Preparation of true nitroso compounds

Up to the present, compounds containing active methylene groups have only seldom been converted into true nitroso compounds or their dimers. 

To avoid immediate rearrangement to oximes, it is necessary to avoid the presence of traces of alkali during the reaction, which should if possible, be carried out without a solvent and particularly without a hydroxylic solvent. 

Baeyer obtained some dimeric nitroso compounds in the terpene series by the action of ethyl or pentyl nitrite on terpene ketones in the presence of small amounts of acetyl chloride or by means of pentyl nitrite and a few drops of concentrated hydrochloric acid in light petroleum solution 273 and similar procedures have been used by some other workers.274,275 

Dimeric ethyl l-nitroso-2-oxocyclopentanecarboxylate 274 Ethyl 2-oxocyclopentanecarb-oxylate (10 g) is mixed with ethyl nitrite (6-7 g theor. 5 g) with ice-cooling, and a few drops of acetyl chloride are added. After 10-15 h the colorless dimeric nitroso compound is filtered off and washed with ether. The yield is 60-80%. 

Wieland and Bloch276 obtained dimeric nitroso compounds by leading nitrogen oxides (from arsenic trioxide and nitric acid of d 1.38) into cooled ethereal solutions of 1,3-diketones. The dimeric nitroso compounds from dibenzoyl-methane, benzoyl-p-anisoylmethane, and benzoylacetone were prepared in this way. 

---