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that failed for 20) led successfully to the corresponding (p-Ns pro-
tected) indoline 21a in 24% yield.
As an alternative to prevent double protection of 18, and in
light of the fact that occasional problems in the deprotection of
cyclic p-nitrobenzenesulfonyl amines occur,37 we explored using
o-nitrobenzenesulfonyl chloride as the protecting agent. Although
successful this led to decreased yields in the following steps. Treat-
ment of 18 with o-nitrobenzenesulfonyl chloride gave exclusively
the mono protected derivative 19b in 31% yield (includes 22%
recovered starting material). Conversion of the phenolic hydroxy
moiety to a triflate leaving group led to 22b in 50% yield. The pre-
cursor thus obtained underwent cyclisation to the (o-Ns protected)
indoline 21b in 28% yield.
The application of a Friedel–Crafts protocol to the enantioselec-
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fully achieved. From 2-naphthol in five steps, indolines 21a/21b
were obtained in overall yields of 2.3% (o-nosylate) and 7.4%
(p-nosylate). Indolines 21a/21b could be converted via a few
well-precedented steps into a known intermediate in the pub-
lished synthesis of nitroCBIs: deprotection to the free alcohol,38
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conditions,37 and conversion of the indoline into the correspond-
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To best of our knowledge this is the first report of the intramo-
lecular cyclisation of a sulfonamide into a triflate attached to a ste-
rically demanding naphthalene. Additionally, this is the first time
an organocatalytic approach has been utilised to achieve enanti-
oselectivity in the synthesis of the CBI scaffold.
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Acknowledgements
This work was funded by the Deutsche Forschungsgemeinschaft
(HE6009/1–1).
27. Liu, T.-Y.; Cui, H.-L.; Chai, Q.; Long, B.-J.; Wu, Y.; Ding, L.-S.; Chen, Y.-C. Chem.
Commun. 2007, 2228.
Supplementary data
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