Lipase‐Catalyzed Acetylation of 3‐Substituted 2,3‐Dihydro‐1 H ‐1,4‐benzodiazepin‐2‐ones: Effect of temperature and conformation on enantioselectivity and configuration

Enantioselectivity of acetylation by vinyl acetate/AcOEt catalyzed by immobilized Candida antarctica lipase ( Novozym 435 ) is studied for rac ‐3‐(hydroxymethyl)‐1,4‐benzodiazepin‐2‐ones 7 , 9 , 14 ( n = 1; number of CH 2 groups in the chain at C(3)), 20 ( n = 2), and for prochiral 3,3‐bis(hydroxymethyl) derivative 16 . Enantiomeric excess (ee [%]) is correlated with conformational properties of substrates (relative conformation, energy difference between two boat‐like ground‐state conformations, ring‐inversion barrier) as determined by DNMR and MM2 calculations. (3 S )‐Enantiomers of acetates (+)‐ 8 , (+)‐ 10 , (+)‐ 15 , and (+)‐ 21 were preferentially formed. In the case of the acetate (−)‐ 17 (ee 90.2%), formation of the (3 R )‐enantiomer was favored. C(3)OH Group with hemiaminal‐like character in rac ‐ 3 ( n = 0) cannot be acetylated by any of 23 tested lipases and four esterases. For racemic alcohols 7 , 9 , 14 , and 20 , preferred acetylationof the (3 S )‐enantiomers, present in solution in absolute ( M )‐conformation, was established; only in prochiral diol 16 ( n = 1) the CH 2 OH group in the ( pro ‐ R )‐position is prevalently acetylated, presumably due to the binding to the enzyme, in absolute ( P )‐conformation. Temperature dependence of enantioselectivity revealed inverse correlation of the E value of rac ‐ 9 , and ee values for prochiral 16 , with T [K], indicating prevalent contribution of the enthalpy term to enantioselection. Absolute conformation ( M / P ) and absolute configuration at C(3) of all products was determined by combining CD and 1 H‐NMR data.