CONVERSION OF N-ACYL-2,3-DIHYDRO-4-PYRIDONES TO 4-CHLORO-1,2-DIHYDROPYRIDINES USING THE VILSMEIER REAGENT - SYNTHESIS OF (-)-CONIINE AND (PLUS-OR-MINUS)-LUPININE

The full details are given of a study on the conversion of dihydropyridones of the type 3 to 4-chloro-1,2-dihydropyridines 4 using a Vilsmeier reagent. The use of 1 equiv of Vilsmeier reagent under mild conditions (ClCHCCl2, rt) transformed several racemic N-acyl-2,3-dihydro-4-pyridones 3 to dihydropyridines 4 in very good to excellent yields (83-96%). A C-3 methyl group can be tolerated as was demonstrated in the preparation of 4-chloro-3-methyl-1,2-dihydropyridine 7 from dihydropyridone 6 in 90 % yield. The utility of this conversion was demonstrated in the synthesis of the piperidine alkaloid, (-)-coniine. The synthesis of (-)-coniine was completed in five steps from 4-methoxy-3-(triisopropylsilyl)pyridine in 54% overall yield. When 2,3-dihydro-4-pyridones are treated with excess Vilsmeier reagent, good yields of 4-chloro-3-formyl-1,2-dihydropyridines result. These heterocycles are useful intermediates for alkaloid preparation, as was shown by two syntheses of the quinolizidine alkaloid, (+/-)-lupinine, carried out in three and five steps, respectively.