New chiral oxazolidine-dihydropyridines

2,4,6-Triphenyl-N-methyl-pyridinium ylides can be prepared by deprotonation of the methyl N-substitutent. Usually, aqueous alkali performs well and “one pot” aldol addition of such ylides to aromatic aldehydes leads to N-β-hydroxy pyridinium salts. Such intermediate products cyclize “in situ” to oxazolidine-dihydropyridines1. Encouraged by such preliminary results, we envisaged a preparative route to chiral isoquinuclidines. To this end, four chiral 2,4,6-triphenylpyridinium tetrafluoroborates were prepared from 2,4,6-triphenylpyrilium tetrafluoroborate and enantiomerically pure-aminoalchools2. As expected, treatment of pyridinium salts (I - IV) with aqueous sodium hydroxide (20%m/v) afforded the corresponding oxazolidines-dihydropyridines in moderate yields, as mixtures of diastereoisomers: The Diels-Alder reaction employing N-methylmaleimide as dienophile afforded the corresponding adducts as mixtures of diastereoisomers that could be isolated by column chromatography on silica gel.