Abstract
The easily accessible amino-acid esters were used as efficient chiral auxiliaries in a variety of cycloadditions. Thus, Schiff bases derived from amino-acid esters were employed as mediators of chirality in thermal and TiCl4-catalyzed [2 + 2]-cycloadditions with ketenes and ketene silyl acetals, respectively, in Lewis acid-catalyzed aza Diels-Alder reactions with the Brassard diene and in tandem Mannich-Michael reactions with the Danishefsky diene. Iminium ions, generated in situ in aqueous solution from amino-acid ester hydrochlorides and formaldehyde, served as chiral heterodienophiles in thermal aza Diels-Alder reactions in water. With alpha,beta-unsaturated amides of amino-acid esters, in particular proline esters, asymmetric thermal and lewis acid-catalyzed Diels-Alder reactions and 1,3-dipolar cycloadditions with nitrile oxides were performed. In general, the cycloadditions proceeded with high stereoselectivity. For the removal of the chiral auxiliary from the cycloadducts several methods were developed. The selectively substituted chiral compounds thereby liberated, i.e. beta-lactams, 2-azabicyclo[2.2.1]heptanes and -[2.2.2]octanes, piperidines, alpha,beta-unsaturated 6-membered lactames, 6-membered cyclic enaminones, bicyclo[2.2.1]heptenes, cyclohexenes and DELTA-2-isoxazolines, are versatile synthons for further syntheses, e.g. for the construction of natural products and pharmacologically interesting analogues thereof. This was, for instance, demonstrated by the synthesis of nocardicinic acid, the parent representative of the nocardicins, by the construction of the pyrrolizidine alkaloid (R)-delta-coniceine, the piperidine alkaloid (S)-coniine, and by the development of new routes to the framework of polycyclic indole alkaloids.
