An enzymatic approach to the synthesis of optically pure (3R)- and (3S)-enantiomers of green tea flavor compound 3-hydroxy-3-methylnonane-2,4-dione

Both (3R)- and (3S)-enantiomers of the chiral green tea flavor compound 3-hydroxy-3-methylnonane- 2,4-dione were synthesized by the combined use of acetylacetoin synthase and acetylacetoin reductase from Bacillus licheniformis. The first enzyme was utilized to catalyze the homo-coupling of 2,3- octanedione and obtain the enantioenriched (3R)-3-hydroxy-3-methylnonane-2,4-dione (ee 44%). The NADH-dependent acetylacetoin reductase was then employed for the diastereoselective (de > 95%) C2 carbonyl reduction of the sole (3R)-enantiomer of the above 2,4-dione, thus affording the syn diol (2S,3R)-2,3-dihydroxy-3-methylnonan-4-one in enantiomerically pure form. While this step allowed for the recovery of unreacted, optically pure (3S)-3-hydroxy-3-methylnonae-2,4-dione, the corresponding (3R)-enantiomer was obtained by subsequent TEMPO-mediated oxidation of the syn diol intermediate. Moreover, using the title compounds as analytical standards, predominance of the (3R) enantiomer in the natural flavor compound was finally demonstrated by chiral GC–MS analysis.