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However, investigation of the biological activities and opto-electric properties of these annulated polycyclic indolizines is still largely impeded by the lack of efficient and general synthetic methods. A few highly specific synthetic methods have been reported,butare rather lengthy and givelow over-all yields.16 Recently, isoindolo[2,1-a]quinolin-11-ones have been synthesized by [4+2] cycloadditions of acyliminium cation with alkenes.17Although [3+2] cycloaddition of pyridinium and quinolinium ylides has been applied for the synthesis of indolizines, these reactions were mainly limited to the use of electron deficient alkynes as dipolarophile.18In the case of alkenes, unstable tetrahydroindolizines are formed,which reversibly transform into a betaine intermediate followed by decomposition.19However, two recent approaches have succeeded in extending the [3+2] reaction to the application of alkenes as dipolarophiles. By using fluoroalkenes,fluoroiodoalkenes, and fluorinated vinyl tosylates20–22in the cycloadditions with the pyridinium and quinolinium ylides followed by elimination of hydrogen fluoride and toluenesulfonic acid, fluorinated and unfluorinated indolizines can be prepared. Meanwhile, Hu and co-workers reported that by using a mild dehydrogenative oxidant tetrakispyri- dinecobalt(II) dichromate [Py4Co(HCrO4)2] (TPCD), the tetrahydroindolizines can be in situ dehydrogenated to indo- lizines.23In this way, several 1,2-annulated indolizines were prepared by cycloadditions of pyridinium and quinolinium ylides with naphthoquinones, etc.24We report here a general and versatile synthesis of 1,2-annulated indolizines by one-pot tandem reactions of pyridinium, quinolinium, and isoquinolinium ylides with cyclic 1,2-dichloroalkenes 4–7. We have also shown that the reactions of the N-ylides with 2,3-dichloroindenone 4, 3,4-dichlorocoumarin 5, and 4a,6,7,8a-tetrachloro-1,4-methanonaphthalene-5,8-dione 6 proceed via [3+2] cycloaddition and elimination of two hydrogen chloride molecules, while the reactions of the N-ylides with 2,3-dichloro-1,4-naphthoquinone 7 take place by a novel mechanism involving sequential nucleophilic substitutions and a formal retro-Menschutkin reaction. By these one-pot tandem reactions of the N-ylides with 4–7,1,2-annulated, and1,2-,5,6-,and 1,2-,7,8-bisannulated polycyclic indolizine derivatives can be conveniently and regioselectively prepared from easily accessible starting materials. |
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RXMCDM: 金币+1 2014-04-25 10:09:26
RXMCDM: 金币+1 2014-04-25 10:09:26
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