Platinum-Catalyzed Ring Opening of 1,2-Cyclopropanated Sugars withO-Nucleophiles. Convenient Synthesis of 2-C-Branched Carbohydrates

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.

Theoretical Study on the Mechanism of Rearrangement Reactions of Bicyclic Derivatives of Cyclopropane to Monocyclic Derivatives under the Catalysis of Pt-Salt

In this paper, the mechanistic studies on the isomerization of hydroxyl and silyl derivatives of bicyclic cyclopropanes under the catalytic action of Zeise's salt have been reported. The catalytic activity of both the monomeric and the dimeric forms of Zeise's salt has been studied by applying the high-level quantum mechanical method. Results from this investigation reveal that the reaction goes favorably under the catalysis of the dimeric form of Zeise's salt. The calculated activation barrier for the catalytic process using Zeise's dimer reveals that the rearrangement occurs with an activation barrier of 19-25 kcal mol^-1. Depending on the nature of substituents present on the substrate, formation of various products has been explained. This study also includes the heteronuclear counter part of Zeise's dimer where one of the Pt-metals is replaced by palladium (Pd) and nickel (Ni) successively. The calculated activation barrier using these heteronuclear catalysts is found to be close enough to that calculated for the catalytic pathway using Zeise's dimer.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

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).

Exclusive ring opening of gem-dihalo-1,2-cyclopropanated oxyglycal to oxepines in AgOAc

Treatment of gem-dihalo-1,2-cyclopropanated d-oxyglycal with primary, secondary, and unsaturated alcohols, in the presence of AgOAc, leads to the formation of chloro-oxepines exclusively. Reaction of the resulting 2-chloro-oxepines with excess alcohol in the presence of AgOAc, do not promote further reactions. This result is in contrast to the reactions of d-glucal derived halo-oxepine with alcohols known previously that lead to the formation of furanoses as the major product under similar reaction conditions. Observation of this study consolidates the reactivity differences of gem-dihalo-1,2-cyclopropanated oxyglycals, as compared to gem-dihalo-1,2-cyclopropanated glycals.

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

C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.

Barton radical reactions of 2-C-branched carbohydrates

Barton esters have been introduced into the side chain of carbohydrates with high yields in only a few steps from easily available glycals. Their radical reactions afford 2-C-methyl and 2-C-bromomethyl hexoses, pentoses and disaccharides in good yields in analytically pure form. Since the Barton esters have been synthesized by an oxidative radical addition and their transformations by reductive radical processes, our results demonstrate the power of such reactions in carbohydrate chemistry.

Convenient syntheses and transformations of 2-C-malonyl carbohydrates

2-C-malonyl carbohydrates were synthesized in only few steps and high yields by radical additions of malonates to glycals. For the first time, the undesired formation of nitrates was completely suppressed with anhydrous cerium ammonium nitrate (CAN) as oxidizing agent. A coherent explanation for the high stereoselectivities of the additions to gluco-configured glycals was provided by variation of the substituents in the 3-position. We established steric effects for the face selectivity, and electronic effects strongly influence the reactivity of the double bonds. The scope and limitation of transition-metal-mediated radical reactions in the synthesis of 2-C-branched carbohydrates was thoroughly investigated. Thus, unsaturated disaccharides and benzyl-protected glycals were used as substrates for the first time. Finally, the 2-C-malonyl carbohydrates were transformed into various products by decarboxylation, saponification and reduction, which afforded interesting precursors for C-disaccharides. In this paper we describe the syntheses of more than 40 new 2-C-analogues of carbohydrates, which were isolated in high yields in analytically pure form. Therefore, the transition-metal-mediated radical addition of malonates to glycals offers a simple and convenient entry to such important carbohydrate derivatives.

Simple synthesis of 2-C-branched glyco-acetic acids

Only three steps are required for the selective synthesis of 2-C-branched glyco-acetic acids from glycals by radical addition and decarboxylation.

1,2-migration of 2'-oxoalkyl group and concomitant synthesis of 2-C-branched O-, S-glycosides and glycosyl azides via 1,2-cyclopropanated sugars

Treatment of 2'-oxoalkyl 2-O-Ms(Ts)-alpha-C-mannosides (4, 5, and 6) with base resulted in 1,2-cyclopropanation via an intramolecular SN2 reaction due to their 1,2-trans-diaxial configurations. The 1,2-cyclopropanated sugars (10 and 13) were reacted with various alcohols, thiols, and sodium azide to produce 2-C-branched O- and S-glycosides and glycosyl azides (11, 14-28) in good to excellent yields. In contrast, 1,2-cis 2'-oxoalkyl 2-O-Ms(Ts)-alpha-C-glucoside 9 formed an acyclic conjugated aldehyde (31) under basic conditions, which occurred by 1'-enolation followed by beta-elimination. An intramolecular Michael addition from 31 produced 2-O-Ms-beta-C-glucoside 30 as a major product. However, due to the electron-withdrawing effect exerted by 2-O-Ms compound 31 also undergoes a C2 epimerization to form 32. Thereafter, the intramolecular Michael addition led to the formation of both 1,2-trans 2'-oxoalkyl 2-O-Ms-alpha-C-mannoside 4 and its beta-anomer (33). Because beta-elimination/Michael addition and C2 epimerization are reversible reactions, equilibriums among 9, 31, 30, 32, 33, and 4 were established, which included the transformation of 1,2-cis C-glucoside 9 into 1,2-trans C-mannoside 4. The subsequent 1,2-cyclopropanation of 4 was an irreversible reaction yielding 1,2-cyclopropanated 10 and further conversion to 1,2-migration products (11 and 12).

Efficient Methodology for the Synthesis of 2-C-Branched Glyco-amino Acids by Ring Opening of 1,2-Cyclopropanecarboxylated Sugars

An efficient methodology for the synthesis of 2-C-branched glyco-amino acid derivatives by diastereoselective ring opening of 1,2-cyclopropanecarboxylated sugars in good yields is reported. [reaction--see text]

Stereoselective synthesis of a C-glycosylic compound (a "methyl C-glycoside") through a regioselective free-radical ring-opening reaction. A single-crystal X-ray structure determination

Readily available 3,4,6-tri-O-acetyl-D-glucal was converted to 2,6-anhydro-5,7-O-benzylidene-1,3,4-trideoxy-D-arabino-hept-3-enitol, a methyl C-glycosylic compound. Cyclopropanation of 4,6-O-benzylidene-D-glucal, followed by tributylstannyl radical-mediated regioselective ring opening of the 1,2-cyclopropano sugar led to a 2,6-anhydro-1-deoxyheptose, (a "methyl C-beta-D-glycoside"). The stereochemistry of the 1,2-cyclopropano sugar and the "methyl C-glycoside" were confirmed by single-crystal X-ray diffraction studies.

Cyclizations of enynes catalyzed by PtCl2 or other transition metal chlorides: divergent reaction pathways

1-en-6-ynes react with alcohols or water in the presence of PtCl2 as catalyst to give carbocycles with alkoxy or hydroxy functional groups at the side chain. The reaction proceeds by anti attack of the alkene onto the (eta2-alkyne)platinum complex. The formation of the C-C and C-O bonds takes place stereoselectively by trans addition of the electrophile derived from the alkyne and the nucleophile to the double bond of the enyne. Formation of five- or six-membered carbo- or heterocycles could be obtained from 1-en-6-ynes depending on the substituents on the alkene or at the tether. Although more limited in scope, Ru(II) and Au(III) chlorides also give rise to alkoxy- or hydroxycyclization of enynes. On the basis of density functional theory (DFT) calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. In the presence of polar, nonnucleophilic solvents, 1-en-6-ynes are cycloisomerized with PtCl2 as catalyst. Formation of a platinacyclopentene intermediate is supported by DFT calculations. The reaction takes place by selective hydrogen abstraction of the trans-allylic substituent. Cycloisomerization of enynes containing disubstituted alkenes could be carried out using RuCl3 or Ru(AsPh3)4Cl2 in MeOH.

Intramolecular cyclopropanation of glycals: studies toward the synthesis of canadensolide, sporothriolide, and xylobovide

[reaction: see text] The first examples of copper-catalyzed intramolecular cyclopropanations of glycal-derived diazoacetates are reported. The new cyclopropanes are converted into advanced intermediates for the synthesis of bislactone natural products. Synthetic highlights include the selective monodeprotection of a di-tert-butylsilylene ether and a zinc-mediated ring opening cascade reaction.