Sharpless found that by using NMO as a co-oxidant, a catalytic amount of OSO4 and dihydroquinidine p-chlorobenzoate (DHQD-CLB) or dihydroquinine p-chlorobenzoate (DHQ-CLB), the Selectively achieve the desired catalytic enantioselective reaction as a chiral ligand (82). Under this catalytic condition, trans-stilbene was converted to the corresponding 1,2-diol in 80% yield and 88% ee. However, the ees of diol products obtained under this catalytic condition were initially lower than those produced by stoichiometric reactions. Sharpless explained this discrepancy with two catalytic cycle mechanisms (Scheme 15) (82,83).
Systematic ligand optimization (Figure 2) studies by the Sharpless group led to significant improvements in reaction efficiency. The first recommended cinchona alkaloid ligands were dihydroquinidine and dihydroquinine acetate (Ac) (81) and p-chlorobenzoate (CLB)
They screened for the asymmetric α-hydroxylation of the above cyclic β-ketoesters under various reaction conditions using various cinchona alkaloid derivatives involving cinchonine and peroxides. As a result, they found that dihydroquinine and cumyl hydroperoxide were the most suitable catalysts and oxidants to obtain enantiomerically enriched α-hydroxy-β-ketoesters, respectively. For example, treatment of 1-oxoindan-2-carboxylic acid benzyl ester (44) with 1.2 equivalents of cumyl hydroperoxide (46) in dibromomethane at room temperature in the presence of 20 mol% dihydroquinine (45) gave optical Active α-hydroxylated β-ketoester 47 was obtained in 98% yield with 73% ee (Scheme 15). Notably, the corresponding bulky tert-butyl ester does not undergo α-hydroxylation under optimized reaction conditions
The group subsequently prepared the hydroquinone-derived thiourea 13 and reported the Mannich reaction of N-acyl aldimines with dimethyl malonate, leading to Mannich adducts with excellent enantioselectivity sex
Hydroquinone is a inhibitor of melanin synthesis, serves as a skin whitener and is usually available in cosmetics or on prescription[1].Hydroquinone can be used in the preparation of oxidized light-emitting materials such as lucigenin by inducing the production of hydrogen peroxide in the presence of basic compounds.Hydroquinone (HQ) is the most abundant quinine present in cigarette tar it can enter the circulation via the lungs and can interact with cellular targets throughout the body,
