Hydrolase-catalyzed resolutions of enantiomers

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" Hydrolase-catalyzed resolutions of enantiomers "
A. N. Serreqi

Document Type	:	Latin Dissertation
Language of Document	:	English
Record Number	:	1113394
Doc. No	:	TLpq230607824
Main Entry	:	A. N. Serreqi
Title & Author	:	Hydrolase-catalyzed resolutions of enantiomers\ A. N. Serreqi
College	:	McGill University (Canada)
Date	:	1995
student score	:	1995
Degree	:	Ph.D.
Page No	:	155
Abstract	:	Enantiomerically-pure compounds are becoming increasingly important particularly to the pharmaceutical industry. Enzymes are useful tools in the synthesis of such compounds. To better understand how lipases discriminate between enantiomers of chiral secondary alcohols we obtained x-ray crystal structures of covalent complexes of Candida rugosa lipase with the transition-state analogs (1R)-menthyl hexylphosphonate and (1S)-menthyl hexylphosphonate. These compounds are transition-state analogs for the hydrolysis of menthyl esters. We observed that the transition-state analog of the unfavored (1S)-enantiomer of menthol disrupted the hydrogen bond between Nusd\varepsilonusd2 of histidine 449 and the menthol oxygen. His 449 is part of the catalytic triad in CRL. This may account for the slower reaction of the (1S)-enantiomer of menthol. The crystal structures also identified binding site regions for secondary alcohols in CRL. These regions are common among many lipases and esterases and account for their common enantiopreference toward secondary alcohols. Lipases and esterases can resolve compounds with phosphorus and sulfur stereocenters by hydrolysis of a pendant acetoxy group. Both CRL and cholesterol esterase have high selectivity for (2-acetoxy-1-naphthyl)methylphenylphosphine oxide. They resolved this substrate with and E of 81 and 32 respectively. A synthetic scale resolution of this substrate with subsequent recrystallization and chemical transformation followed by stereospecific reduction gave both enantiomers of (2-methoxy-1-naphthyl)methylphenyl-phosphine with 96-97% ee. This chiral phosphine is potentially useful in asymmetric syntheses. CE is the most selective enzyme for the sulfur substrates tested but these enantioselectivities were moderate, E's ranged from 5 to 25. From the CE resolution of the phosphorus and sulfur compounds and others we propose an empirical model that predicts which enatiomer reacts faster. The model is based on the size of the substituents and their conformational preferences. Crude Aspergillus niger resolves esters of pipecolic acid with an E of 20 4. A simple partial purification of ANL by fractional precipitation with ammonium sulfate increased the enantioselectivity to >100. The partially purified ANL can be used in a synthetic scale resolution of ()-n-octyl pipecolate to give (S)-()-pipecolic acid (93% ee) and (R)-(+)-pipecolic acid (97% ee).
Subject	:	Pharmacology
	:	Pure sciences