Site-Selective Electrophilic
Cyclization and Subsequent Ring-Opening: A Synthetic Route to Pyrrolo[1,2‑<i>a</i>]quinolines and Indolizines
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Abstract
An efficient strategy for the synthesis of pyrrolo[1,2-<i>a</i>]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-<i>endo</i>-<i>dig</i> cyclized products. Quantum chemical calculations between
C–N (Δ<i>E</i><sub>a</sub> = 9.01 kcal/mol)
and C–C (Δ<i>E</i><sub>a</sub> = 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