16,865 research outputs found
A HIGHLY ENANTIOSELECTIVE AND DIASTEREOSELECTIVE ALDOL REACTION FOR ALPHA-HETEROSUBSTITUTED THIOACETATES
No full textBoron enolates derived from alpha-heterosubstituted thioacetates and bearing menthone-derived chiral ligands react with aldehydes to give anti aldols with excellent diastero- and enantiocontrol
ORIGINS OF STEREOSELECTIVITY IN THE ADDITION OF ALLYL AND CROTYLBORONATES TO ALDEHYDES - THE DEVELOPMENT AND APPLICATION OF A FORCE-FIELD MODEL OF THE TRANSITION-STATE
No full textA molecular mechanics model of the transition state for the addition of allyl and crotyl boronates to aldehydes was developed, based on ab initio calculations and on a process of vial and error optimization. The optimized force field reproduces the experimental syn-anti stereoselectivity for the intermolecular addition of E and Z crotylboronates to aldehydes. The force field is used to analyse the stereoselectivity of various synthetically interesting reactions. In particular, the force field is able to reproduce with excellent quantitative agreement the experimental results of intramolecular boronate reactions leading to methyl or benzyloxy substituted cyclohexanol or cyclopentanol derivatives (R.W. Hoffmann, et al.). In addition the force field is able to reproduce the experimental results of variously substituted allylboronates (gamma,gamma-alkyl substituted, alpha,gamma-alkyl substituted, alpha,gamma,gamma-alkyl substituted, gamma-alkoxy substituted) and the Felkin-antiFelkin ratios of allyl, E-crotyl and Z-crotyl addition to chiral aldehydes
Highly enantio- and diastereoselective boron aldol reactions of alpha-heterosubstituted thioacetates with aldehydes and silyl imines
No full textBoron enolates derived from alpha-heterosubstituted thioacetates and bearing menthone-derived chiral ligands react with aldehydes to give anti aldols with excellent diastero- and enantiocontrol. Boron enolates derived from tert-butyl alpha-halothioacetate and bearing menthone-derived chiral ligands react with imines with excellent diastero- and enantiocontrol to give syn alpha-halo-beta-aminothioesters, which can be converted to the corresponding aziridines by simple ring closure during LAH reduction. A key precursor of antibiotics (+)-thiamphenicol and (-)-florfenicol was synthesized. (C) 1997 Elsevier Science Ltd
REAGENT CONTROL IN THE ALDOL ADDITION-REACTION OF CHIRAL BORON ENOLATES WITH CHIRAL ALDEHYDES
No full textBoron enolates bearing menthone-derived chiral ligands are capable of fair to excellent diastereocontrol in their reactions with chiral aldehydes. Thioester-derived (better than ketone derived) enolates are able to control aldol stereochemistry irrespective of the aldehyde preferences
Computer‐Assisted Design of Chiral Boron Enolates: A Novel, Highly Enantioselective Aldol Reaction for Thioacetates and Thiopropionates
No full textUnsubstituted and anti aldol products with excellent diastereo- and enantioselectivity are formed when enolates, generated from the corresponding thioacetates and thiopropionates by using bromoborane 1 and triethylamine, are treated with aldehydes. Bromoborane 1 was designed based on transition state computer modeling and then synthesized
An Article About Albertus C. Van Raalte, Author Unknown, Except for Parts Taken from an Article by Anna C. Post
No full textAn article about Albertus C. Van Raalte, author unknown, except for parts taken from an article by Anna C. Post. The author knew first generation persons in the Holland settlement and therefore, the article has some value.https://digitalcommons.hope.edu/vrp_1890s/1012/thumbnail.jp
Semisynthesis of taxol: A highly enantio- and diastereoselective synthesis of the side chain and a new method for ester formation at C13 using thioesters
No full textORIGINS OF PI-FACE SELECTIVITY IN THE ALDOL REACTIONS OF CHIRAL E-ENOL BORINATES - A COMPUTATIONAL STUDY USING TRANSITION-STATE MODELING
No full textA molecular mechanics transition state model is used to analyse the stereoselectivity of a series of aldol reactions of E-enol borinates of type 3 with aldehydes. The model reproduces the sense and degree of pi-face selectivity for the chiral E-enol borinates 8, 11, 13 and 14 in the Table. Enolates 8 and 10 preferentially attack the re-face of aldehydes, which is explained (Scheme 5) by the aldol addition proceeding through the preferred transition structure TS-A for both electronic and steric reasons. In contrast, enolates 11, 13 and 14 preferentially attack the si-face of aldehydes solely for steric reasons, which is explained by invoking the favoured transition structure TS-B derived from the modelling results. These two transition state models, TS-A and TS-B, which apply to E-enol borinates, differ substantially from the transition state model, TS-C, used for chiral Z-enol borinates with similar substituents. Our force field model of the boron aldol transition state is shown to be useful in understanding the origins of the pi-face selectivity over a wide range of substrates
Richardson, Barbauld, and the construction of an early modern fan club
No full textMPhilMuch has been written about the life and long works of the eighteenth century epistolary novelist, Samuel Richardson, but the prospect of his position as the first celebrity novelist – responsible for courting his own fame as well as initiating his own fan club – has largely been ignored. The body of manuscripts housed at the National Art Library in the Victoria and Albert Museum in London provides the modern scholar with evidence of the skeletal beginnings of an early fan club. This thesis aims to show how these manuscripts were turned into a saleable commodity by the publisher and entrepreneur Richard Phillips, while under the guiding hand of another, slightly later, literary celebrity, Anna Laetitia Barbauld. In order to restore Richardson’s reputation amongst a new nineteenth century audience, Barbauld was required to construct her own idea of him as an eighteenth century celebrity author, and in doing so the insecurities of a self-professed, apparently diffident man, are revealed. Barbauld’s capacious, but heavily edited selection of letters is analyzed in this thesis, providing ample evidence that Richardson’s correspondents were more than just eager letter writers. By using Barbauld’s biography of Richardson this thesis aims to show how she manipulates the genre of life writing in her construction of him.
This thesis offers an alternative reading of how the Richardson manuscripts are viewed, redefining them as not simply a collection of letters, but as a collective entity, deliberately selected and archived as evidence of an early modern fan club, and its celebrity managing director
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