Chiral auxiliaries sulfone synthesis

On March 15, 1996, Wolfgang Oppolzer, Professor at the University of Geneva, Switzerland, died. Of his numerous important contributions to organic synthesis, the sultams, derived from campher sulfonic acid, have found widespread application especially as chiral auxiliaries. 

This zinc-promoted reaction has been used with a variety of carbonyl compounds. Thus, the Luche conditions were applied in a synthesis of (-1-)-muscarine using an aldehyde derived from ethyl lactate [109]. Allyl halide condensation onto a-ketoamides of proline benzyl ester gave good diastereoselec-tivity when performed in the presence of zinc dust and pyridinium p-toluene-sulfonate in a water/THF mixture. In this way, a-hydroxy ketones were obtained with good enantioselectivity after removal of the chiral auxiliary [110]. Reactions of allyl bromide under the Luche conditions with y-aldo esters afforded y-hydroxy esters, which were converted in a one-pot reaction to y-allyl-y-butyro-lactones (Scheme 22) [111]. 

Camphor 93, a bridged monoterpene with one six- and two five-membered rings is available in both enantiomeric forms and was used by Woodward in the synthesis of vitamin B12. Sulfonation on camphor occurs on the bridgehead methyl group by a series of rearrangements described in the workbook. You will see in chapter 27 how Oppolzer s sultam 95 is used as a chiral auxiliary and in chapter 33 how oxaziridines such as 96 are used in asymmetric oxidation. Both are made from camphor-10-sulfonic acid 94. 

Of similar nature are chiral halogenations using auxiliary groups. Typical examples are the conversion of esters to enantiomerically pure halohydrins (precursors to chiral epoxides) using camphor-10-sulfonic acid derivatives583 and the chiral synthesis of a-amino acid synthons via diastereoselective bromination of TV-acyl oxazolidone derivatives584. 

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