Ruthenium-catalyzed enyne metathesis in stereoselective preparation of cyclic 1-amino-1-carboxylic acids

The Schöllkopf chiron and transition metal mediated reactions, a powerful combination for stereoselective construction of cyclic α-quaternary-α-amino acid derivatives

The focus has been on the development of methodology for stereoselective preparation of spiroannulated intermediates of the Schöllkopf chiron and further transformations to cyclic alpha-amino acids. The spiroannulations are effected by Ru(II)-catalysed ring-closing metathesis reactions, by Ru(II)- and Pd(0)-catalysed cycloisomerisations, by Rh(II)-carbenoid cyclisation reactions and by intramolecular aldol condensations. Hydrolytic reactions of the spirane intermediates have provided several groups of highly novel and functionalised five-, six- and seven-membered cyclic alpha-quaternary-alpha-amino acid derivatives as well as alicyclic derivatives. The novel cyclic amino acid derivatives can be regarded as cyclic constrained analogues of corresponding common amino acids, or in some cases as intermediates for further preparation of such amino acids. Some emphasis has been on the preparation of cyclic serine analogues. Major efforts have been on the preparation of cyclic alpha-quaternary bis(alpha-amino acid) derivatives as conformationally constrained dicarba-analogues of cystine.

Enantiomeric impurities in chiral synthons, catalysts, and auxiliaries. Part 3

The enantiomeric excess of chiral reagents used in asymmetric syntheses directly affects the reaction selectivity and product purity. In this work, 84 of the more recently available chiral compounds were evaluated to determine their actual enantiomeric composition. These compounds are widely used in asymmetric syntheses as chiral synthons, catalysts, and auxiliaries. These include chiral alcohols, amines, amino alcohols, amides, carboxylic acids, epoxides, esters, ketones, and oxolanes among other classes of compounds. All enantiomeric test results were categorized within five purity levels (i.e. <0.01%, 0.01% to 0.1%, 0.1% to 1%, 1% to 10%, and >10%). The majority of the reagents tested were determined to have enantiomeric impurities over 0.01%, and two of them were found to contain enantiomeric impurities exceeding the 10% level. The most effective enantioselective analysis method was a GC approach using a Chiraldex GTA chiral stationary phase (CSP). This method worked exceedingly well with chiral amines and alcohols.