Determination of the enantiomeric excess of α-hydroxy acids

Chiral Linker 1. A Novel Chiral Auxiliary Derived from m-Hydrobenzoin as Enantiomerically Pure Linker for Solid Phase Organic Chemistry

“…The influence of LiCl and HMPA, resp., as additives was investigated as well (entries 3, 4). The stereochemical outcome of each reaction was analysed by integration of signals of 1 H NMR spectra of the crude reaction mixture as well as after saponification with LiOH by measuring the optical rotation of the resulting 2-methyl-3-phenylpropionic acid 8 [15] and HPLC-analysis of its L-valine methyl ester derivative [18]. As shown in Table 1, m-hydrobenzoin derived test system 5 induces chirality up to 31-36% de using LDA (entry 1) and 20% de using less basic LHMDS as deprotonating agent (entry 2), resp.…”

“…The loading of the auxiliary attached onto the resin was determined gravimetrically by evaluation of the mass increase of the resin after the binding step and verified by the amount of recovered methyl acetal in the following step when the chiral protecting group was cleaved off the resin. and HPLC-analysis of its L-valine methyl ester derivative [18] After esterification of 10, the resulting test substrate resin 11 was benzylated according to the procedure used above. The stereochemical outcome was again analyzed after Employing recovered resin 10 without any further treatment in a second reaction-cycle (esterification and alkylation) provided 8 without any decrease in the resulting stereoselectivity (entry 6).…”

“…All steps on solid support were monitored by FT-IR spectroscopy employing KBr disks. and HPLC-analysis of its L-valine methyl ester derivative [18] …”

Chiral linker 5: scope and limitations of arylsubstituted m-hydrobenzoins as solid supported open chain chiral auxiliaries for diastereoselective syntheses

“…The stereochemical outcome of the reactions was determined via 1 H NMR of the crude reaction mixtures as well as after saponification via specific rotation measurements of the acids obtained 8c,9 and HPLC-analysis of derivatives accessible by conversion with L-valine methyl ester (Scheme 3). 10 Table 1 shows the results of the diastereoselective alkylations of esters 8a-8e in comparison to selected results of previous investigations already published. Additionally, the coupling constants of the two benzylic hydrobenzoin protons in the substrates 8a-8f are shown as well, following the interpretation model stated in case of the stereoselective reduction of the corresponding phenyl glyoxylates.…”