Polyphosphoric acid catalysis

To obtain compounds unsubstituted at positions 2 and 3, cyclization of the acetals (53) using polyphosphoric acid catalysis gives indoles (81JOC778), benzo[ >]furans, and benzo[ >]thiophenes (54) (71T1253). 

Substituted 2-phenoxyphenylacetic acids readily cyclize under Friedel-Crafts conditions or acid catalysis to give dibenz[Z>,/]oxepin-10(l l//)-ones.71 85,104- 108 When this reaction is carried out in methanolic hydrochloric acid the 10-methoxy-substituted dibenz[6,/]oxepin system 9a can be isolated.109 5-(Nitro-2-phenoxyphenyl)-2-oxopropanoic acid undergoes cyclization in the presence of polyphosphoric acid yielding the carboxylated dibenzoxepin 9b.107... 

Dihydro-17/-pyrrolo[l,2-< ][l,3,5]oxadiazocin-6(5/7)-one ring system 78 can be obtained from 2,3-dihydro-3-hydroxy-2-(pyrrol-l-ylmethyl)-17/-isoindol-l-one 76. No intermediate acyl chloride was observed during the cyclization of 77a, and product 78 (X = O) was isolated in low 15% yield. Yields were not improved after addition of Lewis acids or use of polyphosphoric acid as a condensation reagent (Scheme 16 <1998JHC9>). Similarly, 2,3-dihydro-l/7-pyrrolo[l,2-< ][l,3,5]thiadiazocin-6(577)-one ring system 19 (X = S) was obtained in low yield. Key intermediate 77b was prepared from 76 in one step under acidic catalysis. 

A -Arylhydrazones, derived from enolizable aldehydes or ketones 30, are converted into indoles 31 under Lewis (ZnCl2, BF3) or Bronsted acid (H2SO4, polyphosphoric acid, CH3COOH, HCl in ethanol) catalysis with loss of ammonia Fischer synthesis, E. Fischer 1883) ... 

Mixtures of polyphosphoric acid (PPA) or K-10 clay and Bi(OTf)3 xH20 have been reported to be efficient systems for the rearrangement of indanone oximes (Equation 36) [73]. The apparent synergy between PPA and Bi(OTf)3 xH20 might be rationalized as Bronsted acid assisted Lewis acid (BLA) catalysis. 

The Friedlander quinoline synthesis of 118 was performed in PEG 400 using polyphosphoric acid (PPA) catalysis xmder microwave irradiahon (Scheme 66) [97]. The same reachon was also carried out under thermal conditions (120 °C, 24-48h), but yields of products 118 were lower (50-83%). 

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