B

toluene:CHCl3 7:3,

50%KOH, -20 °C

ryl

(S,S)-45a (Ar = 3,4,5-F3C6H2)

(0.2-1% mol)

toluene,50%KOH

PhR H

Scheme 2.26 Representative examples of phase transfer catalysts (a) and action mechanism (b)

several a-aminoacids [158] and natural products in high optical purity and a beneficial effect on the selectivity was evidenced when electron-withdrawing substituents were present in the molecular structure of the catalysts (Scheme 2.26a). The extension of such reaction to the preparation of a,a-dialkyl-a-aminoacids (Scheme 2.27a)[159] and modified peptides [160] afforded unnatural derivatives potentially useful as enzyme inhibitors or in mechanism elucidation of enzymatic reactions and sensible rate acceleration was sometimes achieved by addition of achiral crown ethers as cocatalysts, for their known ability to extract metal cations [161]. In order to avoid the need of two different chiral binaphtyl moieties in catalysts of type 45, a new generation of onium salts has been developed through the introduction of selected substituent on the 3,3'-positions of binaphthyl scaffold and modification of the amine functionality [162]. Novel derivatives 46 have been reported as effective chiral phase transfer catalysts for

Br"

(S)-46a X = (S)-46b X = (s)-46c X =

N, R P, R N, R

= Bu, Ar = = Bu, Ar = = Me, Ar

3,4,5-F3-C6H2 3,4,5-F 3-C6H2 2,4-(p-CF3Ph)2-C10Hs

(S)-46d X =

N, R

=^_s

Ar = [3,5-(CF3)2C6H3]2C(OH)

citric acid

YV ^fBu + RBr Ph Me citric acid

toluene, CsOH (5 eqv)/H2O, R = Et, Bn, allyl -20 °C

Of Bu

0 0

Post a comment