The ring transformation of pyrylium salts and pyrones into pyridines is a well-known process, involving ammonia or its derivatives as the nitrogen source I). However, much less is known about the transformation of pyrans into nitrogen heterocycle^^-^).
It was reported in recent years that the chemical and photochemical reactivity of 4H-pyrans does not fit into general schemes and, to a great extent, depends upon the substitution pattern of the heterocyclic On the other hand, we have previously described a simple synthesis of alkyl-substituted 2-amino-4H-pyrans8x9). We now report Scheme
that these alkyl-substituted aminopyrans are useful, easily available starting materials for the preparation of functionalized alkylpyridines in a series of ring transformations in which the amino group furnishes the required ring nitrogen.
The preparation of alkyl-substituted 3,4-dihydro-2(1 H)pyridinones 2 is achieved by treating the alkyl-substituted 2-aminopyrans 1 with sulfuric acid. The reaction is succesful with a variety of pyrans (X = CN; Y = CN, COMe, C0,Et) with alkyl groups at either position 4 or 6 and also unsubstituted at position 6. It is, however, very sensitive to the reagent concentration. Too low a concentration leads to incomplete reaction, whereas excessive reagents make it very difficult to isolate the product. Depending upon the substitution pattern, precise concentrations must be used. Given the righ conditions, dihydropyridinones 2 are easily isolated. In the I3C-NMR spectrum of 2a the signals of the ring carn Prod-Yield uct (Yo)
R2
x
Starting
K1
compd.