If the alkene is oriented as shown in Scheme 11, the natural dihydroquinidine (DHQD) ester forces delivery of the hydroxyls from the top face (β-attack). Conversely, dihydorquinoline (DHQ) esters deliver hydroxyls from the bottom face (α-attack).
The reactions are generally carried out in a mixture of tert-butyl alcohol and water at ambient temperature (Scheme 12).
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Scheme 12 |
Features:
- • The reaction is stereospecific leading to 1,2-cis-addition of two OH groups to the alkenes
• It typically proceeds with high chemoselectivity and enantioselectivity
• The reaction conditions are simple and the reaction can be easily scaled up
• The product is always a diol derived from cis-addition.
• It generally exhibits a high catalytic turnover number
• It has broad substrate scope without affecting the functional groups
1.1.4 Aminohydroxylation
Similar to cis-1,2-dihydroxylation, cis-1,2-aminohydroxylation of alkenes has been developed by reaction with chloroamine in the presence of catalytic amount of OsO4. In this process, alkene reacts with chloroamine in the presence of OsO4 to give sulfonamides that is readily converted into the cis-1,2-hydroxyamines by cleavage with sodium in liquid ammonia (Scheme 13). This process provides a direct cis-aminohydroxylation of alkenes, but the major problem is the poor regioselectivity for unsymmetrical alkenes.
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Scheme 13 |