Organic Letters
Letter
along with 15 mol % of Rh2(OAc)4 and reduced reaction
temperature gave significant improvement.
The mechanism for these reactions is proposed to proceed
by Rh2(OAc)4-catalyzed denitrogenation of the 1-ST 7/8 to
form a rhodium carbenoid2 A (Scheme 7). The oxygen lone
Trust and the University of Glasgow, School of Chemistry, as
well as invaluable discussions with, and support from, Prof. J.
Stephen Clark. Dr. Louis J. Farrugia is gratefully acknowledged
for single-crystal structure determination.
REFERENCES
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Scheme 7. Proposed Mechanism
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pair interacts with the carbenoid to form an oxonium species B
which undergoes [2,3]-sigmatropic rearrangement, transferring
the allyl group to form a new C−C bond. This aspect of the
mechanism is supported by the formation of an allene 15 from
a propargylic ether 14 (Scheme 5). The diastereoselectivity is
proposed to arise from the minimization of steric clash between
migrating group and the bulk of the substituent R1. The
improvement in yield and diastereoselectivity compared to the
corresponding rhodium(II)-catalyzed reactions of α-diazoke-
tones is ascribed to increased steric demand of the N-tosylimine
in addition to electronic factors. Finally, it is noteworthy that
the proposed organometallic intermediates A/B (R2 ≠ H)
generate the observed products given the potential for
competitive 1,2-hydrogen shift,4c which would give byproducts
stabilized by conjugation.
In summary, 1-STs bearing allyloxy and propargyloxy
substituents are readily accessed from simple acyclic alkynes.
Upon treatment with Rh2(OAc)4 at elevated temperatures,
these 1-STs undergo denitrogenative rearrangement to give
decorated dihydrofuran-3-imines with excellent diastereoselec-
tivity. Following hydrolysis, 4-substituted 1-STs were converted
to 2,5-trans-disubstituted dihydrofuran-3-ones, giving an
alternative to the formation and rearrangement of α-
diazoketones that avoids the use of diazomethane. This process
was also successfully applied to 4,5-disubstituted 1-STs which
resulted in the diastereoselective formation of products with a
2-tetrasubstituted center. Studies are currently underway to
expand this method, fully elucidate the mechanism, and
investigate its application to the synthesis of important
bioactive molecules.
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ASSOCIATED CONTENT
* Supporting Information
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́
(6) Lorente, A.; Lamariano-Merketegi, J.; Albericio, F.; Alvarez, M.
S
Chem. Rev. 2013, 113, 4567−4610.
Experimental protocols, characterization data, and NMR
spectra. This material is available free of charge via the Internet
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AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
A.B. is a Ramsay Memorial Research Fellow and gratefully
acknowledges support from the Ramsay Memorial Fellowships
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dx.doi.org/10.1021/ol500309x | Org. Lett. 2014, 16, 1660−1663