Four-carbon-tethered pyridones undergo photocycloaddition to give exclusively trans-[4 + 4] products. The presence of a tether alcohol engenders a solvent-dependent diastereoselectivity for the cycloaddition by intramolecular hydrogen bonding to the adjacent pyridone. Following cycloaddition, the alcohol can deliver a carbonyl group to the proximal, hindered amide nitrogen, leading to a very facile amide hydrolysis.
A direct route to branched N-allylpyrimidine analogues is herein reported via the highly regio- and enantioselective asymmetric allylation of pyrimidines with racemic allylic carbonates. With [Rh(COD)Cl]/chiral diphosphine as the catalyst, a range of chiral pyrimidine acyclic nucleosides could be obtained under neutral conditions in good yields (up to 95% yield) with high levels of regio- and enantioselectivities (15:1 to >40:1 B/L and up to 99% ee). Furthermore, chiral pyrimidine acyclic nucleoside bearing two adjacent chiral centers has been successfully synthesized by asymmetric dihydroxylation.
The enantioselective Friedel-Crafts alkylation reaction of β-naphthols with donor-acceptor aminocyclopropane was developed. In the presence of a copper complex derived from Cu(OTf) and bisoxazoline, a series of γ-substituted γ-aminobutyric acid derivatives were obtained with good yields (up to 98 %) and excellent enantioselectivities (up to 98 %). Using this catalytic system, the 2-amino cyclopropane-1,1-dicarboxylate was obtained with high enantiomeric excess (up to 98 %) by an efficient kinetic resolution (s values of up to 90). The Friedel-Crafts alkylation product could be transformed into a tetracyclic 1,3-oxazine derivative.
N-Heterocycle-assisted C–H activation/annulation
reactions have provided new concepts for the construction and transformation
of azacycles. In this work, we disclose a [5+1] annulation reaction
using a novel transformable pyridazine directing group (DG). The DG-transformable
reaction mode led to the construction of a new heterocyclic ring accompanied
by transformation of the original pyridazine directing group via a
C–H activation/1,4-Rh migration/double bond shift pathway,
affording the skeleton of pyridazino[6,1-b]quinazolines
with a good substrate scope under mild conditions. Diverse fused cyclic
compounds can be achieved by derivatization of the product. The asymmetric
synthesis of the skeleton was also realized to afford the enantiomeric
products with good stereoselectivity.
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