Regioisomeric excess

A related situation is found in the case of P-substituted cycloketones here, the electronic difference between the two a-carbons is almost insignificant, resulting in unselective migration upon chemical oxidation. BVMOs have a particularly different behavior, as they can influence the stereo- and/or regioselectivity of the biooxidation. In the latter case, the distribution of proximal and distal lactones is affected by directing the oxygen insertion process either into the bond close or remote to the position of the P-substituent. Consequently, a regioisomeric excess (re) can be defined for this biotransformation, similar to enantiomeric excess or diastereomeric excess values [143]. 

By using a stereochemical model for the transition state of the insertion reaction with nonbonded interactions between a complex of given chirality and the alkene the enantiomeric excesses as well as the regioisomeric excesses of aldehydes from monosubstituted, and 1,1- and 1,2-disubstituted ethylenes can be predicted. The prediction is correct in more than 80% of the cases studied.46-47... 

Table 10 as well as the Tables 11 and 12 show the differences in energy of the transition states responsible for enantiomeric and regioisomeric excess. They have been calculated on the basis of the enantiomeric and isomeric ratios. They correspond to the difference in free activation energies (AAG ) for a single-step formation of the metal-alkyl complex intermediate from the substrate and the catalyst complex. 

The reactions were allowed to proceed for 48 h and the course of the reaction was monitored by gas chromatography. The relative distribution of indole isomers was not changed over time. It was therefore concluded that the isomer distribution was dependent on the difference in the rates of formation of the isomers. The isomer distribution was determined from the integrated peak areas of the isomers in the gas chromatograms. The response used for the PLS analysis was the regioisomeric excess, RE = Amount (%) of major isomer - amount (%) of minor isomer. An account of the identities of the indole isomers is given in Ref.[ll], but we wiU not enter into details of this here, since it is not needed for the PLS analysis. 

The table below specifies the reaction systems used in the study of the Fischer indole reaction. Reaction systems 1-124 were used to establish an initial PLS model. This model was validated by predictions of the remaining systems 125—162. The final model was then accomplished from all 162 reaction systems. The response given, RE, is the regioisomeric excess. 

Table 17A.1 Reaction systems and observed regioisomeric excess...

DFA reacts with a 10-fold excess of styrene derivatives to afford a regioisomeric... 

The diastereoisomeric excess in the high-temperature region (T > Tinv) is dominantly controlled by steric effects of the chiral auxiliaries, whereas in the low-temperature region (T < Tinv), the nature of the olefin has a dominating influence. When the reaction is carried out on 2-methylfuran, a 2 1 regioisomeric... 

Upon prolonged irradiation in the presence of an excess ketone, the monoadducts are converted into 1,5- and l,6-dioxaspiro[3.3]heptanes (e.g., 80a,b) [53]. The regioisomeric 2- and 3-imino-oxetanes could be prepared by photolysis of ketenimines in the presence of aliphatic or aromatic ketones [72]. 

Sharpless and co-workers first reported the aminohydroxyIation of alkenes in 1975 and have subsequently extended the reaction into an efficient one-step catalytic asymmetric aminohydroxylation. This reaction uses an osmium catalyst [K20s02(OH)4], chloramine salt (such as chloramine T see Chapter 7, section 7.6) as the oxidant and cinchona alkaloid 1.71 or 1.72 as the chiral ligand. For example, asymmetric aminohydroxylation of styrene (1.73) could produce two regioisomeric amino alcohols 1.74 and 1.75. Using Sharpless asymmetric aminohydroxylation, (IR)-N-ethoxycarbonyl-l-phenyl-2-hydroxyethylamine (1.74) was obtained by O Brien et al as the major product and with high enantiomeric excess than its regioisomeric counterpart (R)-N-ethoxycarbonyl-2-phenyl-2-hydroxyethylamine (1.75). The corresponding free amino alcohols were obtained by deprotection of ethyl carbamate (urethane) derivatives. 

The chiral diene ester 16 is converted into three stereoisomeric cyclopropane derivatives12 in a diastereomeric ratio of 62 18 20 (40%) and a regioisomeric product (14%). The diastereofa-cial selectivity is 80 20, whereas the simple diastcreoselectivities are 78 22 and >90 10. As expected, the carbene ligand is preferentially transferred to the less hindered side of the diene ester, thus affording the unf/ -isomers in excess. 

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