1.4- Dioxo compounds heterocycles from

The aromatic ring system is conveniently obtained by reaction of a-dicarbonyl compounds with 2,3-diaminopyrazines. The parent heterocycle and simple methyl derivatives have been obtained by this approach. Diethyl oxalate yields the expected dioxo compound 8 from... 

Since 1,2- to 1,6-difunctional opengeneral procedures (see chapter 1), it is useful to consider them as possible starting materials for syntheses of three- to seven-membered heterocycies 1,2-heterocycles can be made from 1,2-difunctional compounds, e.g. olefins or dibromides 1,3-difunctional compounds, e.g. 1,3-dibromides or 1,3-dioxo compounds, can be converted into 1,3-heterocycles etc. 

The usefulness of the Knorr synthesis arises from the fact that 1,3-dioxo compounds and a-aminoketones are much more easily accessible in large quantities than rational 1,4-difunctional precursors. Such practical syntheses are known for several important heterocycles. They are usually limited to certain substitution patterns of the target molecules. 

These include the SSA-catalyzed synthesis of heterocyclic compounds such as xanthenes, coumarins, oxazoles, and so on. Xanthenes are of great therapeutic and biological interest on account of their many biological activities such as anti-inflanunatory, antiviral, antibacterial properties, and so on. Seyyed Hamzeh et al. (2008) carried out the SSA-catalyzed synthesis of aryl-14 f-dibenzo[fl,y]xanthenes from aldehydes and P-naphthol and 1,8-dioxo-octahydro-xanthenes from aldehydes and 1,3-dicarbonyl componnd such as dimedone under solvent-free conditions (Schemes 5.25, 5.26). Reactions were carried out at 80°C. Nazeruddin et al. (2011) documented an effective method for 9,10-dihydro-12-aryl-8 f-benzo[a]xanthenes-ll(12 f)-one derivatives in excellent yield and short reaction time using SSA under solvent-free conditions (Scheme 5.27). 

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