10.1002/anie.201711209
Angewandte Chemie International Edition
COMMUNICATION
intermediate (6), which subsequently produces indolizine (3a)
via a direct radical-rebound step with the regeneration of the
catalytic cycle. Alternatively, thermodynamically more stable
indolizine (3a) might be generated by the ring expansion of in-
situ generated kinetically stable cyclopropene intermediate (7)
via Co(II)-based metalloradical catalyst. However, such
possibility is ruled out by control experiment.[17]
02(0259)/16/EMR-II), Ramanujan (SB/S2/RJN-45/2013) grant
and DST-Young Scientist Grant (SB/FT/CS-141/2014). SR
thanks CSIR for SRF and SKD thanks CSIR for project JRF. We
thank the entire team of NMR and mass facility at CBMR and
the Director, CBMR for research facility.
Keywords: Transannulation • cyclopropanation • Co-carbene •
catalysis • nitrogen-heterocycles
Towards this end, we were curious about the extension of our
developed metalloradical-catalyzed one-pot strategy for the
cyclopropanations. To our delight, subjecting the standard
conditions, cyclopropanations[ 18 ] were appeared to be very
general affording good yields and diastereoselectivity (Table 6).
[1]
[2]
For leading reviews on transannulation, see: (a) B. Chattopadhyay, V.
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Table 6. Scope of Cyclopropanationa
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1,2,3-triazoles, see: ref. 2.
[3]
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aReactions were conducted using 0.5 mmol of substrate (1) and 1.0 equiv.
alkene (11). dr was based on crude GC/MS analysis. Isolated yields.
bReaction was conducted at 50 oC and isolated as inseparable mixture.
[8]
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Remarkably,
while
styrene-type
of
alkenes
provide
diastereoselectivity (trans/cis) ranging from 80/20 to 85/15,
sulfonyl-group bearing alkenes gave single trans isomer (Table
6, 12h & 12i). We anticipated that the stereochemistry of this
exclusive trans isomer is controlled by the delocalized single
electron over the highly electron-withdrawing SO2-group. The
major trans-isomers (12b-12g) have been isolated as single
diastereoisomer via column chromatographic separation and the
minor isomers were failed to isolate.
In conclusion, we have developed a Co(II)-based metalloradical
activation approach, which undergoes radical addition either to
alkynes for denitrogenative transannulation or to alkenes for
cyclopropanation to afford indolizines and cyclopropanes,
respectively. The reactions are operationally simple and tolerate
a wide range of substrates. This is the first example of the
radical activation mechanism for the denitrogenative
transannulation, which is distinct from the earlier reported ionic
mechanism. The proposed metalloradical activation mechanism
is supported by various controlled experiments. Importantly, the
utility of this developed method has been showcased by the
short total synthesis of () monomorine.
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Acknowledgements
SR and SKD contributed equally to this work. This work was
supported
by
CSIR-EMR-II
grant
(Sanction
No.
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