ORGANIC
LETTERS
2007
Vol. 9, No. 18
3651-3653
A New Efficient Synthesis of
Pyranoquinolines from 1-Acetyl N-Aryl
Cyclopentanecarboxamides
Qian Zhang,* Zhiguo Zhang, Zihong Yan, Qun Liu,* and Tunyu Wang
Department of Chemistry, Northeast Normal UniVersity, Changchun, 130024, China
Received June 29, 2007
ABSTRACT
A new efficient synthesis of pyrano[2,3-b]quinoline derivatives is developed via the H2SO4-mediated tandem cyclization/ring-opening/recyclization
reaction of readily available 1-acetyl N-aryl cyclopentanecarboxamides, during which a novel ring-cleavage fashion of the cyclopentane unit
is involved and possible mechanisms are discussed.
Pyranoquinoline alkaloids, widely distributed among plants
of the Rutaceae family, have attracted considerable attention
because of their broad pharmacological and biological
properties.1 The development of efficient syntheses of
pyranoquinolines has been the focus of much research for
several decades and continues to be an active and rewarding
research area. However, most of the existing methods suffer
from limited scope or availability of starting materials, or
require multistep procedures.1,2 In this Letter, we describe
an efficient synthesis of pyrano[2,3-b]quinolines 3 from the
readily available 1-acetyl N-aryl cyclopentanecarboxamides
1 via a novel tandem cylization/ring-opening/recyclization
reaction (Scheme 1).
originating from the inherent ring strain. Cyclopropanes/
cyclobutanes activated by electron-withdrawing substituents
are capable of undergoing a variety of ring-cleavage
reactions.3-5 In contrast, the ring-cleavage reactions of the
activated cyclopentanes and their applications in organic
synthesis are very rare owning to the stability of the five-
membered cyclopentane ring.6 Although in the works of
Langer and co-workers,6a,b hydroxyspiro[5.4]decenones could
be transformed to bicyclo[4.4.0]deca-1,4-dien-3-ones in the
presence of trifluoroacetic acid, the ring-cleavage of the
cyclopentane unit was through a well-known Wagner-
Meerwein rearrangement. Herein, the first ring-cleavage
fashion of activated cyclopentanes, which is similar to that
of activated cylcopropanes,3,4 is provided and a terminal
Small carbon ring compounds, cyclopropanes and cy-
clobutanes, are extremely versatile building blocks in organic
synthesis due to their ready accessibility and good reactivity
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10.1021/ol701536q CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/03/2007