MTPA esters

Alternatively, complexation with lanthanide shift reagents allow the signals of the MTPA ester to be resolved and used to determine enantiomeric... 

Assign as many proton signals as possible with respect to each of the (R)-and (iS )-MTPA esters. 

In the original paper, the authors performed the reaction using commercially available bakers yeast from a supermarket or bakery. Initially a trial run using similar quantities of Sigma dried yeast resulted in an extremely vigorous initial fermentation, so the quantity of dry yeast was reduced by factor of 5. The contributors assessed the enantiomeric excess of the alcohol by formation of the (+)-MTPA ester and examination of the 19F NMR spectrum. However, the value obtained for the optical rotation was consistent with that reported in the literature. 

The absolute configuration of many of the alcohol products was assigned by the (/ )-( +)-MTPA-Eu(fod)3 method (17). For example, for PhCHOH— CH2OMe, the major isomer formed was the R-( - )-carbinol as shown by a larger LIS value corresponding to MTPA ester 33. 

Optical purity checked by NMR analysis of the MTPA ester. 

Molar ratio LAH 90 ketone = 1.0 2.30 0.64. Reaction at -65°C with heterogeneous reagent used immediately on preparation. Analysis based on NMR of MTPA esters in presence of Eu(fod)3 shift reagent. 

Mole ratio amino alcohol LAH = 4.1 1.8. Reaction carried out at - 70°C with insoluble, freshly prepared complexes in ether. Analysis by GC of MTPA ester derivatives. 

Microbial reduction of prochiral cyclopentane- and cyclohexane-1,3-diones was extensively studied during the 1960 s in connection with steroid total synthesis. Kieslich, Djerassi, and their coworkers reported the reduction of 2,2-dimethylcyclohexane-l,3-dione with Kloeokera magna ATCC 20109, and obtained (S)-3-hydroxy-2,2-dimethylcyclohexanone. We found that the reduction of the 1,3-diketone can also be effected with conventional baker s yeast, and secured the hydroxy ketone of 98-99% ee as determined by an HPLC analysis of the corresponding (S)-a-methoxy-a-trifluoromethylphenylacetate (MTPA ester).(S)-3-Hydroxy-2,2-dimethy1cyc1ohexanone has been proved to be a versatile chiral non-racemic building block in terpene synthesis as shown in Figure 1. 

MTPA esters and amides are easily prepared and contain only a few clear signals derived from the reagent. MTPA fulfills close to all the above mentioned requirements that a CDA, for NMR measurements, is expected to have. 

H-NMR spectra15c of esters of 3,3-dimethyl-2-butanol with three different chiral selector acids are depicted in Figure 2. The two diastereomers can be clearly distinguished in each case. Note the different order of chirality of MTPA esters compared to the other esters. 

Test-tube Preparation of MTPA Esters and Amides General Procedure15c ... 

S) MTPA ester with large" (L3) and small" (LJ) substituents... 

Figure 6. MTPA ester of marine terpenoid sipholenol (partial structure). No correlation of all separated H signals required by Kakisawa s model.

Table 3. H-NMR Chemical Shift Differences (AS, CDC13) for Diastereomeric Mandelic. O-Methylmandelic, and a-Methoxy-j-lrifluoromethylphenylacetic (MTPA) Esters of Known Configuration (Data from Mosher s work15 )...

Diastereomeric MTPA esters of secondary alcohols can be differentiated by their relative methoxy proton signal shifts, e.g., the (7 ,i )-diastereomer 1 shows a much stronger lanthanide induced shift than the (/ ,.S)-isomer417. 

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