Residual stereoisomerism

Accordingly for 63, residual stereoisomerism expresses itself as residual enantiomerism — observable here in the form of residual optical activity — as long as the intramolecular motions are restricted to the low energy Berry process. However, once the high energy Berry modes, passing through conformations 66 and 67, come into existence, any residual stereoisomerism will be annihilated, concomitant, of course, with total racemization. 

Guenzi, A., Johnson, C.A., Cozzi, F., and MizJow, K. Dynamic gearing and residual stereoisomerism in labeled bis(9-triptycyl)methanc and related molecules. Synthesis and... 

Hosseini, M. W., Lehn, J.-M., and Mertes, M. P. Efficient molecular catalysis of ATP-hydrolysis by protonated macrocyclic polyamines. Helv. Chim. Acta 1983, 66, 2454. Guenzi, A., Johnson, C.A., Cozzi, F., and Mizlow, K. Dynamic gearing and residual stereoisomerism in labeled bis(9-triptycyl)methane and related moicciiles. Synthesis and... 

Johnson C.A., Guenzi A, and Mislow K. 1981. Restricted gearing and residual stereoisomerism in bis(l,4-dimethyl-9-triptycyl)methane. /.Am. Chem. Soc. 103 6240-6242. 

Stereoisomerism in either the alkamine nucleus or the acyl residue has a considerable effect on the pharmacological action of the tropeines and cocaines. Differences in activity of tropine and i/i-tropine and their benzoyl derivatives have been mentioned already, and there seems to be a consensus of opinion that the i/i-cocaines (alkyl- or aryl- acyl esters of 0-ecgonine) are less toxic and more potent local anfesthetics than the corresponding cocaines, derived from 1-ecgonine. ... 

The challenge lay in the stereochemicaUy correct synthesis of the polyketide part of the molecule. Starting from L-serine (89) (Chart 6) by C2-elongation steps, reduction of the obtained keto functions including adequate protection and deprotection, and introduction of the salicylic acid residue the four stereoisomeric 3,5-diols (90) were obtained. Comparison of the H-NMR data with those of anachelin (10) showed that the isomer with 3R,5S,6S) configuration was the correct starting material. 

Each macrocycHzahon could, in principle, lead to a mixture of diastereoisomers, depending on how the EtOOC residues at the two methano bridge C atoms are oriented with respect to each other (in-in, in-out, and out-out stereoisomerism) [92]. Usually, only out-out stereoisomerism has been observed so far. One exception is the in-out isomer 81. 

Each of the scaffolds reported in Scheme 24 can be used for the production of a stereo-isomeric sublibrary based on the appropriate peptide sequence. For example, with the sequence A-B-C-D-E and scaffold (1) two types of stereoisomeric sublibraries can be prepared. One type includes the sublibraries A and B of Scheme 26 in which within a given sequence the configuration of each residue is successively inverted thus, retaining the identical connectivity as in the parent linear peptide. In the second type 265 of sublibraries C and D (Scheme 26) the direction of the amide bond is inverted and hence the connectivity is not maintained. In most members of these sublibraries the overall conformation of the scaffold is maintained and therefore these components constitute stereoisomeric sublibraries of the parent library. Conversely, by introducing amide bond surrogates such as reduced amide bonds1465 or thioamide bonds 260,466 the conformation of the scaffolds are changed and their conformational flexibility enhanced. 

Bioactive sequences of up to six amino acid residues known to assume (1- or "/-turns in the bioactive conformation are suitable for such libraries. If the sequence is short, residues have to be added in a manner to retain the desired physicochemical properties of the peptide (e.g., to short polar active sequences hydrophobic residues are preferentially added and vice versa). The choice of the scaffold depends on the number of structure-inducing amino acids such as Gly or Pro present in the native sequence. In absence of such residues scaffolds (1) or (4) (Scheme 24) are selected, whereas if Gly or Pro is present alternative scaffolds can be considered. Then the components of the four stereoisomeric sublibraries of Scheme 26 (or their equivalents if other scaffolds are chosen) are synthesized according to procedures described in the preceding sections. 

Nearly all biological compounds with a chiral center occur naturally in only one stereoisomeric form, either d or L. The amino acid residues in protein molecules are exclusively L stereoisomers. D-Amino acid residues have been found only in a few, generally small peptides, including some peptides of bacterial cell walls and certain peptide antibiotics. 

The 20 amino acids commonly found as residues in proteins contain an a-carboxyl group, an a-amino group, and a distinctive R group substituted on the a-carbon atom. The a-carbon atom of all amino acids except glycine is asymmetric, and thus amino acids can exist in at least two stereoisomeric forms. Only the... 

Further model-building experiments have shown that an a helix can form in polypeptides consisting of either L- or D-amino acids. However, all residues must be of one stereoisomeric series a D-amino acid will disrupt a regular structure consisting of L-amino acids, and vice versa. Naturally occurring L-amino acids can form either right- or left-handed a helices, but extended left-handed helices have not been observed in proteins. 

This residue is a mixture of stereoisomeric dicyclohexyl-18-crown-6 polyethers which may be contaminated with some unchanged dibenzo-18-crown-6 polyether and with alcohols arising from hydrogenolysis of the polyether ring. The submitter reports that this residue is sufficiently pure for many purposes such as the preparation of complexes with potassium hydroxide which are soluble in aromatic hydrocarbons. 

The formation of methyl (S)-thiiranecarboxylate (5) is of interest,3 since in this case the leaving group is a diazo residue originating from an amino group. The starting material is therefore the chiral amino acid ester, methyl (R)-cystein-ate, and the stereoelectronic requirements of the reaction lead to an optically pure product of predictable stereoisomeric form (Expt 8.2). 

This reaction has now been applied to a very great number of substituted allylic alcohols, and the mechanistic and stereoisomeric features of the reaction are becoming clearer.11 In broad outline, it would appear that the initial step is an alkoxy-exchange reaction between two alkoxy residues in the titanium complex and the two hydroxyl groups in the tartrate ester, thus ... 

After ozonation, the solvent usually was removed at 30 °C under rotation. The residue was either distilled in vacuo or crystallized or purified by column chromatography on silica gel. n-Pentane, with increasing amounts of ether (up to 1.5% ), served for the elution. Those mixtures of stereoisomeric ozonides which could not be separated pre-paratively were analyzed by their NMR spectra using a Varian A 60 spectrometer. The results are summarized in Tables II—V. More details can be found in Ref. 3. 

Many enzymes show stereoisomeric specificities. For example, human a-amylase catalyzes the hydrolysis of glucose from the linear portion of starch but not from cellulose. Starch and cellulose are both polymers of glucose, but in the former the sugar residues are connected by q (1 4) linkages, whereas in the latter they are con-... 

Ribonucleosides (47) or synthetic analogs (48) containing a )3-d-glucopyranosyl, instead of the /J-D-ribofuranosyl, residue are amenable to periodate cleavage of their sugar moieties, to give nucleoside dialdehydes (49). Nitromethane cyclization of such dialdehydes proceeds well, and affords stereoisomeric mixtures of (3-deoxy-3-nitro-/3-D-hexopyranosyl)pyrimidines or (3-deoxy-3-nitro-/3-D-hexopy-ranosyl)purines (50). 

The key processes which controlled the stereochemistry were as follows. Firstly, the catalytic reduction of (36) gave two stereoisomeric products, of which (37), in which hydrogen had been added on the side opposite to the bulky indole residue and cis [thus ensuring that hydrogens at C(21), C(20), and C(15)... 

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