Pergamon Tetrahedron 55 (1999) 7305-7318 TETRAHEDRON Conformational Analysis of MNCB (MBNC) Esters and Amides: Promising Chiral Reagents for Stereoselective Applications Shamil K. Latypov a, Albert V. Aganov b*, Satoshi TaharaC, Yukiharu Fuknshi e* a Instituteof Organic& PhysicalChemistry, Arbuzovstr. 8, ga,an~ 420083, Tatarstan,RussianFederation b PhysicalFaculty,KaT~nState University, Kremlevskaya str. 8, 420008, ]~flT$1n~ Tatarstan,RussianFederation c Depaganent of AppliedBioscience,Facultyof Agriculture, Hokkaldo University, Kita-ku,Sappero 060, Japan. Received 1 February 1999; revised 29 March 1999; accepted 15 April 1999 Al~rgxcr: Calculations (MM, AMI/PM3, ab initio) and DNMR experiments indicate that 2-(2'- methoxy-l'-naphthyl)-3,5-dichlorobanzoi¢ acid (lVlNCB) and 2'-methoxy-l,l'-binaphthyl-2-carboxylic acid (MBNC) esters and amides exist in solution as two conformersin equilibrium: one of them (sp) has synperiplanar Ar~-Ar2 and ~ bonds, and anotherone (ap) has antiperiplanar bonds. Due to the close populationsof the forms only a part of large shieldingeffects attributedto the ap form is transformedinto observedhigh field shifts in NMR experiments.Thus, the low populationof the "ap" conformerand the low selectivityof the aryl ring anisotropic influence on the alcohol (amine)moi- ety are limitationsof the efficiencyof MNCB (MBNC) in their use as reagents for absolute stereochem- istry determinationof alcohols and amines by NMR. The way to increase the efficiency of this type of reagent has been revealed on the basis ofthase findings. © 1999 Elsevier ScienceLtd. All rights reserved. INTRODUCTION The growth of interest in stereoselective synthesis and drug design necessitates effective and reliable methods to control enantiomeric purity and absolute stereochemistry of chiral compounds. Therefore essential efforts have been directed towards developing methods to analyze optically active compounds) Up to now the NMR based method 2 is one of the most attractive ones: it is based on the derivatization of enantiomers (indistinguishable in terms of NMR) into diastereomers by corresponding chiral derivatizing reagents (CDR) followed by analysis of their NMR spectra. Since the first pioneering work of Raban and Mislow several chiral reagents were proposed and were found to be useful in determination of absolute configuration of secondary alcohols. 3 All of them contain a chiral center and an aryl ring, the asymmetric, anisotropic influence 4 of which in diastereomers leads to the dif- ferentiation of esters in NMIL Differences were interpreted using empirical models and rules for determination of absolute configuration were formulated. Later, in a similar way, several new classes of chiral compounds were studied by NMR: primary and sec- ondary alcohols, primary amines and carboxylic acids,s Recently the main factors that determine the efficiency of the method in the case of secondary alcohols were established and the basic rules for designing new reagents were formulated. 6 It was shown that sm~mes and populations of the main forms are of great importance in this method. New reagents and preferable condi- tions were proposed on the basis of these findings. At the same time, during the last years several examples of axially chiral reagent applications were dem- 0040-4020/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. Pll: S0040-4020(99)00356-7