Steric Effect of Coordination

Stereoselectivity in the reaction of acyclic ketone 270 is different from that of the cyclic ketone 256. The acetate in 271, prepared by reduction of the ketone 270 to alcohol with LiAlH and acetylation, was displaced with Me A1 from the exo side to give 272 with retention of the stereochemistry. No racemization of benzyl cation was observed. However, reaction of 270 with MeLi gave 274. The OH group of 274 was removed with hydride from the less hindered side as shown by 275 to give 276 with 

Complexation of 278 with naphthalene-Cr(CO)3 gave one isomer 279 with high diastereoselectivity (89 11) due to coordination effect of OH group. After hydrolysis of the acetal, 279 was converted to 281 via 280. The acetate 281 was converted to the syn product 282 by displacement with Me3Al in a similar way (271-272) as described before. On the other hand, 283 was obtained from 280, and the anti product 284 was prepared stereoselectively by selective displacement with hydride [61,67] 

6 Asymmetric Synthesis using Chiral Cr(CO)3-arene Complexes 

Cr(CO)3 coordinates from either the top or bottom side of aromatic rings, bearing two different substituents in ortho or meta position, so that the enantiomers 285 and 286 are obtained. Optical resolution of the enantiomers is carried out by recrystallization, or column chromatography. The racemic complex of benzyl alcohol derivative 287 was separated to 288 and 289 by lipase-catalysed acetylation [68]. Enzymes recognize Cr(CO)3 as a bulky group. Chiral Cr(CO)3-arene complexes are used for asymmetric synthesis [68a]. 

As reactions occur from the face opposite to the metal, highly enantioselective 

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