External protonation enantioselective

The alternative method shown in equation 18, using a chiral alcohol, like (S,S)-97, as the chiral Brpnsted acid, affords a high level of facial selectivity for the enantioselective protonation of lithium enolate 95 to give the required 2-methyltetralone (5)-93 in yields and enantiomeric purity similar to those of equation 17 °. The concept of internal versus external proton delivery has been probed ° . The use of disulfonamide (R,R)-9S as chiral Brpnsted acid leads to the (R)-93 enantiomer (equation 19), whereas using as chiral scaffold (R,R)-99, the (V,(V -dilithio salt of (R,R)-99 and acetic acid (as external proton source) gives (S)-93 (equation however, in poorer yield and enantiomeric... 

To minimize this troublesome process, the concentration of the intermediate has to be kept as low as possible by means of catalytic tautomer-ization of the dienol. To make the chiral discrimination as great as possible, a strong interaction between the prochiral intermediate and the chiral environment is needed. When the chiral inductor is introduced into the starting molecule itself, diastereoselective photodeconjugations can be observed. If an external chiral inductor is used, enantioselective protonations are obtained. This approach, which may be catalytic if the chiral auxiliary is not consumed in the process, can take advantage of the acidic property of enols [27] and of the possibility of an acid- or base-catalyzed tautomer-ization of enols [28]. 

Enantioselective protonation of the prochiral photodienol performed by use of an external chiral source of protons (pathway a)... 

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