Karl-Heinz Pfoertner

Topochemical photoreactions of aromatic retinoids

Abstract The aromatic retinoid 1a exists in three crystal modifications. The centric α- 1a yields the centrosymmetric photocycloadduct 2 , whereas the γ-modification of 1a does not react photochemically. Deviations from this commonly accepted pattern are the occurrence of a second centric modification α1- 1a instead of the usual β-type and of two different photocycloadducts 3 and 4 derived therefrom in a ratio of 5 : 1. This ratio results from the probability of the excited molecule to react with its neighbours which are shifted against each other in the crystal lattice. Substitution of the ethyl ester group in 1a by a di-ethylamide ( 1 d) leads to E/Z-isomerization only, though the shortest distance of 3.66 A between two molecules is suitable for a [2+2]-photocycloaddition across the centre of symmetry. The packing diagrams of α- 1a , α1- 1a , and of 1d as well as the structures of 1a , 1d , and 2 have been determined by X-ray analyses, whereas the structures of 3 , 4 , and 5 could be deduced from 1H- and 13C-NMR data including homo- and heteronuclear 2D and inter-proton Overhauser experiments.

A facile access to substituted indeno[1,7-bc]furans

Abstract The reaction of 4-oxo-β-ionone 1 with 2 eq. HCN from NaCN in MeOH leads in excellent yield stereospecifically to a cis-fused tricyclic ring system of the indeno[1,7-bc]furan type 5. Acid catalyzed hydrolysis of the imino group in 5 gives lactone 6a which was used to determine the basic structure of the new indeno[1,7-bc]furanx by X-ray analysis. The new compounds were fully characterized by 1H and 13C NMR and the assignments by NOE (6a) and 1H-detected one-bond and multiple-bond 1H, 13C-COSY (6a–c).

The 1,3-carbalkoxy shift. An unusual photorearrangement

Abstract Irradiation of the 2-oxoindeno[1,7-bc]furan-6-carbonitrile 1 in protic solvents with light of 254 nm induces a 1,3-carbalkoxy shift. Supported by X-ray analyses of 2 and 3a it could be shown that the resulting 3,7-dioxo-3H-1,3a-ethano-isobenzofuran-9-carbonitriles 2 and 3 are distinguished by the sterical arrangement of the substituents at C-atom 9. In MeOH 9α-carbonitrile 2 and in t.-BuOH 9β-carbonitrile 3 is the main product. In contrast to t.-BuOH the primary reaction in MeOH is overlapped by typical ketone photochemistry of the newly formed 7-oxo group. Lack of any photoreaction of 1 in aprotic solvents indicates an ionic mechanism rather than the involvement of free radicals. All new compounds were fully characterized by 1H and 13C NMR and the assignments supported by NOE and 1H-detected one-bond and multiple bond 1H, 13C-COSY.

Novel photosensitizers for the E/Z-isomerization of trienes. Part 1. Syntheses and application

Uranine and its thioxanthene and selenoxanthene analogues, as well as the disodium salts of o-(6-hydroxy-3-oxo-3H-xanthen-9-yl)benzenesulphonic acid and its thioxanthene and selenoxanthene analogues, show different photosensitizer properties in the E/Z-isomerization of trienes. This has been verified by applying them to the technically useful conversion of tachysterol into previtamin D. The syntheses of the novel photosensitizers with thioxanthene and selenoxanthene moieties are also described.

Novel photosensitizers for the E/Z-isomerization of trienes. Part 2. Mechanistic and photophysical aspects

To investigate the reaction mechanism, the disodium salts of o-(6-hydroxy-3-oxo-3H-xanthen-9-yl)benzenesulphonic acid and its thioxanthene analogue were selected from a class of photosensitizers firstly applied to the E/Z-isomerization of trienes. As an example of the latter the conversion of tachysterol into previtamin D was chosen. A presentation of the photophysical properties of six sensitizers with xanthene, thioxanthene and selenoxanthene moieties and a description of the aggregation phenomena of the above-mentioned two compounds completes this study.