Chiral Lewis Acids as Stoichiometric Reagents

Chiral borane reagents similar to those described above were independently reported by Yamamoto and co-workers in the same year (Eq. 2) [2]. These reagents promote the reaction of juglone and a variety of dienes in the synthesis of anthracycli-nones. The borane reagent used is trimethyl borate. Different tartrate derivatives have been tried and (i ,f )-(+)-tartaric acid diamide has proved to be the most efficient. The best results are obtained in the reaction of juglone and (triethylsiloxy)buta- 

3-diene promoted by a combination of trimethyl borate and (l ,i )-(-i-)-tartaric acid diarylamide (CONH(w-tolyl))—reaction for 12 h at room temperature gives the chiral adduct in 73 % chemical yield and with 92 % ee. The use of an equimolar amount of chiral boron is necessary and, as a consequence, the regio- and stereoselectivity are good. The alkyl and diamide tartrate derivatives, however, are not so effective. This might be explained by the structure of the intermediate which is formed. The hydrogen-bond between the amide hydrogen and the naphthoquinone carbonyl 

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Chiral B(III) Lewis Acids 163 5.4.1 Chiral Lewis Acids as Stoichiometric Reagents... 

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