APPLICATIONS OF MICROWAVE ENERGY IN ORGANIC CHEMISTRY. A REVIEW

Microwave-assisted Oxidation of Alcohols by Pyridinium Chlorochromate

An efficient and mild methodology for the oxidation of alcohols to the corresponding carbonyl functions is described using pyridinium chlorochromate under microwave irradiation.

Rapid hydrolysis of bile acid conjugates using microwaves: retention of absolute stereochemistry in the hydrolysis of (25R) 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oyltaurine

In recent years, defects of bile acid synthesis caused by disorders of peroxisome biogenesis have led to increased interest in C27 bile acids. In humans, while the majority of bile acids are C24 carboxylic acids, the presence of increased concentrations of C27 bile acids and their metabolites in hereditary diseases associated with peroxisomal dysfunction can serve as a useful marker for the intensity of the metabolic disorder. Our present studies describe an efficient method for the rapid hydrolysis of C27 and C24 bile acid conjugates using a commercial microwave oven. The advantages of this method include freedom from racemization, minimal activation, mild reaction conditions, and the highly stereocontrolled nature of the reaction, thus allowing for free bile acid recovery in high yield. For example, when (25R) 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oyl taurine, a major compound present in the bile of Alligator mississippiensis, was deconjugated with 4% NaOH/diethylene glycol or 1 M LiOH/propylene glycol in the microwave oven for 4-6 min, 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid (THCA) was obtained in 81% yield with retention of configuration at C-25. It is suggested that present studies will be helpful in delineating the absolute stereochemistry of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoyl-CoA oxidase, the peroxisomal enzyme that catalyzes the first step in the oxidation of THCA.

Rapid hydrogenation of unsaturated sterols and bile alcohols using microwaves

This paper describes an operationally simple, rapid hydrogenation of unsaturated sterols and bile alcohols in a domestic microwave oven. This has been achieved by the addition of catalytic amounts of Pd/C in methylene chloride/propylene glycol solvents in the presence of ammonium formate followed by microwave irradiation. It is suggested that this methodology will be helpful in the identification of saturated and unsaturated sterols with different side-chain structures in rare diseases: sitosterolemia, cerebrotendinous xanthomatosis (CTX), as well as atherosclerosis and diabetes mellitus. Sterols, such as cholesterol, campesterol, sitosterol, and bile alcohols with unsaturated side chains, were converted to their reduced congeners with high yield and purity.