12 research outputs found
Synthesis of the C1âC10 Fragment of Madeirolide A
The synthesis of a fully elaborated
C1âC10 fragment of madeirolide
A has been achieved via a strategy based on a series of stereospecific
processes. The concise synthetic route also features an iridium-catalyzed
visible light induced radical cyclization for construction of the
THP ring and a palladium-catalyzed glycosylation for formation of
the Îą-cineruloside linkage
Rhodium-Catalyzed Tandem AdditionâCyclizationâRearrangement of Alkynylhydrazones with Organoboronic Acids
Transition
metal-mediated catalysis routinely enables substrates
of multiple Ď-systems to be efficiently coupled with various
carbon nucleophiles along with simultaneous ring formation. This transformation,
however, remains unexplored in connection with pericyclic processes.
Reported here is a protocol for cycloalkene synthesis based on the
merger of rhodium catalysis and a retro-ene reaction. The approach
allows alkyne-tethered hydrazones and organoboronic acids to undergo
a cascade of additionâcyclizationârearrangement reactions
to provide cycloalkene products. The process is initiated by the rhodium-catalyzed
additionâcyclization and completed with the allylic diazene
rearrangement. The reaction can also be rendered asymmetric by using
chiral diene ligands for the rhodium catalyst, whereby enantioselective
addition to the CîťN bond establishes the Cî¸N stereocenter
whose chirality is transferred to an allylic Cî¸H center via
suprafacial rearrangement
Rhodium-Catalyzed Oxygenative [2 + 2] Cycloaddition of Terminal Alkynes and Imines for the Synthesis of βâLactams
A rhodium-catalyzed
oxygenative [2 + 2] cycloaddition of terminal alkynes and imines has
been developed, which gives β-lactams as products with high <i>trans</i> diastereoselectivity. In the presence of a RhÂ(I) catalyst
and 4-picoline <i>N</i>-oxide, a terminal alkyne is converted
to a rhodium ketene species via oxidation of a vinylidene complex
and subsequently undergoes a [2 + 2] cycloaddition with an imine to
give rise to the 2-azetidinone ring system. Mechanistic studies suggest
that the reaction proceeds through a metalloketene rather than free
ketene intermediate. The new method taking advantage of catalytic
generation of a ketene species directly from a terminal alkyne provides
a novel and efficient entry to the Staudinger synthesis of β-lactams
under mild conditions
Tandem DielsâAlder and Retro-Ene Reactions of 1âSulfenyl- and 1âSulfonyl-1,3-dienes as a Traceless Route to Cyclohexenes
A pericyclic
approach for the synthesis of six-membered ring structures
is described. The method employs 1,3-dienes with a 1-sulfur substituent
in a tandem sequence of DielsâAlder and retro-ene reactions.
In this pairing of [4 + 2] cycloaddition and 1,5-sigmatropic rearrangement,
1-sulfenyl-1,3-dienes engage in DielsâAlder reactions with
electron-deficient dienophiles. Subsequently, the sulfenyl group of
the cycloadducts is oxidized and unmasked to form allylic sulfinic
acids, which undergo sterospecific reductive transposition via sulfur
dioxide extrusion. The sequence can also include an inverse electron
demand DielsâAlder reaction by using a 1-sulfonyl-1,3-diene.
This combination of two pericyclic events offers novel stereocontrolled
access to cyclohexenes that are inaccessible via a direct [4 + 2]
cycloaddition route