Transformations of 2-[(Methoxy-2-oxoethyl)sulfanyl]-5,6-dihydropyridine under Acid Catalysis: Unexpected Transition to Derivatives of 5,6-Dihydropyridin-2(1H)-one and 2,3,6,7-Tetrahydro-5H-[1,3]thiazolo[3,2-a]pyridine

Derivatives of 2,3,6,7-tetrahydro-5 H -[1,3]thiazolo[3,2- a ]pyridine and 5,6-dihydropyridin-2(1 H )-one were unexpectedly obtained at treating methyl 2-[(6-methyl-3-ethoxy-5,6-dihydropyridin-2-yl)sulfanyl]-acetate with a dilute solution of hydrochloric acid. Their formation resulted from the hydration of 5,6-dihydropyridine at the C=N bond accompanied with the elimination of a methyl 2-sulfanylacetate molecule and the addition of the methyl 2-sulfanylacetate to the second 5,6-dihydropyridine molecule at the C 3 =C 4 bond followed by the elimination of a methanol molecule and the closure of a thiazole ring.