The nitrogen-interrupted Nazarov cyclization can be a powerful method for the stereocontrolled synthesis of sp3-rich N-heterocycles. However, due to the incompatibility between the basicity of nitrogen and the acidic reaction...
This
work describes an acid-promoted cyclization of triaryldivinyl
ketones containing a thiophene moiety in the α-position. Two
cyclization pathways are accessible: one a 4π-Nazarov cyclization
and the other we propose to proceed through a 6π electrocyclic
mechanism. The relative proportion of products from these divergent
pathways is affected by reaction conditions and steric bulk in the
substrate. We present experimental and computational evidence that
when using HCl in dioxane, the 4π-conrotatory electrocyclization
is more favorable, whereas GaCl3 in methylene chloride
shifts the chemoselectivity toward a putative 6π-disrotatory
electrocyclization. DFT calculations suggest that a complex interplay
between kinetic and thermodynamic factors is implicated in the chemodivergent
behavior.
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