Photochemical synthesis of heterocyclic compounds

The aim of our research was to investigate the photocyclisation of N-substituted imide systems, in order to prepare polycyclic heterocyclic photoproducts of potential pharmaceutical interest. The major group of substrates were N-(dialkylaminoalkyl)succinimides. Irradiation of succinimide Mannich bases (A) leads to two diastereoisomers of 1,3-diazabicyclo[3.3,0]octanes in reasonable yields. The relative stereochemistry was assigned on the basis of spectral data. With substrates (e.g. B) derived from unsymmetrical amines, there is strong orientational preference. Substrates (e.g. G) derived from an unsymmetrical succinimide gave a mixture of products; the preferred orientation of cyclisation is to the more hindered C-1 carbonyl group. Using hydantoin substrates (D) which correspond to succinimides in which a methylene group is replaced by a heteroatom, we found that cyclisation occurs only at the C--4 (imide-like) carbonyl group to give diastereoisomers of 1,3,7 - triazabicyclo[3.3,0]octanes. Glutarimide Mannich bases show many parallels with the succinimide analogues. However, pyrrolidin-2-one Mannich bases did not afford photocyclised products. A minor product (E) is formed by cleavage in the amine fragment. N-(Dialkylaminoethyl) derivatives of saturated imides (F) give rise to ring-expanded photoproducts. This is in contrast to phthalimide substrates (G; n = 2), where photoproducts with a new piperazine ring are obtained. Our hypothesis is that an electron-transfer mechanism operates in the aromatic systems, leading to the possibility of medium/large ring formation, but this is not effective in the succinimides which are poorer electron acceptors. To test this, we irradiated a series o f N-(dialkylaminoalkyl) derivatives of imides with a double bond conjugated with the carbonyl groups, and found that these substrates (H) do give photoproducts with new six- or seven-membered rings. Our results with N-(dialkylaminopropyl)phthallmides (G; n = 3) are not entirely in agreement with earlier reports that medium-ring formation is the major photocyclisation process for phthalimide substrates. Further work is required to extend this comparison of the photocyclisation process for saturated, unsaturated and aromatic N-(dialkylaminoalkyl)imides