Pyryliums, Benzopyryliums, Pyrones and Benzopyrones 77Exercises
- Devise a mechanism to explain the formation of 1,3,5-triphenylbenzene from 2,4,6-triphenylpyrylium
perchlorate on reaction with two mole equivalents of Ph 3 P=CH 2. - Suggest structures for the compounds in the following sequence: 2-methyl-5-hydroxy-4-pyrone reacted
with MeOTf → C 7 H 9 O 3 TfO– (a salt), then this with 2,2,6,6-tetramethylpiperidine (a hindered base) →
C 7 H 8 O 3 , a dipolar substance, and then this with acrylonitrile → C 10 H 11 NO 3. - Draw a mechanism for the conversion of 4-pyrone into 1-methylpyridin-4-one on reaction with
methylamine. - Deduce structures for the pyrylium salt formed by the following sequence: pinacolone (Me 3 CCOMe)
condensed with pivaldehyde (Me 3 CCH=O) giving C 11 H 20 O, which is then reacted with pinacolone in the
presence of NaNH 2 , generating C 17 H 32 O 2 and this with Ph 3 C ClO 4 in AcOH giving a pyrylium salt. - Deduce structures for the pyrones formed by the following sequences: (i) PhCOCH 3 with PhCKCCO 2 Et
in the presence of NaOEt; (ii) PhCOCH 2 COCH 3 with two mole equivalents of NaH then with methyl
4-chlorobenzoate. - ‘Dehydroacetic acid’ (ethyl 2-hydroxy-4-pyrone-3-carboxylate) was fi rst synthesised in 1866 by react-
ing ethyl acetoacetate (MeCOCH 2 CO 2 Et) with hot NaHCO 3. Recalling the synthesis of coumalic acid by
condensation of two three-carbon units, suggest a mechanism for the formation of dehydroacetic acid. - Deduce the structures of the intermediate and the fi nal product in the sequence: salicylaldehyde heated
with MeOCH 2 CO 2 Na and Ac 2 O→ C 10 H 8 O 3 , this then with one mole equivalent of PhMgBr → C 16 H 14 O 3
and fi nally this with HCl → C 16 H 13 O 2 Cl.