1,3-butadienyl ethers, cycloaddition

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... 

A Study of the stereochemical results of cycloaddition reactions between butyl glyoxylate and IfijS-butadienyl ethers containing different sugar units revealed that the proportion of stereoisomeric products formed depends more on the preferential approach of the carbonyl reagent to one of the faces of the butadienyl system than on the endo-addition rule. 

Butyl glyoxylate reacts normally only with l ,3-butadienyl ether 289. However, David and co-workers have found that with the more active dienophile, diethyl mesoxalate, lZ,3-butadienyl ether (290) could also enter into cycloaddition, although the reaction was appreciably slower. The resulting cycloadducts 293 and 294 were decarboethoxylated according to the Krapcho method, furnishing the a-o (295) and a-L (296) as the main products. 

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