The reaction of preformed mixed acetals derived from
(α-hydroxyalkyl)dimethylallylsilane with a
number of aromatic and aliphatic aldehydes in the presence of Lewis
acids results in a highly
diastereoselective intramolecular allylation reaction. The
reaction proceeds through a cyclic
synclinal SE
‘ addition of the allylsilane to an
intermediate oxocarbenium ion. The reaction occurs
exclusively by an intramolecular process as determined by means of a
cross-over experiment. The
relative stereochemistry was determined by the conversion of one of the
allylation products to a
known (stereodefined) aldol-type product. A greater degree of
diastereoselectivity is obtained by
in-situ formation of an oxocarbenium ion from
(α-hydroxyhexyl)dimethylallylsilane and an aldehyde
in the presence of boron trifluoride etherate. The
diastereoselectivity of the in-situ allylation reaction
typically exceeds 100:1 in favor of the syn adduct.
However, reactions with electron rich aryl
aldehydes resulted in a diminished degree of diastereoselectivity.
The initial product of the in-situ reaction is an unstable silyl fluoride which is readily hydrolyzed
to a silanol derivative upon
reaction with methanolic potassium hydroxide. The overall yield of
the two-step process is greater
than 80%. A method for the synthesis of more highly substituted
(α-alkoxyalkyl)dimethylallylsilanes
by allyl anion displacement of methoxide from silicon is also
described. The methyl siloxane
derivatives were obtained by ozonolytic cleavage of an unsubstituted
allyl group in methanol.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.