English to Russian Tech/Engineering Chemistry; Chem Sci/Eng

Здесь будет "но где" или "за исключением где"?

Accordingly, groups (pairs) of compounds exist (e.g., compounds 1 and 4; 2 and 5; 3 and 6; 10 and 11; 14 and 15; 18 and 21; 29 and 30; 34 and 42; and 39 and 42) where a single diastereomer was isolated and tested, ***but where*** the absolute stereochemistry about the “*” is arbitrarily defined.