4 research outputs found
Synthesis and biological activity of methylated derivatives of the Pseudomonas metabolites HHQ, HQNO and PQS
Selectively methylated analogues of naturally occurring 2-heptyl-4(1H)-quinolones, which are alkaloids common within the Rutaceae family and moreover are associated with quorum sensing and virulence of the human pathogen Pseudomonas aeruginosa, have been prepared. While the synthesis by direct methylation was successful for 3-unsubstituted 2-heptyl-4(1H)-quinolones, methylated derivatives of the Pseudomonas quinolone signal (PQS) were synthesized from 3-iodinated quinolones by methylation and iodine–metal exchange/oxidation. The two N- and O-methylated derivatives of the PQS showed strong quorum sensing activity comparable to that of PQS itself. Staphylococcus aureus, another pathogenic bacterium often co-occurring with P. aeruginosa especially in the lung of cystic fibrosis patients, was inhibited in planktonic growth and cellular respiration by the 4-O-methylated derivatives of HQNO and HHQ, respectively
Boron-Enabled Geometric Isomerization of Small Alkene Fragments via Selective Energy Transfer Catalysis
The mammalian visual cycle epitomizes the importance of complex polyenes in biology. However, isomerization-based strategies to enable the sterodivergent construction of these important biomolecules from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low molecular precursors. Herein, we report a general ambiphilic C3 scaffold that can be isomerized and bi-directionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-15 borylacrylates is contingent on the participation of the boron p-orbital in the substrate chromophore. Rotation of the C(sp2)-B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.<br /
Direct Observation of Triplet States in the Isomerization of Alkenylboronates by Energy Transfer Catalysis
Alkenylboronates
are versatile building blocks for stereocontrolled
synthesis owing to the traceless nature of the boron group that can
be leveraged to achieve highly selective geometric isomerization.
Using thioxanthone as an inexpensive photocatalyst, the photoisomerization
of these species continues to provide an expansive platform for stereodivergent
synthesis, particularly in the construction of bioactive polyenes.
Although mechanistic investigations are consistent with light-driven
energy transfer, direct experimental evidence remains conspicuously
absent. Herein, we report a rigorous mechanistic investigation using
two widely used alkenylboronates alongside relevant reference compounds.
Through the combination of irradiation experiments, transient absorption
spectroscopic studies, kinetic modeling, and DFT calculations with
all isomers of the model compounds, it has been possible to unequivocally
detect and characterize the perpendicular triplet generated by energy
transfer. Our results serve not only as a blueprint for mechanistic
studies that are challenging with organic sensitizers, but these guidelines
delineated have also enabled the development of more sustainable reaction
conditions: for the first time, efficient organocatalytic isomerization
under sunlight irradiation has become feasible
A desilylative approach to alkyl substituted C(1)-ammonium enolates : application in enantioselective [2+2] cycloadditions
Funding: The research leading to these results has received funding from the CSC-St Andrews PhD Scholarship Scheme (Y.W.), the Newton Fellowship Programme (H.L.) and the Engineering and Physical Sciences Research Council, University of St Andrews, and the EPSRC Centre for Doctoral Training in Critical Resource Catalysis (CRITICAT) for financial support [Ph.D. studentship to W. H.; Grant code: EP/L016419/1].The catalytic generation of C(1)-ammonium enolates from the corresponding α-silyl-α-alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α-unsubstituted and α-alkyl substituted C(1)-ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]-cycloaddition processes with perfluoroalkylketones (31 examples, up to 96% yield and >99:1 er), as well as selective [2+2]-cycloaddition with trifluoromethyl enones (4 examples, up to 75% yield and >99:1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)-α-silyl-α-alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero-order kinetics.Publisher PDFPeer reviewe