Diels-Alder adducts have been prepared by the action of the reactive intermediate, 2,2,2-trichloroethyl nitrosoformate, on a series of simple conjugated dienes, and the reductive de-acylation and cleavage of these adducts have been investigated. The methods developed in this study have been applied to a new preparation of 14f-aminocodeinone via the cycloadduct of 2,2,2-trichloroethyl nitrosoformate with thebaine. Nitrosyl hydride, HNO, produced by the periodate oxidation of hydroxyl amine, has been trapped by N-benzyloxycarbonylnorthebaine and N-t-butoxycarbony1northebaine to form the corresponding cycloadducts. These thermally labile compounds have been characterised by formation of their N-acetyl derivatives. The intramolecular ene reactions of C-nitrosocarbonyl compounds and nitrosoformates have been studied. The cyclisation of 1-nitrosocarbonylhept- 4-ene, generated by the thermal cleavage of its adduct with 9,10-dimethylanthracene, did not proceed cleanly. However, five olefinic nitrosoformates, generated by the thermal cleavage of their cyclopentadiene adducts, have been shown to undergo intramolecular ene reactions to give cyclic hydroxamic acids. The nitrosofoznates of 3,3-dimethtiylallyl alcohol, 2-methylprop-2-en-l-ol, 4-methylpent-3-en- 1-01,3-methylcyclohex-3-en-1-ol, and 4-methylcyclohex-3-en-1-ol, cyclised to give 5,6,6,7, and 6-membered ring compounds respectively. In each case only one mode of cyclisation was observed.