Synthesis of Indeno-Fused Derivatives of Quinolizinium Salts, Imidazo[1,2-<i>a</i>]pyridine, Pyrido[1,2-<i>a</i>]indole, and 4<i>H</i>-Quinolizin-4-one via Benzannulated Enyne−Allenes

The benzannulated enediynyl propargylic alcohol 8 was prepared from 1-bromo-2-iodobenzene by
two consecutive Sonogashira cross-coupling reactions. The subsequent transformation to mesylate
9 followed by treatment with 4-substituted pyridines 10 then furnished the benzannulated enediynes
11. On exposure of 11 to triethylamine, the indeno-fused quinolizinium salts 15 were produced in
quantitative yield. Presumably the reaction proceeded through a 1,3-prototropic rearrangement to
form the benzannulated enyne−allenes 12, which then underwent either a concerted Diels−Alder
reaction or a two-step process involving a Schmittel cyclization reaction to form biradical 13 followed
by an intramolecular radical−radical coupling to afford 14. A subsequent prototropic rearrangement
then produced 15. Similarly, 21a and 21b were produced from 19a and 19b, respectively. The use
of the Sonogashira reaction for cross-coupling between 1-iodo-2-(phenylethynyl)benzene (7) and
1-(2-propynyl)-1H-imidazole (25) followed by treatment of the resulting adduct with potassium tert-butoxide gave the indeno-fused imidazo[1,2-a]pyridine 24 in 98% yield. Similarly, the indeno-fused
pyrido[1,2-a]indole 32 and 4H-quinolizin-4-one 35 were obtained by starting from 7 for cross-coupling
with 1-(2-propynyl)-1H-indole (30) and 1-(2-propynyl)-2(1H)-pyridinone (33), respectively, followed
by treatment with potassium tert-butoxide.