Diastereofadal selectivity

Jeroncic, L. O. Cabal, M.-P. Danishefsky, S. J. Shulte, G. M. On the diastereofadal selectivity of Lewis acid catalyzed C-C bond forming reactions of conjugated cyclic enones bearing electron-withdrawing substituents at the y position./. Org. Chem. 1991, 56, 387-395. 

The reaction pathways of conjugate addition of Me2CuLi and Me2CuLiTiCl have been studied for acrolein [79] and cydohexenone [80] with the aid of density functional methods, and fit favorably with the NMR properties of intermediates, kinetic isotope effects [81], and the diastereofadal selectivity. A similar medianism also operates in this reaction, as summarized in Scheme 10.5. The rate-determining step of the reaction (TScc) is the C C bond formation caused by reductive dimi-nation from Cu to give Cuf... 

When two of the reaction components are chiral (any combination of substrate, reagent, catalyst, or solvent), the chirality elements of each reactant will operate either in concert (matched pair) or in opposition (mismatched pair) and together influence the stereochemical outcome of the reaction. In this case, the reaction is subject to double asymmetric induction. Unless the diastereofadal selectivities of both chiral reactants are in opposition and identical in magnitude, the ratio C D 1. Applications of double asymmetric induction in synthesis will be discussed in Chapter 5. 

The present homoallylation with isoprene under Ni-EtsB catalysis shows marginal success for the reaction with aliphatic aldehydes. Results are siun-marized in Table 6. Primary alkyl aldehydes (bearing no a-substituents) and sterically less-hindered secondary alkyl aldehydes undergo the homoallylation successfully to provide the expected products in good yields with excellent stereoselectivity (runs 1-5). The results in runs 3-5 indicate that the present reaction shows almost no diastereofadal selectivity with respect to the a-stereocenters of secondary alkyl aldehydes. Sterically demanding aldehydes, such as cyclohexanecarbaldehye and pivalaldehyde, provide the... 

The retrosynthetic plan (Scheme 10) indicates that the dienophile would be a carboxyketene equivalent, CII, which implies the diastereofadal selectivity would be controlled entirely by the diene component Cl. At the outset of this work, there were few examples where chiral dienes had been employed in intermolecular Diels-Alder reactions upon which the investigators could rely for precedents [28, 29]. However, seminal studies had been undertaken by Franck and co-workers who reported that the sorbaldehyde diene CHI reacted with N-phenylmaleimide to give a 7.3 1 mixture of cycloadducts CIV and CV (Scheme 10b) [29]. Thus, the major product resulted from a like process (i.e., preferential attack from the r -face) and these workers developed a set of rules based on their experimental observations. 

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