Person: Park, Yongho
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Park
First Name
Yongho
Name
Park, Yongho
3 results
Search Results
Now showing 1 - 3 of 3
Publication Macrocyclic bis-thioureas catalyze stereospecific glycosylation reactions(American Association for the Advancement of Science (AAAS), 2017-01-12) Park, Yongho; Harper, Kaid; Kuhl, Nadine; Kwan, Eugene; Liu, Richard; Jacobsen, EricCarbohydrates are involved in nearly all aspects of biochemistry, but their complex chemical structures present long-standing practical challenges to their synthesis. In particular, stereochemical outcomes in glycosylation reactions are highly dependent on the steric and electronic properties of coupling partners; thus, carbohydrate synthesis is not easily predictable. Here we report the discovery of a macrocyclic bis-thiourea derivative that catalyzes stereospecific invertive substitution pathways of glycosyl chlorides. The utility of the catalyst is demonstrated in the synthesis of trans-1,2-, cis-1,2-, and 2-deoxy-β-glycosides. Mechanistic studies are consistent with a cooperative mechanism in which an electrophile and a nucleophile are simultaneously activated to effect a stereospecific substitution reaction.Publication Activation of Weak Nucleophiles in Anion-Binding Catalysis(2016-09-14) Park, Yongho; Jacobsen, Eric N.; Balskus, Emily P.; Woo, Christina M.Anion-binding catalysis has emerged as a powerful principle for the development of highly enantioselective transformations. This strategy relies on the ability of dual hydrogen-bond donors to promote anion abstraction from neutral substrates to generate cationic electrophiles such as iminium ions and oxocarbenium ions. Activation of nucleophiles in anion-binding reactions can further expand the scope of both electrophiles and nucleophiles in this mode of catalysis. The research described in this dissertation explores the use of thiourea catalysts to activate weak nucleophiles in two distinct reactions. In Chapter 1, a diastereoselective glycosylation reaction of glycosyl halides is reported. The transformation is catalyzed by macrocyclic bis-thiourea catalysts to afford β-glycosides. Experimental and computational evidence indicate a stereospecific, invertive mechanism in which thiourea moieties facilitate leaving group departure and the amide carbonyl group of the catalyst activates alcohol nucleophiles via general base catalysis. In Chapter 2, an enantioselective aza-Sakurai cyclization of chlorolactams is described. The reaction is effected by an electron-rich thiourea catalyst to provide an efficient entry into indolizidine and quinolizidine frameworks. Structure-enantioselectivity relationship studies and mechanistic analysis point to a dual role of the catalyst wherein the thiourea moiety of the catalyst is engaged in both generation of electrophile and Lewis base activation of allylsilane.Publication Sensitive and Accurate 13C Kinetic Isotope Effect Measurements Enabled by Polarization Transfer(American Chemical Society (ACS), 2016-12-29) Kwan, Eugene; Park, Yongho; Besser, Harrison; Anderson, Thayer; Jacobsen, EricPolarization transfer is demonstrated as a sensitive technique for the measurement of isotopic fractionation of protonated carbons at natural abundance. This method allows kinetic isotope effects (KIEs) to be determined with substantially less material or shorter acquisition time compared with traditional experiments. Computations quantitatively reproduce the KIEs in a Diels-Alder reaction and a catalytic glycosylation. The glycosylation is shown to occur by an effectively concerted mechanism.