2-C-Alkynyl and 2-Ccis-Alkenyl β-Mannosides with Acetal Protected γ-Aldehyde Functionality via 2-Uloside Alkynylation and Lindlar Hydrogenation

Benzyl 3,4,6-tri-O-benzyl-β-D-arabino-hexos-2-ulo-1,5-pyranoside was subjected to mannoselective ketone alkynylation with propiolaldehyde dibenzyl acetal, resulting in the formation of a 2-C-alkynyl β-mannoside bearing a γ-dibenzyl acetal functionality. Subsequent transacetalization of the acetal moiety with methanol and 1,3-dihydroxypropane and acetylation of position 2, respectively, gave 4 different 2-C-alkynyl branched mannosides. Lindlar hydrogenation of the latter under optimized conditions in dimethylformamide afforded a series of 2-C-cis-alkenyl mannosides. X-ray molecular structures of benzyl 3,4,6-tri-O-benzyl-β-D-arabino-hexos-2-ulo-1,5-pyranoside and of the branched glycoside benzyl 3,4,6-tri-O-benzyl-2-C-((Z)-3,3-dibenzyloxyprop-1-en-1-yl)-β-Dmannopyranoside are reported.


Introduction
Carbon-branched carbohydrates and especially 1,2-annulated carbohydrates have been the target of considerable synthetic efforts in recent years, for such sugar derivatives may have potential medical application as glycosidase inhibitors [1][2][3][4].In the context of our current project regarding the development of synthetic strategies towards bicyclic annulated carbohydrate derivatives [5,6], we became interested in the synthesis of 2-C-cis-alkenyl substituted β-mannosides bearing a γ-formyl moiety.Such C-branched glycosides having an additional aldehyde group were thought to be useful synthetic precursors for 1,2-annulated glycosides.Therefore, we contemplated a reaction sequence in which a 2-keto-glucoside (2-uloside) is manno-selectively alkynylated with propiolaldehyde dibenzyl acetal and subsequently partially hydrogenated.Initially, we next anticipated a synthetic route featuring dihydroxylation of the cis-alkene and subsequent deprotection/hemiacetal formation for the synthesis of reducing bycyclic sugar derivatives.Due to the observed inertness of the side chain alkene moiety under several osmium-catalyzed dihydroxylation conditions, most probably caused by the sterical crowdedness of the dibenzyl acetal function, derivatives of the alkynylation product bearing less bulky acetal protecting groups (dimethyl acetal and dioxane) were synthesized and hydrogenated to give the corresponding cis-alkenes.In addition, acetylation of the axial C2-OH function was performed to study a possible adverse effect of the free hydroxyl group in the planned dihydroxylation reactions.Here, we wish to report on the detailed synthesis of a series of four variably protected 2-C-alkynyl branched mannosides as well as 2-C-cis-alkenyl branched mannosides derived thereof.

Results and Discussion
Our synthetic sequence started by subjecting benzyl β-glucoside 1 [7] to Dess-Martin oxidation to afford the corresponding perbenzylated 2-uloside 2 (Scheme 1).Previously, Fokt et al. [8] described the synthesis of 2 via glycosylation reactions with a 2-ulosyl bromide.However, based on our previously encountered difficulties with comparable glycosylation reactions [5] (i.e., low yields and the formation of side products) we anticipated our approach via Dess-Martin oxidation of 1 to be more efficient.Indeed, 2-uloside 2 was obtained in this way in an excellent 96% yield.Due to the high crystallinity of 2-uloside 2, X-ray structural analysis was performed in order to complete the analytical data of 2. Compound 2 crystallized as thin needles upon diffusion of n-pentane into a solution of 2 in ethyl acetate/chloroform (2:1).The space group was found to be P2 1 and the asymmetric unit contained one molecule of 2. The molecular structure (Figure 1) revealed a carbonyl bond length of 1.204 Å (C2-O21) as well as carbonyl angles of 115.3 Å (C1-C2-C3) and 122.3 • (C1-C2-O21 and C3-C2-O21).
The carbohydrate system was found to comprise a distorted 4 C 1 conformation.Our synthetic sequence started by subjecting benzyl β-glucoside 1 [7] to Dess-Martin oxidation to afford the corresponding perbenzylated 2-uloside 2 (Scheme 1).Previously, Fokt et al. [8] described the synthesis of 2 via glycosylation reactions with a 2-ulosyl bromide.However, based on our previously encountered difficulties with comparable glycosylation reactions [5] (i.e., low yields and the formation of side products) we anticipated our approach via Dess-Martin oxidation of 1 to be more efficient.Indeed, 2-uloside 2 was obtained in this way in an excellent 96% yield.Due to the high crystallinity of 2-uloside 2, X-ray structural analysis was performed in order to complete the analytical data of 2. Compound 2 crystallized as thin needles upon diffusion of n-pentane into a solution of 2 in ethyl acetate/chloroform (2:1).The space group was found to be P21 and the asymmetric unit contained one molecule of 2. The molecular structure (Figure 1) revealed a carbonyl bond length of 1.204 Å (C2-O21) as well as carbonyl angles of 115.3 Å (C1-C2-C3) and 122.3° (C1-C2-O21 and C3-C2-O21).The carbohydrate system was found to comprise a distorted 4 C1 conformation.Subsequently, alkynylation of 2 with propiolaldehyde dibenzyl acetal [9] and n-butyllithium gave 2-C-alkynyl mannoside 3a in 85% yield.The attack of the nucleophile solely occurred from the equatorial side and no diastereomeric byproduct could be detected.Similarly, high stereoselectivities were previously observed for a wide variety of nucleophilic addition reactions to β-2-ulosides [5,[10][11][12].In order to expand the synthetic applicability of the thus prepared branched mannosides we performed a series of transacetalizations affording products with variably orthogonally protected side chain formyl moieties.Thus, iodine-catalyzed [13] and acid-catalyzed transacetalization of 3a with methanol and 1,3-dihydroxypropane, respectively, afforded dimethyl acetal 3b (88%) and 1,3dioxane 3c (84%) (Scheme 1).Acetylation of the tertiary 2-OH function under standard conditions [14] gave acetate 3d in 81% yield.The subsequent partial hydrogenation of 3a employing Lindlar's catalyst and H2 [15] was initially performed in ethyl acetate as well as methanol.However, only minor hydrogenation of the starting material was observed in these solvents over several days.We attributed this finding to the sterical crowdedness of the alkyne moiety in 3a caused by the bulky dibenzyl acetal and the other benzyl protecting groups in the glycoside.Changing the solvent to 1,4dioxane had only a slightly accelerating effect.In contrast, using DMF as the solvent resulted in a smooth conversion of alkynes 3a-3d to afford the corresponding cis-alkenes 4a-4d in typically 2-5 days and in good yields (81%-93%).Although the partial hydrogenation of alkynes with Lindlar catalyst has been used frequently, solvent effects as well as effects of sterical crowdedness seem to be hardly assessable, and the conditions often need rigorous optimization regarding the individual reaction system [16][17][18][19].Therefore, we believe the protocol described above might be of further use for the partial hydrogenation of comparable branched alkyne systems.The vicinal alkene coupling constants J7,8 of glycosides 4 were found to be in a region between the anticipated values for cis-and trans-alkenes (J7,8 = 12.4 (4a), 12.4 (4b), 12.1 (4c), 12.4 (4d) Hz).Furthermore, 2D NMR experiments (H,H-NOESY) as well as X-ray structural analysis (vide infra) were utilized for additional structural elucidation and final proof of the structures of 4a-4d.
Originally we anticipated a synthetic route to 1,2-annulated glycosides based on the synselective catalytic osmoylation of cis-alkenes of type 4.However, 4a, 4c and 4d, respectively, appeared to be remarkably inert under various osmoylation conditions [20].Even stoichiometric amounts of Subsequently, alkynylation of 2 with propiolaldehyde dibenzyl acetal [9] and n-butyllithium gave 2-C-alkynyl mannoside 3a in 85% yield.The attack of the nucleophile solely occurred from the equatorial side and no diastereomeric byproduct could be detected.Similarly, high stereoselectivities were previously observed for a wide variety of nucleophilic addition reactions to β-2-ulosides [5,[10][11][12].In order to expand the synthetic applicability of the thus prepared branched mannosides we performed a series of transacetalizations affording products with variably orthogonally protected side chain formyl moieties.Thus, iodine-catalyzed [13] and acid-catalyzed transacetalization of 3a with methanol and 1,3-dihydroxypropane, respectively, afforded dimethyl acetal 3b (88%) and 1,3-dioxane 3c (84%) (Scheme 1).Acetylation of the tertiary 2-OH function under standard conditions [14] gave acetate 3d in 81% yield.The subsequent partial hydrogenation of 3a employing Lindlar's catalyst and H 2 [15] was initially performed in ethyl acetate as well as methanol.However, only minor hydrogenation of the starting material was observed in these solvents over several days.We attributed this finding to the sterical crowdedness of the alkyne moiety in 3a caused by the bulky dibenzyl acetal and the other benzyl protecting groups in the glycoside.Changing the solvent to 1,4-dioxane had only a slightly accelerating effect.In contrast, using DMF as the solvent resulted in a smooth conversion of alkynes 3a-3d to afford the corresponding cis-alkenes 4a-4d in typically 2-5 days and in good yields (81%-93%).Although the partial hydrogenation of alkynes with Lindlar catalyst has been used frequently, solvent effects as well as effects of sterical crowdedness seem to be hardly assessable, and the conditions often need rigorous optimization regarding the individual reaction system [16][17][18][19].Therefore, we believe the protocol described above might be of further use for the partial hydrogenation of comparable branched alkyne systems.The vicinal alkene coupling constants J 7,8 of glycosides 4 were found to be in a region between the anticipated values for cisand trans-alkenes (J 7,8 = 12.4 (4a), 12.4 (4b), 12.1 (4c), 12.4 (4d) Hz).Furthermore, 2D NMR experiments (H,H-NOESY) as well as X-ray structural analysis (vide infra) were utilized for additional structural elucidation and final proof of the structures of 4a-4d.
Originally we anticipated a synthetic route to 1,2-annulated glycosides based on the syn-selective catalytic osmoylation of cis-alkenes of type 4.However, 4a, 4c and 4d, respectively, appeared to be remarkably inert under various osmoylation conditions [20].Even stoichiometric amounts of osmium tetroxide at elevated temperatures did not result in dihydroxylation of the double bond.For example, dibenzyl acetal 4a did not react over several days when stirred at 70 • C in acetone/water/t-butyl alcohol with 1.2 equivalents of osmium tetroxide and 5 vol % of pyridine as an additive.Dimethyl acetal 4b only resulted in acetal hydrolysis and subsequent decomposition.
Molbank 2016, 2016, M916 3 of 9 osmium tetroxide at elevated temperatures did not result in dihydroxylation of the double bond.For example, dibenzyl acetal 4a did not react over several days when stirred at 70 °C in acetone/water/tbutyl alcohol with 1.2 equivalents of osmium tetroxide and 5 vol % of pyridine as an additive.
Dimethyl acetal 4b only resulted in acetal hydrolysis and subsequent decomposition.Both glycosides 4a and 4b but not glycosides 4c and 4d exhibited each remarkably complex 1 H and 13 C-NMR spectra in CDCl3 (Figure 2; for the full spectra see the Supporting Information).We attributed these unusually complex spectra to a conformational isomerism caused by constrained rotation of the single bonds adjacent to the bulky alkene moiety.In DMSO-d6 as the solvent compound 4a showed a simpler NMR spectrum and no rotamers were observed.Similar distinct solvent dependencies of conformational isomerism have been reported in the literature [21].In the case of compound 4b, rotamers were still observable in DMSO-d6, albeit to a much lesser extent.NMR measurements performed at elevated temperatures (50 °C) failed because of decomposition.The structure of glycoside 4a could finally be unambiguously confirmed by X-ray crystallography.Glycoside 4a formed thin needles suitable for X-ray analysis by slow diffusion of n-pentane into a solution of the compound in ethyl acetate.The X-ray structure revealed compound 4a to crystallize in the triclinic space group P1.The asymmetric unit contained four molecules of 4a.An examination of the molecular structure (Figure 3) confirmed the expected 4 C1 conformation of the carbohydrate system.The olefinic bond length (C421-C422: 1.325 Å ) and torsion angle (C402-C421-C422-C423: 3.0°) were found to be as anticipated for an isolated cis-alkene.The sterically crowded side chain dibenzyl acetal moiety was found to shield both sides of the alkene double bond as illustrated by the torsion angles involving the acetal oxygen atoms and the alkene carbon atoms (C421-C422-C423-O40A: 94.3° and C421-C422-C423-O421: −144.1°).
In conclusion, we have described a methodology for the straightforward synthesis of 2-Calkynyl and 2-C-cis-alkenyl β-mannosides with an acetal-protected γ-aldehyde functionality.The synthesis comprises the manno-selective alkynylation of a benzylated 2-uloside and subsequent partial hydrogenation of the intermediate branched alkynes utilizing Lindlar's catalyst.DMF as the solvent of choice was found to have a distinctly accelerating effect on the partial hydrogenation, and this finding could be of further use for the Lindlar hydrogenation of similar sterically crowded alkyne systems.The 2-C-branched carbohydrates reported here might be valuable building blocks for synthetic carbohydrate chemistry due to the potential orthogonality of the side chain carbonyl protection.For example, C-glycosyl-substituted (Z)-acrolein systems might be accessible from the described compounds.Both glycosides 4a and 4b but not glycosides 4c and 4d exhibited each remarkably complex 1 H and 13 C-NMR spectra in CDCl 3 (Figure 2; for the full spectra see the Supporting Information).We attributed these unusually complex spectra to a conformational isomerism caused by constrained rotation of the single bonds adjacent to the bulky alkene moiety.In DMSO-d 6 as the solvent compound 4a showed a simpler NMR spectrum and no rotamers were observed.Similar distinct solvent dependencies of conformational isomerism have been reported in the literature [21].In the case of compound 4b, rotamers were still observable in DMSO-d 6 , albeit to a much lesser extent.NMR measurements performed at elevated temperatures (50 • C) failed because of decomposition.The structure of glycoside 4a could finally be unambiguously confirmed by X-ray crystallography.Glycoside 4a formed thin needles suitable for X-ray analysis by slow diffusion of n-pentane into a solution of the compound in ethyl acetate.The X-ray structure revealed compound 4a to crystallize in the triclinic space group P1.The asymmetric unit contained four molecules of 4a.An examination of the molecular structure (Figure 3) confirmed the expected 4 C 1 conformation of the carbohydrate system.The olefinic bond length (C421-C422: 1.325 Å) and torsion angle (C402-C421-C422-C423: 3.0 • ) were found to be as anticipated for an isolated cis-alkene.The sterically crowded side chain dibenzyl acetal moiety was found to shield both sides of the alkene double bond as illustrated by the torsion angles involving the acetal oxygen atoms and the alkene carbon atoms (C421-C422-C423-O40A: 94.3 • and C421-C422-C423-O421: −144.1 • ).
In conclusion, we have described a methodology for the straightforward synthesis of 2-C-alkynyl and 2-C-cis-alkenyl β-mannosides with an acetal-protected γ-aldehyde functionality.The synthesis comprises the manno-selective alkynylation of a benzylated 2-uloside and subsequent partial hydrogenation of the intermediate branched alkynes utilizing Lindlar's catalyst.DMF as the solvent of choice was found to have a distinctly accelerating effect on the partial hydrogenation, and this finding could be of further use for the Lindlar hydrogenation of similar sterically crowded alkyne systems.The 2-C-branched carbohydrates reported here might be valuable building blocks for synthetic carbohydrate chemistry due to the potential orthogonality of the side chain carbonyl protection.For example, C-glycosyl-substituted (Z)-acrolein systems might be accessible from the described compounds.

General Methods
NMR spectra were recorded with a Bruker Avance 400 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) and calibrated for the solvent signal ( 1 H: CDCl3: 7.26 ppm; acetone-D6: 2.05 ppm; DMSO-d6: 2.50 ppm; 13 C: CDCl3: 77.16 ppm; acetone-d6: 29.84 ppm; DMSO-d6: 39.52 ppm).For diastereotopic methylene protons, the signal appearing at higher chemical shift was designated Ha, the one appearing at lower chemical shift values was designated Hb.NMR spectra of compounds 2-4 are given in the Supporting Information.ESI-TOF-HRMS spectra were measured with a Bruker maXis 4 g spectrometer (Bruker Daltonik GmbH, Bremen, Germany), ESI-FTICR-HRMS spectra with a Bruker Apex II spectrometer and FAB-spectra with a Finnigan MAT spectrometer model TSQ 70 (Finnigan MAT, Bremen, Germany).Elemental analysis was performed with a HEKAtech Euro 3000 CHN analyzer (HEKAtech GmbH, Wegberg, Germany).Optical rotations were measured with a Perkin-Elmer Polarimeter 341 (Perkin Elmer Inc., Waltham, MA, USA) in a 10 cm cuvette at 20 °C.Melting points were determined with a Büchi Melting Point M-560 apparatus (BÜ CHI Labortechnik GmbH, Essen, Germany).Reactions were monitored by TLC on Polygram Sil G/UV silica gel plates from Macherey-Nagel.Detection of spots was effected by charring with H2SO4 (5% in EtOH), staining by spraying the plates with (NH4)2MoO4/Ce(SO4)2 in diluted sulfuric acid or by inspection of the TLC plates under UV light.Preparative chromatography was performed on silica gel (0.032-0.063 mm) from Macherey-Nagel (Macherey-Nagel GmbH & Co. KG, Düren, Germany).All solvents were purchased in technical grade and purified by distillation.For the reactions, the following solvents were dried according to standard methods and stored over molecular sieves 4 Å : CH2Cl2, THF and DMF: P4O10; toluene: Na/benzophenone.Dess-Martin periodinane was prepared via the potassium bromate method [22].Propiolaldehyde dibenzyl acetal was prepared according to the method

General Methods
NMR spectra were recorded with a Bruker Avance 400 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) and calibrated for the solvent signal ( 1 H: CDCl3: 7.26 ppm; acetone-D6: 2.05 ppm; DMSO-d6: 2.50 ppm; 13 C: CDCl3: 77.16 ppm; acetone-d6: 29.84 ppm; DMSO-d6: 39.52 ppm).For diastereotopic methylene protons, the signal appearing at higher chemical shift was designated Ha, the one appearing at lower chemical shift values was designated Hb.NMR spectra of compounds 2-4 are given in the Supporting Information.ESI-TOF-HRMS spectra were measured with a Bruker maXis 4 g spectrometer (Bruker Daltonik GmbH, Bremen, Germany), ESI-FTICR-HRMS spectra with a Bruker Apex II spectrometer and FAB-spectra with a Finnigan MAT spectrometer model TSQ 70 (Finnigan MAT, Bremen, Germany).Elemental analysis was performed with a HEKAtech Euro 3000 CHN analyzer (HEKAtech GmbH, Wegberg, Germany).Optical rotations were measured with a Perkin-Elmer Polarimeter 341 (Perkin Elmer Inc., Waltham, MA, USA) in a 10 cm cuvette at 20 °C.Melting points were determined with a Büchi Melting Point M-560 apparatus (BÜ CHI Labortechnik GmbH, Essen, Germany).Reactions were monitored by TLC on Polygram Sil G/UV silica gel plates from Macherey-Nagel.Detection of spots was effected by charring with H2SO4 (5% in EtOH), staining by spraying the plates with (NH4)2MoO4/Ce(SO4)2 in diluted sulfuric acid or by inspection of the TLC plates under UV light.Preparative chromatography was performed on silica gel (0.032-0.063 mm) from Macherey-Nagel (Macherey-Nagel GmbH & Co. KG, Düren, Germany).All solvents were purchased in technical grade and purified by distillation.For the reactions, the following solvents were dried according to standard methods and stored over molecular sieves 4 Å : CH2Cl2, THF and DMF: P4O10; toluene: Na/benzophenone.Dess-Martin periodinane was prepared via the potassium bromate method [22].Propiolaldehyde dibenzyl acetal was prepared according to the method

X-ray Crystallography
Crystals of 2 were grown by slow diffusion of n-pentane into a solution of the compound in ethyl acetate/chloroform (2:1; 20 mg, 750 µL) at ambient temperature.Crystals of 4a were grown by diffusion of n-pentane into a solution of 4a in ethyl acetate (20 mg, 500 µL) at ambient temperature.X-ray data were collected on a Bruker SMART APEX II DUO diffractometer (Bruker AXS Advanced X-ray Solutions GmbH, Karlsruhe, Germany) using a Cu Kα radiation (λ = 1.54178Å) or Mo Kα radiation (λ = 0.71073 Å).Corrections for absorption effects were applied using SADABS [23].All structures were solved by direct methods using SHELXS and SHELXL for structure solution and refinement [24][25][26].Complete crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data, CCDC 1505647 (for 2) and CCDC 1505648 (for 4a).Copies of this information may be obtained free of charge from The Cambridge Crystallographic Data Centre.

Figure 1 .
Figure 1.Molecular structure of 2. Hydrogen atoms as well as a disorder involving the O51 benzyl group are omitted for clarity.Ellipsoids are drawn at a 50% probability level.Red = oxygen, gray = carbon.

Figure 1 .
Figure 1.Molecular structure of 2. Hydrogen atoms as well as a disorder involving the O51 benzyl group are omitted for clarity.Ellipsoids are drawn at a 50% probability level.Red = oxygen, gray = carbon.

Figure 3 .
Figure 3. Molecular structure of 4a.Hydrogen atoms are omitted for clarity.Ellipsoids are drawn at a 50% probability level.Red = oxygen, gray = carbon.

Figure 3 .
Figure 3. Molecular structure of 4a.Hydrogen atoms are omitted for clarity.Ellipsoids are drawn at a 50% probability level.Red = oxygen, gray = carbon.

Figure 3 .
Figure 3. Molecular structure of 4a.Hydrogen atoms are omitted for clarity.Ellipsoids are drawn at a 50% probability level.Red = oxygen, gray = carbon.