Styrenes asymmetric aminohydroxylations

TABLE 4-16. Influence of Ligand and Solvent on the Regioselectivity in Asymmetric Aminohydroxylation Reaction of Four Styrene Substrates7 9b... 

The catalytic asymmetric aminohydroxylation of a variety of styrene derivatives, vinyl aromatics, and some other olefins using osmium tetroxide... 

Table 8 Asymmetric aminohydroxylation of styrene and methyl cinnamate proceeding in the second catalytic cycle...

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. 

Solvent selection can markedly influence the product ratios of a reaction. In the catalytic asymmetric aminohydroxylation of styrene, the ratio of the acetamide products rose from 0.9 1 to 6.1 1 when the reaction was run in aqueous acetonitrile instead of aqueous n-PrOH (Figure 4.11) [34]. Solvent polarity can influence asymmetric induction, particularly during peptide coupling. When the racemic azlactone 15 was condensed with L-lysine methyl ester (16, Figure 4.12), the D,L-product predominated in relatively nonpolar solvents, and the L,L-product predominated in polar solvents and at lower temperatures [35]. 

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