Microwave irradiation oxime synthesis

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-... 

The scope and efficiency of [4+2] cycloaddition reactions used for the synthesis of pyridines continue to improve. Recently, the collection of dienes participating in aza-Diels Alder reactions has expanded to include 3-phosphinyl-l-aza-l,3-butadienes, 3-azatrienes, and l,3-bis(trimethylsiloxy)buta-l, 3-dienes (1,3-bis silyl enol ethers), which form phosphorylated, vinyl-substituted, and 2-(arylsulfonyl)-4-hydroxypyridines, respectively <06T1095 06T7661 06S2551>. In addition, efforts to improve the synthetic efficiency have been notable, as illustrated with the use of microwave technology. As shown below, a synthesis of highly functionalized pyridine 14 from 3-siloxy-l-aza-1,3-butadiene 15 (conveniently prepared from p-keto oxime 16) and electron-deficient acetylenes utilizes microwave irradiation to reduce reaction times and improve yields <06T5454>. 

In 1992, Puciova and Toma published the synthesis of a broad range of oximes by microwave irradiation. They observed that the application of microwave heating enhanced... 

Touaux, B., Texier-Boullet, F., and Hamelin, 1.1998. Synthesis of oximes, conversion to nitrile oxides and their subsequent 1,3-dipolar cycloaddition reactions under microwave irradiation and solvent-free reaction conditions. Heteroatom Chemistry, 9 351-54. 

Combs et al. have described an unprecedented in situ thermal reduction of the N-0 bond on microwave irradiation at 200 °C for 20 min in the synthesis of 2,4,5-triarylimidazoles from the parent keto-oximes and aldehydes [41]. A similar method has been extended to a short synthesis of lepidiline B (Scheme 10.18) and trifenagrel illustrating the utility of microwave technology [42]. 

Iminyl radicals, generated by microwave irradiation of ( -phenyl oxime ethers such as 224, cychzed onto the indole 2-position to afford indolopyridine derivatives (225) [117]. The most likely pathway involves a 6-endo cyclization and was further exploited into a formal synthesis of neocryptolepine from gramine in five steps. 

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