Gunther Seitz

Syntheses and evaluation of halogenated cytisine derivatives and of bioisosteric thiocytisine as potent and selective nAChR ligands.

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.

Effects of cytisine on hydroxyl radicals in vitro and MPTP-induced dopamine depletion in vivo

The potential new iron-chelator cytisine and the radical scavenger N-tert-butyl-alpha-(2-sulfophenyl) nitrone (S-PBN) were incubated in a Fenton system and hydroxyl radical formation was measured with the salicylate trapping assay. Both cytisine and S-PBN reduced hydroxyl radical formation in a concentration-dependent manner. For in vivo studies, C57BL/6 mice were injected repeatedly with cytisine (0.5 mg/kg or 2.0 mg/kg s.c.) or saline seven days before and after a single 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injection (30 mg/kg s.c.). Seven days after MPTP treatment alone dopamine levels were significantly reduced to 12% of the control values (p < 0.001), whereas MPTP + cytisine treatment (2 mg/kg) led to more than twofold higher dopamine levels (p < 0.01) compared with MPTP alone. We have shown for the first time that cytisine attenuates hydroxyl radical formation in vitro and reduces MPTP-induced dopamine depletion. Thus, cytisine may be useful for the treatment of Parkinsons Disease where the chelation of iron ions could prevent neuronal cell death.

Novel B-ring modified allocolchicinoids of the NCME series: design, synthesis, antimicrotubule activity and cytotoxicity

Two new series of allocolchicinoids mimicking the structure of (-)-N-acetylcolchinol O-methyl ether (2, NCME) were synthesized and evaluated for their abilities to inhibit tubulin assembly. Possible antitumor properties resulting thereof were evaluated in vitro on the human MCF-7 breast cancer cell line. The first series of NCME-derivatives was brought about by extending the seven membered B-ring to novel semisynthetic variations with a nitrogen containing eight-membered B-ring similar, for example, to the artificial, potent steganacin aza-analogue 3. In the second series the seven-membered B-ring of NCME (2) was modified by annulation with a heterocyclic ring system. The racemic ketone 7a serving as key precursor involved in the syntheses of all the target NCME variants 9-13 and 15, 16 was easily transformed into the eight-membered B-ring lactams 9 and 10 via a Beckmann rearrangement of the corresponding E-oxime 8. The tetrazole annulated congener 11 was prepared via azidotrimethylsilane-mediated Schmidt rearrangement. Treatment of educt 7a with Brederecks reagent led to the enamino ketone 14, which was easily converted into the pyrazole- or pyrimidine-annulated allocolchicinoids 15 and 16. Remarkably, all the allocolchicinoids 9-13 with an azocin-B-ring affected the tubulin/microtubule equilibrium only moderately. In contrast, the novel heterocycle annulated seven membered B-ring variants 15 and 16 proved to be highly potent tubulin-inhibitory, antimitotic agents. Interaction with tubulin occured at concentrations similar to those observed for colchicine (1) or the lead NCME (2). In all cases the antiproliferative effects correlated roughly with the inhibition of tubulin assembly.

Intramolekulare diels-alder-reaktionen mit substituierten 1,2,4,5-tetrazinen und 1,2,4-triazinen

Abstract 1,2,4,5-tetrazines as well as 1,2,4-triazines carrying sidechain dienophiles react in an intramolecular Diels-Alder cycloaddition with inverse electron demand to yield novel bicyclic pyridazines and pyridines.

Intramolekulare Diels-Alder-reaktionen mit 1,2,4-triazinen und 1,2,4,5-tetrazinen zu neuen Siebenring-anellierten pyridinen und pyridazinen.

Abstract Suitable substituted 1,2,4-triazines as Mell as 1,2,4,3-tetrazines carrying 7-membered ω-alkyne sidechain dienophiles react in an intramolecular Diels-Alder cycloaddition mith inverse electron demand to yield the corresponding novel bicyclic pyridines pyridazines, respectively.

A new and efficient synthetic route to enantiopure (+)-anatoxin-a from (−)-cocaine hydrochloride

Abstract The potent nAChR agonist (+)-anatoxin-a was efficiently synthesized in enantiomerically pure form starting from a readily available confiscated grade (−)-cocaine hydrochloride. The eight-step synthesis providing novel extensions to existing methodology proceeded with 26% overall yield and with stereochemical integrity of the relevant original stereogenic centers. Key steps were an effective ring expansion of the (+)-2-tropinone 2 to the 9-azabicyclo[4.2.1]nonanone 5 with TMSCHN2/Al(CH3)3 and the introduction of the required methyl ketone side chain in masked form by reacting the corresponding enol triflate 6 with ethyl vinyl ether/Pd(OAc2) under Heck reaction conditions.

Novel allocolchicinoids with an eight membered B-ring : Design, synthesis and inhibition of tubulin assembly

Several B-ring variations of O-methyl androbiphenyline (8), newly accessible from (-)-(M,7S)-colchicine via photooxygenation and subsequent endoperoxide-transformation, were synthesized and evaluated for their inhibitory effects on tubulin assembly in vitro. The amino-allocolchicinoid (9), a key compound in this study, was transformed to the highly potent ketone 10 and by oxidation with H2O2/Na2WO4 to a mixture of syn/anti-oximes, like 11 and 12. These could easily be transformed to hitherto unknown allocolchicinoids 13 and 14 with an eight membered B-ring lactam obtained via a Beckmann rearrangement. Surprisingly both do not notably affect tubulin assembly, despite obvious structural similarities with active analogues of the thiocolchicine- and azasteganacin-series.

4-Phenyl-1,2,4-Triazoline-3,5-Dione: A Novel Dehydrogenating Agent for Dihydropyridazines

Abstract We describe a novel and most convenient method for dehydrogenating various annulated dihydropyridazines, being formed by inverse [4+2] cycloadditions of cyclic alkenes with 1,2,4,5-tetrazines, with the easily accessible 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) as dehydrogenating agent.

[4+2]-Cycloadditionen mit tetrabromcyclopropen: Ein neuer syntheseweg zu polyfunktionalisierten indolen, benzofuranen und benzothiophenen

Zusammenfassung Diels-Alder reactions of the 2-vinylhetarenes 1a–c with tetrabromocyclopropene 3 are shown to give the functionalised indole, benzofuran and benzothiophene 10a–c respectively by selective cleavage of the C-1/C-3 bond of the intermediate 4 (X=Br). Additionally, a novel access to the corresponding carbaldehydes 12a–c with satisfying overall yields is described.

Synthesis, crystal structure and magnetic properties of the first structurally characterized 1,2-dithiocroconato-containing Cu(II) complex, [Cu(bpca)(H2O)]2[Cu(1,2-dtcr)2]·2H2O

Abstract The first crystal and molecular structure of a transition metal complex containing 1,2-dithiocroconate (1,2-dtcr, dianion of 1,2-dimercaptocylopent-1-ene-3,4,5-trione), [Cu(bpca)(H2O)]2[Cu(1,2-dtcr)2]·2H2O (where bpca is the bis(2-pyrdidylcarbonyl)amide anion), has been determined by single crystal X-ray diffraction methods. The compound crystallizesin the monoclinic syste, space group P21/c, with a = 11.661(3), b = 20.255(6), c = 8.265(3) A , s = 107.26(2)° and Z = 2. The structure is formally built of [Cu(1,2-dtcr)2]2− and [Cu(bpca)(H2O)]+ ions and water of hydration. The copper atom of the anion is situated at a crystallographic inversion centre, bonded to four sulfur atoms in a planar, approximately square arrangement. In the cation the copper equatorial plane is formed by the three nitrogen atoms of the bpca ligand and a water oxygen atom. In addition there is a very weak axial bond to one of the sulfur atoms of a 1,2-dtcr ligand in the anion. Through these latter weak bonds each anion is connected to, and sandwiched between, two cations, resulting in neutral, trinuclear, centrosymmetric formula units. The triple-decker molecules are arranged in stacks along the crystallographic a-axis creating close contacts between the terminal copper atoms and bpca groups of the neighbouring molecules. This intermolecular interaction is, however, too weak to define the structure as a chain compound. The distance between adjacent copper atoms within the trinuclear unit is 4.189(1) A, while the shortest intra-stack metal-metal separation between terminal copper atoms is 5.281(1) A. Variable-temperature magnetic susceptibility measurements in the temperature r.2–140 K reveal that a Curie law is followed; with three non-interacting copper(II) ions in the formula unit.