Carbocyclic α,β-diamino acids: asymmetric Strecker synthesis of stereomeric 1,2-diaminocyclohexanecarboxylic acids

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.

A diastereo- and enantioselective synthesis of alpha-substituted anti-alpha,beta-diaminophosphonic acid derivatives

Highly diastereo- and enantioselective additions of alpha-nitrophosphonates to imines catalyzed by a chiral Brønsted acid are described.

A diastereo- and enantioselective synthesis of alpha-substituted syn-alpha,beta-diamino acids

Highly diastereo- and enantioselective additions of substituted alpha-nitroesters to imines have been developed. High diastereoselection relies on the finding that the combination of chiral proton catalyst 2b and alpha-nitro aryl esters bearing 2,6-disubstitution combine to raise substrate-controlled diastereoselection to >20:1 in favor of the syn diastereomer. Furthermore, the chiral catalyst provides enantioselection to the 99% level through control of the addition step in which the azomethine pi-faces are differentiated. The bifunctional chiral protic acid catalyst enables these reactions to proceed without separate preactivation of either substrate, leading to a straightforward synthetic protocol for the formation of alpha,beta-diamino phenyl alanine derivatives.

Stereoselective Quaternization of α-Amino Phenylacetonitriles Mediated by a Remote Sulfinyl Group

Enantiomerically pure alpha-substituted alpha-amino phenylacetonitriles have been readily prepared from 2-p-tolylsulfinylbenzaldimines following a two-step sequence: a moderately stereoselective hydrocyanation of the imines and a completely stereoselective quaternization of the resulting diastereoisomeric mixture of alpha-amino phenylacetonitriles with different alkylating or acylating reagents in the presence of KHMDS. Theoretical calculations support a stereoselectivity control exerted by the remote sulfinyl group, as long as it is responsible for the conformational preferences of the benzyllithium intermediates, which suffer the attack of the electrophiles to the less hindered diastereotopic face.

Bicyclic glutamic acid derivatives

For the second-generation asymmetric synthesis of the trans-tris(homoglutamic) acids via Strecker reaction of chiral ketimines, the cyanide addition as the key stereodifferentiating step produces mixtures of diastereomeric alpha-amino nitrile esters the composition of which is independent of the reaction temperature and the type of the solvent, respectively. The subsequent hydrolysis is exclusively achieved with concentrated H(2)SO(4) yielding diastereomeric mixtures of three secondary alpha-amino alpha-carbamoyl-gamma-esters and two diastereomeric cis-fused angular alpha-carbamoyl gamma-lactams as bicyclic glutamic acid derivatives, gained from in situ stereomer differentiating cyclisation of the secondary cis-alpha-amino alpha-carbamoyl-gamma-esters. Separation was achieved by CC. The pure secondary trans-alpha-amino alpha-carbamoyl-gamma-esters cyclise on heating and treatment with concentrated H(2)SO(4), respectively, to diastereomeric cis-fused angular secondary alpha-amino imides. Their hydrogenolysis led to the enantiomeric cis-fused angular primary alpha-amino imides. The configuration of all compounds was completely established by NMR methods, CD-spectra, and by X-ray analyses of the (alphaR,1R,5R)-1-carbamoyl-2-(1-phenylethyl)-2-azabicyclo[3.3.0]octan-3-one and of the trans-alphaS,1S,2R-2-ethoxycarbonylmethyl-1-(1-phenylethylamino)cyclopentanecarboxamide.

Rigid nonproteinogenic cyclic amino acids as ligands for glutamate receptors:trans-Tris(homoglutamic) acids

The second-generation asymmetric synthesis of the trans-tris(homoglutamic) acids reported herein proceeds via Strecker reaction of chiral ketimines, obtained from condensation of racemic 2-ethoxycarbonylmethylcyclopentanone and commercially available (S)- and (R)-1-phenylethylamine, respectively. In the key stereodifferentiating step, the cyanide addition leads to mixtures of diastereomeric alpha-amino nitrile-esters, the composition of which is independent of the reaction temperature and the type of the solvent, respectively. Hydrolysis of the alpha-amino nitrile-esters with concentrated H(2)SO(4) yielded diastereomeric mixtures of secondary alpha-amino amido-esters, which after separation were hydrogenolyzed and hydrolyzed each to the enantiomeric trans-1-amino-2-carboxymethylcyclopentanecarboxylic acids. Their configuration was completely established by NMR methods, CD spectra, and X-ray analysis of the trans-1S,2R-configured secondary alpha-amino amido-ester. In receptor binding assays and functional tests, trans-1S,2R-1-amino-2-carboxymethylcyclopentanecarboxylic acid hydrochloride was found to behave as a selective mGluR(2)-antagonist without relevant binding properties at iGluRs.

Highly Stereoselective Addition of Organometallic Reagents to N-tert-Butanesulfinyl Imines Derived from 3- and 4-Substituted Cyclohexanones

Addition of alkyl or aryl Grignard reagents to N-sulfinyl imines derived from 3- and 4-substituted cyclohexanones proceeds with good yields and with excellent diasteroselectivity. The selectivity of the reaction is controlled by the ring substituent rather than the sulfinyl group stereochemistry, and therefore racemic tert-butanesulfinamide can be employed.