45 research outputs found
Quantitative Prediction of Rate Constants for Aqueous Racemization To Avoid Pointless Stereoselective Syntheses
Racemisation has a large impact upon the biological properties of molecules but the chemical scope of compounds with known rate constants for racemisation in aqueous conditions was hitherto limited. To address this remarkable blind spot, we have measured the kinetics for racemisation of 28 compounds using circular dichroism and 1H NMR spectroscopy. We show that rate constants for racemisation (measured by ourselves and others) correlate well with deprotonation energies from quantum mechanical (QM) and group contribution calculations. Such calculations thus provide predictions of the second-order rate constants for general-base-catalyzed racemisation that are usefully accurate. When applied to recent publications describing the stereoselective synthesis of compounds of purported biological value, the calculations reveal that racemisation would be sufficiently fast to render these expensive syntheses pointless
ChemInform Abstract: The Use of Bis(trimethylsilyl)acetamide and Bis(trimethylsilyl)urea for Protection and as Control Reagents in Synthesis
ChemInform Abstract: A Convenient Alternative Route to Trimethylsilyl Fluorosulfonate for Use in Catalytic Reactions.
ChemInform Abstract: The Use of Bistrimethylsilylacetamide and -urea in Trimethylsilyl Trifluoromethanesulfonate Catalyzed Reactions.
Aspects of some alkylation reactions
Friedel-Crafts reactions of the (-)-8-phenylmenthyl and (+ )-trans-2-(acumyl)
cycJohexyl aryl hydroxy acetates catalysed by trimethylsilyl triflate (TMSOTf)
and equivalents in the presence of electron rich heterocycles gave the expected
diarylacetates in high de 88%, via a planar cation.
The interaction of trifluoromethanesulfonic (triflic) acid (TfOH) with either bistrimethylsilyl
-acetamide (BSA) or -urea (BSU) can be used to generate
stoichiometric amounts of trimethylsilyl triflate (TMSOTf) "in situ" , the system can
be used efficiently to remove triflic acid from TMSOTf and to generate catalytic
amounts of TMSOTf from TfOH for use in a range of trimethylsilyl triflate
(TMSOTf) catalysed reactions.
Similarly the interaction of fluorosulfonic acid (FSA) with either bis-trimethylsilyl
-acetamide (BSA) or -urea (BSU) can be used to generate catalytic amounts of
trimethylsilylfluorosulfonate (TMSOFs) "in situ" for use in a wide range of reactions
as an alternative to trimethylsilyltriflate (TMSOTf)
Scandium(III) trifluoromethanesulfonate and copper(II) trifluoromethanesulfonate
can be used to catalyse aromatic alkylation with arylhydroxyacetates. Scandium(III)
trifluoromethanesulfonate also proved to be a recyclable catalyst for these reactions.
A number of Pictet-Spengler cyclisation reactions were also catalysed by
Scandium(III) trifluoromethanesulfonate and copper(II) trifluoromethanesulfonate.
Mannich reactions of a number of caJix[4jresorcinarene derivatives with (R)-(+)-amethylbenzylamine
under alkaline conditions lead to the formation of single
diastereomeric tetrakis(l,3-dihydrobenzoxazine) derivatives in high yields; the
reactions using the (S)-(-)-a-methylbenzylamine afford the enantiomers. The
products react with protic acids to afford equilibrium mixtures of diastereomers
Highly diastereoselective functionalisation of calix[4]resorcinarene derivatives and acid catalysed epimerisation reactions
Mannich reactions of a number of calix[4]resorcinarene derivatives with (R)-(+)-α-methylbenzylamine under alkaline conditions lead to the formation of single diastereomeric tetrakis(1,3-dihydrobenzoxazine) derivatives in high yields; the reactions using the (S)-(-)-α-methylbenzylamine afford the enantiomers. The products react with protic acids to afford equilibrium mixtures of diastereomers.</p