A straightforward and efficient synthesis of the two less explored types of indolocarbazoles has been developed giving rise to highly fluorescent compounds with fluorescence quantum yields around 0.7.
This
work describes the 6-
endo
-
dig
cyclization
of
S
-aryl propargyl sulfides to afford
2
H
-thiochromenes. The substitution at the propargylic
position plays a crucial role in allowing intramolecular silver-catalyzed
alkyne hydroarylation and
N
-iodosuccinimide-promoted
iodoarylation. Additionally, a PTSA-catalyzed thiolation reaction
of propargylic alcohols was developed to synthesize the required
tertiary
S
-aryl propargyl ethers. The applicability
of merging these two methods is demonstrated by synthesizing the retinoic
acid receptor antagonist AGN194310.
The combination of organolithium chemistry with gold catalysis has enabled the development of a synthetic strategy for accessing polysubstituted indoles and carbazoles from readily available starting materials. This method is based on a "back-to-front" approach from ketopyrroles, generated by intramolecular carbolithiation of N,N-bis-(2-lithioallyl)amines that evolve into 3,4-bis(lithiomethyl)dihydropyrrole intermediates capable of reacting with carboxylic esters and Weinreb amides. These ketopyrroles have demonstrated to be excellent precursors of mono or bis(alkynols)pyrroles that, under gold-catalysis, experience a benzannulation reaction providing access to regioselectively substituted indoles or carbazoles.
Propargylic glycols, 2-alkynyl-1-(indol-3-yl)-1,2-diols, react with thiols undergoing a complex but selective gold-catalyzed transformation that gives rise to α-indol-3-yl α-((Z)-2-thioalkenyl) ketones. The sequence is triggered by the regioselective thiolation of indolyl diols followed by an attack of the sulfur instead of the indole over the activated alkyne. The final compounds are obtained in remarkably high yields and arise from simple starting materials such as indolyl acyloins, ethynyl magnesium bromide and thiols.
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