MATRIX SYNTHESIS OF CHIRAL CARBOXY DERIVATIVES - ALDEHYDE CHIRAL DISCRIMINATION DUE TO A 2-POINT COORDINATION OF MG2+

To control stereoselectivity in aldol-like reactions with chiral carbo hydrate templates, we studied the interaction between completely prote cted dialdo compounds and magnesium enediolates of arylacetic acids. D iastereomeric mixtures of the highly functionalized acids obtained wer e esterified to isolate individual methyl uronates. It was found that all the diastereomeric esters exhibit Cotton effects of the same posit ive sign in the 220-230 nm region and so possess the same S configurat ion of the aryl chiral center C(6). Chiral center C(5) configurational assignments were performed using IR and ORD spectroscopy. We separate d and specified four pairs of diastereomeric methyl uronates. It follo ws that the precursory acids have the same 5R, 6S (major isomers) and 5S, 6S (minor isomers) configurations. A tentative mechanism for com plexation and possible models of Mg2+-protected dialdose intermediate complexes has been proposed. We have concluded that a kind of orbital steering is realized, accompanied by some ''tuning'' of molecular asse mbly conditioned by two-point coordination between Mg2+ and potential cation-binding sites in the substrate molecules. Thus it has been demo nstrated that reasonable diastereo-selectivity can be achieved even th rough the use of small matrix molecules using rather small functional groups, which do not impose any stringent steric requirements. (C) 199 3 Wiley-Liss, Inc.