NITROSATION IN ORGANIC-CHEMISTRY - GENERAL SYNTHESIS OF ALPHA-NITROSO KETONE ACETAL DIMERS AND ALPHA-OXIMINO KETONE ACETALS AND MECHANISM OF THEIR FRAGMENTATION REACTIONS

The reaction of 1-methoxycyclohexene with methyl nitrite either in sulfur dioxide solution in the presence of sulfuric acid, boron trifluoride etherate, or some other acid catalyst, or more effectively in an excess of methyl nitrite as solvent, provides l, l-dimethoxy-2-nitrosocyclohexane dimer in essentially quantitative yield. Other cyclic and open chain enol ethers behave similarly, providing the corresponding α-nitroso ketone acetal dimers in good yields. Transformation of the a-nitroso ketone acetal dimers to the corresponding α-oximino ketone acetals can be easily achieved either by heating in a suitable solvent or in the presence of acid or base catalysts. α-Oximino ketone acetals undergo reaction with ortho esters to give an oxime-ortho ester adduct, which in the presence of an acid catalyst provides an oxime-alkoxycarbonium ion that undergoes rapid fragmentation. For example, l, l-dimethoxy-2-oximinocyclohexane reacts with trimethyl orthoformate to give the corresponding oxime-orthoformate adduct, which then yields methyl 5-cyanopentanoate and trimethyl orthoformate. This cleavage of α-oximino ketone acetals can be carried out even with a catalytic amount of an ortho ester, or more effectively with dialkoxyalkylcar-bonium tetrafluoroborates as catalysts. α-Nitrosocyclohexanone dimethyl acetal dimer underwent a more facile fragmentation to the methyl 5-cyanopentanoate than the α-oximinocyclohexanone dimethyl acetal under the same reaction conditions, suggesting that the previously discussed nitrosolysis reaction involves fragmentation of the a-nitroso ketone acetal monomer rather than the fragmentation of the α-oximinocyclohexanone acetal. © 1979, American Chemical Society. All rights reserved.