Synthesis of biologically active iminosugars using unconventional methods - microwave and ultrasonic irradiation
Alfred L. Loomis was the first to introduce ultrasound to chemistry in 1927. He realized that intense sound waves brought in chemical reactions would alter normal chemical response. When reliable generators became available in the 1980s, there was a renewed interest in ultrasonics.
The effects of ultrasound on chemical transformations are not the result of any direct coupling of the sound field with the chemical species involved on a molecular level. The ultrasonic wavelength frequency is far-off from the vibration frequency of the bonds between atoms in a molecule: ultrasonic frequency is in the range of kHz or MHz compared with GHz for bonds between atoms in molecules. The reason why ultrasounds are able to produce chemical effects is through the phenomenon of cavitation: generation, growing and violent collapse of cavitation bubbles with enormous energy locally delivered. Cavitation in a liquid occurs due to the stresses induced in the liquid by the passing of a sound wave.
The energy liberated in the breakdown (collapse) of a bubble is adequate for excitation, ionization and dissociation of molecules.
Under ultrasonic irradiation chemical reactions that take place under conventional conditions are accelerated, or even yield totally different products. The reason for this can be due to either physical or chemical effects of cavitation.
The physical effects can enhance the reactivity of a catalyst by enlarging the surface area, or accelerate a reaction by proper mixing of reagents. Furthermore, the collapse causes a couple of strong physical effects outside the bubble: shear forces, jets and shock waves.
Ultrasounds affect the reaction rates because of the formation of highly reactive radical species during cavitation. Ultrasound activation, when applied to carbohydrates, is known to favour formation of carbon-heteroatom bonds, acetalization reactions, glycosylation reactions, formation of carbon-carbon bonds by reaction of aldehyde functions of carbohydrates with organometallic nucleophiles and improved selectivity in the removal of protecting groups.