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Synthesis of Indeno-Fused Derivatives of Quinolizinium Salts, imidazo[1,2-a]pyridine, pyrido[1,2-a]indole, and 4h-quinolizin-4-one via Benzannulated Enyne-Allenes

Synthesis of Indeno-Fused Derivatives of Quinolizinium Salts, imidazo[1,2-a]pyridine, pyrido[1,2-a]indole, and 4h-quinolizin-4-one via Benzannulated Enyne-Allenes

Weixiang Dai, Jeffrey L Petersen, Kung K Wang

J Org Chem. 2005 Aug 19;70(17):6647-52.

PMID: 16095282

Abstract:

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.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP16156584	2-Propynyl methanesulfonate		16156-58-4	Price

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