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New methodology has been developed for the Lewis acid catalyzed synthesis of malonamides. First, the scandium(III)-catalyzed addition of diverse nucleophiles (e.g., indoles, N,N-dimethyl-m-anisidine, 2-ethylpyrrole, and 2-methylallylsilane) to coumarin-3-carboxylates has been developed to afford chromanone-3-carboxylates in high yields as a single diastereomer. Upon investigating a subsequent lanthanum(III)-catalyzed amidation reaction, a new multicomponent reaction was designed by bringing together coumarin-3-carboxylates with indoles and amines to afford indolylmalonamides, which were identified to exhibit fluorescent properties. The photophysical properties for selected compounds have been analyzed, including quantum yield, molar absorptivity, and Stokes shift. Synthetic studies of several reaction byproducts involved in the network of reaction equilibria for the three-component reaction provide mechanistic insight for the development of this methodology.

Glycosyl esters constitute a widespread family of natural products such as anti-tumor agents,1 antibiotics,2 terpenoid glycosides,3 plant pigments4 and other substances.5 It has been shown that aromatic carboxylic acids are metabolized by humans as glycosyl esters,6 and D-glucopyranosyl fatty acid esters were found to inhibit the growth of leukemia cells7 and plant growth.8 The synthesis of a prodrug in the g form of a glycosyl ester has been reported.9 In synthetic chemistry glycosyl acetates are frequently used glycosyl donors in Lewis-acid catalyzed glycosylation reactions10 and other transformations.11

Unprotected (naked) galactopyranosides were used for the first time in the direct regio- and stereo-selective synthesis of β-(1→6)-linked oligosaccharides in 75–90% yield by coupling them with acetobromosugars in the presence of silver carbonate in dichloromethane. β-(1→4)-Linked oligosaccharides (yield 5–10%) were the by-products.

Base treatment of O-benzyl protected C-2- or C-4-ulopyranosyl compounds (4 alpha, 4 beta, and 11) by either 10% Et(3)N or 1% K(2)CO(3) in MeOH initiated a beta elimination to afford alpha,beta-unsaturated C-ulopyranosyl compounds (5 alpha, 5 beta, and 12), which further rearranged in a stereocontrolled manner to multifuctionalized alpha,beta-cyclopentenones (6 and 14) in 70-80% yield. Both C-alpha- and C-beta-2-ulosides (5 alpha and 5 beta) produced the same cyclopentenone 6, indicating that a 1,2-enolate is formed prior to the cleavage of the C-5--O bond. Because 6 is racemic, it was probably formed by the intramolecular cycloaldolization of two equally populated enantiomeric intermediates. When treated with 90% Et(3)N in MeOH, 5 alpha yielded almost exclusively 15 (isomer of 6), which was formed by a migration of the double bond in 5 alpha during the previously described rearrangement. Thus either 6 or 15 was the major product, depending on the base used.

2‘-Aldehydes and 2‘-ketones of α-C-glycosides, including the gluco-, galacto-, and manno- series, were epimerized exclusively to their β-anomers in good-to-excellent yields under basic conditions and in the presence of zinc acetate. The β-stereoselectivity is independent of the neighboring group at 2-O-substitution of sugar substrates. Therefore, this provides a particularly useful method for the preparation of manno-β-C-glycosides. The epimerization is likely initiated by the formation of Zn-enolate that is stabilized by intramolecular chelation to the pyranose ring-oxygen to form a syn chair-boat structure. Due to the activation generated by the Zn−O coordination, fission of the C1−O bond occurs, leading to opening of the pyranose ring, which is spontaneously followed by a change in conformation. The more stable anti chair-boat transition state is favored, and the subsequent hetero-intramolecular Michael addition results in the formation of β-C-glycoside in a ring-closure step.

In order to further characterize the conformational epitope of GBSPIII, we synthesized various oligosaccharides with the GBSPIII-related structures by a tailor-assembly synthetic scheme and a more traditional block-wise chemo-enzymatic approach. The oligosaccharides were used to probe the conformational epitope of GBSPIII using number of complementary techniques. The protective epitope of GPSPIII was further defined as length-dependent and conformational. The results of the studies confirmed that two repeating units (2RU) is the minimum binding unit and the epitope optimization mainly takes place between chain length 2RU to 7RU. Epitope optimization and multivalency were observed between 7RU and 20RU. The data support our hypothesis that the conformational epitope is an extended helical segment of the GBSPIII. GBSPIII exists mainly in the random coil form, which structurally mimics short oligosaccharide self-antigens, but it can infrequently and spontaneously form extended helices. Although not prevalent in GBSPIII the immune system preferentially selects these helical epitopes because they are unique to the polysaccharide.

Treatment of 2'-carbonyl-alpha-C-glycopyranosides of gluco, galacto, manno, 2-deoxy, and 2-azido sugars with 4% NaOMe resulted in anomeric epimerization to give their respective beta-anomers in good to excellent yields. The epimerization of the 2'-aldehyde of alpha-C-galactopyranoside (10) in deuterium methanol, which afforded the beta-anomer with exclusive deuterium replacements at the 1'-position, excluded the possibility of the exo-glycal as being involved as an intermediate. When 2'-aldehyde (36) and 2'-ketone (41) of 2,3-di-O-benzyl-alpha/beta-l-C-arabinofuranoside were used as substrates we were able to obtain the respective equatorial alpha-C-arabinopyranosides (37 and 42). These observations confirmed that the epimerization involves an acyclic alpha,beta-unsaturated aldehyde or ketone, which is formed by the enolation of 2'-carbonyl-alpha-C-glycoside with subsequent beta-elimination. Thereafter an intramolecular hetero-Michael cycloaddition occurs, leading to the formation of thermodynamically controlled stable products, which were exclusively the equatorial C-glycopyranosides, except in the case of 2'-carbonyl-C-furanosides, where a mixture of two anomers was obtained.

Per-0-acetylated glycopyranoses were converted by methylthiotrimethylsilane in the presence of boron trifluoride or trimethylsilyl trifluoromethanesulfonate to the corresponding methyl 1,2- trans -1-thioglycopyranosides in a highly stereoselective process.

Aryl and acyl hydrazines were converted to azides in excellent yields by an equimolar amount of nitrosyl tetrafluoroborate or nitrosyl sulfate.

The methyl glycoside (1) of the α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → portion of the group-specific, common antigen polysaccharide of Group B Streptococci has been synthesized. The key compound of the synthesis was methyl 3,4-di-O-benzyl-α-L-rhamnopyranoside (4). Controlled acetolysis of 4 gave 1,2-di-O-acetyl-3,4-di-O-benzyl-α-L-rhamnopyranose (10), which upon reaction with 4 under catalysis by trimethylsilyl trifluoromethanesulfonate (TMS-Tf) yielded the α-(1 → 2)-linked dirhamnoside. Subsequent rhamnosylation followed by deprotection gave 1. TMS-Tf was also used in rhamnosylation reactions with tetra-O-acetyl-α-L-rhamnopyranose. Acetylation of acceptor alcohols was found to be a major side reaction in the TMS-Tf assisted glycosylations. One-dimensional nuclear Overhauser enhancement differential spectroscopy indicated that, in solution, 1 takes up a bent instead of an extended conformation.