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The novel acidic amino acids 6a-c, 7, and 8 have been synthesized via 1,3-dipolar cycloadditions, using nitrile oxides and alkynes. The prepared compounds are heterocyclic analogues of glutamic acid with differing chain lengths. One of these compounds, (RS)-2-amino-3-(3-carboxy-5-methyl-4- isoxazolyl)propionic acid (ACPA, 8), was shown in [3H]AMPA binding studies to be more active than AMPA itself (IC50 = 20 nM compared to IC50 = 79 nM for AMPA). No affinity for NMDA receptors (NMDA-sensitive [3H]glutamic acid binding) was found, and only weak affinity in [3H]kainic acid binding (IC50 = 6.3 microM) was detected. The excitatory activity in rat cortical wedge also showed that ACPA was more potent than AMPA (EC50 = 1.0 microM compared to EC50 = 3.5 microM for AMPA). The depolarizing effect of ACPA could be fully antagonized by the selective non-NMDA antagonist 6-cyano-7-nitro-quinoxazoline-2,3-dione (CNQX), but was unaffected by the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (AP5).

The syntheses of several N-methyltropan-3-ylindoles, designed as conge- ners of ibogaine, are described. The synthetic approach to N-methyltro- pan-3-yl-2-indole revealed that the tropanyl 3'-center was quite sensi- tive to acid-catalyzed epimerization. The carbocyclic analog, N-methyl- 2-[bicyclo[3.2.1]-oct-3-yl]indole, also underwent this rearrangement. However, N-methyltropan-3-yl-3-indole was insensitive to acid or base, even under more vigorous conditions. This simple isomerization is quite rare for 2-substituted indoles, especially for cases where the center of reaction is not additionally activated, and normally only takes place under extreme reaction conditions. The mechanism of this reaction was investigated using ab initio molecular orbital calculations, NMR spec- troscopy, and deuterium labeling studies. The difference in reactivity is suggested to arise from a decrease in the relative energy of the exocyclic enamine tautomer due to the presence of increased strain in the endo bicyclic 2-substituent. The title compounds displayed modest pharmacological activity in a variety of biological assays

The enantiomers of the indolyl-2-oxoacetamide 5 were found to have 5-HT3 receptor partial agonist activity with (R)-5 having higher potency than (S)-5.

The synthesis and biochemical evaluation of a series of oxadiazole derivatives of imidazobenzodiazepines related to the benzodiazepine antagonist Ro 15-1788 (2a) are reported. Although the oxadiazole ring is seen as an isosteric replacement for the ester linkage, significant differences in structure-activity trends were observed. Specifically, oxadiazoles 9-12 invariably had increased receptor efficacy (as witnessed by measurements of the GABA shift) relative to the corresponding ester. Additionally, and in direct contrast to the classical agonists such as diazepam, affinity for the benzodiazepine receptor was enhanced by a 7- rather than 8-halo substituent. The results are discussed in terms of a six-point receptor-binding model originally based on the X-ray structure of 2a. For comparison, the crystal structures of two representative oxadiazole derivatives, 10h and 12o, having a 6-oxo and 6-phenyl group, respectively, were determined and the data incorporated into a modified binding model to account for the greater efficacy of these compounds. It is concluded that the antagonist behavior of 2a relies upon the hydrogen-bond-acceptor properties of the ester carbonyl oxygen whereas for the oxadiazole series this site is localized at the imidazole nitrogen.