H.-J. Hansen

Photochemistry of some heterocyclic systems

Whereas 2H-benztriazoles are photochemically stable, 1H-benztriazoles yield biradicals after spliting of nitrogen. These biradicals attack aromatic substrates such as anisole, toluene, fluorobenzene or benzonitrile preferentially in the o- and p-positions with formation of the corresponding o-aminobiphenyls (cf. schemes 2-5). With a deficiency of aromatic substrate, attack at the o-position is strongly favored (cf. Figures 1, 2). Thereafter the very complex photochemistry of 3,5-diaryl-2-isoxazolines will be discussed (cf. schemes 7-9). This led to the discovery of the photochemical cycloaddition between 3-phenyl-2H-azirines and aldehydes, with formation of 3-oxazolines (cf. schemes 10, 11). The nitrile methylene ylides produced photochemically from azirines react, mainly in good yield, with substrates which contain cumulative (cf. schemes 15, 16) or activated (cf. schemes 12,13) double bonds. Noteworthy, is the photochemical incorporation of carbon dioxide, leading to the little known isoazlactones (scheme 15). After a short discussion of the photochemistry of the sydnones (cf. schemes 19-23), which has been only briefly investigated so far, the photolysis of indazoles, benzisoxazoles, and anthranils in strongly acid solution will be discussed (cf. schemes 24-27). These photochemical reactions give rise to aryl azenium ions or aryl oxenium ions, which react with the solvent SH preferentially in the position para to the nitrogen or oxygen, respectively.

RECENT DEVELOPMENTS IN AROMATIC SIGMATROPIC REARRANGEMENTS AND RELATED REACTIONS

Recent developments of some aromatic pericyclic reactions are reviewed. The transition state geometry of the aromatic Claisen and Cope rearrangements is discussed. Whereas a chair-like geometry is favoured normally, the stereochemistry of the products resulting from the thermal rearrangement of methyl-substituted threo-2-(α-methylallyl)-cyclohexa-3,5-dien-l-ones indicates that a boat-like geometry is involved to an extent of 40 − 90%. Bridged biphenyl and binaphthyl allyl and propargyl ethers, which possess a ground state conformation favourable for a [3s, 3s) -rearrangement are transformed thermally, even at temperatures between 25 − 130°, into dienones and their further rearrangement products. A new 2-allyl phenol → coumaran rearrangement, as well as a one step synthesis of a twistadienone, is described. Propargyl aryl and cycloheptatrienyl ethers yield, by multistep rearrangement processes, new cyclic compounds. For example, 2,6-dimethylphenyl ether on heating, followed by treatment with acid, is transformed into the 1,2,4-trimethyl-tropyliumion in good yield. Some photochemical reactions of tricyclic ketones, resulting from the thermal rearrangement of propargyl aryl ethers, are mentioned.