Sulfur chiral, diastereoselective control with

The reaction works well in most solvents, but diastereoselectivity control is better in polar solvents due a higher reversibility of betaine formation. The best chiral sulhde organocatalysts are those with the sulfur moiety in a cyclic structure. Compound 271 produces the best enantio- and diastereoselectivities with metal complex CuCacac) (Scheme 7.60). It enables the use of substoichiometric amounts of chiral sulhde keeping yields acceptable with excellent diastereo and enantioselectivities. The reaction is very sensitive to the concentration of the sulhde [206]. 

The fact that these adducts were diastereoisomerically pure (deduced from the reported melting points and NMR spectra) suggests that these reactions took place with a complete control of the diastereoselectivity. Bearing this in mind, in 1989 Waldner [64] synthesized optically pure sulfoxide 55 (by MCPBA oxidation of the corresponding chiral thioether and further separation from the epimeric mixture so obtained) and studied its reactions with dienes and aza-dienes (Scheme 29). A high 7r-facial selectivity (almost complete with cyclopen-tadiene and azadienes) was observed in all cases, the approach of the diene from the dienophilic face supporting the lone pair of electrons at sulfur being favored. In the case of the reaction with cyclopentadiene, the endo-selectivity was complete as well. 

While high stereoselection has been achieved in radical reactions which occur in a-position146 to a center substituted with a chiral auxiliary, diastereofacial control in the addition of achiral radicals to the P carbon is, in general, difficult to achieve.147 In connection with this, Toru et al. reported extremely high P-stereoselection in the addition of tertiary, secondary, and even primary alkyl radicals to chiral a-sulfinyl cyclopentanones in 1993.148 The effectiveness of the diastereoselective addition of achiral radicals has been shown to depend on the size of the substituent at the sulfmyl sulfur. Bulky chiral arylsulfmyl groups show excellent diastereoselectivi-ties (> 98 < 2). 

The chiral auxiliary approach. Poncas group has worked with chiral sulfur moieties like 10-methylthioisoborneol, Oppolzer s camphorsultam or chiral oxazolidinones which gave excellent results in stereo control and yields [95-100]. Recently they have reported that chiral alkynylthiols like 46 exhibit excellent diastereoselectivities in both inter- and intramolecular PKR, and have used this approach for the synthesis of 47, an intermediate in the synthesis of (+)-15-nor-pentelenene (48) (Scheme 15) [101,102]. 

Lithium A number of Michael-type additions of organolithiums have been described. This is also the case of the reaction of aryllithiums with nitroalkene (389). Deprotonation of the benzylic fluoride (455) with LDA, directed by the neighbouring sulfoxide group, generated the benzyllithium intermediate (456), which underwent addition to the Michael acceptor R CH=CHY (Y = CO2BU, S02Ph R = Ar, alkyl, alkenyl) in a diastereoselective manner controlled by the chiral sulfur to afford the 5yn-configured product (457) with <99 1 dr ... 

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