Solvent-Free Protection under Microwave Irradiation

2 Solvent-Free Protection under Microwave Irradiation 

T pe of reaction protection Reaction condition solvent-free 

Keywords L-galactono-1,4-lactone, aldehyde, montmorillonite, microwave irradiation, acetals 

References M. Csiba, J. Cleophax, A. Loupy, J. Malthete, S. D. Gero, Tetrahedron Lett., 34, 1787 (1993). 

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New oxidation reactions of organic substances and the reasons for their acceleration under microwave irradiation have recently been investigated. In particular, in a search for highly efficient oxidation procedures, Chakraborty and Bordoloi [53] used pyridinium chlorochromate (PCC) under the action of microwave irradiation for oxidation of protected a-glucofuranose 62 to the corresponding ketone 63 (99%) much more quickly (10 min) than using the conventional technique (4 h under reflux) (Scheme 12.29) and with an easier work-up procedure. They found the oxidation can also be performed with moist PCC under solvent-free conditions and with the same yield. 

Protection of Alcohols as TMS Ethers. Several new methods have been developed for the protection of alcohols as TMS ethers. For example, TMS silyl ethers of alcohols and phenols can be prepared efficiently by treatment of the alcohol or phenol with TMSCl and catalytic amount of imidazole or iodine under the solvent-free and microwave irradiation conditions. This transformation proved to be reversible. Under the same microwave conditions, treatment of the silyl ether in methanol and in the presence of catalytic amount of iodine releases the parent alcohol in quantitative yield. 

Oxone adsorbed on silica gel or alumina is a very effective oxidant for the selective oxidation of primary and secondary amines to hydroxylamines without overoxidation. These reactions can even be accomplished under solvent-free conditions and with very short reaction times with heating or microwave irradiation (eq 53). Pyridine and trialkylamines were also readily oxidized to their A-oxides. It is suggested that the hydroxylamines are protected from overoxidation because of their strong adsorption to the silica gel or alumina surface. 

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