APPLICATIONS OF MICROWAVE ENERGY IN ORGANIC CHEMISTRY. A REVIEW

Controlled Microwave Heating in Modern Organic Synthesis

Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

Diastereoselective Synthesis of Cularine Alkaloids via Enium Ions and an Easy Entry to Isoquinolines by Aza-Wittig Electrocyclic Ring Closure

In preliminary communications, we reported the diastereoselective synthesis of cularine and sarcocapnine via the intramolecular ring closure of nitrenium and oxenium ions, a new highly diastereoselective reductive methylation with (+)-8-phenylmenthyl chloroacetate followed by reduction with sodium borohydride, and a facile entry to the isoquinoline precursors by aza-Wittig electrocyclic ring closure. We now report the full details of the syntheses of (+)-O-demethylcularine, (+)-cularine, (+)-sarcocapnidine, (+)-sarcocapnine, and (+)-crassifoline and describe different methods of synthesis of their precursors.

A comparative study on the synthesis of 3′-aryl-3-methyl/2, 3-dimethylspiro[6,7,8,9-tetrahydro-5H-benzo[a]cyclohepten-5, 2′-(tetrahydro[1,3] thiazolane)]-4′-ones using microwave irradiation and conventional methods

A convenient one pot synthesis of 3′-aryl-3-methyl/2,3-dimethylspiro[6,7,8,9-tetrahydro-5 H-benzo[a]cyclohepten-5,2′-(tetrahydro[1,3]thiazolane)-4′-ones under microwave irradiation, as well as conventional methods, is described. Microwave irradiation has resulted in the reduction of time from hours to minutes. Antimicrobial activity of these compounds was studied and that some compounds gave positive results.

Isocyanates of N alpha-[(9-fluorenylmethyl)oxy]carbonyl amino acids: synthesis, isolation, characterization, and application to the efficient synthesis of urea peptidomimetics

The Curtius rearrangement of Fmoc-amino acid azides 1 was carried out in toluene by refluxing the solution for 30 min. The resulting isocyanates 2 have been isolated as crystalline solids and are fully characterized by IR, (1)H NMR, (13)C NMR, and mass spectra. They are found to be stable for several months when stored at 4 degrees C. The acyl azides of Asp, Glu, Ser, Tyr, and Lys with side-chain protection having tert-butyl, benzyl, and Boc groups were also converted to the corresponding isocyanates 2h-m. The rearrangement of Fmoc-amino acid azides in toluene to isocyanates 2 under microwave irradiation was also accomplished. The direct exposure of solid azides to microwaves for 60 s led to the completion of the rearrangement. The resulting isocyanates, after recrystallization, were found to be analytically pure. The scale-up of the rearrangement, under microwave irradiation as tested up to 0.75 mol, posed no problems and led to the isolation of the isocyanates in 91-96% yield. The utility of isocyanates as building blocks in the synthesis of urea peptides 4 is demonstrated. Further, the coupling of isocyantes 2 directly with N,O-bis(trimethylsilyl) derivatives of amino acids 6 resulted in urea peptide acids 7 with good yield in high purity. Thus, the synthesis of urea peptide acids 7d-g containing Asp, Glu, Ser, and Tyr with a free side-chain functional group have been carried out.

Temperature-sensitive photochemical aromatic substitution on 4-nitroanisole

A temperature-sensitive photochemical nucleophilic aromatic substitution on 4-nitroanisole by a hydroxide ion in homogeneous solutions, in a two-phase system under phase-transfer catalysis conditions, and in the microwave field is reported. It was found that reaction regioselectivity dramatically changes with temperature in the region of -20 to 196 degrees C. The quantum yield of the 4-methoxyphenol formation was found to be temperature independent, in contrast to that of the 4-nitrophenol formation, suggesting that there is a temperature dependent process occurring after the partitioning between replacement of the nitro group and the methoxy group has taken place. The reaction was also investigated by using quantum chemical calculations. A technique for microwave-assisted photochemical synthesis is proposed as an efficient and practical tool for organic synthesis.

Tetramethylammonium dichloroiodate: an efficient and environmentally friendly iodination reagent for iodination of aromatic compounds under mild and solvent-free conditions

Tetramethylammonium dichloroiodate (1, TMADCI) as a mild and efficient iodination reagent was prepared. Iodination of different aromatic compounds with this reagent takes place fast and with high yields under solvent-free conditions.