A divergent synthetic strategy allowed access to several members of a new class of helicenes, the "expanded helicenes", which are composed of alternating linearly and angularly fused rings. The strategy is based on a three-fold, partially intermolecular [2+2+n] (n = 1 or 2) cycloaddition with substrates containing three diyne units. Investigation of aggregation behavior, both in solution and in the solid state, revealed that one of these compounds forms an unusual homochiral, π-stacked dimer via an equilibrium that is slow on the NMR time scale. The versatility of the method was harnessed to access a selenophene-annulated expanded helicene that, in contrast to its benzannulated analogue, exhibits long-range π-stacking in the solid state. The new helicenes possess low racemization barriers, as demonstrated by dynamic H NMR spectroscopy.
A longstanding challenge in fundamental functional group
interconversion
has been the direct transformation of benzene into pyridine via nitrogen
insertion and carbon deletion. Herein, we report a protocol for the
transformation of aryl azides, easily accessible from their corresponding
anilines, to 2-aminopyridines using blue light and oxygen. Mechanistic
studies corroborate that the arene to pyridine conversion is achieved
by nitrogen insertion into the benzene ring followed by oxidative
carbon extrusion.
Cyclobutenes
are highly useful synthetic intermediates as well
as important motifs in bioactive small molecules. Herein, we report
a regio-, chemo-, and enantioselective synthesis of cyclobutenes from
olefins using N-sulfonyl-1,2,3-triazoles as vicinal
dicarbene equivalents or alkyne [2 + 2] cycloaddition surrogates.
Terminal and cis-olefins can be transformed into
enantioenriched cyclopropanes via rhodium catalysis. Then, in one
pot, treatment of these intermediates with tosyl hydrazide and base
effects diazo formation followed by rhodium-catalyzed ring expansion
to yield enantioenriched cyclobutenes. These cyclobutenes can be transformed
into highly substituted, enantioenriched cyclobutanes, including structures
relevant to natural product scaffolds.
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