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Description:
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Asymmetric hydrogenation as a general route to polypropionates has been
explored for allylic alcohols , acids , and derivatives , which has led to the generation of
2 ,4 -dimethylated hexane derivatives . Quantitative yields in most cases and
enantioselectivities greater than 98 % were obtained . A remarkable stereofacial inversion
was observed when an ester or acid was present in the allylic position instead of an
alcohol or alcohol derivative . This led to the construction of all four diastereomers of
the hexanol series from a single enantiomer of hydrogenation catalyst . Also described
are an attempted synthesis of ( - ) -lardolure , a formal synthesis of the methyl ester portion
of the preen gland extract from the domestic goose Anser anser , and a total synthesis of
an extract from the fungi A . cruciatus . The synthesis of these compounds demonstrated
shortcomings of the known catalyst system showing enantioselectivities for
polymethylated compounds was high , while diastereoselectivity was low . Methodology
to develop new N -heterocyclic carbene catalysts was developed using the cyanide
coupling of aldimines to generate electronically tunable 1 ,3 ,4 ,5 -tetraaryl complexes , and
X -ray , IR , and calculations were used to elucidate their electronic characteristics . These
studies indicate that the 4 ,5 -positions have as great an influence on the metal -ligand
bond as the 1 ,3 -positions . In addition , they are among the most electron -donating 2 -
metalated N -heterocyclic carbenes found thus far . An intrinsic relationship between
catalytic activity and electron donating ability was found in transfer hydrogenations . |