Synthesis of enantiopure carbohydrate mimetics by Lewis acid catalyzed rearrangement of 1,3-dioxolanyl-substituted 1,2-oxazines

Article abstract of DOI:10.1002/anie.200501127

(Chemical Equation Presented) Under control: Lewis acids induce an unexpected rearrangement of 1,3-dioxolanyl-substituted 1,2-oxazines to provide bicyclic compounds in a stereocontrolled manner. They are precursors for enantiopure carbohydrate mimetics co

Full text of DOI:10.1002/anie.200501127

Angewandte
Chemie
Heterocycles
DOI: 10.1002/anie.200501127
Synthesis of Enantiopure Carbohydrate Mimetics
by Lewis Acid Catalyzed Rearrangement of 1,3-
Dioxolanyl-Substituted 1,2-Oxazines**
Ahmed Al-Harrasi and Hans-Ulrich Reißig*
Dedicated to Professor Helmut Vorbrüggen
on the occasion of his 75th birthday
Enantiomerically pure 3,6-dihydro-2H-1,2-oxazines, which
are easily available by [3+3] cyclization of lithiated
alkoxyallenes with aldonitrones,[1] are versatile intermediates
for the stereoselective synthesis of a range of highly function-
alized compounds. All these products, including polyhydrox-
ylated pyrrolidines, stereodefined amino polyols, and substi-
tuted tetrahydrofuran derivatives,[2] are interesting because of
their potential biological activities, for example, as glycosi-
dase inhibitors.[3] While trying to deprotect 1,2-oxazine syn-3
with Lewis acids we found that a rearrangement to tetrahy-
dropyran-bridged bicyclic 1,2-oxazine 4 occurred in moderate
yield (Scheme 1).[2b] Thereby the acetonide protecting group
of syn-3 was incorporated into product 4. Herein we report
that:
Scheme 1. Reaction conditions: a) THF, ꢀ788C, 2 h (syn-3: 76 %); b)2
+ Et2AlCl, Et2O, then add to 1, ꢀ788C, 2 h (anti-3: 84%); c) SnCl4
(3 equiv), CH3CN, ꢀ308C!RT, 6h ( 4: quant.); d) Me3SiOTf
(0.05 equiv), CH2Cl2, ꢀ308C!RT, 6h ( 5: 79%); e) tBuMe2SiOTf
(3 equiv), CH2Cl2, room temperature, 20 h, then NEt3, 08C, 15 min (6:
quant.); f) tBuMe2SiOTf (3 equiv), CH2Cl2, room temperature, 20 h,
then NEt3, 08C, 15 min (7: 93%). Bn=benzyl; TBS=tert-butyldime-
thylsilyl; Tf=trifluoromethanesulfonyl.
*
this reaction proceeds quite generally with suitably
substituted 3,6-dihydro-2H-1,2-oxazines as starting mate-
rials,
*
the obtained enantiopure bicyclic products can be con-
verted stereoselectively into numerous polyhydroxylated
amino-substituted pyran derivatives,
pected cyclization of syn-3 to 4, which was first observed in
moderate yields in the presence of BF3·OEt2, was optimized
by using a range of Lewis acids and screening the reaction
conditions. Although the rearrangement could be accom-
plished with different Lewis acids, dibutylboron triflate,
trimethylsilyl triflate, and tin tetrachloride proved to be the
best promoters.[5] The conversion of syn-3 into bicyclic
product 4 proceeded quantitatively with SnCl4 in acetonitrile.
We propose that the mechanism of this reaction involves
the coordination of the Lewis acid to the “outer” dioxolane
oxygen atom (O-1) of syn-3, followed by ring opening of the
acetonide unit and intramolecular attackof the generated
carbenium ion at the enol ether unit of the 1,2-oxazine ring.
Cleavage of the (trimethylsilyl)ethyl group—most probably
to give ethene and Me3SiX species[6]—affords the central
carbonyl group of the resulting bicyclic compound 4. This
rearrangement can be classified as an intramolecular aldol-
type addition of an acetal to an enol ether[7] or as a Prins
reaction.[8]
*
in a similar manner, highly functionalized oxepane
derivatives are accessible.
The resulting products may be regarded as carbohydrate
mimetics,[4] which are potentially important building blocks
for the synthesis of biologically active compounds, for
example, oligosaccharide analogues.
The stereodivergent addition of lithiated alkoxyallene 1 to
the d-glyceraldehyde-derived nitrone 2 gives either syn- or
anti-configured 3 in excellent yield (Scheme 1).[1] The unex-
[*] A. Al-Harrasi, Prof. Dr. H.-U. Reißig
Institut für Chemie und Biochemie
Freie Universität Berlin
Takustrasse 3, 14195 Berlin (Germany)
Fax: (+49)30-8385-5367
E-mail: hans.reissig@chemie.fu-berlin.de
[**] This work was generously supported by the Deutsche Forschungs-
gemeinschaft, the Deutscher Akademischer Austauschdienst (fel-
lowship for A.A.H.), the Fonds der Chemischen Industrie, and
Schering AG. We thank B. Bressel and Dr. R. Zimmer for help during
the preparation of this manuscript, W. Münch for HPLC separa-
tions, as well as M. Wiecko and J. Gebers for experimental
assistance.
The rearrangement can also be triggered by catalytic
amounts of trimethylsilyl (TMS) triflate.[9] Treatment of syn-3
with 0.05 equivalents of this mild Lewis acid led to TMS-
protected derivative 5 in 79% yield. To introduce the more
stable tert-butyldimethylsilyl protecting group, syn-3 was
treated with tBuMe2SiOTf (3 equiv) and then with triethyl-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 6227 –6231
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6227

Communications
amine to generate bicyclic product 6. In a similar manner the
rearrangement of diastereomeric 1,2-oxazine anti-3 led to
protected bicyclic 1,2-oxazine 7 (Scheme 1). This compound
can also be obtained by SnCl4-induced rearrangement fol-
lowed by silylation (80% yield). As diastereomeric hetero-
cycles syn- and anti-3 are enantiopure,[10] this should also hold
for the corresponding bicyclic products 4–7.
The incorporation of the acetonide unit into products 4–7
suggested that other “protecting” groups can be employed in
the starting material to obtain differently substituted bicyclic
compounds. Therefore syn-configured 1,2-oxazines 8, 10, and
12, which contain cyclopentanone-, cyclohexanone-, and
benzaldehyde-derived acetals, were synthesized in analogous
procedures (Scheme 2).[11] As expected, treatment of 8 with
Scheme 2. Reaction conditions: a) SnCl4 (3 equiv), CH3CN, ꢀ308C!
RT, 6h; b) tBuMe2SiOTf (3 equiv), NEt3, CH2Cl2, 08C, 1 h, (yields over
two steps: 9: 86 %,11: 70%, 13: 77%); c) tBuMe2SiOTf (3 equiv),
CH2Cl2, room temperature, 20 h, then NEt3, 08C, 15 min, (11, 13:
quant.).
Scheme 3. Reaction conditions: a) NaBH4, EtOH, 08C, 4 h, (14: 97%,
15: 98%, 16: 70%); b) H2, Pd/C, MeOH, room temperature, 1 d, (17:
72%, 18: 96 %,19: 99%, 23: 64% over two steps); c) SmI2, THF,
room temperature (20: quant., 21: 93%, 22: 95%).
tin tetrachloride followed by O-silylation gave spiro com-
pound 9 in very good overall yield. For 1,2-oxazines 10 and 12
the one-step procedure with tert-butyldimethylsilyl triflate as
promoter was chosen, leading directly to the protected
rearrangement products 11 and 13 in excellent yields. The
configuration at the acetal carbon atom of compound 12 did
not affect the outcome: the pure R-configured bicyclic
product 13 was obtained from both diastereoisomers (or a
mixture of the two).[12] This can be expected if a carbenium
ion with an equatorial aryl group is involved in the cyclization
process. The anti-configured compounds underwent similarly
efficient rearrangements.[5]
Several options are available to generate different con-
figurations of the tetrahydropyran derivatives. Bicyclic 1,2-
oxazine 7 (derived from anti-3) was reduced with NaBH4 and
subsequent ring opening with samarium diiodide led to
trans,trans,trans-24 (Scheme 4). Hydrogenolysis gave the
ꢀ
The bicyclic products presented contain an N O bond
whose cleavage should give access to highly functionalized
enantiopure pyran derivatives. With ketones such as 6 this
ring fission was not successful. However, after reduction of
the carbonyl group of 6, 9, or 11 with NaBH4 the resulting
diastereomerically pure alcohols 14–16, respectively (or their
O-protected derivatives),[5] could be opened smoothly.
Hydrogenolysis with hydrogen and palladium on charcoal
did not only cleave the ring of the 1,2-oxazine but also
removed the N-benzyl group to give primary amines 17–19 in
good to very good yields (Scheme 3).[13] Debenzylation can be
avoided by using the milder reducing agent samarium
diiodide.[14] Secondary amines 20–22 were obtained after
short reaction time in almost quantitative yields. These two
reduction steps were used to convert phenyl-substituted
bicyclic compound 13 into pyran derivative 23, which contains
five stereogenic centers.[15]
Scheme 4. Reaction conditions: a) NaBH4, EtOH, 08C, 4 h (82%);
b) SmI2, THF, room temperature (72%); c) DEAD, PPh3, p-nitroben-
zoic acid, benzene, room temperature, 6h, (81%); d) NaN 3, MeOH,
558C, 2 d, (78%); e) SmI2, THF, room temperature (25: 81%); f) H2,
Pd/C, MeOH, room temperature (26: quant.). DEAD=diethylazodicar-
boxylate.
6228 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6227 –6231

Angewandte
Chemie
expected N-debenzylated product.[5] With bicyclic compound
14 as an example, it was demonstrated that Mitsunobu
reaction[16] allows an inversion of configuration of the
secondary hydroxy group and hence the synthesis of all-cis-
substituted carbohydrate mimetics. Both methods of reduc-
tion provided the expected products 25 and 26 in good yields.
Further stereodefined pyran derivatives synthesized from
bicyclic compounds 4 and 6 are presented in Scheme 5.
Scheme 6. Reaction conditions: a) THF, ꢀ788C, 2 h, (31: 49%,
syn/anti=97:3); b) SnCl4 (3 equiv), CH3CN, ꢀ308C!RT, 6h, (76%);
c) tBuMe2SiOTf (3 equiv), NEt3, CH2Cl2, 08C, 1 h, (32: 92%);
d) NaBH4, EtOH, 08C, 4 h, (71%); e) H2, Pd/C, MeOH, room temper-
ature, 1 day, (33: 82%).
Scheme 5. Reaction conditions: a) MsCl, NEt3, CH2Cl2, 08C, 4 h,
(quant.); b) NaBH4, EtOH, room temperature, 3 h, (85%); c) NaN3,
DMF, 908C, 6h, (73%); d) H 2, Pd/C, MeOH, room temperature,
3 days, (27: 81%); e) NaBH4, EtOH, 08C, 4 h, (97%); f) MsCl, NEt3,
CH2Cl2, 08C, 4 h, (quant.); g) KSAc, DMF/toluene, 608C, 6h, ( 28:
64%); h) Me3SO+Iꢀ, nBuLi, ꢀ788C!RT, 12 h, (29: 70%). Ms=mesyl,
DMF=N,N-dimethylformamide.
Mesylation of the primary hydroxy group of 4, followed by
reduction of the carbonyl group with NaBH4 and introduction
of an azido group, generated a new precursor for the
reductive ring opening. Hydrogenolysis furnished pyran
derivative 27 with an intact azido functionality which may
be used for further transformations.[17] Starting from 6, a
thioacetate group can be installed by nucleophilic attackat
the corresponding mesylate, resulting in the bicyclic 1,2-
oxazine 28 in good overall yield. Treatment with trimethyl-
sulfoxonium iodide stereoselectively transformed 6 into
tricyclic compound 29, which contains an epoxide moiety.
Products 27–29 and their precursors offer new options for the
preparation of highly functionalized enantiopure tetrahydro-
pyran derivatives that can serve as carbohydrate mimetics.
In first experiments we examined whether rings larger
than pyrans can be formed by using the rearrangement
presented herein. Schemes 6 and 7 show the reaction path-
ways that led to enantiopure oxepane derivatives in a highly
diastereoselective and surprisingly effective manner. The
reaction of nitrone 30 (which was easily prepared from d-
isoascorbic acid[18]) with lithiated alkoxyallene 1 provided
compound 31 with high syn/anti selectivity. In the presence of
SnCl4 in acetonitrile, 1,2-oxazine 31 afforded the desired
rearrangement product, which after protection of the primary
Scheme 7. Reaction conditions: a) THF, ꢀ788C, 2 h, (35: 71%,
syn/anti>97:3); b) SnCl4 (3 equiv), CH3CN, ꢀ308C!RT, 6h, (55%);
c) tBuMe2SiOTf (3 equiv), NEt3, CH2Cl2, 08C, 1 h, (36: 97%);
d) NaBH4, EtOH, 08C, 4 h, (93%); e) H2, Pd/C, MeOH, room temper-
ature, 1 day, (37: 70%).
hydroxy group gave bicyclic product 32 in good overall yield.
Reduction with NaBH4 followed by hydrogenolysis yielded
oxepane derivative 33, whose constitution and configuration
was confirmed by X-ray crystallographic analysis. In the same
manner, the stereoisomeric oxepane derivative 37 was
prepared from the diastereomeric nitrone 34 (derived from
l-ascorbic acid)[18] via intermediates 35 and 36.[19] Preliminary
experiments proved that suitably substituted 1,2-oxazines as
precursors opened a route to oxacyclooctane derivatives
(oxocanes) in moderate yields.[5]
Our results show that Lewis acid induced rearrangements
of 3,6-dihydro-2H-1,2-oxazines with 1,3-dioxolanyl substitu-
ents and subsequent transformations lead to a variety of
polyhydroxylated amino-substituted pyran and oxepane
derivatives in an efficient and stereocontrolled manner.[20]
The obtained enantiopure oxygen-containing heterocycles
can easily be protected selectively (and orthogonally). There-
Angew. Chem. Int. Ed. 2005, 44, 6227 –6231
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6229

Communications
628; for recent original reports on intramolecular reactions
forming pyran derivatives, see: S. Das, L.-S. Li, S. C. Sinha, Org.
Lett. 2004, 6, 123 – 126; X. Gao, P. P. Seth, J. K. Bitok, N. I. Totah,
Synlett 2005, 819 – 823; K. N. Cossey, R. L. Funk, J. Am. Chem.
Soc. 2004, 126, 12216 – 12217.
fore these analogues of aminodesoxy sugars should be of high
interest for integration into oligosaccharides.[4,21] Their lip-
ophilicity should be strongly influenced by the nature of alkyl
groups R1 and R2 (Scheme 2). These compounds also have
potential as starting materials for the synthesis of carbohy-
drate-based b- or g-amino acids (sugar amino acids) and they
can therefore provide novel peptide analogues.[22] The pyran
derivatives 6 and 7 can be easily prepared in gram scale and
hence they are also candidates for stereodefined scaffolds for
the synthesis of polyfunctionalized compounds. This concept
has been successfully applied to several carbohydrate deriv-
atives.[23]
[8] For a review on Prins reactions, see: B. B. Snider in Compre-
hensive Organic Synthesis, Vol. 2, (Eds.: B. M. Trost, I. Fleming,
C. H. Heathcock), Pergamon, Oxford, 1991, pp. 527 – 561; for
recent studies on intramolecular Prins reactions leading to
bicyclic pyran derivatives, see: Y. S. Cho, H. Y. Kim, J. H. Cha,
A. N. Pae, H. Y. Koh, J. H. Choi, M. H. Chang, Org. Lett. 2002, 4,
2025 – 2028; V. K. Yadav, N. V. Kumar, J. Am. Chem. Soc. 2004,
126, 8652 – 8653; R. Jasti, J. Vitale, S. D. Rychnovsky, J. Am.
Chem. Soc. 2004, 126, 9904 – 9905.
[9] H. Vorbrüggen, Silicon-mediated Transformations of Functional
Groups, Wiley-VCH, Weinheim, 2004.
Received: March 30, 2005
[10] The enantiomeric purity of syn-3 was proved in earlier studies:
R. Pulz, Dissertation, Freie Universitꢀt Berlin, 2002.
Keywords: carbohydrates · heterocycles · rearrangement ·
reduction
.
[11] The corresponding nitrones were prepared analogously to 2,
starting from d-mannitol: A. Dondoni, S. Franco, F. Merchµn, P.
Merino, R. Tejero, Synth. Commun. 1994, 24, 2537 – 2550.
[12] The constitution and configuration of products presented herein
were confirmed by 2D NMR spectra, associated with NOE-
studies; X-ray crystallographic analyses of key compounds 23,
33, and 35 as well as of the precursor of the ring-opened product
27 are available. Details will be presented in a future full
publication.
[1] a) W. Schade, H.-U. Reissig, Synlett 1999, 632 – 634; b) M.
Helms, W. Schade, R. Pulz, T. Watanabe, A. Al-Harrasi, L.
Fisera, I. Hlobilovµ, G. Zahn, H.-U. Reissig, Eur. J. Org. Chem.
2005, 1003 – 1019.
[2] a) R. Pulz, T. Watanabe, W. Schade, H.-U. Reissig, Synlett 2000,
983 – 986; b) R. Pulz, A. Al-Harrasi, H.-U. Reissig, Synlett 2002,
817 – 819; c) R. Pulz, A. Al-Harrasi, H.-U. Reissig, Org. Lett.
2002, 4, 2353 – 2355; d) R. Pulz, W. Schade, H.-U. Reissig, Synlett
2003, 405 – 407; e) R. Pulz, S. Cicchi, A. Brandi, H.-U. Reissig,
Eur. J. Org. Chem. 2003, 1153 – 1156; f) M. Helms, H.-U. Reissig,
Eur. J. Org. Chem. 2005, 998 – 1001.
[13] Detailed studies showed that during hydrogenolyses of 1,2-
oxazines similar to syn- or anti-3, at first the N-benzyl group was
ꢀ
rapidly removed. Thereafter the N O bond was cleaved. See
reference [10] and M. Helms, Dissertation, Freie Universitꢀt
Berlin, 2005.
[3] For selected publications, see: a) Iminosugars as Glycosidase
Inhibitors (Ed.: A. E. Stütz), Wiley-VCH, Weinheim, 1999;
b) T. D. Heightman, A. T. Vasella, Angew. Chem. 1999, 111,
794 – 815; Angew. Chem. Int. Ed. 1999, 38, 750 – 770; c) V. H.
Lillelund, H. H. Jensen, X. Liang, M. Bols, Chem. Rev. 2002, 102,
515 – 553; d) S. Gerber-Lemaire, F. Popowycz, E. Rodríguez-
García, A. T. C. Asenjo, I. Robina, P. Vogel, ChemBioChem
2002, 3, 466 – 470.
ꢀ
[14] For the cleavage of N O bonds with SmI2, see: G. E. Keck, S. F.
McHardy, T. T. Wager, Tetrahedron Lett. 1995, 36, 7419 – 7422;
J. L. Chiara, C. Destabel, P. Gallego, J. Marco-Contelles, J. Org.
Chem. 1996, 61, 359 – 360; see also reference [2c].
[15] In this case the hydride reagent attacks the bicyclic compound 13
from the other face; in contrast to 6, 9, or 11, there is no shielding
alkyl group. The configuration was undoubtedly proven by X-ray
crystallographic analysis at the stage of 23.
[4] For reviews, see: Carbohydrate Mimics—Concepts and Methods
(Ed.: Y. Chapleur), Wiley-VCH, Weinheim, 1998; P. Sears, C.-
H. Wong, Angew. Chem. 1999, 111, 2446 – 2471; Angew. Chem.
Int. Ed. 1999, 38, 2301 – 2324; C.-H. Wong, Acc. Chem. Res. 1999,
32, 376 – 385; B. Werschkun, J. Thiem, Top. Curr. Chem. 2001,
215, 293 – 325; M. H. Postema, J. L. Piper, R. L. Betts, Synlett
2005, 1345 – 1358. Selected recent original reports: C. Kieburg,
K. Sadalapure, T. K. Lindhorst, Eur. J. Org. Chem. 2000, 2035 –
2040; E. R. Palmacci, P. H. Seeberger, Org. Lett. 2001, 3, 1547 –
1550; P. Compain, O. R. Martin, Bioorg. Med. Chem. 2001, 9,
3077 – 3092; G. Hummel, L. Jobron, O. Hindsgaul, J. Carbohydr.
Chem. 2003, 22, 781 – 800; R. B. Hossany, M. A. Johnson, A. A.
Eniade, B. M. Pinto, Bioorg. Med. Chem. 2004, 12, 3743 – 3754; P.
Wipf, J. G. Pierce, N. Zhuang, Org. Lett. 2005, 7, 483 – 485, and
references thererin.
[16] For reviews, see: O. Mitsunobu, Synthesis 1981, 1 – 28; B. R.
Castro, Org. React. 1983, 29, 1 – 162; for the method applied in
this case (cleavage of the ester with sodium azide), see: J. A.
Gꢁmez-Vidal, M. T. Forrester, R. B. Silverman, Org. Lett. 2001,
3, 2477 – 2479.
[17] This azido group should offer excellent possibilities for the
connection to other carbohydrate units, for example, by 1,3-
dipolar cycloaddition with alkynes: A. Dondoni, P. P. Giovan-
nini, A. Massi, Org. Lett. 2004, 6, 2929 – 2932.
[18] Several known procedures were combined: C. Andrꢂ, J. Bolte, C.
Demuynck, Tetrahedron: Asymmetry 1998, 9, 1359 – 1367; E.
Abushanab, P. Vemishetti, R. W. Leiby, H. K. Singh, A. B.
Mikkilineni, D. C.-J. Wu, R. Saibaba, R. P. Panzica, J. Org.
Chem. 1988, 53, 2598 – 2602; see also reference [11].
[19] The oxepane derivatives may be regarded as aminoseptanose
analogues; for septanoses, see: Z. Pakulski, Pol. J. Chem. 1996,
70, 667 – 707; M. R. DeMatteo, N. L. Snyder, M. Morton, D. M.
Baldisseri, C. M. Hadad, M. W. Peczuh, J. Org. Chem. 2005, 70,
24 – 38, and references therein.
[5] A. Al-Harrasi, H.-U. Reissig, unpublished results.
[6] Ethene was trapped with bromine during this conversion.[5] The
rapid fragmentation of the residue R seems to be quite
important for the smooth cyclization to compounds 4–7. With
the 4-methoxy-substituted analogue of syn-3 similar reactions
were observed but owing to the persistence of the methyl group
the outcome was more complicated. Details will be presented in
a future publication.
[20] Carbohydrate derivatives that contain a 2,2-dimethyl group in
the pyran ring are also part of antitumor substrates such as
novobiocin: X. M. Yu, G. Shen, B. S. J. Blagg, J. Org. Chem.
2004, 69, 7375 – 7378, and references therein.
[21] Amino glycosides and their analogues are of interest as ligands
for ribosomal RNA: D. M. Ratner, E. W. Adams, M. D. Disney,
P. H. Seeberger, ChemBioChem, 2004, 5, 1375 – 1383.
[7] For reviews, see: F. Effenberger, Angew. Chem. 1969, 81, 374 –
391; Angew. Chem. Int. Ed. Engl. 1969, 8, 295 – 312; T. H. Chan
in Comprehensive Organic Synthesis, Vol. 2 (Eds.: B. M. Trost, I.
Fleming, C. H. Heathcock), Pergamon, Oxford, 1991, pp. 595 –
6230 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6227 –6231

Angewandte
Chemie
[22] For reviews, see: S. A. W. Gruner, E. Locardi, E. Lohof, H.
Kessler, Chem. Rev. 2002, 102, 491 – 514; A. Dondoni, A. Marra,
Chem. Rev. 2000, 100, 4395 – 4421; for the use of sugar
diaminocarboxylic acids, see: F. Sicherl, V. Wittmann, Angew.
Chem. 2005, 117, 2133 – 2136; Angew. Chem. Int. Ed. 2005, 44,
2096 – 2099.
[23] U. Hünger, J. Ohnsmann, H. Kunz, Angew. Chem. 2004, 116,
1125 – 1128; Angew. Chem. Int. Ed. 2004, 43, 1104 – 1107.
Angew. Chem. Int. Ed. 2005, 44, 6227 –6231
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6231