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Biomedical subjects

Bernd Engels

Publications and source records attributed to Bernd Engels.

24 records · Page 2Linked to original sources
PubMed2003

Theoretical investigation of the photochemical C2-C6 cyclisation of enyne-heteroallenes.

Herein we discuss computations that explain experimental results regarding a highly efficient triplet analogue of the C(2)-C(6) cyclisation of enyne-heteroallenes recently discovered by Schmittel and co-workers.1 To shed some light on the reasons for the differences found between enyne-carbodiimides, enyne-ketenimines and enyne-allenes, we have computed the reaction profiles of the C(2)-C(6) and of the C(2)-C(7) cyclisations for various model compounds, assuming that the reactions take place on the lowest-lying triplet surfaces. Our results nicely explain the differences and the unexpected high efficiency found for the enyne-carbodiimides. The differences between enyne-carbodiimides and enyne-ketenimines prove to be due to differences in the shapes of the corresponding triplet surfaces. In contrast to the enyne-carbodiimides, for which our calculations predict that a direct cyclisation to the biradical intermediates should occur after the vertical excitation, the enyne-ketenimines relax into a local minimum on the triplet surface. As a consequence, further reaction channels are opened. Our computations indicate that enyne-allene compounds do not react because the necessary excitation energy lies outside the range of the employed triplet photosensitizer. Finally, the close agreement between our results and the experimental findings indicates that the underlying reasons for the differences in the photochemical behaviour of enyne-carbodiimides, enyne-ketenimines and enyne-allenes are related to differences in the electronic structures of the parent systems, while substituent effects are less important.

Journal Article↗
PubMed2003

Synthesis, biological activity, and docking studies of new acetylcholinesterase inhibitors of the bispyridinium type.

A novel series of acetylcholinesterase (AChE) inhibitors of the bispyridinium type was synthesized and the inhibitory activity against AChE and butyrylcholinesterase (BChE) measured. In essence, the substitution pattern influenced the inhibitory potency against AChE, where the most active bispyridiniumoxime (TMB-4) was bisbenzyl substituted followed by monobenzyl substituted, bismethyl substituted, and unsubstituted derivatives of TMB-4. Hence, the bisbenzyl ether of TMB-4 was further investigated. In order to obtain diverse lipophilic and electronic properties for these bisbenzyl bispyridinium derivatives (so-called DUO series), the lateral ring substitution was systematically varied. The lowest IC(50) value against AChE found thus far in the DUO series was 0.34 microM. Docking studies were carried out to elucidate the differences in biological activity. A general binding mode for nearly all compounds could be identified by these investigations. In this binding mode, the docked ligands span the narrow, deeply buried active-site gorge, interacting with Trp84 at the bottom of the gorge, Tyr334 or Phe331 halfway down the gorge, and Trp279 at the peripheral anionic site at the mouth of the gorge. For specific ligands, additional interactions were found which helped to explain their deviating activity. Based on the promising characteristics of the novel acetylcholinesterase inhibitors presented, a series of structurally related, optimized candidates will be developed.

Acetylcholinesterase↗
PubMed2002

On the regioselectivity of the cyclization of enyne-ketenes: a computational investigation and comparison with the Myers-Saito and Schmittel reaction.

The Moore (C(2)-C(7)) cyclization and the alternative C(2)-C(6) cyclization of enyne-ketenes belong to the family of biradical cyclization reactions such as the Bergman reaction of ene-diynes, both the cyclizations of enyne-allenes and enyne-cumulenes. The latter garnered substantial interest due to their antitumor efficacy. The mechanisms of both cyclization modes of enyne-ketenes are still unclear, but as the enyne-ketenes can formally be regarded as heteroanalogues of enyne-allenes, both cyclizations are expected to react via biradical routes. Nevertheless, as shown recently for cyclic allenes, the substitution of a methylene group by oxygen can lead to different energetic ordering of the electronic states of the key intermediates. To elucidate the mechanism, the present work investigates the course of both cyclization modes for various model compounds. To reveal general motifs for the large family of biradical cyclizations, a comparison with enyne-allenes is performed.

Journal Article↗
PubMed2002

Computational assessment of the electronic structures of cyclohexa-1,2,4-triene, 1-oxacyclohexa-2,3,5-triene (3delta(2)-pyran), their benzo derivatives, and cyclohexa-1,2-diene. An experimental approach to 3delta(2)-pyran.

The six-membered cyclic allenes given in the title have been studied theoretically by means of an MR-CI approach. For all compounds, the allene structures were found to be the ground states in the gas phase. In the cases of cyclohexa-1,2-diene (1), the isobenzene 2, and the isonaphthalene 7, the most stable structures having a planar allene moiety are the diradicals 1b, 2b, and 7b, representing the transition states for the racemization of 1a, 2a, and 7a and being less stable than the latter by 14.1, 8.9, and 11.2 kcal/mol, respectively. At variance with this order, the 3delta(2)-pyran 4 and the chromene 5 have the zwitterions 4c and 5c as the most stable planar structures, which lie only 1.0 and 5.4 kcal/mol above 4a and 5a, respectively. According to the simulation of the solvent effect, 4c even becomes the ground state of 4 in THF solution. The frontier orbitals of the respective states of 2 and 4 suggest different rates and sites for the reaction with nucleophiles. For the first time, the pyran 4 has been generated and trapped. As a precursor for 4, 3-bromo-4H-pyran (9) was chosen, the synthesis of which was achieved on two routes from 4H-pyran. The treatment of 9 with potassium tert-butoxide (KOt-Bu)/18-crown-6 gave 4-tert-butoxy-4H-pyran as the only discernible product, whether styrene or furan was present, indicating the interception of 4 by KOt-Bu. Finally, the disagreement between the experiment and the theory concerning the heat of formation and the electronic nature of the isobenzene 2 is resolved by demonstrating that the experimental data can provide only an upper limit of the DeltaH(f) degrees value.

Journal Article↗