Lactones kinetic resolution

The 9 — 15 fragment was prepared by a similar route. Once again Sharpless kinetic resolution method was applied, but in the opposite sense, i.e., at 29% conversion a mixture of the racemic olefin educt with the virtually pure epoxide stereoisomer was obtained. On acid-catalysed epoxide opening and lactonization the stereocentre C-12 was inverted, and the pure dihydroxy lactone was isolated. This was methylated, protected as the acetonide, reduced to the lactol, protected by Wittig olefination and silylation, and finally ozonolysed to give the desired aldehyde. 

CHMO is known to catalyze a number of enantioselective BV reactions, including the kinetic resolution of certain racemic ketones and desymmetrization of prochiral substrates [84—87]. An example is the desymmetrization of 4-methylcyclohexanone, which affords the (S)-configurated seven-membered lactone with 98% ee [84,87]. Of course, many ketones fail to react with acceptable levels of enantioselectivity, or are not even accepted by the enzyme. 

The identification of a novel BVMO from Mycobacterium tuberculosis (BVMOMtbs) complements this toolbox, as this particular biocatalyst performs a classical kinetic resolution instead of a regiodivergent oxidation vith complete consumption of substrate [140]. Notably, this enzyme accepts only one ketone enantiomer and converts it selectively to the abnormal lactone while the antipodal substrate remains unchanged (Scheme 9.24) [141]. 

Commercially available lipase PS has been used to obtain kinetic resolution of racemic 4-substituted oxetan-2-ones and 3- and 3,4-disubstituted oxetan-2-ones in organic solvents <00JCS(P1)71>. The enzyme appears to be relatively insensitive to the substituents on the lactone or to the nature of the ring opened products. 

Bolm et al. (130) reported the asymmetric Baeyer-Villiger reaction catalyzed by Cu(II) complexes. Aerobic oxidation of racemic cyclic ketones in the presence of pivalaldehyde effects a kinetic resolution to afford lactones in moderate enan-tioselectivity. Aryloxide oxazolines are the most effective ligands among those examined. Sterically demanding substituents ortho to the phenoxide are necessary for high yields. Several neutral bis(oxazolines) provide poor selectivities and yields in this reaction. Cycloheptanones and cyclohexanones lacking an aryl group on the a carbon do not react under these conditions. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

CPDMO is a new bioreagent for the synthesis of optically pure lactones with excellent enantioselectivity. CPDMO is not only effective in desymmetrization of meso and prochiral compounds (Procedure 2, Section 11.8.2), but excellent in carrying out the kinetic resolution of racemates (Procedure 3, Section 11.8.3). Additional examples of optically pure lactones that can be obtained are summarized in Table 11.4. In the fermenter work (Procedure 4, Section 11.8.4), (R)-2-methyl cyclohexanone was not converted, but evaporated under aeration condition (1 wm). This led to the expected product (5)-7-methyl oxepanone at the end of the experiment. The optically pure lactone could be recovered without sdica-gel chromatography separation. However, the production yield may be improved by using a better condenser. 

In recent work, Chmielewski and co-workers (174) reported the highly stereoselective reaction of ene-lactones with chiral pyrrolidine nitrone (141) to afford tricyclic adducts (Scheme 1.31). A 1 1 mixture of ene-lactone 142 and nitrone 141 provided adduct 143 with an uncharacterized isomer (97 3) (91%) whUe homo-chiral D-glycero (138) gave the adduct 144 as a single diastereomer (88%). A 2 1 mixture of racemic 138 and nitrone 141 afforded a 91 1 mixture of the two possible adducts, representing an effective kinetic resolution of the racemic lactone. 

Langlois and co-workers (179) found the same exo stereochemical preference through double asymmetric induction of a related ene-lactone (1 )-145 with their well-explored and efficient camphor-derived oxazoline nitrone (150-146 (Scheme 1.32). They found the cycloaddition components form a matched pair and allowed kinetic resolution of the racemic lactone in up to 70% enantiomeric excess (ee). They suggest the selectivity for exo adduct 147 arises through destabilization of the endo transition state by a steric clash between dipolarophile ring hydrogens and the bornane moiety. 

Recently, a lipase-catalysed kinetic resolution has been developed to produce L-tert-leucine via a lactone of its A-benzoyl derivative. There is also reputed to be another bioroute to L-terMeucine using transaminase technology. 

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

A highly stereoselective kinetic resolution process for Rh-catalyzed enyne cycloisomerization has also been developed by Zhang et al. [41]. This transformation has enabled the highly enantioselective synthesis of polyfunctionalized tetrahydrofurans and lactones with two or three adjacent stereocenters and it is regarded as a major breakthrough in enynes cycloisomerization and in kinetic resolution (Scheme 7). 

---