Microwave irradiation five-membered heterocycles

Keywords Five-membered heterocycles Fused heterocycles Heterocycles Microwave irradiation Six-membered heterocycles... 

Considering the formation of saturated five-membered heterocycles with two heteroatoms, it is worth to note the possibility to prepare 1,3-dioxolanes, dithiane, oxathianes 148 [93] and dioxolanones 149 [94] by condensation of the corresponding carbonyl compounds under microwave irradiation in acid medium (Scheme 52). The reaction, which is very useful for the protection of carbonyl compounds or for the preparation of useful synthetic intermediates, has also been carried out under batch conditions over Montmorillonite KIO clay in more than 150 g scale, using a 1 L quartz reactor [95]. 

Hence, microwave irradiation in DMA for 12 min at 120 °C or 30 min at 140 °C, depending on the substrate, resulted in 20 or 21 in 90-96% yields, compared to yields around 50% and a 24 h reaction time for conventional heating. As can be seen in Eq. 2 in Fig. 5, two possibilities exist to connect the heteroaryl bromide part with the five-membered heterocycle (indicated by a and fi. Table 2) that results in a six-membered 2-pyridone ring. Still, a very high selectivity was obtained yielding the 2-pyridones 21, in which the new bond had been introduced at Ca. It should be noted that the obtained structures 20 and 21 represent heterocyclic derivatives of 2-quinolones rather than true 2-quinolones. 

General reviews Tierney, J. R. and Lidstrom, R. (Eds), Microwave-Assisted Synthesis , Blackwell, 2005 Microwave irradiation for accelerating organic reactions. Rart I Three-, four- and five-membered heterocycles , El Ashry, E. S. H., Ramadan, E., Kassem, A. A. and Hagar, M., A v. Het-erocycl. Chem., 2005, 88,1 Rart II Six-, seven-membered, spiro, and fused heterocycles , El Ashry, E. S. H., Ramadan, E. and Kassem, A. K,Adv. Heterocycl. Chem., 2006, 90, 1. 

The 1,3-dipolar cycloaddition reaction between nitrones and unsaturated systems under the action of microwave irradiation has been shown to be a powerful method for the synthesis of a wide variety of novel five-membered heterocycles. In 2002, several nitrone-mediated 1,3-dipolar cydoaddition reactions were reported by de la Hoz in the original edition of this book [3jj. Most of the examples provided had been described between 1995 and 2001 [9a, 13b, 84, 92]. 

El Ashry, E. S. H., Ramadan, E., Kassem, A. A., Hagar, M., Microwave Irradiation for Accelerating Organic Reactions. Part I Three-, Four- and Five-Membered Heterocycles, 88, 1. 

This chapter will deal with the appHcation of microwave irradiation for the synthesis of a variety of heterocycles. For a better understanding, the chapter is organized on the basis of the type of heterocycle, in order of increasing complexity, starting fixrm five-membered heterocycles. 

Besides the above, a few more fherapeutically advantaged heterocyclic systems, synthesized by green heating (microwave irradiation) protocol, were found in the literature. In all tiie cases, dielectric heating was applied to S5mthesize the five-membered heterocyclic ring with three heteroatoms. These are summarized in Table 2 (Fig. 13). 

Pyrrole is one of the most prominent heterocycles, having been known for more than 150 years, and it is the structural skeleton of several natural products, synthetic pharmaceuticals, and electrically conducting materials. A simple access to the pyrrole ring system involves the conversion of cyclic anhydrides into five-membered imides. Mortoni and coworkers have described the conversion of 2-methylquinoline-3,4-dicarboxylic acid anhydride to a quinoline-3,4-dicarboximide library by treatment of the anhydride with a diverse set of primary amines under microwave conditions (Scheme 6.180) [341]. The authors studied a range of different conditions, including dry media protocols (see Section 4.1) whereby the starting materials were adsorbed onto an inorganic support and then irradiated with microwaves. For the transforma-... 

This alkylation reaction can be applied to intramolecular alkylation affording cyclic products, as shown in Equations (19)-(21). The reaction of 2-vinylpyridines with 1,5- or 1,6-dienes results in the formation of five- or six-membered carbocycles with good efficiency.20,20a,20b In addition to pyridine functionality, oxozole and imidazole rings can be applied to this intramolecular cyclization. When the reaction is conducted in the presence of a monodentate chiral ferrocenylphosphine and [RhCl(coe)2]2, enantiomerically enriched carbocycles are obtained. A similar type of intramolecular cyclization is applied to TV-heterocycles. The microwave irradiation strongly... 

Abstract In modem aspect, microwave synthesis is an ecofriendly process for the synthesis of heterocyclic compounds. Due to its rapid action to produce products with greater purity and yield, it is now used worldwide. In the present chapter, a survey on the microwave technique and a number of five- and six-membered heterocyclic compounds are reported which are synthesised by scientists under micro-wave irradiations. 

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