Enantioselective Diels-Alder reaction of ( E )-1-trimethylsilyloxy-1,3-butadiene with chiral glyoxylates
Enantioselective Diels-Alder reaction of (E)-1-trimethylsilyloxy-1,3-butadiene (I) with chiral (−)-menthyl (II) and (−)-8-phenylmenthyl (III) glyoxylates in various solvents without or with catalysts was studied. The reaction gave a mixture of trans- and cis-isomers of (−)-menthyl (IV) and (−)-8-phenylmenthyl (V) 2-trimethylsilyloxy-5,6-dihydro-2H-pyran-6-carboxylates. The regioselectivity of the reaction was explained by quantum-chemical calculations, the enantioselectivity was determined using 13 C NMR spectroscopy and the absolute configuration of the addition products was assigned on the basis of chemical correlation with (S)-(−)-dimethyl malate