THE RELATIVE STEREOCHEMISTRY OF SOME α-METHYL-β-SILYLCARBONYL COMPOUNDS PRODUCED BY THE DIASTEREOSELECTIVE ALKYLATION OF β-SILYLENOLATES

Article abstract of DOI:10.1016/0022-328X(84)85181-5

Conjugate addition of lithium bis(dimethylphenylsilyl)cuprate to methyl cinnamate (1), to 4-phenylbut-3-en-2-one (4), and to dec-3-en-2-one (15), followed by methylation of the intermediate enolate, gives largely one diastereoisomer of the α-methyl-β-silylcarbonyl compound (the β-silyl ester (3), 3-methyl-4-dimethyl(phenyl)silyl-4-phenylbutan-2-one (5), and 4-dimethyl(phenyl)silyl-3-methyldecan-3-one (16), respectively).The relative configurations of the two chiral centres in these products are proved to be (RR,SS), (RR,SS), and (RS,SR), respectively, by conversion of the ester 3 into the ketone 5, and by Baeyer-Villiger oxidation of the ketone 5, its diastereoisomer (RS,SR)-3-methyl-4-dimethyl(phenyl)silyl-4-phenylbutan-2-one (8), and the ketone 16 to the corresponding acetates ((RR,SS)-1-phenyl-1-dimethyl(phenyl)silylprop-2-yl acetate (9), (RS,SR)-1-phenyl-1-dimethyl(phenyl)silylprop-2-yl acetate (12), and (RR,SS)-3-dimethyl(phenyl)silylnon-2-yl acetate (17)).Fluoride ion-catalysed elimination of the silyl and acetate groups is not stereospecific when the silyl group is benzylic (9 and 12), but is stereospecifically anti for the saturated acetate (17).Reduction of the acetates 9, 12 and 17 followed by syn-Peterson elimination gives the alkenes E-phenylpropene (11), Z-phenylprop-1-ene (14) and E-non-2-ene (19).