Induction 1,4-asymmctric

The influence of 1,2-asymmctric induction on the exchange of diastereotopic bromine atoms has also been investigated22,23. Thus, treatment of the / -silyloxydibromo compound 15 with butyllithium at — 110°C in the presence of 2-methylpropana led to products 17-19 after the reaction mixture was warmed to 20 °C. The distribution of the products indicates that the diastereomeric lithium compounds 16 A and 16B were formed in a ratio of 84 16, with 16A being kinetically favored by 1,2-asymmetric induction. Formation of the m-configurated epoxide (cis,anti-18) was slowed to such an extent that its formation was incomplete and a substantial amount of the parent bromohydrin 17 remained. The analogous m.yyn-configurat-ed epoxide was not observed. Presumably for sterie reasons, the parent bromohydrin did not cyclize to the epoxide but instead led to the ketone 1923. 

Chiral 1,3-diols. Chiral p-alkoxy aldehydes react with compounds of the type RTiCI, at - 78° to form derivatives of 1,3-diols with high 1,3-asymmetric induction (equation I). High 1.3-asymmctric induction is also observed in aldol condensations of these aldehydes catalyzed by TiCL (equations II and III). 

Moreover, 1,3-asymmctric induction strongly depends on the nature of the homoallylie substituent. Fluorine and oxygen substituents are trmethyl group causes reversal of the stereochemistry and the m-product predominates15. 

Thus, the new center is formed with 1,3-anti selectivity, apparently with no reference to the configuration at the a-carbon. This behavior can be understood, at least in part, by reference to the cyclic transition state 8 (see p 4005). Whereas the /(-substituent has a strong reason to take the equatorial position, a substituent on the a-carbon should have no strong preference. It is interesting that in both these reactions the selectivity is higher than for the corresponding a-unsubstituted example (d.r. 96 4 for the formation of r/-2.6-dimethyl-3,5-heptanediol, see p 4005). A similar predominance of 1.3-asymmctric induction has been noted for both of the other methods involving intramolecular hydride transfer (p 4005)42 4 8. 

Disubstituted butan-4-olides.2 Products of this type can be obtained in high enantioselectivity by addition of Grignard reagents to y-keto esters, (R)-l, of the chiral auxiliary B1NOL. The reaction can proceed with high 1,7-asymmctric induction. 

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