Chiral hydrobenzoin-derived

A more readily available Fe-complex for asymmetric Diels-Alder reaction is from chiral hydrobenzoin.Dienophiles derived from f7/ ,61 )-l-pivaloyloxyspiro[4.4]nonan-6-ol 5 give chiral adducts on reaction with dienes. A representative of axially chiral dienophiles is 65 which gives Diels-Alder reactions with high endo- and diastereoselectivity. An access to dihydropyranyl ethers by the hetero-Diels-Alder reaction, C2-syiiunetric Cu(II) complexes such as and yttrium tri[(7 )-l,l -binaphth-2,2 -diyl]phosphonate are... 

Schuster, C. (2004) Meso-hydrobenzoin derivatives as new chiral auxiliaries and... 

Scheme 7.24. Resin-bound m-hydrobenzoin-derived chiral auxiliary for stereoselective reduction of phenylglyoxylates.

Rovis and co-workers have extended the application of redox transformations to generate chiral acylazolium species from a-haloaldehydes 245 and the NHC derived from pre-catalyst 247, allowing the desymmetrisation of mei o-hydrobenzoin 246 to give ester 248 in good yield and enantioselectivity (Scheme 12.54) [30]. 

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

Both the (RR)- and (5S)-enantiomers of hydrobenzoin (Figure I4a) have been made to react (110, 117, 118) successfully in a (2 + 2) cyclization with ethylene glycol ditosylate in the presence of bases (e.g., NaOH in dioxane or NaH in MA -dimethylformamide) to give the enantiomeric tetiaphenyl-18-crown-6 derivatives (RRRR)-73 and (SSSS)-73. The corresponding optically pure tetra-anisyl, tetra-a-naphthyl, and tetra-/3-naphthyl-18-crown-6 derivatives 74, 75, and 76, respectively, have also been prepared in similar fashion (119). It should be noted that (RRRR)- and (,SSSS)-73 have also been obtained as a result of a base-promoted (1 + 1) cyclization (120) between the chiral extended diol and... 

Chiral Ti complex, derived from hydrobenzoin dilithium salt and TiCl4, can be used for the asymmetric Diels-Alder reaction of several dienes with fumarate [53] (Eq. 8A.30). However, attempted use of acrylate as dienophile resulted in low enantioselectivity. 

Simultaneously and independently, Cullis and Lowe developed a second general methodology for the synthesis of, 0, 0-labeled chiral phosphate monoesters (76, 77). This synthesis relies upon the synthesis of a cyclic hydrobenzoin triester of the alcohol or phosphoric acid followed by hydrogenolysis to liberate the isotopically labeled monoester product (Fig. 2). Hydrobenzoin, chiral by virtue of stereospecific labeling with 0 and 0, is the source of the two specified oxygen isotopes, and O is derived from H2 0 via P OCL. The reader is referred to the articles by Cullis and Lowe for details of the synthesis. 

Enantiomer selective coloration of optically active amines, our important project, was realized by chiral azophenol crown 4 incorporated with two units of optically active hydrobenzoin. The synthetic route is shown in Scheme 2. Reaction of 2,6-bis(bromomethyl)-l,4-dimethoxybenzene 22, which is derived from hy-droquinone monomethylether 19 by a three-step procedure, with the dibutyltin derivative 26 of optically active dihydrobenzoin gives optically active podand 23 in 63% yield. Cyclization of 23 with the ditosylate of polyethylene glycol, followed by oxidation with ceric ammonium nitrate (CAN) and treatment with dinitrophenylhy-drazine, affords the desired chiral azophenol crowns 4n. 

Diketones. Chiral 1,2-diols have been extensively utilized as ligands and chiral auxiliaries for asymmetric synthesis. The CBS reduction of 1,2-diketones, such as benzil and heterocyclic derivatives furnishes optically active hydrobenzoins with excellent ee (>99 % ee) and good de (64-86 % de) 63) (Figure 8), whereas the reagent 1 afforded hydrobenzoin in 70 % ee with 34 % de (Cho, B. T., unpublished results). 

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