxidation of diacetyl- and dibenzyl-protected l-rhamnal and d-arabinal under homogeneous conditions.[a]
Entry
Glycal
Time [h]
Product ratio a:b[b]
Yield [%][c]
1
2
3
4
12: R¼Ac, R1 ¼OAc, R2 ¼H, R3 ¼Me
13: R¼Bn, R1 ¼OBn, R2 ¼H, R3 ¼Me
14: R¼Ac, R1 ¼H, R2 ¼OAc, R3 ¼H
15: R¼Bn, R1 ¼H, R2 ¼OBn, R3 ¼H
3
20: 1.5: 1
21: 5.1: 1
22:> 50: 1
23:> 50: 1
95
90
95
96
1.5
1.5
1.5
[a]
Reagents and conditions: i) MTO (2%), UHP (3 equivs.), MeOH, room temperature; ii) Pyridine, Ac2O, room temperature,
15 h.
[b]
[c]
1
Calculated by integration of the H NMR spectra of the crude mixtures.
Isolated yields after purification by flash column chromatography (all conversions>98%).
the diastereoselectivity of the epoxidation was calculat- The results are shown in Tables 1 and 2. In all cases com-
1
ed by integration of the H NMR signals of the crude plete conversions and high yields (90–97%) of the corre-
product mixture after acetylation of the free C-2 OH. In- sponding methyl glycosides were obtained.
deed, configuration at C-2 is determined in the epoxida-
Concerning the diastereoselectivity, epoxidation of
tion step. Attack from the bottom gives the a-epoxide triacetyl d-galactal (10) proceeded with a considerably
(Figure 4), which ultimately affords the gluco deriva- greater selectivity with respect to the corresponding glu-
tives 16a and 17a. Conversely, attack from the top gives cal derivative 8 (entry 3 vs. 1, Table 1). This shows that
a b-epoxide, which leads to the manno derivatives 16b the configuration at C-4 plays a major role in determin-
and 17b.
ing the degree of stereoselection, with the OR group at
The diastereoselectivity is governed mainly by steric C-4 directing the attack of oxygen preferentially to the
factors, with the epoxidation occurring preferentially opposite face when it is placed in an axial position. Sur-
at the face of the double bond opposite to the OR group prisingly however, the b-d-galactopyranoside 18a[42] was
at C-3. As expected on this basis, the epoxidation-meth- formed with a higher degree of selectivity than com-
anolysis of tri-O-benzylglucal (9) afforded the glucose pound 19a[43] (entry 3 vs. 4, Table 1). Tribenzyl-d-galac-
derivative 17a with a much higher preference compared tal (11) behaved anomalously, affording compound 19
to triacetylglucal 8 (entry 2 vs. 1, Table 1).
with a lower selectivity, even when compared to glucal
Encouraged by these results, we investigated the 9. Moreover, the epoxidation reaction was much quicker
scope of this epoxidation-ring opening sequence by sub- than that for the acetyl derivative 10, indicating that oth-
jecting glycals 10–15 to the same reaction conditions. er factors must play a role in the oxidation of galactal 11.
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an erosion of the stereospecificity in the ring opening
of the intermediate epoxide, suggesting that in the aque-
ous medium alternative ring opening pathways, or epi-
merization of thefinalmethylglycosides at the anomeric
center, might occur. On comparison with the alternative
epoxidation methods of glycals based on DMDO[2] or
MCPBA/KF,[4] the MTO/UHP procedure showed a low-
er level of stereoselectivity, except in the oxidation of d-
arabinal derivatives. However, we have shown that the
stereoselectivity in the MTO-catalyzed epoxidation of
glycals is strongly affected by the solvent used and the
presence of donor additives.[37]
Having ascertained the efficiency and generality of
the domino oxidation-methanolysis of glycals under ho-
mogeneous conditions with UHP as the oxidant and
MTO as the catalyst, we turned our interest to test the
performance of catalysts I–VII for carrying out the
same reactions under heterogeneous conditions. These
catalysts have already shown their ability in oxidizing
a number ofdifferent substrates, offering the advantages
ofheterogeneouscatalysts, such asthe easy and practical
work-up of the reaction, with recovery by simple filtra-
tion, and recyclability without any substantial loss of ac-
tivity over several successive runs.[18,29,38]
Figure 4. Favoured intermediate epoxides.
Glycals 9 and 11–13 were selected as representative
substrates for the oxidations with heterogeneous cata-
Diacetyl-l-rhamnal (12) and dibenzyl-l-rhamnal (13) lysts I–VII, in order to evaluate the generality of this
led to similar diastereoselectivity results as their glucal transformation with a considerable variety of substrates.
counterparts 8 (entry 1, Table 2 vs. entry 1, Table 1) The selection of substrates which showed lower selectiv-
and 9 (entry 2, Table 2 vs. entry 2, Table 1), respectively. ities with the homogeneous catalyst was chosen in order
The epoxidation occurred preferentially, in both cases, to study the effects of the heterogeneous catalysts on the
anti to the OR group at C-3 and the intermediate a-ep- selectivity of the reaction. As a general procedure, the
oxides were formed preferentially (Figure 4). These ep- glycal (0.2 mmol) to be oxidized and UHP (2.0–4.0
oxides afforded the derivatives 20a[44] and 21a[45] via SN2 equivs., see Tables 3–6) were added to a suspension of
ring opening with MeOH at the anomeric carbon. The freshly prepared catalysts I–VII (loading factor 1, that
minor derivatives 20b[46] and 21b[47] are derived from is 1 mmol MTO per gram of resin) in MeOH (1.0 mL),
the diastereomeric b-epoxides. Finally, the MTO-cata- and the mixture was stirred at 258C. The oxidation re-
lyzed epoxidation of d-arabinal derivatives occurred sults are summarized in Tables 3–6, entries 1–4 of
with high stereoselectivity. Indeed, the methyl glyco- each Table for catalysts I–IV and entries 6, 8, and 10
sides 22a[48] and 23a,[49] derived from the corresponding for catalysts V, VI, and VII, respectively. In this latter
b-epoxide intermediates (Figure 4), were obtained as case, the corresponding reactions with homogeneous
the only products from arabinals 14 and 15, respectively complexes 5–7 were also carried out as references (Ta-
(entries 3 and 4, Table 2). These latter results, presuma- ble 3–6, entries 5, 7 and 9, respectively). In absence of
bly, were due to the hindrance offered by the pseudoax- the catalyst, less than 5% substrate conversion took
ial OR groups at C-4 of d-arabinal, thus securing a com- place under otherwise identical conditions.
plete stereoselectivity in the epoxidation.
The results of the reactions summarized in Tables 3–6
A comparison of these results with those reported by show that most of the catalysts were effective in accom-
Quayle and co-workers with the use of aqueous H2O2,[32] plishing the oxidation of benzylglycals 9, 11, and 13 and
where only glucal and galactal derivatives have been led to complete conversions under the appropriate ex-
used as substrates, allows us to establish that our proce- perimental conditions. Conversely, the supported or mi-
dure with UHP in MeOH affords generally higher prod- croencapsulated heterogeneous catalysts I–VII, as well
uct yields. From the point of view of stereoselectivity of as the amine-MTO complexes 5–7, failed to convert
epoxidation, while the glucals 8 and 9 gave similar ratios, completely the less reactive diacetylrhamnal 12. How-
a considerably higher selectivity was observed by us in ever, excellent conversions in the 90% range were ach-
the reaction of galactal 10 (7.5:1) with respect to that re- ieved with many catalysts in 1–2 days. As expected,
ported in aqueous H2O2 (3:1). Moreover, that proce- both the reactions with heterogenized catalysts I–VII
dure caused in some cases (e.g., in the oxidation of 9) and complexed MTO 5–7 were considerably slower
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Table 3. Oxidation of tribenzyl-d-glucal (9) under heterogeneous conditions.[a]
Entry
Catalyst
UHP (equivs.)
Conversion [%]
Time [h]
a:b Ratio[b]
1
2
3
4
5
6
7
8
9
PVP-2%/MTO I
PVP-25%/MTO II
PVPN-25%/MTO III
PS-2%/MTO IV
4.0
3.0
1.0
2.0
2.0
4.0
2.0
4.0
2.0
4.0
71
>98
>98
>98
>98
85
>98
50
>98
98
72
42
18
22
22
64
27
27
20
47
5.0: 1
6.0: 1
7.3: 1
6.0: 1
7.7: 1
8.0: 1
5.6: 1
7.5: 1
6.8: 1
6.6: 1
5
PS/5 V
6
PS/6 VI
7
10
PS/7 VII
[a]
Reagents and conditions: i) catalyst (corresponding to 1% MTO), UHP, MeOH, room temperature; ii) pyridine, Ac2O,
room temperature, 15 h.
Calculated by integration of the H NMR spectra of the crude mixtures.
[b]
1
Table 4. Oxidation of tribenzyl-d-galactal (11) under heterogeneous conditions.[a]
Entry
Catalyst
UHP [equivs.]
Conversion [%]
Time [h]
a:b Ratio[b]
1
2
3
4
5
6
7
8
9
PVP-2%/MTO I
PVP-25%/MTO II
PVPN-25%/MTO III
PS-2%/MTO IV
4.5
2.5
1.5
2.5
3.5
5.5
1.5
3.5
5.0
4.5
>98
>98
>98
>98
>98
84
>98
86
94
47
24
21
24
47
73
18
52
72
73
3.2: 1
3.8: 1
1.3: 1
2.6: 1
2.1: 1
1.6: 1
4.9: 1
5.0: 1
2.2: 1
1.5: 1
5
PS/5 V
6
PS/6 VI
7
10
PS/7 VII
84
[a]
Reagents and conditions: i) catalyst (corresponding to 1% MTO), UHP, MeOH, room temperature; ii) pyridine, Ac2O,
room temperature, 15 h.
Calculated by integration of the H NMR spectra of the crude mixtures.
[b]
1
than those catalyzed by MTO itself, required much lon- tion of other substrates) with polymer-supported meth-
ger reaction times and, often, a higher excess of oxidant. ylrhenium trioxide systems, the increase of reactivity
Among the poly(4-vinylpyridine)/MTO systems I–III, a with the pyridine N-oxide catalyst III is rather unusu-
rough order of reactivity III>II>I was generally fol- al.[18,29] Among the microencapsulated polystyrene/
lowed, suggesting an increase of the reaction rate with MTO systems IV–VII, the catalysts V–VII, containing
a higher value of reticulation grade of the matrix, and the MTO complexes 5–7, reacted more sluggishly than
on passing from pyridine to pyridine N-oxide units. catalyst IV.
While a faster reaction with the increase of reticulation
Concerning the selectivity of the reaction, all the cat-
grade is in agreement with our previously reported re- alysts employed displayed high levels of facial diastereo-
sults on the selective epoxidation of olefins (and oxida- selectivity in the epoxidation step, affording ultimately
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Table 5. Oxidation of diacetyl-l-rhamnal (12) under heterogeneous conditions.[a]
Entry
Catalyst
UHP [equivs.]
Conversion [%]
Time [h]
a:b Ratio[b]
1
2
3
4
5
6
7
8
9
PVP-2%/MTO I
PVP-25%/MTO II
PVPN-25%/MTO III
PS-2%/MTO IV
5.5
2.5
2.0
4.0
3.0
4.0
2.0
5.5
2.5
3.0
67
92
94
91
94
87
90
77
92
88
53
25
23
54
30
53
24
70
21
44
1.8: 1
2.0: 1
1.9: 1
1.8: 1
1.8: 1
1.8: 1
2.5: 1
2.7: 1
2.5: 1
3.5: 1
5
PS/5 V
6
PS/6 VI
7
10
PS/7 VII
[a]
Reagents and conditions: i) catalyst (corresponding to 1% MTO), UHP, MeOH, room temperature; ii) pyridine, Ac2O,
room temperature, 15 h.
Calculated by integration of the H NMR spectra of the crude mixtures.
[b]
1
Table 6. Oxidation of dibenzyl-l-rhamnal (13) under heterogeneous conditions.[a]
Entry
Catalyst
UHP [equivs.]
Conversion [%]
Time [h]
a:b Ratio[b]
1
2
3
4
5
6
7
8
9
PVP-2%/MTO I
PVP-25%/MTO II
PVPN-25%/MTO III
PS-2%/MTO IV
4.0
2.0
1.5
4.0
2.5
3.5
1.5
3.0
3.0
3.0
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
46
22
18
20
24
46
6
45
26
24
5.2: 1
6.0: 1
5.3: 1
5.0: 1
5.5: 1
7.5: 1
6.2: 1
5.9: 1
6.2: 1
6.3: 1
5
PS/5 V
6
PS/6 VI
7
10
PS/7 VII
[a]
Reagents and conditions: i) catalyst (corresponding to 1% MTO), UHP, MeOH, room temperature; ii) pyridine, Ac2O,
room temperature, 15 h.
Calculated by integration of the H NMR spectra of the crude mixtures.
[b]
1
products a which exceeded consistently their isomers b. bly the selectivity previously observed by using MTO.
Usually, slight variations of selectivity were observed For example, the selectivity for the oxidation-methanol-
within a family of catalysts, and also on comparing the ysis of glucal 9 increased from 5.3:1 with MTO to 8.0:1
heterogeneous reactions with those carried out under with catalyst V (Table 3, entry 6). Analogously, the facial
homogeneous conditions with the related catalysts. No diastereoselectivity increased from 1.7:1 to 5.0:1 with
generalized trend can be drawn, in the sense that none catalyst VI for galactal 11 (Table 4, entry 8), from
of the catalysts performs uniformly best with all the sub- 1.5:1 to 3.5:1 with catalyst VII for rhamnal 12 (Table 5,
strates. However, optimized conditions can be identified entry 10), and from 5.1:1 to 7.5:1 with catalyst V for
for each substrate, which allow us to enhance considera- rhamnal 13 (Table 6, entry 6). The complexation of
482
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Adv. Synth. Catal. 2006, 348, 476 – 486

Methyltrioxorhenium-Catalyzed Epoxidation-Methanolysis of Glycals
FULL PAPERS
were prepared according to literature procedures.[39] NMR
spectra were recorded on a Varian Mercury 400 (1H,
400 MHz) or a Bruker (1H, 200 MHz) spectrometer. Chroma-
tographic purifications were performed on columns packed
with silica gel, 230–400 mesh, for flash technique; Rf values re-
fer to the eluent mixture used for the purification.
MTO with amines, while decreasing the reactivity, re-
sulted generally in higher selectivities for the oxidation
reaction, either under homogeneous or heterogeneous
conditions. Among the poly(4-vinylpyridine)/MTO sys-
tems I–III, slight variations of selectivity were ob-
served, with the catalyst II affording generally the
most selective oxidation. To resume, broad screening
of the homogeneous (MTO and MTO complexes 5–7)
and heterogeneous (supported or microencapsulated
I–VII) catalysts allowed us to define for each glycal sub-
strate the optimal reaction conditions, either in terms of
conversion or stereoselectivity.
Characterization data for products 16–23 can be found in
the Supporting Information.
À
Preparation of Heterogeneous MTO Catalysts I IV
Poly(4-vinylpyridine)/MTO (PVP-2%/MTO I, PVP-25%/
MTO II, and PVPN-25%/MTO III) and polystyrene/MTO
(PS-2%/MTO IV) catalysts were prepared as previously re-
ported.[18] In summary, MTO (77 mg, 0.3 mmol) was added to
a suspension of the appropriate resin (600 mg) in ethanol
(4 mL), or tetrahydrofuran in the case of polystyrene. The mix-
ture was stirred for 1 h using a magnetic stirrer. Coacervates
were found to envelope the solid core dispersed in the medium
and hexane (5 mL) was added to harden the capsule walls. The
solvent was removed by filtration, and the solid residue was
washed with ethyl acetate and finally dried under high vacuum.
In every case, MTO was completely included into the polymer.
This result was confirmed by spectroscopic analysis of the res-
idue obtained after evaporation of the organic layers. The cat-
alysts were used without any further purification.
Conclusion
Structurally diversified and differently protected glycals
8–15 underwent a mild and facile oxidation by means of
UHP in MeOH catalyzed by MTO. The nucleophilic sol-
vent caused the immediate ring opening of the epoxide
formed in situ by SN2 attack at the anomeric carbon.
The methyl glycosides formed were directly acetylated
to 16–23a, b for a more convenient analysis of the reac-
tion mixtures and characterization of the products. Ho-
mogeneous MTO-amine complexes 5–7 and heteroge-
neous poly(4-vinylpyridine)/MTO compounds I–III,
and microencapsulated polystyrene/MTO systems IV–
VII were also tested and demonstrated their effective-
ness ascatalysts for the oxidation step. The facial diaster-
eoselectivity ofthe oxidation ranged from satisfactory to
excellent depending on the substrate and could be opti-
mized by ample screening of the catalysts. The heteroge-
neous catalysts V–VII based on MTO-amine com-
plexes 5–7 microencapsulated in polystyrene generally
afforded the best stereoselectivities. Under optimized
conditions, all the substrates displayed synthetically
meaningful selectivities, up to 100% for arabinal deriv-
atives. Polymer-supported MTO compounds, which
can be easily recovered by filtration from the reaction
mixture and used for successive transformations, con-
firmed their high versatility and broaden their use as cat-
alysts for the oxidation of organic compounds.
Synthesis of Lewis Base Adducts of MTO, Compounds
5–7
Lewis base adducts of MTO 5–7 containing the nitrogen li-
gands 2-aminomethylpyridine, trans-1,2-diaminocyclohexane
and 1-phenylethylamine, respectively, were prepared follow-
ing a synthetic procedure previously reported in the litera-
ture.[38] As a general procedure, 1.0 mmol of the appropriate
monodentate ligand 7, or 0.5 mmol of bidentate ligand 5 or 6,
were added to 1.0 mmol of MTO in toluene (10 mL) at room
temperature. A yellow precipitate was immediately formed.
The reaction mixture was concentrated, cooled to À358C
and the precipitate isolated by filtration.
Synthesis of Microencapsulated Lewis Base Adducts
À
of MTO, Compounds V VII
The process described here constitutes a novel domi-
no reaction, which allows us to convert glycals into
methyl glycosides with complete conversion and excel-
lent yields, under environmentally friendly experimen-
tal conditions and with the use of simple work-up proce-
dures, employing a number of different homogeneous
and heterogeneous MTO-based catalysts.
Microencapsulated Lewis base adducts of MTO, compounds
V–VII, were prepared following a modified procedure previ-
ously reported for the synthesis of polystyrene/MTO cata-
lyst.[38] In summary, to a suspension of 600 mg of polystyrene
in 4 mL of tetrahydrofuran (THF) was added 0.3 mmol of the
appropriate adduct 5–7, and the mixture was stirred for 1 h us-
ing a magnetic stirrer. Coacervates were found to envelope the
solid core dispersed in the medium and 5.0 mL of hexane were
added to harden the capsule walls. The solvent was removed by
filtration, and the solid residue was washed with ethyl acetate
and finally dried under high vacuum. In each case, MTO com-
plexes had completely become bound to the polymer. This re-
sult was confirmed by spectroscopic analysis of the residue ob-
tained after evaporation of the organic layers. The catalysts
were used without any further purification.
Experimental Section
General Remarks
All commercial products were of the highest grade available
and were used without any further purification. Glycals 8–15
Adv. Synth. Catal. 2006, 348, 476 – 486
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asc.wiley-vch.de
483

Andrea Goti et al.
FULL PAPERS
tion by flash column chromatography on silica gel (petroleum
ether-AcOEt, 7:5) afforded a mixture of compounds 16a and
16b (Rf ¼0.50) as a colorless oil, 92% overall yield.
Oxidation of Glycals. General Procedures
(a) Homogeneous oxidation with MTO; General procedure: A
10-mL reaction flask was charged sequentially with MTO
(0.01 mmol), MeOH (1 mL), and UHP (1.5 mmol). The stirred
solution became yellow due to the formation of peroxy species
and after 5 minutes the glycal (0.5 mmol) was added. The reac-
tion mixture was stirred at room temperature until no more
starting material was detected by TLC. After removed the sol-
vent under reduced pressure the crude reaction mixture was
added with CH2Cl2 and the undissolved urea filtered off to af-
ford the crude reaction mixture as a pale yellow oil.
To an ice-cooled solution of the crude mixture in dry pyri-
dine (1 mL), acetic anhydride (0.5 mL) was added dropwise.
After stirring 15 hours at room temperature, the mixture was
concentrated under vacuum to afford the crude product mix-
ture as a pale yellow oil. The crude was analyzed by 1H NMR
spectroscopy in order to determine the selectivity of the oxida-
tion, then the products were purified by flash column chroma-
tography. The product ratios and NMR characterization of the
products obtained from the reactions with MTO are reported
below and in the Supporting Information. All the products
have been identified by comparison with literature data.
(b) Homogeneous oxidation with MTO complexes 5–7;
General procedure: To the suspension of the appropriate cata-
lysts 5–7 (0.01 mmol) and UHP (2.5 mmol) in MeOH (1.0 mL)
was added the substrate (1.0 mmol) to be oxidized. The reac-
tion mixture was stirred at room temperature until no more
starting material was detected by TLC, or the reaction did
not progress further. After removal of the solvent under re-
duced pressure, the crude reaction mixture was added with
CH2Cl2 and the undissolved urea filtered off to afford the crude
reaction mixture as a pale yellow oil.
To an ice-cooled solution of the crude product in dry pyri-
dine (1 mL), acetic anhydride (0.5 mL) was added dropwise.
After stirring 15 hours at room temperature the mixture was
concentrated under vacuum to afford the crude product mix-
ture as a pale yellow oil. The crude material was analyzed by
1H NMR spectroscopy in order to determine the selectivity
of the oxidation, then the products were purified by flash col-
umn chromatography.
(c) Heterogeneous oxidation with catalysts I–VII; General
procedure: To the suspension of the appropriate catalysts I–
VII (corresponding to 1.0% in weight of MTO, loading factor
1.0) and UHP (4.0 mmol) in MeOH (1.0 mL) was added the
substrate (1.0 mmol) to be oxidized. The reaction mixture
was stirred at room temperature until no more starting materi-
al was detected by TLC, or the reaction did not progress fur-
ther. At the end of the reaction the catalyst was recovered by
filtration and washed with MeOH. After removal of the sol-
vent under reduced pressure, the crude reaction mixture was
added with CH2Cl2 and the undissolved urea filtered off to af-
ford the crude as a pale yellow oil.
Epoxidation-Methanolysis of 3,4,6-Tribenzyl-d-glucal (9):
Reaction time: 15 h. Product ratio: 17a/17b¼5.3:1. Purifica-
tion by flash column chromatography on silica gel (petroleum
ether-AcOEt, 4:1) afforded pure 17a (Rf ¼0.32) and the prod-
uct 17b (Rf ¼0.36), contaminated with 17a, as colorless oils,
87% overall yield.
Epoxidation-Methanolysis of 3,4,6-Triacetyl-d-galactal
(10): Reaction time: 19 h. Product ratio: 18a/18b¼7.5:1. Puri-
fication by flash column chromatography on silica gel (ethyl
ether-petroleum ether, 2:1) afforded pure 18a (Rf ¼0.35) and
the product 18b (Rf ¼0.40), contaminated with 18a, as colorless
oils, 97% overall yield.
Epoxidation-Methanolysis of 3,4,6-Tribenzyl-d-galactal
(11): Reaction time: 3.5 h. Product ratio: 19a/19b¼1.7:1. Puri-
fication by flash column chromatography on silica gel (petro-
leum ether-ethyl ether, 2:1) afforded pure 19a (Rf ¼0.16) and
the product 19b (Rf ¼0.23), contaminated with 19a, as colorless
oils, 90% overall yield.
Epoxidation-Methanolysis of 3,4-Diacetyl-l-rhamnal (12):
Reaction time: 3 h. Product ratio: 20a/20b¼1.5:1. Purification
by flash column chromatography on silica gel (petroleum
ether-AcOEt, 7:2) afforded pure products 20a (Rf ¼0.21)
and 20b (Rf ¼0.26) as colorless oils, 95% overall yield.
Epoxidation-Methanolysis of 3,4-Dibenzyl-l-rhamnal (13):
Reaction time: 1.5 h. Product ratio: 21a/21b¼5.1:1. Purifica-
tion by flash column chromatography on silica gel (petroleum
ether-AcOEt, 5:1) afforded mixtures of products 21a (Rf ¼
0.36) and 21b (Rf ¼0.41), each one contaminated with the iso-
mer, as colorless oils, 90% overall yield.
Epoxidation-Methanolysis of 3,4-Diacetyl-d-arabinal (14):
Reaction time: 1.5 h. Compound 22a was obtained with a
selectivity>50:1. Purification by flash column chromatogra-
phy on silica gel (petroleum ether-AcOEt, 2:1) afforded
pure 22a (Rf ¼0.42) as a colorless oil, 95% yield.
Epoxidation-Methanolysis of 3,4-Dibenzyl-d-arabinal (15):
Reaction time: 1.5 h. Compound 23a was obtained with a
selectivity>50:1. Purification by flash column chromatogra-
phy on silica gel (petroleum ether-AcOEt, 3:1) afforded
pure 23a (Rf ¼0.34) as a colorless oil, 96% yield.
Acknowledgements
The authors thank MIUR (PRIN 2002 and PRIN 2003) and
INSTM (PRISMA 2004) for financial support. A grant from
Ente Cassa di Risparmio di Firenze, Italy, for a 400 MHz
NMR spectrometer is gratefully acknowledged.
References and Notes
To an ice-cooled solution of the crude product in dry pyri-
dine (1 mL), acetic anhydride (0.5 mL) was added dropwise.
After stirring 15 hours at room temperature the mixture was
concentrated under vacuum to afford the crude product as a
pale yellow oil. The crude material was analyzed by 1H NMR
spectroscopy in order to determine the selectivity of the oxida-
tion, then the products were purified by flash column chroma-
tography.
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484
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