1.4- Dihydropyridines, formation from ethyl

A new class of fluoroalkyled pyridine herbicides has been synthesized from ethyl 4,4,4-trifluoroacetoacetate. The synthetic route involves formation of dihydropyridine intermediate which upon oxidation or dehydrofluorination produces the desired pyridine compounds. Hydrolysis of the ester groups followed by derivatization of the resulting acids yields a variety of herbicidally active pyridine compounds. 

The addition of nucleophiles to 1-acylpyridinium salts has surfaced as a powerful method for the synthesis of substituted pyridines. The 1-acylpyridinium salts are formed in situ by adding an acyl chloride to a pyridine in an aprotic solvent such as tetrahydrofuran. The formation of the 1-acylpyridinium salt is very rapid and will occur in the presence of various organometallics without significant competition from the reaction of the nucleophile and the acyl chloride. The addition of ethyl chloroformate to a mixture of pyridine and ethylmagnesium bromide gives 1,2- and 1-4-dihydropyridines 29 and 30 in a ratio of 64/36. Although these dihydropyridine intermediates can be aromatized with hot sulfur to 2- and 4-alkylpyridines, the poor regioselectivity makes this procedure unattractive. 

A total synthesis of the racemic form of the cyclophane alkaloid lythranthridine (31) has been described (Scheme 5), starting from the diol (30), synthesized earlier. The total synthesis of ( )-carpaine (32) has been reported preliminarily (Scheme 6) it uses a new reaction between l-acyl-l,2-dihydropyridines and singlet oxygen, involving the formation of an adduct, which is then cleaved by stannous chloride in the presence of ethyl vinyl ether. °... 

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