Racemic α-epimerizable and unfunctionalized aldehydes have been converted into enantiomerically enriched mixtures through a sequence of (i) a conversion into the diastereoisomeric 3-substituted 1-phenyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines by reaction with the (R)- or (S)-1-(α-aminobenzyl)-2-naphthol (Betti's base), (ii) an acid promoted crystallization-induced diastereoisomer transformation (CIDT), and (iii) a clean cleavage of the dihydro-1,3-naphthoxazinic ring of the enriched diastereoisomer, easily collected by filtration, allowing the recovery of the enantiomerically enriched aldehydes and the chiral auxiliary.
carried out employing an in situ prepared ruthenium/axially chiral phosphine catalyst (up to 98% ee). The olfactory activity of the single enantiomers has been evaluated. The fragrances (S)-(+)- and (R)-(–)-canthoxal [(S)-(+)- and (R)-(–)-3-(4-methoxyphenyl)-2-methylpropanal] and (+)- and (–)-Silvial® [(+)- and (–)-3-(4-isobutylphenyl)-2-methylpropanal] have been synthesized in high enantiopurity via a simple