Oximes microwave-assisted reactions

This conversion is generally achieved by converting the aldehyde into oxime followed by its dehydration by a variety of reagents. One pot synthesis has also been developed , but these methods suffer from a number of drawbacks. In the microwave assisted reaction, the aldehyde is converted into oxime by reaction with hydroxylamine hydrochloride and potassium fluoride on alumina... 

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

This protocol could be extended to a range of different ,/i-unsaturated carbonyl compounds and either activated or deactivated aryl iodides [22], An application of related Heck chemistry to the synthesis of methylated resveratrol (3,4, 5-trihydroxy-( )-stilbene) is shown in Scheme 6.4 [23]. The phytoalexin resveratrol exhibits a variety of interesting biological and therapeutic properties, among them activity against several human cancer cell lines. Botella and Najera have shown that the trimethyl ether of resveratrol (Scheme 6.4) can be rapidly prepared by microwave-assisted Heck reaction of the appropriate aryl iodide and styrene derivatives, using the same oxime-derived palladacycle as indicated in Scheme 6.3. 

A dimeric 4-hydroxyacetophenone oxime-derived palladacycle was used by Najera et al. as a very efficient precatalyst for selective and microwave-assisted arylations of acrolein diethyl acetal, furnishing 3-arylpropanoate products [45]. In the presence of Cy2NMe and tetrabutylammonium bromide (TBAB), higher reaction rates and slightly improved yields were realized under microwave conditions (Scheme 7). 

In an example of Green Chemistry, the microwave-assisted multi-component reaction of cinnamic aldehyde oxime, 4-chloroacetophenone, and ammonium acetate (ratio 1 2 1,110°C, 8 min) leads to a product A in 87% yield. 

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