DHQ-PHAL

Olefins of the /ranr-disubstituted type have given diols with excellent enantiomeric purities when dihydroxylated with the (DHQD)2-PHAL/(DHQ-PHAL pair of chiral ligands with osmium tetroxide (see Table 6D.3 [16,26,29,31,35,40,41,46-48]). All the entries but one in Column 9 for diols obtained with these ligands exceed 90% ee (or 90% de). From other entries in Table 6D.3, particularly those of Column 3 for earlier stoichiometric ADs with the DHQD-CLB ligand, good enantioselectivities are anticipated for the dihydroxylation of most trans-disubstituted olefins when the PHAL ligands are used. 

Similarly, an intramolecular variant utilizing carbamates 26 derived from allylic alcohols has been developed using an amine like Hunigs base (ethyl diisopropylamine) as additive [21], The products were obtained with complete regio- and diastereocontrol, but surprisingly, only in racemic form when chiral ligands like (DHQ PHAL, being established for the AA, were employed. 

The strategy is impressively simple the phthalazine derivative 15 can readily be prepared from quinine in one step. Being a divinyl derivative, it can be submitted as a cross-linking unit in the radical polymerization of methyl methacrylate (MMA) or 2-hydroxy methacrylate (HEMA). Thereby, an immobilized (DHQ-PHAL) derivative 16 is obtained, which is suited for the asymmetric dihydroxylation of frantr-stilbene (>99 % ee) and ( )-cinnamic acid methyl ester (>99 % ee. Table 1). The insoluble catalyst can be recovered by simple filtration, and its repeated... 

Figure 7.6 (DHQ)PHAL, (QN)2PHAL and (QN)2PHAL(4-OH) ligands for AD reactions.

The reactions with a combination of (DHQ)2-PHAL [or (DHQD)2-PHAL] and /V-halosulfo-namides can be successfully applied to trans-olefins. Especially when the substrates are a,j3-unsaturated esters, high regioselectivity as well as good enantioselectivity is realized (Scheme 55).210,211 The use of an /V-halosulfonamide bearing a smaller A-substituent increases the enantioselectivity.211 n-Propanol/water (1 1) is the solvent of choice. Aminohydroxylation of silyl enol ethers has been successfully performed with DHQD-CL or (DHQD)2-PYR, to give the corresponding a-amino ketones.212... 

Entry Substrate (DHQD)2-PHAL 96a (DHQ)2-PHAL 96b c2-dhqd 99 c2-dhq 99... 

Wang et al.36 have used the chiral catalyst (DHQ)2 PHAL (see Chapter 4 for the structure) for the asymmetric synthesis of the taxol side chain. Optically enriched diol was obtained at 99% ee via asymmetric dihydroxylation. Sub-... 

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