Microwave irradiation, aryl diketones

A regioselective synthesis of pyrazoloquinolines 225 by the multicomponent reaction of 3-aryl-5-aminopyrazole 222 with 1,3-diketones and aromatic aldehydes in ethanol in the presence of Et3N under microwave irradiation at 150°C (Scheme 3.62, reaction i) was described in [193,194]. In the presence of a strong base such as EtONa (KOH), the three-component treatment proceeded via a different pathway and led to novel and unusual reaction products—quinolizi-nones 226 (Scheme 3.62, reaction ii) [193]. The formation of only one diaster-eomeric pair from two possible ones with trans relative configuration of the... 

Lindsley and coworkers prepared functionalized quinoxalines and heterocyclic pyrazines in excellent yields from 1,2-diketone intermediates under the action of microwave irradiation (Scheme 10.50) [104]. In addition to being a general method for synthesis of a variety of aryl/heteroaryl 1,2-diamines and 1,2-diketones, this procedure suppresses the formation of the polymeric species characteristic of traditional thermal conditions. 

Epoxycydohexanone is an unusual substrate for the Heck reaction. The reactivity of this molecule under Heck coupling conditions is most likely attributed to its in-situ isomerization to 1,2-cyclohexanedione. The 1,2-diketone is subsequently reacting as an olefin via the enol tautomer. Thus, within 5 to 30 min of directed microwave irradiation of the aqueous PEG mixture, with less than 0.05% palladium acetate and no phosphine ligand, up to 72% C3-arylated product was isolated (Scheme 15.78) [148]. 

The diketone 20 was synthesized from the aldol condensation of aryl methyl ketones 19 and benzil. The benzilic acid type rearrangement of 4-aryl-l,2-diphenyl-2-butene-l,4-diones (20) to 5-aryl-3,3-diphenyl-2,3-dihydro-l. -2-pyrrolones (21) has been accomplished under microwave irradiation using ammonium formate in PEG-200. ... 

A number of microwave-assisted multicomponent methods for the synthesis of imidazoles have been reported [68-71 ]. The irradiation of a 1,2-diketone and aldehyde with ammonium acetate in acetic acid for 5 min at 180 °C in a single-mode reactor provides alkyl-, aryl-, and heteroaryl-substituted imidazoles 39 in excellent yield (Scheme 14) and this method has been used for the rapid and efficient preparation of two biologically active imidazoles, lepidiline B and trifenagrel [68]. 

---