Dihydroquinidine p-chlorobenzoate

After the "asymmetric epoxidation" of allylic alcohols at the very beginning of the 80 s, at the end of the same decade (1988) Sharpless again surprised the chemical community with a new procedure for the "asymmetric dihydroxylation" of alkenes [30]. The procedure involves the dihydroxylation of simple alkenes with N-methylmorpholine A -oxide and catalytic amounts of osmium tetroxide in acetone-water as solvent at 0 to 4 °C, in the presence of either dihydroquinine or dihydroquinidine p-chlorobenzoate (DHQ-pClBz or DHQD-pClBz) as the chiral ligands (Scheme 10.3). 

This procedure has been modified to become an effective catalytic procedure in which iV-methyl-moipholine A -oxide is used as the secondary oxidant. In this manner, ( )-stilbene has been converted into (+)-r/irco-hydrobenzoin (55% yield after two reciystallizations, >99% ee) on a one molar scale, by treatment with osmium tetroxide (0.002 mol equiv.) and iV-methylmoipholine 1 -oxide (1.2 mol equiv.) in aqueous acetone in the presence of dihydroquinidine p-chlorobenzoate (0.134 mol equiv.). The latter compound can be recovered in 91% yield. 

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