LIPASE-CATALYZED ACETYLATION OF 3-SUBSTITUTED 2,3-DIHYDRO-1H-1,4-BENZODIAZEPIN-2-ONES - EFFECT OF TEMPERATURE AND CONFORMATION ON ENANTIOSELECTIVITY AND CONFIGURATION
A. Avdagic et al., LIPASE-CATALYZED ACETYLATION OF 3-SUBSTITUTED 2,3-DIHYDRO-1H-1,4-BENZODIAZEPIN-2-ONES - EFFECT OF TEMPERATURE AND CONFORMATION ON ENANTIOSELECTIVITY AND CONFIGURATION, Helvetica chimica acta, 81(8), 1998, pp. 1567-1582
Enantioselectivity of acetylation by vinyl acetate/AcOEt catalyzed by
immobilized Candida antarctica lipase (Novozym 433) is studied for rac
-3-(hydroxymethyl)-1,4-benzodiazepin-2-ones 7, 9, 14 (n = 1; number of
CH2 groups in the chain at C(3)), 20 (n = 2), and for prochiral 3,3-b
is(hydroxymethyl) derivative 16. Enantiomeric excess (ee [%]) is corre
lated with conformational properties of substrates (relative conformat
ion, energy difference betweeen two boat-like ground-state conformatio
ns, ring-inversion barrier) as determined by DNMR and MM2 calculations
. (3S)-Enantiomers of acetates (+)-8, (+)-10, (+)-15, and(+)-21 were p
referentially formed. In the case of the acetate (-)-17 (ee 90.2%), fo
rmation of the (3R)-enantiomer was favored. C(3)-OH Group with hemiami
nal-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 acetylation of the (3S)-enantiomers, present in solution
in absolute (M)-conformation, was established; only in prochiral diol
16 (n = 1) the CH2OH group in the (pro-R)-position is prevalently ace
tylated, presumably due to the binding to the enzyme, in absolute (P)-
conformation. Temperature dependence of enantioselectivity revealed in
verse correlation of the E value of rac-9, and ee values for prochiral
16, with T [K], indicating prevalent contribution of the enthalpy ter
m to enantioselection. Absolute conformation (MIP) and absolute config
uration at C(3) of all products was determined by combining CD and H-1
-NMR data.