Esters a,P unsaturation

Here we discuss several examples of the multicomponent processes involving aminoazoles, aldehydes, and other organic components such as mercaptoacids, haloacetic acids and their ester, a,p-unsaturated imines, etc., which were not incorporated into the previous two sections of the review. 

Dipolar cycloaddition reactions between nitrile oxides and aUcenes produce 2-isoxazolines. Through reductive cleavage of the N—O bond of the 2-isoxazohnes, the resulting heterocycles can be readily transformed into a variety of important synthetic intermediates such as p-hydroxy ketones (aldols), p-hydroxy esters, a,p-unsaturated carbonyl compounds, y-amino alcohols, imino ketones and so forth (7-12). 

Examples of intramolecular transformations209 include dienone-a,(3-unsaturated ester cyclizations, e.g. (275— 276),209a>b and ester-a, p - unsaturated ester annulations, e.g. (277—>278 Scheme 91).209c Pertinent to the success of these multibond formation protocols is the selective use of the initiating carbanion (or enolates), the counter cation and the initial acceptor. 

Walphos (138), developed by Sturm at the University of Vienna and optimized at Solvias, is derived from the Ugi amine 54.174175 Walphos can be electronically fine-tuned as various phosphine groups are introduced in separate steps of the synthesis. Walphos catalysts have been used to reduce enamide esters to a-amino esters, P-keto esters, a,P-unsaturated carboxylic acids, and itaconate esters with enantioselectivities >90%.176... 

In 1962 Renson et al. reported that a variety of selenol esters can be synthesized by the reaction of the appropriate acyl chlorides with selenols in the presence of pyridine. - Selenols can easily be prepared from elemental selenium and the corresponding Grignard reagents. In addition to simple selenol esters, a,P-unsaturated selenol esters (4) and (5) and o-substituted aromatic selenol esters (6) have been obtained by this method, as shown in Scheme 1. Butyl selenol esters are generally colorless or light yellow liquids, whereas the phenyl or substituted phenyl selenol esters are white solids, which are easily purified by recrystallization. More recently other groups have used Renson s method to synthesize similar selenol esters. " ... 

In analogy to allyl halogenides, iodobenzene and other aromatic iodo derivatives can be reacted with Ni(CO)4 and acetylene at reaction temperatures above 100 °C with formation of y-ketoacids or their esters [418, 442], which may be considered as hydrolysis products of p,y-unsaturated y-lac-tones or as hydrogenation products of a,p-unsaturated y-ketoacids or -esters. a,p-Unsaturated y-ketoacids or -esters will be hydrogenated under the reaction conditions, but do not take up carbon monoxide because of the presence of electrophilic substituents. Besides allyl halogenides, allyl alcohols, -ethers and -esters may also be reacted to give unsaturated acids or esters. 

Aliphatic esters a,p-Unsaturated esters Aromatic esters... 

---