CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

quantitative assessment of the 1,2-asymmetric induction. . Moreover the reaction pathway
is advantageous compared to the one starting from the Cram configuration since it directly
leads to a staggered conformation in the product.
Nu -
Nu -
22
M
O
S
R
M
R
S
O
23
L
Felkin-Anh
28
L
anti-Felkin-Anh
29
Scheme I.3.10
Many addition recations to carbonyl compounds proceed via cyclic six-membered
transition states. The course of such reactions can be understood by the Zimmerman-
Traxler model, 25 which requires the arrangement of the reaction partners in a chair like
transition state, placing the large substituents equatorially, if possible. This model is
usually strictly followed in ene, allylation, and aldol reactions.
Enneerrggyy ccoonnssi iddeerraat tioonnss inn i aassyymmeet trri icc ssyynnt thheessi iss. . Asymmetric synthesis can involve i) the
selective displacement of an enantiotopic ligand (Scheme I.3.11a) or (ii) the selective
addition of a reagent to the Re/Si-face of a π-bond (Scheme I.3.11b). 26a The energy profile
of such reactions suggests favourable formation of racemates as the transition states R # and
S # are enantiomeric and therefore, are isoenergic. As a consequence, the rate of formation
of the R-isomer is equal to that of the S-isomer, and the reaction affords a racemic mixture.
Thus, for the development of an asymmetric synthesis, the transition state must be
diastereomeric so that potential energies are different. This would ensure that the
16