Enantioselective C-C Bond Construction: Part Three of Three
Sulfones are chemical chameleons, electron-withdrawing groups that are also good leaving groups. Tamio Hayashi of Kyoto University took advantage of this in designing (J. Am. Chem. Soc. 2003, 125, 2872. ) an enantioselective method for the construction of ternary stereogenic centers. The Rh-catalyzed conjugate addition of the aryl Ti species 2 to the unsaturated sulfone proceeds with high enantioselectivity. Subsequent β-hydride elimination proceeds, as expected, away from the newly-formed ternary center. Readdition of Rh-H followed by reductive elimination of the sulfone then gives the alkene 3.
There has been a continuing effort to make the Baylis-Hillman reaction a catalytic asymmetric process. Scott Schnauss of Boston University recently reported (J. Am. Chem. Soc. 2003, 125, 12094. ) an elegant solution to this problem, based on the use of BINOL-derived Bronsted acids as catalysts. The product hydroxy enones such as 6 are interesting in themselves, and also as substrates for further transformation, for instance by Claisen rearrangement.
Prochiral α-substituted enones such as 7 are inexpensive starting materials. Patrick Walsh of the University of Pennsylvania recently reported (J. Am. Chem. Soc. 2003, 125, 9544. ) a catalytic enantioselective procedure for the 1,2-addition of dialkyl zinc reagents to such enones. The chiral catalyst is a sulfonamide derived from 1,2-diaminocyclohexane. The tertiary allylic alcohols are useful products, difficult to prepare by other methods. Even more exciting is the observation that addition of oxygen to the reaction mixture directly converts the tertiary alkoxide to the epoxide 9 with high diastereocontrol. Subsequent Lewis acid-catalyzed rearrangement of the epoxide 9 then gives the ketone 10. The overall process sets the absolute configuration of two stereogenic centers. The construction of cyclic quaternary stereogenic centers is particularly noteworthy.