Protecting-group free synthesis of glycoconjugates displaying dual fungicidal and plant defense-eliciting activities

A straightforward synthesis of carbohydrate templated isoxazolidines is described, by reaction of unprotected glycosylhydroxylamines (operating as 1,3-dipoles) with methyl acrylate using microwave activation. Rhamno- and erythro-isoxazolidines are recognized by plant cells, resulting in a strong ROS-production as a plant immune response, and exert a high antifungal activity against Botrytis cinerea.

Operationally simple enzymatic deprotection of C-3 position on 3,4,6-tri-O-acetyl-d-glucal

The new strategies to obtain selectively protected hydroxyl function on sugar derivatives are still of the high value for the progress of glycochemistry and organic synthesis. Herein, we describe an interesting enzymatic deprotection strategy that was applied to the most commonly used glycal derivative - 3,4,6-tri-O-acetyl-d-glucal. The procedure is operationally simple, easy to scale-up and the biocatalyst might be effortlessly recycled from the reaction mixture. Resulting product - 4,6-di-O-acetyl-D-glucal we then challenged to synthesize two glycal synthons armed with 3 different protecting group - a synthetic target difficult to achieve with traditional methods.

Broadening the reaction scope of unprotected aldoses via their corresponding nitrones: 1,3-dipolar cycloadditions with alkenes

Condensation reactions of unprotected tetroses and pentoses with hydroxylamines afforded nitrones, which were easily converted to densely functionalized isoxazolidines in the presence of electron-poor alkenes. The 1,3-dipolar cycloaddition occurred with good facial discrimination of the chiral nitrone but under rather low endo/exo control. Stereochemistry of isomers was ascertained by chemical correlation with known derivatives from the literature. Microwave activation appeared as the most efficient reaction mode, affording the expected adducts within several minutes whereas hours were needed under standard heating. Alternatively, the transformation proved also possible under high pressure conditions by using a hand pump system, avoiding any energy source. Although water could not be used as the solvent, leading to hydrolysis of the nitrone substrate, a large variety of organic solvents proved efficient. The method has potential use in the preparation of non-ionic carbohydrate-based amphiphiles.