Enantiomeric purity, racemic lactones

Interestingly, enzymes are chiral catalysts and their potential for enantio-selective polymerization has been investigated [93]. Several examples are reported where a racemic mixture of lactones is polymerized by enzymatic polymerization to afford the corresponding optically active polyester [93]. For instance, lipase CA (Novozym 435) catalyses the ROP of racemic 4-methyl-s-caprolactone and 4-ethyl-s-caprolactone in bulk at 45 °C and 60 °C to afford (S )-eiuiched poly(4-methyl-e-caprolactone) and poly(4-ethyl- -caprolactone) with an enantiomeric purity higher than 95% [153]. 

Owing to their common enzymatic pathways, chiral aroma compounds from fruits and other natural sources should be characterized by origin specific enantiomeric ratios. Indeed, from freshly harvested strawberries y-decalactone and y-dodecalactone have been detected with high enantiomeric purity favouring the (4R)-configurated y-lactones y-Cjo(4R >98 % ee), y-C,2(4R >99 % ee) [28]. On the other hand, racemic y-decalactone in aroma relevant amounts has not yet been observed in fruits. Thus, the detection of racemic y-decalactone from fruit-containing food indicates the addition of nature-identical y-C 10-lactone. 

Enantiomerically pure lactonic pheromones 607-611, of the carpenter bee, blacktailed deer, Japanese beetle, rove beetle, and Oriental hornet, respectively, have been synthesized from racemic cyano alcohols of type 612. The key to the success of the overall approach is the facile separation of diastereomeric carbamates derived from cyano alcohols of type 612 by automated multigram LC. The chosen approach also facilitates the assignment of absolute configurations to the lactone enantiomers and their precursors. In the case of 607, direct determination of enantiomeric purity and absolute configuration is also possible using the chiral solvating agent 2,2,2-trifluoro-l-(9-anthryl)ethanol [439]. 

Synthetic highlights A critical step in the synthesis of the compound involves non-hydrolytic anomalous lactone ring-opening by TMSl. To avoid racemization, the Mitsunobu reaction was applied in the formation of the ether bond. Optimization of the Mitsunobu reaction was finally achieved without loss of enantiomeric purity. 

The first exploitation of microbial BVase to produce chiral lactones from cyclic racemic bicyclo[2.2.1]hept-2-en-7-ones (Fig. 10) was reported by Ouazzani et al. [17]. A report of preparative scale bioconversion of cyclic ketone was made [26]. The preparative scale bioconversion of using Acinetobacter TD 63 provides lactones in high optical purity [27]. Subsequently [28], whole-cell bioconversions of bicyclic lactones was made, using Acinetobacter sp. NCIB 9871 and Pseudomonas sp. NCIB 9872 to produce file same chiral synthons. In all cases, the racemic ketone was oxidized, and equal amounts of the regioisomeric lactones (—)-(i5, 5/ )-2-oxabicyclo[3.3.0]oct-6-en-3-one and (—)-(iR,55)-3-oxabicyclo[3.3.0]oct-6-en-3-one were produced in high enantiomeric purity. 

In such cases the enantiomeric excess is crystallized with a higher purity than that of in the initial composition, in a certain range of temperature. For example, the common intermediate of the synthesis of several prostaglandines is a lactone (PGL). Its enantiomeric mixtures (eeo) can be enriched by crystallization (eesoUd) from melt, while the racemic ratio (liquid residue, eeuquid) can be recovered for repeated resolution.20... 

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