Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 314

Carbohydrates are readily available in enantiopure form. They and their deriva-
tives have been widely utilized in asymmetric synthesis as chiral starting materials,
chiral auxiliaries, chiral reagents, and chiral ligands for metal catalysts. In the past
decade, carbohydrate-based ketones [9-12, 14, 15] have been shown to be highly
effective organocatalysts for asymmetric epoxidation of olefins with very broad
substrate scope. The hydroxyl groups can be used as handles for chiral controlling
elements and as precursors to carbonyl groups. In 1996, d-fructose-derived ketone
1 (Scheme 15.2) was reported to be a highly effective epoxidation catalyst for a
wide variety of trans - and trisubstituted olefins [46, 47]. Ketone 1 can be readily
synthesized in large quantities from d-fructose by ketalization with acetone and
oxidation of the resulting alcohol (Scheme 15.2) [47-49]. Efforts have also been
made to develop a synthesis of l-fructose from l-sorbose to allow easy access to
the enantiomer of ketone 1 ( ent - 1, Scheme 15.3) [50, 51]. Alcohol 2, obtained from
the ketalization of l-sorbose with 2,2-dimethoxypropane and SnCl 2 , is converted
into mesylate 3, which undergoes a series of transformations in one pot to furnish
l-fructose. Ent - 1 can be readily synthesized from l-fructose via ketalization and
oxidation.

Scheme 15.2 Synthesis of ketone 1.

Scheme 15.3 Synthesis of ent - 1.

A proposed catalytic cycle for ketone 1 -mediated epoxidation is outlined in
Scheme 15.4. Ketone-mediated epoxidations are often sensitive to the reaction pH
and are usually carried out at pH 7-8 since Oxone autodecomposes readily at high
pH [52, 53]. However, ketone 1 was found to rapidly decompose in the presence
of Oxone at pH 7-8, presumably via Baeyer-Villiger oxidation to form lactones 8