Chemistry Reference
In-Depth Information
Part I
Carbohydrate Auxiliaries
General Remarks on the Use of Pseudo-Enantiomers of Carbohydrate Tools
The application of pseudo-enantiomers is of great importance when carbohydrat-
ederived chiral tools are used: While d-monosaccharides are easily available from
the
chiral pool
, the corresponding l-enantiomers are mostly expensive and in some
cases even unavailable. For the preparation of a pseudo-enantiomer of a given
carbohydrate tool, a carbohydrate scaffold with opposite configuration at relevant
stereocenters is chosen. These relevant stereocenters are usually those directly
involved in the events determining the direction of the asymmetric induction, that
is, the one(s) carrying the substrate (in the case of carbohydrate auxiliaries), coor-
dinating metal centers (in the case of carbohydrate ligands), or shielding one face
of a substrate. The remaining stereocenters, which are further from the reacting
sites, are neglected and may have any configuration. Thus the synthesis of a
pseudo-enantiomeric tool can start from other l-monosaccharides, which are more
readily available, that is, l-rhamnose and l-fucose and even d-carbohydrates may
be employed. By this approach the preparation of a real enantiomer from an
expensive l-enantiomer of a d-carbohydrate can be avoided altogether. Attractive
and powerful as this approach may be it is important to note that choosing suitable
a pseudo-enantiomeric auxiliary offering high levels of stereoinduction is by no
means trivial. Usually, several tentative pseudo-enantiomers can be envisioned for
a given carbohydrate tool but which of them - if any - gives high levels of stereoin-
duction cannot, unfortunately, be predicted. Therefore, finding suitable pseudo-
enantiomers remains a process of trial and error. With this in view, unsurprisingly,
some highly efficient carbohydrate tools have remained without any suitable
pseudo-enantiomer (e.g., the Duthaler-Hafner reagent, Chapter 7).
