The <i>β</i>‐Lactone Route to a Totally Stereoselective Synthesis of Carnitine Derivatives

Abstract

Abstract The syntheses of the enantiomerically pure, carnitine‐related β‐lactones 10 and 12 starting from various carnitine precursors of opposite configuration (or carnitine itself) are described. ( R )‐3‐Chlorocarnitine ( 20 ) has also been directly prepared from ( S )‐carnitine ( 14 ) and has been cyclized to 12 by a second inversion of configuration of the stereogenic centre. By nucleophilic attack at the carbonyl carbon, the β‐lactone carnitine derivatives have been converted into esters, amides and guanidino congeners. Following this route, it is possible to obtain the biologically active isomer ( R )‐carnitine ( 1 ) starting from the otherwise useless industrial by‐product ( S )‐carnitine ( 14 ). Nucleophilic attack by selected ambidient nucleophiles at the β‐carbon of the same β‐lactone derivatives results in a second inversion of configuration of the stereogenic centre. Besides aminocarnitine ( 3 ), chiral acetylcarnitine ( 2 ) and acetylthiocarnitine ( 5 ) have been synthesized in homochiral forms following this latter procedure.