Synthesis of Sucrose-Mimicking Disaccharide by Intramolecular Aglycone Delivery

Rare sugars are known for their ability to suppress postprandial blood glucose levels. Therefore, oligosaccharides and disaccharides derived from rare sugars could potentially serve as functional sweeteners. A disaccharide [α-d-allopyranosyl-(1→2)-β-d-psicofuranoside] mimicking sucrose was synthesized from rare monosaccharides D-allose and D-psicose. Glycosylation using the intermolecular aglycon delivery (IAD) method was employed to selectively form 1,2-cis α-glycosidic linkages of the allopyranose residues. Moreover, β-selective psicofuranosylation was performed using a psicofuranosyl acceptor with 1,3,4,6-tetra-O-benzoyl groups. This is the first report on the synthesis of non-reducing disaccharides comprising only rare d-sugars by IAD using protected ketose as a unique acceptor; additionally, this approach is expected to be applicable to the synthesis of functional sweeteners.

Because of the reducing sugar structure of our target compound (1), retrosynthetic disconnection for glycosylation can be designed in two ways: by forming β-Dpsicofuranoside or α-D-allopyranoside.In previous studies on the selective βpsicofuranosylation, Ueda et al. and Yamanoi et al. reported the synthesis of psicose derivatives as donors with leaving groups (benzyl phthalate and acetate, respectively) and optimized the glycosylation conditions by investigating protecting groups in the presence of TMSOTf and Sc(OTf)3, respectively [22][23][24].To date, only a few studies have used psicose derivatives with hydroxy groups as the required acceptors for allopyranosylation to synthesize pseudo sucrose disaccharides.We decided to use 1,3,4,6-tetra-O-benzoyl  [12,13].(B) 2,3-cis β-L-Fructosylation by IAD with trimethoxybenzyl ether [14].(C) β-Mannosylation by the NAP-IAD method [15].(D) β-L-Rhamnosylation by the NAP-IAD method [16].(E) α-Allosylation with psicose derivatives by the NAP-IAD method (this study).Red arrow indicates the intramolecular transfer of the aglycon.Blue and red double arrows indicate cis configurations introduced in the substrates and formed as the products, respectively.The newly formed cis-glycosides were colored in red.
Because of the reducing sugar structure of our target compound (1), retrosynthetic disconnection for glycosylation can be designed in two ways: by forming β-D-psicofuranoside or α-D-allopyranoside.In previous studies on the selective β-psicofuranosylation, Ueda et al. and Yamanoi et al. reported the synthesis of psicose derivatives as donors with leaving groups (benzyl phthalate and acetate, respectively) and optimized the glycosylation conditions by investigating protecting groups in the presence of TMSOTf and Sc(OTf) 3 , respectively [22][23][24].To date, only a few studies have used psicose derivatives with hydroxy groups as the required acceptors for allopyranosylation to synthesize pseudo sucrose disaccharides.We decided to use 1,3,4,6-tetra-O-benzoyl psicose as the acceptor because of the simplicity of its one-step derivation from psicose according to a reported method [25,26].
In this study, we synthesized a pseudo sucrose disaccharide (1) by α-D-allosylation using NAP-IAD between the rare sugars D-allopyranose and D-psicofuranose.

Results and Discussion
We first attempted the stereoselective α-D-allosylation using the NAP-IAD glycosylation method.The D-allosyl donor with an NAP group at the C-2 position was synthesized from D-allose in six steps.As shown in Scheme 1, the first step involved the acetylation of the hydroxy group of D-allose with Ac 2 O in pyridine.The peracetate was then converted to the thioglycoside as an α/β glycoside mixture (α/β ratio of 23:77) at a yield of 66%, resulting in the recovery of the required β-glycoside-rich fraction.Deacetylation of 2, followed by the regioselective formation of 4,6-O-benzylidene acetal on the resultant tetraol 3, selectively yielded the desired compound 4. In this step, the α-thioglycoside was completely removed as this anomeric functionality was required for the subsequent general activation with MeOTf in the IAD reaction.However, this process resulted in a yield of 29% over two steps, with 2,3-O-benzylidene acetal and 2,3,4,6-di-O-benzylidene obtained as byproducts at 6% and 7% yields, respectively.The remaining hydroxy groups of β-thioglycoside 4 at the C-2 and C-3 positions were protected regioselectively.Initially, the hydroxy group at the C-2 position was protected as an NAP ether, affording the resultant compound 5 at a 52% yield.The hydroxy group at the C-3 position was protected as a ben-zyl ether to afford 6 at a 99% yield.Following this, the D-allosyl donor 6 was oxidized using 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) in the presence of 1,3,4,6-tetra-O-benzoyl-Dpsicofuranose (8) as the acceptor.This reaction produced mixed acetal isomers 8, forming a simple diastereomeric mixture related to naphthylidene acetal.The major acetal isomer was then isolated, and its subsequent intramolecular glycosylation was performed using MeOTf and 2,6-di-tert-butyl-4-methylpyridine (DTBMP) in 1,2-dichloroethane.The major isolated compound was an unexpected 1,2-O-naphthylidene acetal of the donor moiety (10) at a 49% yield.The 1,2-O-benzylidene-type cyclization from 1,2-cis glycoside was possible owing to the formation of 1,2-cis-glycoside in the IAD [27].The configuration of allose may support the formation of 1,2-O-benzylidene acetal when the 3-OH of 1,2-cis-allopyranoside was protected as a Bn ether, which created a steric hinderance to the naphthylmethyl group at the C-2 position.As previously reported, the activation of ketosidic bonds is largely influenced by the stereochemistry in the sugar ring, which is not clearly understood [28].This may have occurred because of the initial nucleophilic attack of the oxygen atom to the resultant naphthylmethyl cation at the 2-position of D-psicofuranoside in the 1,2-Onaphthylidene acetal formation, leading to the undesired cleavage of the C-O glycosidic bond of 1,2-cis-D-psicofuranoside. This cleavage likely occurred concomitantly with the enhancement of neighboring group participation by one of the carbonyl oxygens of the benzoyl groups within the D-psicofuranoside moiety.To produce the desired disaccharide derivative, it is necessary to trap the cation species initially on the naphthylmethyl group after glycosidic bond formation.The enhancement effect on the cleavage of the glycosidic bond has been also found in the case of the super arming effect of 2-O-benzoylated glycosyl donor reported by Demchemko [29,30], although we speculate that the electron withdrawing group as the protective group on the psicose stabilizes the psicosidic linkage simply by the disarming effect.Previous studies utilized (TMS) 3 SiH for the in situ reductive trapping of the benzylic cation of the NAP ether [20,21].However, the side reaction could not be suppressed, suggesting that the cation species resulting from the fragmentation of D-psicofuranoside is more stable than the benzylic cation.
To prevent the unfavorable formation of 1,2-O-naphthylidene acetal, we attempted IAD using the D-allosyl donor 7, which contained a trimethyl silyl (TMS) group at the C-3 position to trap the naphthylmethyl cation in situ by immediate acetal formation [21].The trimethylsilylation of derivative 5 proceeded with a yield of 99%.The desired mixed acetal 11 was generated by oxidizing derivative 7 with DDQ, which was confirmed using thin layer chromatography (TLC), MALDI-TOF MS, and NMR spectroscopy.Meanwhile, the actual glycosylation reaction was carried out using MeOTf and DTBMP in 1,2-dichloroethane without purifying the mixed acetals 11.This reaction afforded the 1,2-cis-D-allosyl-(1→2)β-D-psicofuranose derivative 12, with a 2,3-O-naphthylidene acetal group on the D-allosyl moiety (19%).In this reaction, 1,2-O-naphthylidene formation was not detected by MALDI-TOF MS.This result indicates the importance of introducing the nucleophilic 3-O-TMS ether to intramolecularly trap benzylic cation species, rather than intermolecularly trapping them using (TMS) 3 SiH.In this reaction, 35% of the psicose acceptor was recovered.Based on the TLC analysis, it is possible that the tethered intermediate does not form, that the acetal is cleaved even in the presence of DTBMP as a proton trap, or that the product is cleaved during formation.The coupling constant between the protons at the C-1 and C-2 positions of the allose in derivative 12 is relatively large.However, in the structures obtained using conformational analysis, the dihedral angles of the protons between the C-1 and C-2 protons in the α-alloside were approximately 37 • -41 • , regardless of the conformers derived from the naphthylidene group, suggesting that the product was an α-anomer (Figure 3).To prevent the unfavorable formation of 1,2-O-naphthylidene acetal, we attempted IAD using the D-allosyl donor 7, which contained a trimethyl silyl (TMS) group at the C-3 position to trap the naphthylmethyl cation in situ by immediate acetal formation [21].The trimethylsilylation of derivative 5 proceeded with a yield of 99%.The desired mixed acetal 11 was generated by oxidizing derivative 7 with DDQ, which was confirmed using thin layer chromatography (TLC), MALDI-TOF MS, and NMR spectroscopy.Meanwhile, proton trap, or that the product is cleaved during formation.The coupling constant between the protons at the C-1 and C-2 positions of the allose in derivative 12 is relatively large.However, in the structures obtained using conformational analysis, the dihedral angles of the protons between the C-1 and C-2 protons in the α-alloside were approximately 37°-41°, regardless of the conformers derived from the naphthylidene group, suggesting that the product was an α-anomer (Figure 3).To obtain all conformational isomers of derivative 12, a conformational search for compounds was performed in the CONFLEX 9 software using molecular mechanics force field (MMFF94s) calculations with a search limit of 1.0 kcal/mol.
The final target compound 1 was synthesized from compound 7 in four steps without purification.The protecting groups of the coupling products were removed by a short treatment (<1 min) with trifluoroacetic acid (TFA) to prevent glycosidic bond cleavage; nevertheless, the yield of the target compound was very low (4%).Based on the TLC analysis, it is possible that the disaccharide formed is cleaved into monosaccharides.Subsequent removal of benzoyl groups was achieved by treatment with sodium methoxide in methanol, quantitatively affording the target disaccharide as a benzoate adduct.The formation of this adduct was confirmed by 1 H and 13 C NMR spectroscopy as well as mass spectrometry.High resolution MS indicated the formation of the desired disaccharide (calcd.for C12H22NaO11 [M + Na] + 365.1054; found 365.1046).It has been suggested that the disaccharide, which has a cis-oriented oxygen functionality around the glycosides, exhibits a strong ionophore-like ability to uptake metal ions such as Na + ions along with counter anions such as BzO − .However, the NMR spectra of the final compound indicates that it does not release the salt during purification.
The glycosidic linkages of the allosyl residue in the products were determined based on the 1 JC-H coupling constant (170 Hz for β or >170 Hz for α) [31,32].The 1 JC-H coupling of the deprotected disaccharide was measured at 180 Hz, indicating the formation of an αallosidic linkage.The anomeric configurations of the psicosyl residue were confirmed with reference to a previous study by Morimoto et al. [33].The 13 C chemical shift of the C- To obtain all conformational isomers of derivative 12, a conformational search for compounds was performed in the CONFLEX 9 software using molecular mechanics force field (MMFF94s) calculations with a search limit of 1.0 kcal/mol.
The final target compound 1 was synthesized from compound 7 in four steps without purification.The protecting groups of the coupling products were removed by a short treatment (<1 min) with trifluoroacetic acid (TFA) to prevent glycosidic bond cleavage; nevertheless, the yield of the target compound was very low (4%).Based on the TLC analysis, it is possible that the disaccharide formed is cleaved into monosaccharides.Subsequent removal of benzoyl groups was achieved by treatment with sodium methoxide in methanol, quantitatively affording the target disaccharide as a benzoate adduct.The formation of this adduct was confirmed by 1 H and 13 C NMR spectroscopy as well as mass spectrometry.High resolution MS indicated the formation of the desired disaccharide (calcd.for C 12 H 22 NaO 11 [M + Na] + 365.1054; found 365.1046).It has been suggested that the disaccharide, which has a cis-oriented oxygen functionality around the glycosides, exhibits a strong ionophore-like ability to uptake metal ions such as Na + ions along with counter anions such as BzO − .However, the NMR spectra of the final compound indicates that it does not release the salt during purification.
The glycosidic linkages of the allosyl residue in the products were determined based on the 1 J C-H coupling constant (170 Hz for β or >170 Hz for α) [31,32].The 1 J C-H coupling of the deprotected disaccharide was measured at 180 Hz, indicating the formation of an α-allosidic linkage.The anomeric configurations of the psicosyl residue were confirmed with reference to a previous study by Morimoto et al. [33].The 13 C chemical shift of the C-2 carbon of β-psicoside was recorded as 109.2ppm (Table 1).In a previous study, a value of 111.9 Hz was reported when the chemical shift was referenced to the signal corresponding to the methyl group of sodium 3-(trimethylsilyl)[2,2,3,3-2 H 4 ] propionate (TSP-d 4 ).The overall chemical shift values shifted downfield by approximately 2 ppm compared to those observed in D 2 O. Additionally, the stereoisomer α-D-allosyl-(1→2)-α-D-psicofuranoside was synthesized using the conventional glycosylation method (NIS/TfOH) with ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-allopyranoside, and the NMR spectra of the two stereoisomers were compared (Supporting Information Schemes S1 and S2, and Table S1).The 13 C chemical shift of the C-2 carbon in the deprotected products obtained via NAP-IAD glycosylation was recorded as approximately 106 ppm, which did not match the reference data.The 13 C chemical shifts of the C1 peaks of methyl α-D-glucopyranoside and trehalose (α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside) were recorded as 101 ppm [34] and 94 ppm [35], respectively.This difference in the chemical shift value may have been caused by the reduced shielding effect of oxygen between the two sugar moieties in trehalose compared to that of a simple aglycone.Hence, the conformation of the psicofuranoside bond was determined to be β, and the NAP-IAD reaction was confirmed to result in β-psicofuranosylation.
Table 1. 1   Chemical shifts are referenced to the internal HOD (δ 4.80 ppm) for 1 H NMR and a native scale for 13 C NMR.

General Methods
1 H NMR and 13 C NMR spectra were recorded on a JEOL (Akishima, Japan) ECS-400 spectrometer at 400 and 100 MHz, respectively.The 1 H NMR chemical shifts were referenced to the signals of Me 4 Si as the internal standard (0.00 ppm in CDCl 3 and HDO, and 4.80 ppm in D 2 O).The 13 C NMR chemical shifts were referenced to the signals of the solvent [δ C (CDCl 3 ) 77.0] and native scale (D 2 O).Assignments were aided by COSY, TOCSY, and 1 H- 13 C correlation experiments.All reactions were monitored by TLC using a glass plate coated with silica gel 60F254 (0.2 mm thickness, Merck KGaA, Darmstadt, Germany).Silica gel column chromatography was performed using 60N silica gel (Kanto Chemical, Tokyo, Japan).Anhydrous solvents (superdehydrated grade) were purchased from FUJIFILM Wako Pure Chemical Corp (Osaka, Japan).Optical rotations were measured with a JASCO (Hachioji, Japan) P2200 polarimeter.High resolution mass spectra (HRMS) were recorded on Bruker (Billerica, MA, USA) micrOTOF II and Thermo Scientific (Waltham, MA, USA) Exactive Plus spectrometers using electrospray ionization in acetonitrile or methanol.MALDI-TOF MS was carried out using an Autoflex Speed mass spectrometer (Billerica, MA, USA).

Synthesis of Compound 4
To allose derivative 2 (3.0 g, 7.65 mmol), a 1 M solution of sodium methoxide in methanol (0.77 mmol, 770 µL) in a mixture of tetrahydrofuran (15.0 mL) and methanol (15.0 mL) was added at 0 • C and stirred at room temperature for 10 h.The reaction solution was neutralized using an Amberlyst at 0 • C.After diluting the reaction solution with methanol, filtration with a cotton plug was carried out, and the solvent was removed to afford the white powder 3 (3.2g), which was used without further purification.Benzaldehyde dimethyl acetal (0.96 mL, 6.43 mmol) and 2,4,6-trichloro [1,3,5]triazine (0.30 g, 1.61 mmol) were added to the obtained powder 3 (1.2g, 5.36 mmol) in DMF (30.0 mL) at 0 • C and stirred at room temperature for 2.5 h.The reaction solution was diluted with ethyl acetate and washed with saturated aq.NaHCO 3 and brine.The organic layer was dried with Na 2 SO 4 and concentrated under reduced pressure.The resulting reaction mixture was purified by silica gel column chromatography using a hexane-ethyl acetate (2/3, v/v) mixture to afford derivative 4 (483 mg, 1.55 mmol, 29%).R f = 0.49 (hexane/ethyl acetate = 2/3, v/v);

Synthesis of Compound 5
To the allose derivative 4 (483 mg, 1.55 mmol), dibutyltin(IV) oxide (426 mg, 1.71 mmol) in toluene (30.0 mL) was added and stirred at 120 • C for 2 h.The reaction solution was cooled to room temperature and the solvent was removed.To a solution of the resulting reaction mixture in toluene (30.0 mL), 2-bromomethylnaphthalene (483 mg, 2.44 mmol), tetrabutylammonium iodide (860 mg, 2.33 mmol), and cesium fluoride (354 mg, 2.33 mmol) were added at room temperature for 3 h.The reaction solution was cooled to room temperature and diluted with ethyl acetate.The reaction mixture was filtered through Celite and washed with saturated aq.NaHCO 3 and brine.The organic layer was dried with MgSO 4 , filtered, and evaporated in vacuo.The resulting residue was purified by silica gel column chromatography using a toluene-ethyl acetate (3/2, v/v) mixture to afford compound 5 tethering intermediate 9 obtained after post-treatment was dissolved in 1,2-dichloroethane (3.0 mL), and in the presence of 4 Å molecular sieves (300 mg), 1 M MeOTf solution (1.33 mL, 1.33 mmol) was added at 0 • C.After starting the reaction for 1 day, triethylamine (24 µL, 0.22 mmol) was added at 0 • C to quench the reaction.The reaction solution was diluted with ethyl acetate, filtered through celite, and washed with saturated aq.NaHCO 3 and brine.The organic layer was dried over Na 2 SO 4 and the solvent was removed under reduced pressure.The resulting residue was purified by gel filtration chromatography with chloroform to give allose derivative 10 (27 mg, 49%).R f = 0.50 (hexane/ethyl acetate = 3/2, v/v); [α] 24  D 161.59 (c 0.30, CHCl 3 );