Capsaicin and resiniferatoxin are natural products which act specifically on a subset of primary afferent sensory neurons to open a novel cation-selective ion channel in the plasma membrane. These sensory neurons are involved in nociception, and so, these agents are targets for the design of a novel class of analgesics. Although synthetic agonists at the capsaicin receptor have been described previously, competitive antagonists at this receptor would be interesting and novel pharmacological agents. Structure-activity relationships for capsaicin agonists have previously been rationalized, by ourselves and others, by dividing the capsaicin molecule into three regions--the A (aromatic ring)-, B (amide bond)-, and C (hydrophobic side chain)-regions. In this study, the effects on biological activity of conformational constraint of the A-region with respect to the B-region are discussed. Conformational constraint was achieved by the introduction of saturated ring systems of different sizes. The resulting compounds provided agonists of comparable potency to unconstrained analogues as well as a moderately potent antagonist, capsazepine. This compound is the first competitive antagonist of capsaicin and resiniferatoxin to be described and is active in various systems, in vitro and in vivo. It has recently attracted considerable interest as a tool for dissecting the mechanisms by which capsaicin analogues evoke their effects. NMR spectroscopy and X-ray crystallography experiments, as well as molecular modeling techniques, were used to study the conformational behavior of a representative constrained agonist and antagonist. The conformation of the saturated ring contraint in the two cases was found to differ markedly, dramatically affecting the relative disposition of the A-ring and B-region pharmacophores. In agonist structures, the A- and B-regions were virtually coplanar in contrast to those in the antagonist, in which they were approximately orthogonal. A rationale for agonist and antagonist activity at the capsaicin receptor is proposed, based on the consideration of these conformational differences.

Pilocarpine isosteres have been synthesized and characterized with regard to their in vitro muscarinic properties. The results indicate that the carbonyl oxygen of the lactone function of pilocarpine is of primary importance for agonist activity with the ether oxygen being of lesser or secondary importance. An X-ray structure determination for the hydrogen O,O'-ditoluoyltartrate salt of thiolactone pilocarpine isostere 2a has been performed. This compound has an unusual pharmacological profile exhibiting M1-agonist selectivity as well ass presynaptic antagonism. As a result this compound is also viewed as having therapeutic potential for Alzheimer's disease. A model for the binding of pilocarpine and other muscarinic agonists to the third transmembrane helix of the human m1 muscarinic receptor has been developed.

Cyclic Phenyl Carbamates and Their Action on AcetylcbolinesteraseSeveral six-, seven-, and eight-membered cyclic phenyl carbamates of the miotin type have been synthesised and their in uitro potency as inhibitors of acetylcholinesterase determined. The eight-membered rings were found to be the most potent and are comparable with physostigmin or miotin. It was concluded that, for maximum potency, the orientation of the carbamate group relative to the aromatic plane has to be close to orthogonal.Einleitung. ~ Acetylcholinesterase (AChE, EC 3.1.1.7) ist eine regulierende Serinesterase, welche die stimulierende Wirkung des Neurotransmitters Acetylcholin (ACh) am postsynaptischen Rezeptor abbricht. Durch Blockieren der Esterase mit einem Inhibitor wird die Dauer eines Nerv-Impulses verlangert, was bei einer Hypofunktion des cholinergen Systems (z. B. Alzheimer, Miastenia gravis) therapeutisch verwendet werden kann [ 11.Der Wirkungsmechanismus der AChE bei der Spaltung von ACh und von pseudoirreversiblen Inhibitoren vom Miotin-Typ ist schon langst bekannt und sehr gut untersucht worden [2]. Das 'aktive Zentrum' der AChE besteht aus drei Haupt-Interaktionsstellen: einer sog. Ester-Region (oder 'site'), einer anionischen Region und einem lipophilen Mittelteil (s.

The diastereoisomeric 2-substituted 2-azabicyclo[2.2.l]hept-5-enes 2 4 were prepared by aza-Diels-Alder reaction of cyclopentadiene with the corresponding methaniminium ions. Their relative configurations were deduced using 'H,'H-ROESY experiments, and their absolute configurations were assigned from the crystal structure of the aziridinium derivative (-)-5. The absolute configuration of (+)-1, i.e. (lR), was assigned by CD spectroscopy.Introduction. -Following Grieco and Larsen's pioneering report of the aza-DielsAlder reaction between cyclopentadiene and N-benzylmethaniminium ion, formed in situ in aqueous medium [I], many groups exploited and extended this reaction [2]. Analogous diastereoselective reactions were described using amino acids [3] [4] as chiral sources.The absolute configurations of the aza-Diets-Alder adducts with chiral l-phenylethylamine were, however, unknown. In the course of our syntheses of muscarinic agonists [5] and terpene alkaloids [6], we needed the aza-Diels-Alder adducts in enantiomerically pure form and knowledge of their absolute configuration. Thus, we assigned the relative configurations of the adducts using 2D-ROESY experiments and confirmed the validity of the NMR method by independent assignment of the absolute configuration of aziridinium bromide (-)-5 by X-ray crystallography. With this knowledge in hand, the absolute configurations of the enantiomers of 1 could be deduced by CD spectroscopy.

Introduction. -The 1-azabicyclo[2.2. llheptane (1) was first prepared by Prelog and Clemo [l] in 1936. In the last few years, there has been great interest in this skeleton for preparing rigid structural analogues of biogenic amines (for acetylcholine analogues, see [2a-k]; for serotonine analogues, see [21-n]). In derivatives of 1 in which the 3-position is substituted with an ester isostere, the gauche conformation of acetylcholine is mimicked. The 3-exo-(l,2,4-oxadiazolyl) derivatives of 1-azabicyclo[2.2. llheptane were claimed to be among the most potent muscarinic agonists known [3]. When we started our work in this area, all chiral compounds in this series had been reported as racemates [2]. Very recently, methods for resolving l-azabicyclo[2.2.l]heptane-3-carboxylates as their diastereoisomeric amides [4] were disclosed, as well as the synthesis of (+)-(3S,4R)-ethyl l-azabicyclo[2.2.l]heptane-3-carboxylate [5]. For a suitable comparison of the biological activity of these and similar compounds with that of other muscarinic agonists in our models [6], we needed the compounds in enantiomerically pure form and knowledge of their absolute configuration.We, therefore, synthesised both enantiomers of 3-( 1,3-dithian-2-ylidene)-l-azabicyclo[2.2. llheptane ((+)-and (-)-5), a key intermediate in the synthesis of a large number of cholinergic agonists [2a], and determined their absolute configurations both using CD spectroscopy and X-ray crystallography.