Recent advances in the stereoselective synthesis of carbohydrate 2-C-analogs

Utilizing ESI-MS/MS and ion mobility spectrometry for structural characterization and isomer differentiation of 1, 2-unsaturated (1-4) linked disaccharide derivatives and their 2-C-functionalized analogues

RATIONALE

The 1, 2-unsaturated derivatives of (1-4) linked disaccharides serve as versatile building blocks for synthesizing biologically active compounds. Distinguishing between four pairs of stereoisomers in mixtures presents a challenging task. In this study, disaccharide derivatives are analyzed as alkali metal adducts using electrospray ionization tandem mass spectrometry (ESI-MS/MS), both as individual compounds and in mixtures by ion mobility mass spectrometry (IMS).

METHODS

Electrospray ionization (ESI) in combination with tandem mass spectrometry (MS/MS) in positive mode was employed to differentiate lithium adduct ions of hexa-acetyl/hexa-benzyl-D-lactals, and hexa-acetyl/hexa-benzyl-D-maltals, along with their corresponding 2-C-branched malonates. The high resolving power of trapped ion mobility spectrometry (TIMS) with the imeX™ functionality rapidly identified different metal adducts (Li, Na, Cs) as individual isomers and separated mixtures of stereoisomers. The measured collisional cross section (CCS) values were analyzed in relation to predicted CCS values.

RESULTS

MS/MS spectra of the [M + Li]+ ions of glycal disaccharide analogues exhibited typical cross-ring and glycosidic bond cleavages. Collision-induced dissociation (CID) spectra provided insights into their fragmentation behavior, allowing differentiation of (1-4) linked disaccharides. TIMS technology delivered adjustable ion mobility resolution for suitable separation of the four sets of stereo isomeric compounds. However, accurately predicting CCS values to differentiate between respective isomeric pairs using the SigmaCCS program for sodium adducts is only partially achievable.

CONCLUSIONS

ESI CID spectra of [M + Li]+ adduct ions for individual glycal disaccharide analogues facilitate the discrimination between alpha and beta (1-4) linked unsaturated disaccharides and their 2-C-branched analogues. Through optimized experimental conditions, complete baseline ion mobility separation of stereoisomer pairs of the Cs adducts is achieved.

Simple Synthesis of 1,2-Dideoxy-2-Vinyl Carbohydrates by Tin-Free Radical Reactions of Xanthates

Vinyl-substituted carbohydrates have been synthesized from glycals derived from hexoses and pentoses. Key step is the radical reaction of xanthates in the presence of triethylborane, a non-toxic reagent. The mechanism has been investigated by isolation of various side products, which speak for a reversibility of the cyclopropylmethyl radical ring-opening. Compared to reactions with tributyltin hydride, higher regioselectivities in favor of the 2-vinyl-substituted sugars have been obtained. Yields are slightly lower with triethylborane, but all products have been isolated in analytically pure form. The new reaction is applicable to benzyl- and silyl-protected carbohydrates, which makes free sugars accessible as well. Overall, more than 15 1,2-dideoxy-2-vinyl carbohydrates have been synthesized from simple precursors in only few steps.

One-Pot Synthesis of 2-C-Branched Glycosyl Triazoles by Integrating 1,2-Cyclopropanated Sugar Ring-Opening Azidation and CuAAC Reaction

A series of 2-C-branched glycosyl triazoles including triazole-tethered oligosaccharides and glycopeptides were synthesized in one pot from 1,2-cyclopropanated sugars or 2'-acetonyl-2-O-Ts-C-furanosides, NaN₃, and alkynes using PEG-400 as a single solvent. Nucleophilic ring-opening azidation of 1,2-cyclopropanated sugars (or 2'-acetonyl group 1,2-migration-azidation of C-furanosides) obtained glycosyl azides, which upon reaction with alkynes under CuAAC conditions achieved glycosyl triazoles in good yields and high stereoselectivity without the need to change the solvent and isolate any intermediates.

C2-ketonylation of carbohydrates via excited-state palladium-catalyzed 1,2-spin-center shift

C2-ketonyl-2-deoxysugars, sugars with the C2-hydroxyl group replaced by a ketone side chain, are important carbohydrate mimetics in glycobiology and drug discovery studies; however, their preparation remains a vital challenge in organic synthesis. Here we report the first direct strategy to synthesize this class of glycomimetics from readily available 1-bromosugars and silyl enol ethers via an excited-state palladium-catalyzed 1,2-spin-center shift (SCS) process. This step-economic reaction features broad substrate scope, has a high functional group tolerance, and can be used in late-stage functionalization of natural product- and drug-glycoconjugates. Preliminary experimental and computational mechanistic studies suggested a non-chain radical mechanism involving photoexcited palladium species, a 1,2-SCS process, and a radical Mizoroki-Heck reaction.

Electrochemical Bromination of Glycals

Herein, the convenient one-step electrochemical bromination of glycals using Bu₄NBr as the brominating source under metal-catalyst-free and oxidant-free reaction conditions was described. A series of 2-bromoglycals bearing different electron-withdrawing or electron-donating protective groups were successfully synthesized in moderate to excellent yields. The coupling of tri-O-benzyl-2-bromogalactal with phenylacetylene, potassium phenyltrifluoroborate, or a 6-OH acceptor was achieved to afford 2C-branched carbohydrates and disaccharides via Sonogashira coupling, Suzuki coupling, and Ferrier rearrangement reactions with high efficiency. The radical trapping and cyclic voltammetry experiments indicated that bromine radicals may be involved in the reaction process.

Advances in Pd-catalyzed C-C bond formation in carbohydrates and their applications in the synthesis of natural products and medicinally relevant molecules

Advances in the Pd-catalyzed synthesis of C-glycosides and branched sugars are summarized herein and the strategies are categorized based on named reactions or types of sugar moieties involved in the reactions. These include cross-coupling reactions, C-H activations, and carbonylative cross-coupling reactions. Applications of Pd-catalyzed C-glycosylation reactions are discussed in the synthesis of natural products and biologically active molecules such as bergenin, papulacandin D, and SGLT2-inhibitors. Important mechanistic cycles are drawn and the mechanisms for how Pd-activates the sugar moieties for various coupling partners are discussed. The directing group-assisted C-glycosylation and some intramolecular C-H activation reactions are also included.

Electrochemical asymmetric synthesis of biologically active substances

Electrically driven oxidation and reduction reactions are well-established methods for synthesis even in the chemical industry, but asymmetric versions are still few. The mild conditions used, atom efficiency and low cost make these reactions a very attractive alternative to other methods of synthesis. Very fine tuning can be achieved based on minute changes in potentials, allowing only one functional group in a molecule to react in the presence of several others, which is ideal for applications in total synthesis. In this review, the literature in the field of asymmetric synthesis of biologically active substances over the last 10 years is surveyed.

Radical Clock Probes to Determine Carbohydrate Radical Stabilities

Carbohydrate radical stabilities in the 1- and 2-position have been determined by a radical clock approach, starting from cyclopropanated sugars with xanthates as precursors. Various hexoses and pentoses afforded 1-deoxy sugars as main products, indicating that anomeric radicals are more stable than radicals in the 2-position. An additional influence of the configurations on radical stabilities has been observed. Our results should be interesting for the understanding of 1,2-radical rearrangements in carbohydrate chemistry and offer an easy access to deoxy-vinyl sugars.

Synthesis and glycosidase inhibition of conformationally locked DNJ and DMJ derivatives exploiting a 2-oxo-C-allyl iminosugar

The synthesis and glycosidase inhibition profile of a series of bicyclic analogs of DNJ and DMJ displaying a similar hydroxyl pattern and a distinct conformation is described.

Synthesis and conformational analysis of linear homo- and heterooligomers from novel 2-C-branched sugar amino acids (SAAs)

Sugar amino acids (SAAs), as biologically interesting structures bearing both amino and carboxylic acid functional groups represent an important class of multifunctional building blocks. In this study, we develop an easy access to novel SAAs in only three steps starting from nitro compounds in high yields in analytically pure form, easily available by ceric (IV) mediated radical additions. Such novel SAAs have been applied in the assembly of total nine carbopeptoids with the form of linear homo- and heterooligomers for the structural investigations employing circular dichroism (CD) spectroscopy, which suggest that the carbopeptoids emerge a well-extended, left (or right)-handed conformation similar to polyproline II (PPII) helices. NMR studies also clearly demonstrated the presence of ordered secondary structural elements. 2D-ROESY spectra were acquired to identify ⁱ⁺¹ NH ↔  ⁱ C ₁ H, ⁱ C ₂ H correlations which support the conformational analysis of tetramers by CD spectroscopy. These findings provide interesting information of SAAs and their oligomers as potential scaffolds for discovering new drugs and materials.

Crystal structures of three bicyclic carbohydrate derivatives

The title compounds, [(1R,3R,4R,5R,6S)-4,5-bis-(acet-yloxy)-7-oxo-2-oxabi-cyclo[4.2.0]octan-3-yl]methyl acetate, C14H18O8, (I), [(1S,4R,5S,6R)-5-acet-yloxy-7-hy-droxy-imino-2-oxobi-cyclo-[4.2.0]octan-4-yl acetate, C11H15NO6, (II), and [(3aR,5R,6R,7R,7aS)-6,7-bis-(acet-yloxy)-2-oxo-octa-hydro-pyrano[3,2-b]pyrrol-5-yl]methyl acetate, C14H19NO8, (III), are stable bicyclic carbohydrate derivatives. They can easily be synthesized in a few steps from commercially available glycals. As a result of the ring strain from the four-membered rings in (I) and (II), the conformations of the carbohydrates deviate strongly from the ideal chair form. Compound (II) occurs in the boat form. In the five-membered lactam (III), on the other hand, the carbohydrate adopts an almost ideal chair conformation. As a result of the distortion of the sugar rings, the configurations of the three bicyclic carbohydrate derivatives could not be determined from their NMR coupling constants. From our three crystal structure determinations, we were able to establish for the first time the absolute configurations of all new stereocenters of the carbohydrate rings.

Synthesis of 2-Thiocarbohydrates and Their Binding to Concanavalin A

A convenient and general synthesis of 2-thiocarbohydrates via cerium ammonium nitrate oxidation of the thiocyanate ion is described. Radical addition to glycals proceeds with excellent regio- and good stereoselectivities in only one step, deprotection affords water-soluble 2-thio saccharides. Binding studies to Con A have been performed by isothermal titration calorimetry (ITC) and saturation transfer difference (STD) NMR spectroscopy. The 2-thiomannose derivative binds even stronger to Con A than the natural substrate, offering opportunities for new lectin or enzyme inhibitors.

Cobalt(II) Porphyrin-Catalyzed Intramolecular Cyclopropanation of N-Alkyl Indoles/Pyrroles with Alkylcarbene: Efficient Synthesis of Polycyclic N-Heterocycles

A protocol on chemoselective cobalt(II) porphyrin-catalyzed intramolecular cyclopropanation of N-alkyl indoles/pyrroles with alkylcarbenes has been developed. The reaction enables the rapid construction of a range of nitrogen-containing polycyclic compounds in moderate to high yields from readily accessible materials. These N-containing polycyclic compounds can be converted into a variety of N-heterocycles with potential synthetic and biological interest. Compared to their N-tosylhydrazone counterparts, the use of bulky N-2,4,6-triisopropylbenzenesulfonyl hydrazones as carbene precursors allows cyclopropanation to occur under milder reaction conditions.

Methyl-α-D-2-selenonyl Pent-2-enofuranoside: A Reactive Selenosugar for the Diversity Oriented Synthesis of Enantiomerically Pure Heterocycles, Carbocycles, and Isonucleosides

The construction of vinyl selenone on a furanoside led to a highly reactive synthetic intermediate methyl-α-D-2-selenonyl pent-2-enofuranoside composed of a masked aldehyde, an electron-deficient double bond along with an excellent leaving group. This new Michael acceptor on reactions with different nucleophiles afforded bicyclic azasugars, cyclopropanated carbohydrate, dihydrofuran- and dihydroisoxazole- substituted furanosides, and isonucleosides in moderate to good yields. Hydrolysis of the hemiacetal linkage of some of these modified carbohydrates afforded enantiopure aziridines, nitrocyclopropane, and dihydrofuran.

Synthesis of 1,2-cis-2-C-branched aryl-C-glucosides via desulfurization of carbohydrate based hemithioacetals

1-C and 2-C-branched carbohydrates are present as substructures in a number of biologically important compounds. Although the synthesis of such carbohydrate derivatives is extensively studied, the synthesis of 1,2-cis-2-C-branched C-, S-, and N-glycosides is less explored. In this article a synthetic strategy for the synthesis of 1,2-cis-2-C-branched-aryl-C-glucosides is reported via a hydrogenolytic desulfurization of suitably orientated carbohydrate based hemithioacetals. 1,2-cis-2-Hydroxymethyl and 2-carbaldehyde of aryl-C-glucosides have been synthesized using the current strategy in very good yields. The 2-carbaldehyde-aryl-C-glucosides have been identified as suitable substrates for the stereospecific preparation of 2,3-unsaturated-aryl-C-glycosides (Ferrier products).

Simple synthesis of conformationally fixed glycosamine analogues by beckmann rearrangement at the carbohydrate ring

Conformationally fixed carbohydrate analogues are promising small-molecule inhibitors for hydrolases like O-GlcNAcase (OGA); however, their synthesis usually requires many steps. Herein we describe cycloadditions of dichloroketene to various glycals and subsequent Beckmann rearrangements, which offer an easy and stereoselective entry to glycosamine derivatives in good yields. The reactions are applicable for hexoses, pentoses, and disaccharides, and transformations to the corresponding imidates proceed smoothly. First biological tests reveal that such imidates indeed inhibit human OGA.

A general metal-free approach for the stereoselective synthesis of C-glycals from unactivated alkynes

A novel metal-free strategy for a rapid and α-selctive C-alkynylation of glycals was developed. The reaction utilizes TMSOTf as a promoter to generate in situ trimethylsilylacetylene for C-alkynylation. Thanks to this methodology, we can access C-glycosides in a single step from a variety of acetylenes , i.e., arylacetylenes and most importantly aliphatic alkynes.

Stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl4: an efficient synthesis of multi-substituted perhydrofuro[2,3-b]furans and perhydrofuro[2,3-b]pyrans

An efficient stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl₄ is described.

One-pot synthesis of fluorescein based β-aminoglycosylketones and their biological and material applications

Facile one-pot synthesis of fluorescein based β-aminoglycosylketones was carried out using 4,6-O-protected-C-glycoside, fluorescein-monoaldehyde and aromatic amines in the presence of potassium carbonate as a catalyst.