Stereocontrolled Synthesis of (2 R ,3 S )‐2‐Methylisocitrate, a Central Intermediate in the Methylcitrate Cycle

2‐Methylisocitrate (=3‐hydroxybutane‐1,2,3‐tricarboxylic acid) is an intermediate in the oxidation of propanoate to pyruvate (=2‐oxopropanoate) via the methylcitrate cycle in both bacteria and fungi ( Scheme 1 ). Stereocontrolled syntheses of (2 R ,3 S )‐ and (2 S ,3 R )‐2‐methylisocitrate (98% e.e.) were achieved starting from ( R )‐ and ( S )‐lactic acid (=(2 R )‐ and (2 S )‐2‐hydroxypropanoic acid), respectively. The dispiroketal (6 S ,7 S ,15 R )‐15‐methyl‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadecan‐14‐one ( 2a ) derived from ( R )‐lactic acid was deprotonated with lithium diisopropylamide to give a carbanion that was condensed with diethyl fumarate ( Scheme 3 ). The configuration of the adduct diethyl (2 S )‐2‐[(6 S ,7 S ,14 R )‐14‐methyl‐15‐oxo‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadec‐14‐yl]butanedioate ( 3a ) was assigned by consideration of possible transition states for the fumarate condensation ( cf. Scheme 2 ), and this was confirmed by a crystal‐structure analysis. The adduct was subjected to acid hydrolysis to afford the lactone 4a of (2 R ,3 S )‐2‐methylisocitrate and hence (2 R ,3 S )‐2‐methylisocitrate. Similarly, ( S )‐lactic acid led to (2 S ,3 R )‐2‐methylisocitrate. Comparison of 2‐methylisocitrate produced enzymatically with the synthetic enantiomers established that the biologically active isomer is (2 R ,3 S )‐2‐methylisocitrate.