Selectivity in Reduction of Monocyclic Molecules

The actual experimental results can be explained by using models 290 and 291, the two envelope conformations available to 2-substituted cyclopentanones (sec. 1.5.B). In 290, approach of a reagent to the carbonyl at an angle of 110° (the Biirgi-Dunitz trajectory-sec. 6.6.A)285 leads to an interaction with Ha (path b), making path a preferred and this leads to the trans product, which appears to be contradicted by inspection of 291, where approach via path a is inhibited by the R group. Since the R group will be pseudo-equatorial in the 

The Cram model does not predict the correct stereochemistry of reduction products derived from cyclohexanone derivatives. anti-Periplanar hyperconjugative effects have been invoked to explain the observed results. The Felkin-Ahn model suggests an interaction such as that shown in 297. 91 An alternative proposal was put forth by Cieplak, shown in 298, 92 and it was found that the incipient bond (the one being formed) was electron deficient, in line with Cieplak s proposal. Cieplak s model has led to some controversy, and a 

UAIH4 LiAlH(OtBu)3 NaBH4 KBH4 LiAlH(OMe)3 Ip2BH  

Reprinted in part with permission from Ashby, E.C. Sevenair, J.P. Dobbs, F.R. J. Org. Chem., 1971, 36, 197. Copyright 1971 American Chemical Society. 

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