Site-Selective Electrophilic Cyclization and Subsequent Ring-Opening: A Synthetic Route to Pyrrolo[1,2‑a]quinolines and Indolizines

An efficient strategy for the synthesis of pyrrolo­[1,2-a]­quinolines and indolizines from pyranoquinolines via site-selective
electrophilic cyclization and subsequent opening of pyran ring using
silver/iodine under mild reaction conditions is described. This approach
involves the preferential attack of the pyridyl nitrogen over aryl
ring and leads to the formation of 5-endo-dig cyclized products. Quantum chemical calculations between
C–N (ΔEa = 9.01 kcal/mol)
and C–C (ΔEa = 31.31 kcal/mol)
bond formation were performed in order to rationalize the observed
site selectivity. Structure of the products were confirmed by the
X-ray crystallographic studies. Iodo-substituted compounds generated
by the electrophilic iodocyclization were further diversified via
Pd-catalyzed cross-coupling reactions.

本文描述了一种从吡喃喹啉出发，通过位点选择性的亲电环化和银/碘催化的吡喃环开环反应，高效合成吡啰啉[1,2-a]喹啉和吲哚啉的策略。该方法优先攻击吡啶氮而非芳环，从而得到5-endo-二环化产物。为阐释所观察到的位点选择性，进行了C-N键（ΔEa = 9.01 kcal/mol）和C-C键（ΔEa = 31.31 kcal/mol）形成过程中的量子化学计算。通过X射线晶体学对产物的结构进行了确认。通过亲电碘环化生成的碘代化合物，进一步通过钯催化的交叉偶联反应进行了多样化。