Salicylaldehydes, microwave-assisted

Synthesis of the benzopyran ring has also been performed by microwave-assisted copper-catalysed cross coupling of an aryl iodide with terminal alkynes, in the presence of copper(I) iodide/triphenylphosphine (Scheme 3.35)56. An alternative approach involving microwave heating of mixtures of salicylaldehyde and various derivatives of ethyl acetate in the presence of piperidine has enabled rapid Knoevenagel synthesis of coumarin derivatives (Scheme 3.35)57. 

Real-time in situ Raman analysis of microwave-assisted organic reactions has also been reported [40, 41]. Raman spectroscopy provides a combination of high selectivity along with the ability to conduct analysis directly through the wall of glass reaction vessels and in monitoring the Knoevenegel condensation reaction of salicylaldehyde and benzyl acetoacetate to form 3-acetylcoumarin, observations were made inside the microwave compartment of the synthesizer. 

Microwave acceleration of reactions is a valuable tool for organic synthesis [39], and various specialized instruments are now commercially available. Tye and co-workers have reported the microwave-assisted Petasis borono-Mannich reaction of arylboron-ic acids and primary or secondary amines with either glyoxylic acid or salicylaldehyde [40]. Optimized reaction conditions employed didiloromethane as solvent, and microwave assisted heating at 120 °C for 10 min. Products were obtained in generally modest yields (10-83%), in part due to incomplete reaction conversion imder the reported conditions. 

Microwave-assisted Petasis borono-Mannich reactions of arylboronic acids, secondary amines and salicylaldehyde have been carried out in CH2CI2 at 120 °C for 10 min [40], The main problem encountered here was incomplete conversions, resulting in modest adduct yields (23-76%). The reaction of a primary amine (p-ani-sidine) under the same conditions failed to produce the desired adducts, giving instead only the imines (cf. reactions with glyoxylic acid). 

---