CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

The new stereogenic centre formed at a diastereotopic site is controlled by the nearby
stereogenic unit. A good example is provided by the conjugate addition of enoate 10 in
presence of vinylmagnesium bromide and CuI, leading to adduct 11 in moderate yield
(Scheme I.3.2). 17
NBn 2
i
NBn 2
H 3 C
CO 2 Et
H 3 C
CO 2 Et
10 11
i) vinylmagnesium bromide, CuI, TMSCl, HMPA, THF, -78 o C.
Scheme I.3.2
A disadvantage of this method is based on the fact that this method deals with mere
addition of an additional stereogenic unit to an already enentiomerically pure substrate, and
hence an enantiomerically pure substrate is necessary.
b) Auxilliary controlled methods.
In this method, the ‘chiral auxiliary’ is deliberately attached to an achiral substrate
in order to direct the reaction of an incoming group and can be removed once it has served
its purpose. This approach leads to the formation of a mixture of diastereomers because of
the presence of an additional stereogenic centre of the auxiliary. After removing the
auxiliary, the final product is obtained in very high enantiomeric excess. An example is the
diastereoselective alkylation of propionic acid (12) via its amide formation with ([1S, 2S]-
(+))-Pseudoephedrine to give 15 in >97% ee (Scheme I.3.3). 18
10