Achiral Amine

The [3+2]-cycloaddition or [2+2]-cycloaddition of OSO4 and alkene followed by rearrangement forms the cyclic osmate ester. Tertiary amines (achiral ligand) such as 4-dimethyl-aminopyridine (DMAP) or pyridine (Py) accelerate the addition reaction (Scheme 7.24). 

Scheme 43.26 Asymmetric domino double Michael addition catalyzed by chiral secondary amine/achiral Bronsted base.

Glycine is the simplest ammo acid and the only one m Table 27 1 that is achiral The a carbon atom is a chirality center m all the others Configurations m ammo acids are normally specified by the d l notational system All the chiral ammo acids obtained from proteins have the l configuration at their a carbon atom meaning that the amine group IS at the left when a Fischer projection is arranged so the carboxyl group is at the top... 

Nitrogen chirality may also be produced by the action of an achiral peroxyacid on a Schiff base containing a chiral amine (75JOC3878). In this case the oxaziridine contains a configurationally known centre of chirality relative to this, absolute configurations of the centres of chirality at nitrogen and carbon, and thus the complete absolute configuration of the molecule, can be determined (see Section 5.08.2.2). 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

In another example of enantioselective distillation, it was the enantiomeric mixture to resolve itself which contributed to create a chiral environment. Thus, non-racemic mixtures of a-phenylethylamine were enantiomerically enriched by submitting to distillation different salts of this amine with achiral acids [199]. 

To understand how this method of resolution works, let s see what happens when a racemic mixture of chiral acids, such as (+)- and (-)-lactic acids, reacts with an achiral amine base, such as methylamine, CH3NH2. Stereochemically, the situation is analogous to what happens when left and right hands (chiral) pick up a ball (achiral). Both left and right hands pick up the ball equally well, and the products—ball in right hand versus ball in left hand—are mirror images. In the same way, both ( H- and (-)-lactic acid react with methylamine equally... 

The Gabriel synthesis represents another indirect but highly valuable approach to amines. Trost has demonstrated a method for the asymmetric ring-opening of butadiene monoepoxide by use of one equivalent of phthalimide, 7t-allylpalladium chloride dimer, and the chiral bisphosphine 22 (Scheme 7.37). The dynamic kinetic asymmetric transformation proceeded through a putative achiral intermedi-... 

Simple diastereoselection has also been reported for Strecker syntheses using pentanedial or butanedial with primary amines or hydrogen cyanide which affords 2,5-dicyanopiperidine or 2,5-dicyanopyrrolidine derivatives, respectively35. In both cases, the achiral svn-diastereomers are slightly preferred. At 70 °C, the pure rA-compounds can be partially transformed to the /rfflM-isomcrs. 

High enantioselectivities may be reached using the kinetic controlled Michael addition of achiral tin enolates, prepared in situ, to a,/i-unsaturated carbonyl compounds catalyzed by a chiral amine. The presence of trimethylsilyl trifluoromethanesulfonate as an activator is required in these reactions236. Some typical results, using stoichiometric amounts of chiral amine and various enolates are given below. In the case of the l-(melhylthio)-l-[(trimethylsilyl)thio]ethene it is proposed that metal exchange between the tin(II) trifluoromethanesulfonate and the ketene acetal occurs prior to the 1,4-addition237,395. 

A number of (S)- or (R)-2-aminoamides were independently obtained by running reactions between chiral 2-bromoamides 1 and an achiral primary or secondary amine, in both sets of conditions (ref. 5). Accordingly, either a diastereoisomeric mixture or a single diastereoisomer in high diastereoisomeric excess, resulted respectively when an (S,R)- or (S)-2-bromoamide is treated with a chiral amine, again in the two sets of conditions. 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

Influence of Achiral Tertiary Amines on the Enantioselective Hydrogenation of a,p-diketones Over Cinchonidine-Pt/Al203 Catalyst... 

Enantioselective hydrogenation of 2,3-butanedione and 3,4-hexanedione has been studied over cinchonidine - Pt/Al203 catalyst system in the presence or absence of achiral tertiary amines (quinuclidine, DABCO) using solvents such as toluene and ethanol. Kinetic results confirmed that (i) added achiral tertiary amines increase both the reaction rate and the enantioselectivity, (ii) both substrates have a strong poisoning effect, (iii) an accurate purification of the substrates is needed to get adequate kinetic data. The observed poisoning effect is attributed to the oligomers formed from diketones. 

Pt/Al2C>3-cinchona alkaloid catalyst system is widely used for enantioselective hydrogenation of different prochiral substrates, such as a-ketoesters [1-2], a,p-diketones, etc. [3-5], It has been shown that in the enantioselective hydrogenation of ethyl pyruvate (Etpy) under certain reaction conditions (low cinchonidine concentration, using toluene as a solvent) achiral tertiary amines (ATAs triethylamine, quinuclidine (Q) and DABCO) as additives increase not only the reaction rate, but the enantioselectivity [6], This observation has been explained by a virtual increase of chiral modifier concentration as a result of the shift in cinchonidine monomer - dimer equilibrium by ATAs [7],... 

Amines where one of the four groups is a nonbonding electron pair are achiral... 

Sn(OTf)2 can function as a catalyst for aldol reactions, allylations, and cyanations asymmetric versions of these reactions have also been reported. Diastereoselective and enantioselective aldol reactions of aldehydes with silyl enol ethers using Sn(OTf)2 and a chiral amine have been reported (Scheme SO) 338 33 5 A proposed active complex is shown in the scheme. Catalytic asymmetric aldol reactions using Sn(OTf)2, a chiral diamine, and tin(II) oxide have been developed.340 Tin(II) oxide is assumed to prevent achiral reaction pathway by weakening the Lewis acidity of Me3SiOTf, which is formed during the reaction. 

The 1,2-diol is liberated easily from cyclic osmate ester by either reductive or oxidative hydrolysis.213 Importantly, the ligand acceleration has been utilized extensively for the production of chiral 1,2-diols from (achiral) olefins using optically active amine bases (such as L = dihydroquinidine, dihydroquinine and various chiral diamine ligands).215... 

Phosphoramidite ligands based on TADDOL (36) and on D-mannitol (37) [74] have also been used (Scheme 28.11). However, the enantioselectivities reported for the hydrogenation of a-dehydroamino acids and itaconates were generally lower compared to the ligands based on BINOL. A different strategy is the use of ligands 38a-g based on the achiral diol catechol, and chiral amines [75]. 

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