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However, all attempts of cyclization failed with the Boc-pro-
tected bisindoles. Two modifications changed things for the
better regarding the cyclization: replacement of the Boc- by a
non-coordinating TIPS-protecting group and the use of indo-
lines instead of sensitive indoles. Cyclopentane anellation by
SnCl4-induced cyclization of phenylvinylcarbinols became
possible, at least for the sterically congested β-cyclocitral
adduct 40 of N-TIPS-indoline. The less sterically demanding
substrate 44 gave lower yields. Experiments employing Au(I)
and Pt(II) catalysts point at how to continue, since tricycles 48,
49, and 50 were obtained in good yields. We will now investi-
gate the synthesis and Au(I) and Pt(II)-catalyzed cyclizations of
TIPS-protected bisindolines, following a modified retrosyn-
thesis of raputindole A (1).
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Supporting Information
Supporting Information File 1
Experimental procedures and NMR spectra.
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We thank Merck KGaA (Darmstadt, Germany) for the generous
gift of chromatography materials. The BASF Group (Ludwigs-
hafen, Germany) and Honeywell Specialty Chemicals Seelze
GmbH (Seelze, Germany) are thanked for the donation of sol-
vents.
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