Synthesis of new aza-C -disaccharides using cycloaddition reaction of five member chiral cyclic nitrones to alkenes derived from glucose and galactose

Aza-C -disaccharides were obtained in high yields using as the key reaction for the C-C bond formation the 1,3-dipolar cycloaddition of sugar derived cyclic nitrones with sugar derived alkenes. The observed diastereoselectivity of the reactions is rationalized on the basis of substituent induced steric factors and hydrogen bond formation.


Introduction
Azasugars have attracted much interest in recent years due to their ability to act as glycosidase inhibitors and hence to have potential applications in the treatment of metabolic diseases (diabetes), cancer and viral diseases. 1Since many glycosidase display selectivity towards the aglycon part of the inhibitor, the attachment of a second aglycon-mimicking sugar unit to an azasugar is expected to enhance its selectivity and effectiveness as inhibitor.In this context azasugars linked to common monosaccharides by a nonhydrolyzable C-C bond, namely the aza-C-disaccharides are promising substrates as selective glycosidase inhibitors and have become attractive synthetic targets. 2,3ince the synthesis of the first aza-C-disaccharide by Johnson and co-workers several synthetic approaches towards this class of compounds have been reported including, as the key step for the C-C forming reaction, Suzuki couplings, Barbier reactions, Wittig olefinations, double reductive aminations, cross-aldolization reaction, Nosaki-Kishi couplings, aldol condensation reactions. 3ecently, we have described the synthesis of the branched chain aza-C-disaccharide 3 applying as the key step the stereoselective cycloaddition of the chiral open chain nitrone 1 to the sugar derived alkene 2. 4 Although nitrone cycloadditions have been found extended applications in sugar chemistry, 5 to the best of our knowledge there are only two more references dealing with their usage for the synthesis of aza-C-disaccharides. 6 In an effort to expand the scope of this approach we initially examined the reactions of the nitrone 1 with the monosubstituted alkenes 4 and 5 derived from D-glucose and D-galactose respectively. 7However, these alkenes gave inseparable mixtures of diastereoisomers in low yields, probably as a result of their low reactivity due to the absence of the activating ester group.Therefore we focused our approach to the five membered chiral nitrones 6 and 7, which are expected to be more reactive.Furthermore these nitrones offer a ready pyrrolidine ring for the construction of the aza-C-disaccharide skeleton.

Results and Discussion
Nitrone 7 was prepared from D-ribose and its synthesis and applications to the preparation of pyrrolidine and pyrrolizidine derivatives have been previously described by us. 8Nitrone 6 was prepared from D-arabinose and its synthesis and application as intermediate for the total synthesis of pyrrolizidine alkaloids hyacinthacine A 2 and 7-deoxycasuarine have been previously described by different research groups. 9Nitrones 6 and 7 have been also used by us for the synthesis of bicyclic isoxazolidine nucleoside analogues. 10Τhe reactions between nitrones 6 and 7 with alkenes 4 and 5 were carried out by refluxing equimolecular amounts of the reactants in dry dichloromethane under an argon atmosphere until the disappearance of the starting compounds (about 3 or 4 days).The reactions of both nitrones 6 and 7 with the alkene 4 were highly diastereoselective and only one diastereoisomer 8 and 12 respectively was obtained from each reaction in 80-85 % yield (Scheme 1).The reaction of nitrone 6 with alkene 5 was also diastereoselective and only isomer 10 was isolated in 82 % yield from the reaction after column chromatography, although traces of a second isomer were detected in the crude reaction mixture.However, the reaction of nitrone 7 with alkene 5 resulted in an inseparable mixture of two diastereomers 14a and 14b in a ratio 3:2 as it was determined from its 1 H NMR spectrum.
Reductive cleavage of the N-O bond of the obtained isoxazolidine cycloadducts 8, 10 and 12 carried out by catalytic hydrogenation over Raney Ni gave almost quantitatively the corresponding aza-C-disaccharide derivatives 9, 11 and 13 respectively (Scheme 1).This reductive cleavage was also attempted to the mixture of compounds 14 with the expectation to separate the reduction products.However an inseparable mixture of aza-C-disaccharides 15a and 15b was obtained.
The structure elucidation of the obtained cycloadducts was mainly based on their spectral data.In particular the discrimination between the possible stereoisomers was based on 1 H NMR data. 1 H NMR assignments, where it was possible, were confirmed by double resonance experiments.The proposed regiochemistry is in accordance to the well established regiochemistry of cycloadditions of nitrones with monosubstituted alkenes as dipolarophiles where the formation of 5-substituted isoxazolidines predominates, 11 and it is strongly supported by the chemical shift of isoxazolidine methylene protons which appear as ddd or multiplets at δ 2.27-2.56,characteristic values of 4-isoxazolidine protons.For 5-substituted isoxazolidines there are four possible diastereomers arising from the exo/endo approach of dipolarophile and also from the Re and the Si face of the reacting nitrone (Scheme 2).Between these four possible diastereomeric structures the obtained products 8, 10 and 12 were assigned as Re-exo cycloadducts.
In compound 8 the two methylene isoxazolidine protons (3΄-H) appear at δ 2.32 (ddd, J gem = 12.8 Hz, J 3΄H 1 ,3a΄H = 8.2 Ηz, J 3΄H 1 ,2΄H = 5.9 Ηz) and 2.45 (ddd, J gem = 12.8 Hz, J 3΄H 2 ,3a΄H = 4.1 Ηz, J 3΄H 2 ,2΄H = 7.7 Ηz) each one having one large and one smaller coupling constant with its neighbouring protons indicative that it is trans to one of them and cis to the other, as it holds for an exo adduct in which the 3a΄-Η and 2΄-H of the isoxazolidine ring have a trans disposition.
The approach from the Re-face of the nitrone which leads the 3a΄-Η to be on the same side with 5΄-H was evidenced by NOE measurements as depicted in Figure 1.The significant mutual enhancements between 3a΄ with 5΄-H as well those between 3΄-H 2 and 4΄-H strongly support the proposed structure.The 1 H NMR spectrum of compound 10 in CDCl 3 was not very helpful, since most of the peaks crucial for structure determination were overlapped.Fortunately, in the 1 H NMR in CDCl 3 / C 6 D 6 solution (see experimental part) some of the peaks were resolved and permitted NOE measurements, which support the proposed structure (Figure 1).Thus, the mutual NOE enhancements between 3-H 2 with both 4-H and 2-H show that these protons are on the same internal side of the ring system, as it comes out from a Re-exo approach.This is further evidenced from the mutual NOE increments between 2-H and 6-H.In the 1 H NMR spectrum of compound 12 many of the crucial peaks were also overlapped.Several solvent mixtures were tested and the best resolution was obtained in CDCl 3 / C 6 D 6 3:1 mixture.Even in this mixture there were many overlappings, but the two 8-H and 8a-H were resolved and their measured coupling constants give evidence for the proposed structure.Thus, 8a-H appears at δ 3.82 as dd with J 8aH,8H 1 = 8.0 Hz and J 8aH,8bH = 2.4 Hz, whereas J 8aH,8H 2 is zero (see Scheme 1 for numbering).The small coupling constant between 8a-H and 8b-H is indicative that these protons are in trans disposition, as it holds for cycloadducts from the Re-face.The two 8-H appear at δ 2.39 (ddd, J gem = 13.0Hz, J 8H 1 ,8aH = 8.0 Ηz, J 8H 1 ,7H = 5.8 Ηz) and 2.56 (dd, J gem = 13.0Hz, J 8H 2 ,7H = 8.9 Ηz).The 8-H 2 at δ 2.56 with a zero coupling constant with 8a-H and so trans to it exhibits the larger coupling constant with 7-H (8.9 versus 5.8 of 8-H 1 ), indicative that that 8-H 2 is cis to 7-H.Thus, 8a-H and 7-H should be trans to each other, as it holds for exo-adducts.The assigned stereochemistries are also in line with the well documented behavior of cyclic nitrones to react in most cases preferentially via exo transition states 12 and with our previous results on the reactions of nitrones 6 and 7 with other dipolarophiles.8b, 10 In particular, it has been proved that nitrones 6 and 7 give Re-exo cycloadducts as major products and Si-exo as minor ones.Nitrone 6 having a more restricted Si-face, since substituents in both 3-and 5positions are oriented towards this face, is more diastereoselective and give higher ratios of the major Re-exo adducts or give them as sole products in the case of more bulky substituents.In nitrone 7 the acetonide moiety lying towards the Si-face induces selectivity to the Re-face.However, the methoxycarboxy methylene group at the 5-position being in this face near the reaction center reduces the face selectivity so that both Re-exo and Si-exo adducts are obtained.Based on the known behavior of nitrone 7 we propose structures 14a (Re-exo cycloadduct) and 14b (Si-exo cycloadduct) for the obtained unseparable diastereoisomers from the reaction of nitrone 7 with alkene 5.
Contrary to the behavior of nitrone 7 with the D-galactose derived alkene 5 and also with other alkenes, 8b its reaction with the D-glucose derived alkene 4 exhibits high diastereoselectivity and the diastereoisomer from the Re-exo approach is exclusively formed.This behavior is nicely explained by examination of molecular models.Thus, it becomes obvious that in the transition state of the Re-exo approach leading to a ladle shaped tricyclic skeleton, the free hydroxy group of the sugar moiety being in the same site of ester group of nitrone can form a hydrogen bond with the carbonyl oxygen stabilizing this transition state (Scheme 3).

Conclusions
1,3-Dipolar cycloadditons of cyclic sugar derived nitrones with sugar derived alkenes can lead conveniently and in high yields to aza-C-disaccharides.The diastereoselectivity of the reactions is controlled by the substitution pattern of both the nitrone and the alkene.The preferred stereochemical process is governed by steric factors and comes from the less hindered Re-exo transition state whereas stereoisomers from the Si-exo transition state can be also formed in the case of the less hindered nitrone 7.However, the formation of an hydrogen bond between the

Experimental Section
General Procedures.Mps are uncorrected and were determined on a Kofler hot-stage microscope.IR spectra were recorded on a Perkin-Elmer 297 spectrometer. 1H NMR spectra were recorded at 300 MHz on a Bruker 300 AM spectrometer and 13 C NMR spectra at 75.5 MHz on the same spectrometer, and are quoted relative to tetramethylsilane as internal reference, in deuteriochloroform solutions, unless otherwise stated.High resolution mass spectra (HRESI) were obtained with a 7 T APEX II spectrometer.Elemental analyses were performed with a Perkin-Elmer CHN 2400 automatic analyzer.Optical rotations were measured with a A. KRÜSS Optronic P3002, operating at 589 nm (l = 1dm, 25 ºC).Column chromatography was carried out on Merck Kieselgel (particle size 0.063-0.200mm) and solvents were distilled before use.

Scheme 3 .
Scheme 3. Re-exo transition state for the formation of compound 12.