Title of Invention

CHEMOENZYMATIC PROCESS FOR THE PREPARATION OF IMINOCYCLITOLS

Abstract

The present invention discloses a chemoenzymatic process for the preparation of an iminocyclitol corresponding to formula (I), (II), (III) or (IV), wherein: R1 and R2 are the same or different, and independently selected from the group consisting of: H, OH, hydroxymethyl, methyl, ethyl, butyl, pentyl, hexyl, octyl, isopropyl, isobutyl, 2-methylbutyl, and benzyl; R3 is selected from the group consisting of: H, hydroxymethyl, hydroxyethyl, ethyl, butyl, pentyl, hexyl, octyl, dodecyl, isobutyl, isopropyl, isopentyl, 2- methylbutyl, benzyl, and phenylethyl; n: O or 1; the process comprising: (i) an aldol addition catalyzed by a D-fructose-6-phosphate aldolase enzyme (FSA) and an acceptor aminoaldehyde; and (ii) an intramolecular reductive amination of the addition adduct obtained in step (i) with H2, in the presence of a metallic catalyst, optionally being carried out said step (ii) in the presence of an aldehyde of formula R3 -CHO, wherein R3 is as defined above.

Full Text

Chemoenzymatic process for the preparation of iminocyclitols FIELD OF THE INVENTION The present invention relates to a chemoenzymatic process for the preparation of iminocyclitols. The synthesized products can be used as dietary supplements and functional ingredients for the food industry, as well as therapeutic agents (e.g. in the treatment of diabetes). BACKGROUND ART Polyhydroxylated compounds, such as oligosaccharides, complex carbohydrates and lipid and protein conjugates thereof, are molecules of great importance in biochemical processes of biological recognition such as cell adhesion, viral infections, cell differentiation in organ development and metastasis (Koeller, K. M., Wong, C. H., Nat. Biotechnol. 18 (2000) 835). Thus, the enzymes involved in their synthesis or degradation, glycosyltransferases and glycosidases respectively, constitute inhibition or activation targets (according to Kolter, T., Wendeler, M., Chembiochem 4 (2003) 260) since they are involved in metabolic disorders and diseases, such as type II diabetes, hepatitis B and C, Gaucher's disease, Fabry's disease, cystic fibrosis, colon cancer, or viral infections including HIV (Asano, N., J. Enzyme Inhib. 15 (2000) 215; Asano, N., Glycobiology 13 (2003) 93R; Fiaux, H., Popowycz, F., Favre, S., Schutz, C, Vogel, P., Gerber-Lemaire, S., Juillerat-Jeanneret, L, J. Med. Chem. 48 (2005) 4237). Among the polyhydroxylated compounds which are inhibitors of glycosyltransferases and glycosidases, the types of iminocyclitols which stand out are pyrrolidines, piperidines, indolizidines, pyrrolidines, nortropanes, and seven-membered polyhydroxylated iminocyclitols), among others, some of them being powerful inhibitors of glycosidases and glycosyltransferases. (Asano, N., J. Enzyme Inhib. 15 (2000) 215; Asano, N., Glycobiology 13 (2003) 93R; Lillelunh, V. H., Jensen, H. H., Liang, X., Bols, M., Chem. Rev. 102 (2002) 515; Compain, P., Martin, O. R., Curr. Top. Med. Chem. 3 (2003) 541; Mehta, G., Lakshminath, S., Tetrahedron Lett. 43 (2002) 331; Moris-Varas, F., Qian, X. -H., Wong, C. -H., J. Am. Chem. Soc. 118 10 (1996) 7647; Fuentes, J., Olano, D., Pradera, M. A., Tetrahedron Lett. 40 (1999)4063; Li, H. Q., Bleriot, Y., Chantereau, C, Mallet, J. M., Sollogoub, M., Zhang, Y. M., Rodriguez-Garcia, E., Vogel, P., Jimenez-Barbero, J., Sinay, P., Org. Biomol. Chem. 2 (2004) 1492; Lin, C. C, Pan, Y. S., Patkar, L. N., Lin, H. M., Tzou, D. L. M., Subramanian, T., Bioorg. Med. Chem. 12 (2004) 3259; Godin, G., Gamier, E., Compain, P., Martin, O.R., Ikeda, K., Asano, N., Tetrahedron Lett. 45 (2004) 579). Some derivatives such as miglitol and miglustat are drugs marketed for the treatment of type II diabetes (Piatt, F. M., Butters, T. D., Drugs 63 (2003) 2435). Some natural iminociclytols or plant extracts containing them have also been described as functional ingredients in the food industry or as dietary supplements. Thus, US20010018090 discloses the use of 1-deoxynojirimicin or an analogue thereof as a calorie reducing agent that may be incorporated in food or beverage; US20060222720 discloses an anorectic agent containing aqueous solvent extracts of Vernonia cinerea and mulberry as active ingredients; WO2004037001 discloses the addition of mulberry extracts to a sacharide containing food for regulating blood sugar levels. So far, the chemoenzymatic strategies for the synthesis of iminocyclitols disclosed are based on the use of aldolases, enzymes capable of catalyzing stereoselective aldol condensation reactions between aldehydes and ketones. (Von der Osten, C.H., Sinskey, A. J., Barbas, C. F., Ill, Pederson, R. L., Wang, Y. F., Wong, C. H., J. Am. Chem. Soc. 111 (1989) 3924, Romero, A., Wong, C. H., J. Org. Chem. 65 (2000) 8264, Look, G. C, Fotsch, C. H., Wong, C. H., Ace. Chem. Res. 26 (1993) 182, Machajewskif, T. D., Wong, C. H., Angew. Chem. Int. Ed. 39 (2000) 1353; patent (US005329052A)). Among known aldolases, the dihydroxyacetonephosphate (DHAP)-dependent aldolases have focused the attention due to four reasons: 1) their availability, either because some of them are commercially available or because their preparation is relatively easy from modified E. coli, 2) their high stereoselectivity, 3) their wide structural tolerance for the acceptor aldehyde, and 4) their stereogenic ability. DHAP-dependent aldolases (DHAP-aldolases) catalyze reversible DHAP aldol addition with an acceptor aldehyde, obtaining α,β-dihydroxyketones with two new stereogenic centres. It is especially interesting to note that the four stereocomplementary DHAP-aldolases (Figure 1) are already known: D-fructose-1,6-diphosphate aldolase (FruA); L-rhamnulose-1-phosphate aldolase (RhuA); L-fuculose-1-phosphate aldolase (FucA); and D-tagatose-1,6-diphosphate aldolase (TagA). Advantageously, these biocatalysts have some ability to control the aldol addition stereochemistry, the configuration of the new generated stereogenic centres depending on the enzyme and not on the reagents. The general chemoenzymatic synthetic scheme of iminocyclitols synthesis using DHAP-aldolases is shown in Figure 2. The critical step of this scheme is the aldol addition of DHAP to aminoaldehydes or synthetic equivalents thereof catalyzed by DHAP-aldolases. In this step two stereogenic centres, whose configuration depends on the enzyme, are generated, although there are numerous examples wherein, depending on the substrate, the enzyme looses selectivity, obtaining diastereomeric products. The following step is a hydrolysis of the phosphate moiety of the aldol adduct by an acid phosphatase. Finally, the Cbz removal and the transformation to iminocyclitol is generally carried out in one step. The preparation of the dihydroxyacetonephosphate (DHAP) is a critical step of this synthesis. The chemical synthesis of dihydroxyacetonephosphate is carried out through five steps with overall yields about 60% (Figure 3) according to Jung et al. disclosure (Jung, S. -H., Jeong, J. -H., Miller, P., Wong, C.-H., J. Org. Chem. 59 (1994) 7182). Multienzyme systems for "in situ" generation of DHAP are an alternative approach. These are sophisticated processes demanding a very fine control of the reaction conditions and the presence of components in the reaction mixture which can hinder the isolation and purification of the final product (Fessner, W. D., Sinerius, G., Angew. Chem. Int. Ed. 33 (1994) 209; Charmantray, F., El Blidi, L, Gefflaut, T., Hecquet, L, Bolte, J., Lemaire, M., J. Org. Chem. 69 (2004) 9310, Sanchez-Moreno, I., Francisco Garcia-Garcia, J., Bastida, A., Garcia Junceda, E., Chem. Commun. (2004) 1634). In the patent (US005329052A) and as it is disclosed by Von der Osten et al. (Von der Osten, C. H., Sinskey, A. J., Barbas, C. F., Ill, Pederson, R. L, Wang, Y. F., Wong, C. H., J. Am. Chem. Soc. 111 (1989) 3924), dihydroxyacetone is used in the presence of arsenic salts as a substitute of DHAP for enzymatic aldol addition. Although the process is simplified, the use of arsenic salts is not applicable due to their toxicity and, therefore, their environmental and health danger. Already disclosed chemoenzymatic synthesis of iminocyclitols use about 8 steps from the acceptor aldehyde: two of them are enzymatic steps, the aldol addition of DHAP to the aldehyde, and the phosphate ester hydrolysis; and 6 chemical steps for DHAP synthesis, and the formation of the corresponding iminocyclitols. If the reaction is carried out with multienzyme systems, it requires two more enzymes: one for the formation of the key intermediate, DHAP, and another for regenerating the enzymatic phosphorilation reagent. Therefore, these are strategies with a lot of steps or very sophisticated and therefore with a limited industrial applicability. SUMMARY OF THE INVENTION An object of the invention is a chemoenzymatic process for the preparation of an iminocyclitol corresponding to formula (I), (II), (III) or (IV): wherein: - R1 and R2 are the same or different, and independently selected from the group consisting of: H, OH, hydroxymethyl, methyl, ethyl, butyl, pentyl, hexyl, octyl, isopropyl, isobutyl, 2-methylbutyl, and benzyl; -R3 is selected from the group consisting of: H, hydroxymethyl, hydroxyethyl, ethyl, butyl, pentyl, hexyl, octyl, dodecyl, isobutyl, isopropyl, isopentyl, 2-methylbutyl, benzyl, and phenylethyl; - n: Oor 1; - the configurations of the carbon atoms to which R1 and R2 substituents are attached in the iminocyclitol of formula I are, the same or different, and independently selected from R and S; and - the stereogenic centres (*) in the iminocyclitol of formula II, III or IV are, the same or different, and independently selected from R and S, characterized in that it comprises the following steps: i) an aldol addition catalyzed by a D-fructose-6-phosphate aldolase enzyme (FSA), between the dihydroxyacetone (DHA) and an acceptor aminoaldehyde, the acceptor aminoaldehyde corresponding to formula V, VI or VII: - R1, R2, and n are as defined above; - the configurations of the carbon atoms to which R1 and R2 substituents are attached in formula V and the stereogenic centre of formulae VI and VII are the same or different, and independently selected from R and S; and - Cbz represents a benzyloxycarbonyl group; and ii) an intramolecular reductive amination of the addition adduct obtained in step (i) with H2, in the presence of a metallic catalyst; optionally being carried out said step (ii) with an aldehyde of formula R3-CHO, wherein R3 is as defined above, resulting in a double reductive amination. DETAILED DESCRIPTION OF THE INVENTION The present inventors have found that it is possible to synthesize iminocyclitols by a process based on the use of the D-fructose-6-phosphate aldolase enzyme, hereinafter FSA, as a biological catalyst for the aldol addition reaction between dihydroxyacetone and an aminoaldehyde of formula V, VI or VII. FSA ability for catalyzing the aldol addition between DHA and glycolaldehyde, D,L-glyceraldehyde-3-phosphate, D-glyceraldehyde and D-erythrose is known (Schurmann, M.; Sprenger, G. A. J. Biol. Chem. (2001) 276 11055). Furthermore, it is known that the active site of the FSA includes an arginine residue which is essential in order to satisfactorily arrange its natural substrate and give rise to the enzymatic reaction. The nature of the enzymatic active site determines the nature of compounds to be used as substrate and from Schurman et al., (supra) it can be concluded that the best acceptor substrates are the hydrophilic aldehydes described therein. Surprisingly, the present inventors have found that although the aminoaldehydes of formula (V), (VI) and (VII) have quite different physico-chemical properties, being highly hydrophobic compared with the substrates known in the prior art, the aldol addition is carried out efficiently. Furthermore, in Schurmann et al., (supra) the aldol addition adducts produced by FSA were not totally characterized by spectroscopic techniques nor the stereochemistry of the adducts was resolved. Therefore, it was not possible to deduce that the aminoaldehydes of formulae V, VI and VII were FSA substrates nor the final stereochemistry of the reaction was that suitable for the products of the present invention. Additional advantages derived from the use of FSA are the following: - FSA uses dihydroxyacetone (DHA) for the aldol addition reaction instead dihydroxyacetone phosphate (DHAP), saving 5 synthetic steps with regard to DHAP-dependent aldolases methodology as described above, -The step of the enzymatic hydrolysis of the phosphate group by acid phosphatase is avoided and, therefore, synthetic steps and production costs are diminished, -FSA preparation and purification is simple and low cost, -FSA is stable as a biocatalyst at 4 °C for at least seven months without losing activity, and -It does not use nor generate toxic residues. As mentioned above, DHAP-aldolases can use DHA in the presence of arsenic salts, a high-toxicity product, harmful for the health and the environment. Finally, another advantage of these biocatalysts is that they have some ability to control aldol addition stereochemistry. Thus, the configuration of the new generated stereogenic centres depends on the enzyme and not on the aldol addition reagents. Therefore, an object of the invention is a chemoenzymatic process as defined above. The elimination of the amine protecting group, and the intramolecular reductive amination in this stage (ii) may occur in one-pot reaction. Preferably, it occurs in a one-pot reaction. A preferred embodiment is the process of the invention wherein the iminocyclitol is an iminocyclitol of formula I which is selected from the group consisting of: miglitol; miglustat; D-fagomine; 1-deoxynojirimycin; N-substituted derivatives thereof, such as A/-butyl-D-fagomine; and 1,4-dideoxy-1,4-imino-D-arabinitol. Preferably, the iminocyclitol is selected from the group consisting of: D-fagomine; 1-deoxynojirimycin; A/-butyl-D-fagomine; and 1,4-dideoxy-1,4-imino-D-arabinitol. More preferably, the iminocyclitol is selected from the group consisting of: D-fagomine; 1-deoxynojirimycin; and N-butyl-D-fagomine. Another preferred embodiment of the invention is the process wherein the aminoaldehyde of (i) is a type V protected aminoaldehyde belonging, as an illustration and without limiting the scope of the invention, to the following group: N-Cbz-3-aminopropanal, and (S)-N-Cbz-3-amino-2-hydroxypropanal A particular embodiment of the invention is the process of the invention wherein the FSA enzyme used in step (i) corresponds to E. coli FSA with SEQ ID N02. D-fructose-6-phosphate aldolase (FSA) enzyme used in the present invention has been cloned in E. coli MC4100 strain, derived from E. coli K-12 strain (Schurmann, M.; Sprenger, G. A. J. Biol. Chem. (2001) 276 11055; Casadaban, M. J. (1976) J. Mol. Biol. 104, 541 -555) and subsequently purified. Thus, the preferred FSA enzyme used consists in the wild type which naturally occurs in said microorganism and with an aminoacid sequence corresponding to SEQ ID N02. Any other wild type n-fructose-6-phosphate aldolase (FSA) enzyme can be isolated and identified in other microorganisms due to the information and processes existing in the state of the art. Therefore, other embodiment of the present invention is the process wherein the FSA enzyme is an enzyme with an analogous sequence to SEQ ID NO2, isolated from a microorganism other than E. coli. As used herein, the term "analogous" intends to include any aminoacid sequence which can be isolated from a microorganism and have the aldol addition ability between the dihydroxyacetone (DHA) and an acceptor aldehyde of formula V, VI, or VII (Figure 4). Generally, an analogous aminoacid sequence is substantially homologous to the previously cited aminoacid sequence. As used herein, the expression "substantially homologous" means that the aminoacid sequences in question have an identity degree of at least 30%, preferably of at least 85%, or more preferably of at least 95%. Other particular object of the invention is the process of the invention wherein the metallic catalyst used in step (ii) belongs, as an illustration and without limiting the scope of the invention, to the following group: Pd, Pt, Rh, and combinations of Pd and sodium cyanoborohydride (NaCNBH3). DESCRIPTION OF DRAWINGS Figure 1.- Stereochemistry of DHAP-aldolases. Figure 2.- General scheme of the chemoenzymatic synthesis of iminocyclitols. Nequiv: protected amine orazide, such as benzyloxycarbonyl-NH-, tert-butyloxycarbonyl-NH-, 9-fluorenylmethoxycarbonyl-NH-, phenylacetyl-NH-, and azido. a) DHAP-aldolase, b) acid phosphatase; c) intramolecular reductive amination with H2 in the presence of a metallic catalyst or a reductive agent. Figure 3.- Synthesis scheme of dihydroxyacetone phosphate (DHAP). a) HC(OEt)3, H2SO4 cat., EtOH, b) CI(O)P(OPh)2, anhydrous pyridine, DMAP cat., c) H2 50 psi, crystalline PtO2, EtOH, d) H20, 65 °C, e) aqueous NaOH up to pH=7, f) Dowex H+, 65 °C. Figure 4.- Scheme of the exemplified reactions of the use of the process of the invention for the synthesis of D-fagomine, N-butyl-D-fagomine, and 1-deoxynojirimycin: a) FSA, b) Pd/C H2 pressure 50 psi, and c) CH3CH2CH2COH, Pd/C H2 pressure 50 psi. EXAMPLES Next, five examples illustrate the use of this process for the preparation of D-fagomine, A/-butyl-D-fagomine, 1-deoxynojirimycin, and 1,4-dideoxy-1,4-imino-D-arabinitol (DAB). The general scheme of the reactions detailed next is shown on Figure 4. Example 1.- D-Fagomine synthesis Step 1) Preparation of the aldol addition adduct. Starting aldehyde, N-Cbz-3-aminopropanal, was obtained from 3-aminopropanol by conventional processes disclosed by Espelt et al. (Espelt, L, Parella, T., Bujons, J., Solans, C, Joglar, J., Delgado, A., Clapes, P., Chem. -Eur. J. 9 (2003)4887; Ocejo, M., Vicario, J. L, Badia, D., Carrillo, L., Reyes, E., Synlett (2005)2110). N-Cbz-3-aminopropanal (2.1 g, 22.9 mmol) was dissolved with dimethylformamide (40 mL) in a reactor of 250 mL of volume and equipped with orbital stirring. Dihydroxyacetone (4.7 g, 22.9 mmol) and FSA enzyme in raw powder (2.09 g, 3445 U) were added to this solution dissolved with boric borate buffer 50 mM pH 7 (155 mL). The mixture was left to react under orbital stirring (120 rpm) at 4 °C for 24 hours. The reaction conversion at this point was greater than 98%. Next, MeOH (200 mL) was added to the reaction mixture, appearing a precipitate which was separated by centrifugation. The supernatant was purified by reverse phase liquid chromatography. Pure fractions were pooled, the solvent was evaporated obtaining 4.7 g of a white solid (yield 69%, diastereomeric excess 99%). The preparation and purification of FSA were carried out from crude protein extract from the fermentation and cell disruption by thermal treatment at 75 °C for 40 minutes (Schurmann, M., Sprenger, G. A„ J. Biol. Chem. 276 (2001) 11055; Thorell, S., Schurmann, M., Sprenger, G. A„ Schneider, G., J.Mol. Biol. 319 (2002) 161; Schurmann, M., Sprenger, G. A., J. Mol. Catal. B-Enzym. 19 (2002) 247). The protein extract was obtained from E. coli MC4100 strain, derived from E. coli K-12 strain (Casadaban, M. J. J. Mol. Biol. (1976) 104,541), which comprises the coding sequence of E. coli FSA protein (SEQ ID N01). Cloning, ligation to a plasmid, and transformation into an E. coli strain are described in detail in Schurmann, M., Sprenger, G. A., J. Biol. Chem. 276 (2001) 11055; The FSA protein thus obtained (SEQ ID N02) retains its activity while protein impurities precipitate, being separated by simple filtration or centrifugation. Step 2) Deprotection and intramolecular reductive amination. The adduct obtained in the last step (373 mg, 1.26 mmol) was dissolved in ethanol/water 1:9 (50 mL). The solution was kept under H2 atmosphere at 50 psi of pressure in the presence of palladium over carbon (100 mg). At these conditions, the elimination of Cbz group and the intramolecular reductive amination proceeded simultaneously for 12 hours. Alternatively, the adduct obtained in the last step (373 mg, 1,26 mmol) was dissolved in ethanol/water 1:9 (50 mL) in the presence of sodium cyanoborohydride (NaCNBH3) (50 mg). The solution was kept under H2 atmosphere at room pressure in the presence of palladium over carbon (100 mg). At these conditions, the elimination of the Cbz group was carried out by the palladium action and the intramolecular reductive amination by the presence of NaCNBH3. Both reactions proceed simultaneously for 6 hours. Next, the reaction mixture was filtered over deactivated alumina and the filtrate was evaporated obtaining 164 mg of D-fagomine solid (yield 89%). [α]D22 + 20.4 (c 1.0 in H2O); δH (500 MHz; D2O; 22 °C) 3.86 (1H, dd, J 11.8 and 3.0, 7-H), 3.66 (1H, dd, J 11.8 and 6.5, 7-H), 3.56 (1H, ddd, J 11.5, 9.0 and 5.0,4-H), 3.21 (1H t, J 9.5 and 9.5, 3-H), 3.06 (1H, ddd, J 12.9, 4.4 and 2.3, 6-H), 2.68 (1H, dt, J 12.94,12.92 and 2.70, 6-H), 2.61 (1H, ddd, J 9.68, 6.44 and 2.97, 2-H), 2.01 (1H, tdd, J 13.0, 4.9, 2.5 and 2.5, 5-H) y 1.48 ppm (1H, dq, J 13.0,12.9,11.5 and 4.5, 5-H); δC (101 MHz; D20; 22 °C) 72.9, 72.7, 61.1,60.9, 42.6 and 32.1. Example 2.-N-Butyl-D-Fagomine synthesis Step 1) Preparation of the aldol addition adduct as in the previous section. Step 2) Deprotection and double reductive amination. The adduct resulting from the previous step (150 mg, 0.51 mmol) and butanal were dissolved in ethanol/water 7:3 (10 mL). Palladium over carbon (50 mg) was added to this solution and the mixture was left to react under H2 at 50 psi for 12 hours. Next, the process was similar to that of the previous example, and 52 mg of N-butyl-D-fagomine solid were obtained (yield 52%) after the purification of the reaction crude by a silica column using MeOH/CHCI3 mixtures as eluents. [α]D22 = -24.5 (c 1.2 in MeOH); δH(500 MHz, D20, 22°C) 3.90 (1H, dd, J 12.7, and 2.4, 7-H), 3.82 (1H, dd, J 12.7 and 2.9, 7-H), 3.45 (1H, ddd, J 11.5, 9.1 and 5.1, 4-H), 3.30 (1H, t, J 9.4, 3-H), 2.90 (1H, td, J 12.2, 3.5 and 3.5, 6-H), 2.73 (1H, ddd, J 13.3, 11.2 y 5.4, 8-H), 2.50 (1H, ddd, J 13.3,11.1 and 5.2, 8-H), 2.36 (1H, dt, J 12.6, 12.6 and 2.4, 6-H), 2.16 (1H, td, J 9.8, 2.6 and 2.6 Hz, 2-H), 1.92 (1H tdd, J 12.7, 5.0, 2.5 and 2.5, 5-H), 1.55-1.35 (3H, m, 5-H and 2 x 9-H), 1.30-1.21 (2H, m, 2 x 10-H) and 0.88 (3H, t, J 7.4 and 7.4, 11-Me); 8c, (101 MHz, D20, 22 °C) 73.2, 72.0, 65.8, 58.1, 52.2, 49.1, 30.5, 25.6, 20.4 and 13.3. Example 3.- 1-deoxynojirimycin synthesis Step 1) Preparation of the aldol addition adduct. Starting aldehyde, (S)-N-Cbz-3-amino-2-hydroxypropanal, was obtained from (S)-3-amino-2-hydroxypropanol by a process disclosed by De Luca et al. (De Luca, L, Giacomelli, G., Porcheddu, A., Org. Lett. 3 (2001) 3041). The process was equivalent to that disclosed in Example 1, but in this case with the difference of performing the reaction at 25 °C. Step 2) Deprotection and intramolecular reductive amination. It was performed as in Example 1 obtaining 164 mg (yield 89%) of a white solid of 1-deoxynojirimycin. [α]D22 + 48.0 (C 1.0 at H2O). 1H NMR (500 MHz, D20. δ ppm 3.74 (dd, J = 11.8, 3.00 Hz, 1H), 3.56 (dd, J = 11.9, 6.2 Hz, 1H), 3.4 (ddd, J = 10.96, 9.06, 5.25 Hz, 1H), 3.24 (t, J = 9.1, 9.1 Hz, 1H), 3.18 (t, J = 9.4, 9.4 Hz, 1H), 3.1 (dd, J = 12.3, 5.2 Hz, 1H), 2.54 (hept, J = 9.4, 6.0, 3.0, 1H), 2.43 (dd, J = 12.3, 11.0 Hz, 1H). Example 4.- Synthesis of 1 -deoxynojirimycin Step 1) Preparation of the adduct from aldolic addition. The starting aldehyde, (R,S)-N-Cbz-3-amino-2-hydroxy-propanal, was obtained from (R,S)-N-Cbz-3-amino-2-hydroxy-propanol (1 g, 4.4 mmol) by oxidation with IBX (o-iodoxybenzoic acid). In a 250 mL reactor equipped with orbital shaking and reflux N-Cbz-3-amino-2-hydroxy-propanol (1 g, 4.4 mmol) was dissolved in ethyl acetate (150 mL). To this solution IBX (2,5 g; 2 equivalents) was added and the reaction was kept under reflux for 3 h. The resulting solution was filtered and the ethyl acetate layer washed with 5% (p/v) NaHCO3 and saturated NaCI to eliminate reaction by-products. The ethyl acetate solution which contained (R,S)-N-Cbz-3-amino-2-hydroxy-propanal, was added over an aqueous solution of dihydroxyacetone (510 mg, 5.7 mmol) and crude powder FSA (235 mg, 3445 U) in boric-borate buffer 50 mM pH 8 (250 mL) in a 500 mL reactor. The ethyl acetate was evaporated from the resulting two-phase mixture and this allowed the diffusion of the aldehyde into the aqueous phase. The reaction was then kept under orbital shaking (120 rpm) for 24 hours at 25°C. At this point the reaction conversion was higher than 98%. Then, MeOH (250 mL) was added to the crude reaction mixture and a solid residue was separated by centrifugation. The supernatant was purified by reversed-phase liquid chromatography to obtain a white solid (600 mg, 44 % yield). Step 2) Deprotection and intramolecular reductive amination. The adduct obtained in the previous step (600 mg, 1.91 mmol) was dissolved in ethanol/water 1:4 (80 mL). The solution was kept for 12 h under H2 atmosphere at a pressure of 50 psi in the presence of palladium/charcoal (176 mg). Under these conditions both the elimination of Cbz and the intramolecular reductive amination proceeded simultaneously over a period of 12 hours. Then the crude reaction mixture was filtered over neutral alumina and the filtrate evaporated to obtain a white solid (164 mg, 89% yield). 1H NMR (500 MHz, D20) δ ppm 3.74 (dd, J = 11.8, 3.00 Hz, 1H), 3.56 (dd, J = 11.9, 6.2 Hz, 1H), 3.4 (ddd, J= 10.96, 9.06, 5.25 Hz, 1H), 3.24 (t, J = 9.1, 9.1 Hz, 1H), 3.18 (t, J = 9.4, 9.4 Hz, 1H), 3.1 (dd, J= 12.3, 5.2 Hz, 1H), 2.54 (hept, J =9.4, 6.0, 3.0, 1 H), 2.43 (dd, J = 12.3, 11.0 Hz, 1H). Example 5.- Synthesis of 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) Step 1) Preparation of an adduct from aldolic addition. The starting aldehyde N-Cbz-2-aminoetanal was obtained from 2-aminoetanol by standard procedures such as those described by Espelt, L, Parella, T., Bujons, J., Solans, C, Joglar, J., Delgado, A., Clapes, P., Chem.-Eur. J. 9 (2003) 4887; Ocejo, M., Vicario, J. L, Badia, D., Carrillo, L, Reyes, E., Synlett (2005) 2110. In a 250 mL reactor equipped with orbital shaking N-Cbz-3-aminoetanal (1.51 g, 7.8 mmol) was dissolved in dimethylformamide (8 mL). To this solution dihydroxyacetone (0,71 g, 7.9 mmol) and a lyophilized preparation with FSA activity (0.7 g, 1150 U) dissolved in boric-borate buffer 50 mM pH 7.0 (72 mL) were added. The reaction proceeded under orbital shaking (120 rpm) for 120 hours at 25 °C. At this point the reaction conversion was 49%. Then MeOH (100 mL) was added and a solid residue was separated by centrifugation. The supernatant was purified by reversed-phase liquid chromatography. The pure fractions were collected and the solvent evaporated to obtain a white solid (0.50 g, 23 % yield) Step 2) Deprotection and intramolecular reductive amination. The adduct obtained in the previous step (500 mg, 1.77 mmol) was dissolved in ethanol/water 1:9 (90 mL). The solution was kept for 12 h under H2 atmosphere at a pressure of 50 psi in the presence of Pd/C (204 mg) as catalyst. Then the crude reaction mixture was filtered over neutral alumina and the filtrate evaporated to obtain a white solid (253 mg). The final product was purified from this solid by cation exchange chromatography to obtain a 10 mM aqueous NH3 solution. The pure fractions were pooled and the solvent evaporated to obtain a white solid (129 mg, 10 % global yield, 99% diastereomeric excess). [α]D20 + 26.2 (c 1.0 en H20); [afo20 + 35.2 (c 1.0 en MeOH) 1H NMR (500 MHz, D2O, 22 °C) δ(ppm): 4.35 (m, 1 H, H4), 4.11 (t, J = 3.3 Hz, 1 H, H3), 3.97 (dd, J = 12.2, 4.6 Hz, 1 H, H6), 3.85 (dd, J = 12.2, 8.3 Hz, 1 H, 6H), 3.63 (dd, J = 8.3, 4.2 Hz, 1 H, H2), 3.59 (dd, J = 12.6, 4.8 Hz, 1H, H 5), 3.37 (dd, J = 12.6, 2.7 Hz, 1H, H5). 13C NMR (101 MHz, D20, 22 °C) 5 (ppm): 78.37 (C3), 77.00 (C4), 69.30 (C2), 61.66 (C6), 52.68 (C5) CLAIMS 1- Chemoenzymatic process for the preparation of an iminocyclitol of formula (I), (II), (III) or (IV): wherein: - R1 and R2 are the same or different, and independently selected from the group consisting of: H, OH, hydroxymethyl, methyl, ethyl, butyl, pentyl, hexyl, octyl, isopropyl, isobutyl, 2-methylbutyl, and benzyl; -R3 is selected from the group consisting of: H, hydroxymethyl, hydroxyethyl, ethyl, butyl, pentyl, hexyl, octyl, dodecyl, isobutyl, isopropyl, isopentyl, 2-methylbutyl, benzyl, and phenylethyl; -n: 0 or 1; - the configurations of the carbon atoms to which R1 and R2 substituents are attached in the iminocyclitol of formula I are, the same or different, and independently selected from R and S; and -the stereogenic centres (*) in the iminocyclitol of formula II, III or IV are, the same or different, and independently selected from R and S. characterized in that it comprises the following steps: i) an aldol addition catalyzed by a D-fructose-6-phosphate aldolase enzyme (FSA), between the dihydroxyacetone (DHA) and an acceptor aminoaldehyde the aminoaldehyde corresponding to formula V, VI or VII: wherein - R1, R2 and n are as defined above; - the configurations of the carbon atoms to which R1 and R2 substituents are attached in formula V and the stereogenic centre of formulae VI and VII are the same or different, and independently selected from R and S; and - Cbz represents a benzyloxycarbonyl group; and ii) an intramolecular reductive amination of the addition adduct obtained in step (i) with H2, in the presence of a metallic catalyst, optionally being carried out said step (ii) with an aldehyde of formula R3-CHO, wherein R3 is as defined above, resulting in a double reductive amination. 2. The process according to claim 1, characterized in that the iminocyclitol is selected from the group consisting of: miglitol; miglustat; D-fagomine; 1-deoxynojirimycin; W-butyl-D-fagomine; and 1,4-dideoxy-1,4-imino-D-arabinitol. 3. The process according to claim 2, characterized in that the iminocyclitol is selected from the group consisting of: D-fagomine; 1-deoxynojirimycin; N-butyl-D-fagomine; and 1,4-dideoxy-1,4-imino-D-arabinitol. 4. The process according to claim 3, characterized in that the iminocyclitol is selected from the group consisting of: D-fagomine, 1-deoxynojirimycin, and N-butyl-D-fagomine. 5. Process according to claim 1, characterized in that the aminoaldehyde in (i) is a type V protected aminoaldehyde selected from the group consisting of: N-Cbz-3-aminopropanal, and (S)-N-Cbz-3-amino-2-hydroxypropanal. 6. The process according to claim 1, characterized in that the metallic catalyst used in step (ii) is selected from the group consisting of: Pd, Pt, Rh, and combinations of Pd with sodium cyanoborohydride (NaCNBH3). 7. The process according to claim 1, characterized in that the FSA enzyme has an identity degree with SEQ ID NO: 2 of at least 85%. 8. The process according to claim 7, characterized in that the FSA enzyme has an identity degree with SEQ ID NO: 2 of at least 95%. 9. The process according to claim 8, characterized in that the FSA enzyme consists of the aminoacid sequence of SEQ ID NO2. The present invention discloses a chemoenzymatic process for the preparation of an iminocyclitol corresponding to formula (I), (II), (III) or (IV), wherein: R1 and R2 are the same or different, and independently selected from the group consisting of: H, OH, hydroxymethyl, methyl, ethyl, butyl, pentyl, hexyl, octyl, isopropyl, isobutyl, 2-methylbutyl, and benzyl; R3 is selected from the group consisting of: H, hydroxymethyl, hydroxyethyl, ethyl, butyl, pentyl, hexyl, octyl, dodecyl, isobutyl, isopropyl, isopentyl, 2- methylbutyl, benzyl, and phenylethyl; n: O or 1; the process comprising: (i) an aldol addition catalyzed by a D-fructose-6-phosphate aldolase enzyme (FSA) and an acceptor aminoaldehyde; and (ii) an intramolecular reductive amination of the addition adduct obtained in step (i) with H2, in the presence of a metallic catalyst, optionally being carried out said step (ii) in the presence of an aldehyde of formula R3 -CHO, wherein R3 is as defined above.

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