2- -pyridines, dehydrative cyclisation

A typical pyrilium salt synthesis is illustrated by the preparation of salt 9.12. The precursor to 9.12 is pyran 9.11, available by dehydration of 1,5-diketone 9.10. Note the similarity of this sequence to the Hantzch pyridine synthesis, Chapter 5. Also, the dehydrative cyclisation of a diketone to oxygen heterocycle 9.11 is reminiscent of furan synthesis, Chapter 2. 

Imidazo[l,5-fl]pyridines are synthesised by the dehydrative cyclisation of iV-acyl-2-aminomethyl-pyri-dines or of thioamides, ° which can be prepared and ring closed under very mild conditions. 3-Amino -oxy and -thio derivatives are available via related cyclisations. 

Polyimides are most conveniently prepared by condensation of a bis-aromatic anhydride with a 6w-aromatic amine (Figure 6.13). The first step yields a polyamic acid which can be conveniently dehydrated and cyclised to a polyimide by treatment with acetic anhydride and pyridine under rather mild conditions. This can now be achieved in an organic suspension to yield spherical particulate polyimides [67]. In a typical procedure a polyamic acid precursor is formed in solution in dimethylacetamide (DMAc 10 wt%) by reaction of pyromellitic dianhydride with /7-phenyle-nediamine at room temperature for 2 h. This pre-polymer solution is then suspended in paraffin oil containing a dissolved polymer chosen to function as a steric stabiliser for the DMAc solution. A copolymer of maleic anhydride and octadec-l-ene is very effective. The suspended droplets are kept dispersed by overhead stirring while acetic anhydride and pyridine are added dropwise to induce imidisation, typically reaction proceeds for... 

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