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Phosphoric Acid Catalysis
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
Using a similar strategy, Toste and coworkers were able to generate a chiral fluorination reagent in nonpolar medium and used it for enantioselective fluorocyclization (Figure 3.19).53 Selectfluor, which is typically insoluble in nonpolar solvents, was used as an electrophilic fluorine source to activate the π-bond of enol ether. In the presence of catalytic CPA 6, Selectfluor, and base, the enol ether underwent antiselective fluorination/cyclization to form spiro-fused oxazoline with high diastereo- and enantioselectivity. Consistent with their proposal that CPAs may act as effective PCTs, they found that CPA 6 with lipophilic alkyl chain attached to the 6,6′-positions of the BINOL backbone afforded better enantioselectivity. In addition, nonlinear relationship between the ee of the catalyst and the ee of the fluorocyclization product was observed, suggesting that both tetrafluoroborate counteranions of Selectfluor were exchanged with chiral phosphates before it reacted with the enol ether substrate. Toste’s reports demonstrated the versatility of CPAs as counterion catalysts that could be paired with cationic reagents to facilitate asymmetric reactions.
A highly stereoselective oxidation and an easy one pot elimination methodology for 3-allyl-3-phenylthio-β-lactams
Published in Journal of Sulfur Chemistry, 2022
Suvidha Pandey, Reshma Nagpal, Aarti Thakur, Shamsher S. Bari, Renu Thapar
The conversion possibly takes place by the interaction of 5 with SF.2BF4 in CH3CN at room temperature resulting in the formation of fluorosulfonium salt [41,42], which upon work up gets converted into sulfoxide 6 [42]. The Selectfluor due to its bulky nature approaches the reactant from the least hindered face of the molecule for the transfer of F+ to the thio group, generating fluorosulfonium salt (Figure 1). This fluorosulfonium salt upon hydrolysis during the work up exclusively affords (S)-cis-3-allyl-3-phenylsulfinyl-β-lactam.