MoO2(acac)2-A Mild and Efficient Catalyst for the Deprotection of Acetals

Advancement of the Cleavage Methods of Carbohydrate-derived Isopropylidene and Cyclohexylidene Ketals

Abstract:

Carbohydrates, amino acids, and nucleosides, the fundamental building blocks of complex biomolecules in nature, are essential starting materials for the fabrication of natural and unnatural structural entities, which necessitate the masking and demasking of various functional groups with the utmost chemoselectivity, mildness, and efficiency to avoid unintended bond breaking and formation, as well as associated reactions. Ketals, benzylidene, methoxymethyl, p-methoxybenzyl, silyl ethers, trityl, tert-butyl carbamate, and other functional groups are widely used in modern organic synthesis. In carbohydrate chemistry, the commonly used protecting functionality of isopropylidene and cyclohexylidene ketals necessitates effective methods for selective cleavage. This review summarises different methods for deblocking isopropylidene and cyclohexylidene ketals using inorganic acids, Lewis acid, silica-supported inorganic acids, Amberlite-120 (H+) resin, phosphotungstic acid, Nafion-H, NaBArF4.2H2O, montmorillonite clay, Dowex 50W-X8, camphorsulphonic acid (CSA), ceric ammonium nitrate, molecular iodine, ionic liquids, zeolites and so on.

Dioxomolybdenum(VI) Complexes as Catalytic Neutral Esterification Agents

Esterification reactions of carboxylic acids with different alcohols in neutral nonaqueous media have been carried out in excellent yields using catalytic amounts of dioxomolybdenum(VI) complexes of the type [MoO2L2] (where LH = acetylacetone, salicylaldehyde, o-aminophenol, 8-hydroxyquinoline, and 2-acetyl pyridoxime). The reactions of propionic acid with primary, secondary and tertiary alcohols in the presence of catalytic amounts of MoO2(acac)2 result in higher yields of the esters with primary alcohols, compared to those with secondary or tertiary alcohols. The results of the esterification reactions of propionic acid with methanol and tertiary butanol in the presence of different dioxomolybdenum(VI) complexes suggest that the homogeneous catalysts are better than heterogeneous analogues.

Synthesis of Hydrodipyrrins Tailored for Reactivity at the 1- and 9-Positions

A collection of 33 hydrodipyrrins (9 targets, 21 intermediates, and 3 byproducts) has been prepared. The hydrodipyrrins (dihydrodipyrrins, tetrahydrodipyrrins, and hexahydrodipyrrins) contain a pyrrole ring and a geminal-dimethyl substituted 1-pyrroline (or pyrrolidine) ring. The alpha-substituents on the pyrrole ring (H, Br, CHO) and pyrroline ring (H, CH(3), CH(OR)(2), OMe, SMe) provide different reactivity combinations (Nu(-), E(+)) and 0, 1, or 2 carbon atoms (which can give rise to the bridging meso-carbons in hydroporphyrins). Straightforward access to various hydrodipyrrins should facilitate development of syntheses of diverse hydroporphyrins.