Influence of N-amino protecting group on aldolase-catalyzed aldol additions of dihydroxyacetone phosphate to amino aldehydes

This work examines the influence of N-protecting groups on the conversion and stereoselectivity of dihydroxy acetone phosphate (DHAP) dependent aldolase-catalyzed aldol additions of DHAP to N-protected-3-aminopropanal. Phenylacetyl-(PhAc-), tertbutyloxycarbonyl- ((t)Boc-) and fluoren-9-ylmethoxycarbonyl- (Fmoc-)-3-aminopropanal were evaluated as substrates for D-fructose 1,6-bisphosphate aldolase from rabbit muscle (RAMA), and L-rhamnulose-1-phosphate aldolase (RhuA) and L-fuculose-1-phosphate aldolase (FucA), both from Escherichia coli. Using PhAc and (t)Boc ca. 70% conversions to the aldol adduct were achieved, whereas Fmoc gave maximum conversions of ca. 25%. The stereoselectivity of the DHAP-aldolases did not depend on the N-protected-3-aminopropanal derivative. Moreover, inversion of FucA stereoselectivity relative to that obtained with the natural L-lactaldehyde was observed. Both N-PhAc and (t)Boc adduct product derivatives were successfully deprotected by penicillin G acylase (PGA)-catalyzed hydrolysis at pH 7 and by treatment with aqueous TFA (6% v/v), respectively. However, the corresponding cyclic imine sugars could not be isolated, presumable due to the presence of a highly reactive primary amine and a keto group in the molecule, which lead to a number of unexpected reactions. (c) 2005 Elsevier Ltd. All rights reserved.