We were unable to find an efficient stereochemistry-controlled synthe c route to diester 2 exo,endo and, therefore a empted to isolate the desired stereoisomer from the diastereomeric mixtures of diacids 7 or corresponding dimethyl ethers. We conducted KOH-mediated hydrolysis of dinitrile 6 endo,endo in MeOH that led to the forma on of three diastereomers of acid 7 in an approximate ra o of exo,endo:endo,endo:exo,exo = 4:2:1 (according to H NMR analysis see below). Important, that such a mixture was constantly and reproducibly formed from 6 endo,endo dinitrile under the condi on above. A empts to chromatographically separate the parent acid and its deriva ves were unsuccessful. However, frac onal crystalliza on of the mixture 7 endo,endo and 7 exo,exo diacids from MeCN (required five repe ons) provided 7 exo,endo diastereomer in a prepara ve yield of 16%. The structure of this compound was also confirmed by XRD crystallography, and the esterifica on yielded the desired diester 2 exo,endo (Scheme 3).