Stereospecific approach to the synthesis of ring-A oxygenated sarpagine indole alkaloids. Total synthesis of the dimeric indole alkaloid P-(+)-dispegatrine and six other monomeric indole alkaloids

The first regio- and stereocontrolled total synthesis of the bisphenolic, bisquaternary alkaloid (+)-dispegatrine (1) has been accomplished in an overall yield of 8.3% (12 reaction vessels) from 5-methoxy-d-tryptophan ethyl ester (17). A crucial late-stage thallium(III) mediated intermolecular oxidative dehydrodimerization was employed in the formation of the C9-C9' biaryl axis in 1. The complete stereocontrol observed in this key biaryl coupling step is due to the asymmetric induction by the natural sarpagine configuration of the monomer lochnerine (6) and was confirmed by both the Suzuki and the oxidative dehydrodimerization model studies on the tetrahydro β-carboline (35). The axial chirality of the lochnerine dimer (40) and in turn dispegatrine (1) was established by X-ray crystallography and was determined to be P(S). Additionally, the first total synthesis of the monomeric indole alkaloids (+)-spegatrine (2), (+)-10-methoxyvellosimine (5), (+)-lochnerine (6), lochvinerine (7), (+)-sarpagine (8), and (+)-lochneram (11) were also achieved via the common pentacyclic intermediate 16.

Nature-inspired stereospecific total synthesis of P-(+)-dispegatrine and four other monomeric sarpagine indole alkaloids

Non-phenolic oxidative coupling : The first total synthesis of the C -2 symmetric indole alkaloid P -(+)-dispegatrine (1 ) is reported. A late-stage thallium(III)acetate mediated intermolecular oxidative coupling was employed to construct the C(9)-C(9′) bond with complete regio- and stereocontrol. The exclusive formation of a single atropodiastereomer 12 in this critical step arises due to internal asymmetric induction, as planned. In addition, the first total synthesis of four other monomeric sarpagine indole alkaloids is described.

Brønsted acid mediated cyclization of enaminones. Rapid and efficient access to the tetracyclic framework of the Strychnos alkaloids

The development of an efficient diastereoselective method that permits rapid construction of the tetracyclic core 17 of the Strychnos-Aspidosperma alkaloids is described. Enaminone 16, synthesized in high yield, has been cyclized under the influence of a Brønsted acid to provide the core tetracyclic framework 17 of the Strychnos alkaloids in optically active form or alternatively to the β-ketoester tetrahydro-β-carboline (THBC) unit 18, by varying the equivalents of acid and the molar concentration. Attempts to utilize 18 to form the C₇-C₁₆ bond of the akuammiline related alkaloids represented by strictamine (22), using metal-carbenoid chemistry, are also described.

Regiospecific, enantiospecific total synthesis of C-19 methyl substituted sarpagine alkaloids dihydroperaksine-17-al and dihydroperaksine

The optically active tetracyclic ketone 8 was converted into the pentacylic core 14 of the C-19 methyl substituted N(a)-H sarpagine and ajmaline alkaloids via a critical haloboration reaction. The ketone 14 was then employed in the total synthesis of 19(S),20(R)-dihydroperaksine-17-al (1) and 19(S),20(R)-dihydroperaksine (2). The key regioselective hydroboration and controlled oxidation-epimerization sequence developed in this approach should provide a general method to functionalize the C(20)-C(21) double bond in the ajmaline-related indole alkaloids.

Anxiolytic-like effects of 8-acetylene imidazobenzodiazepines in a rhesus monkey conflict procedure

Conflict procedures can be used to study the receptor mechanisms underlying the anxiolytic effects of benzodiazepines and other GABA(A) receptor modulators. In the present study, we first determined the efficacy and binding affinity of the benzodiazepine diazepam and recently synthesized GABA(A) receptor modulators JY-XHe-053, XHe-II-053, HZ-166, SH-053-2'F-S-CH₃ and SH-053-2'F-R-CH₃ at GABA(A) receptors containing α1, α2, α3 and α5 subunits. Results from these studies suggest that each compound displayed lower efficacy at GABA(A) receptors containing α1 subunits and varying degrees of efficacy and affinity at GABA(A) receptors containing α2, α3 and α5 subunits. Next, we assessed their anxiolytic effects using a rhesus monkey conflict procedure in which behavior was maintained under a fixed-ratio schedule of food delivery in the absence (non-suppressed responding) and presence (suppressed responding) of response-contingent electric shock. Relatively non-selective compounds, such as diazepam and JY-XHe-053 produced characteristic increases in rates of suppressed responding at low to intermediate doses and decreased the average rates of non-suppressed responding at higher doses. XHe-II-053 and HZ-166 also produced increases in suppressed responding at low to intermediate doses, but were ineffective at decreasing rates of non-suppressed responding, consistent with their relatively low efficacy at GABA(A) receptors containing α1 and α5 subunits. In contrast, SH-053-2'F-S-CH₃ and SH-053-2'F-R-CH₃ produced only partial increases in suppressed responding and were ineffective on non-suppressed responding, consistent with their profiles as partial agonists at GABA(A) receptors containing α2, α3 and α5 subunits. These behavioral effects suggest that the anxiolytic and rate-reducing effects of GABA(A) receptor positive modulators are dependent on their relative efficacy and affinity at different GABA(A) receptor subtypes.

Novel positive allosteric modulators of GABAA receptors: do subtle differences in activity at alpha1 plus alpha5 versus alpha2 plus alpha3 subunits account for dissimilarities in behavioral effects in rats?

Over the last years, genetic studies have greatly improved our knowledge on the receptor subtypes mediating various pharmacological effects of positive allosteric modulators at GABA(A) receptors. This stimulated the development of new benzodiazepine (BZ)-like ligands, especially those inactive/low-active at GABA(A) receptors containing the alpha(1) subunit, with the aim of generating more selective drugs. Hereby, the affinity and efficacy of four recently synthesized BZ site ligands: SH-053-2'N, SH-053-S-CH3-2'F, SH-053-R-CH3-2'F and JY-XHe-053 were assessed. They were also studied in behavioral tests of spontaneous locomotor activity, elevated plus maze, and water maze in rats, which are considered predictive of, respectively, the sedative, anxiolytic, and amnesic influence of BZs. The novel ligands had moderately low to low affinity and mild to partial agonistic efficacy at GABA(A) receptors containing the alpha(1) subunit, with variable, but more pronounced efficacy at other BZ-sensitive binding sites. While presumably alpha(1) receptor-mediated sedative effects of GABA(A) modulation were not fully eliminated with any of the ligands tested, only SH-053-2'N and SH-053-S-CH3-2'F, both dosed at 30 mg/kg, exerted anxiolytic effects. The lack of clear anxiolytic-like activity of JY-XHe-053, despite its efficacy at alpha(2)- and alpha(3)-GABA(A) receptors, may have been partly connected with its preferential affinity at alpha(5)-GABA(A) receptors coupled with weak agonist activity at alpha(1)-containing subtypes. The memory impairment in water-maze experiments, generally reported with BZ site agonists, was completely circumvented with all four ligands. The results suggest that a substantial amount of activity at alpha(1) GABA(A) receptors is needed for affecting spatial learning and memory impairments, while much weaker activity at alpha(1)- and alpha(5)-GABA(A) receptors is sufficient for eliciting sedation.

Recent progress in the total synthesis of indole alkaloids

This review describes the most recent synthetic routes directed toward the construction of structurally complex indole alkaloids, many syntheses of which contain the asymmetric Pictet-Spengler reaction as a key stereochemical step. A kinetic and conformational study of the epimerization of cis 1,2,3-trisubstituted tetrahydro-beta-carbolines into their trans counterparts is described, because this is key to complete asymmetric induction in the Pictet-Spengler reaction. A mechanistic study of the enzyme-catalyzed Pictet-Spengler reaction is also included. The total synthesis of the opioid agonist mitragynine, as well as corynantheidol and the oxindole alstonisine is presented. With regard to bisindole alkaloids, the total synthesis of the antileishmanial bisindoles accedinisine and N'-demethylaccedinisne is described.

Antiseizure activity of novel gamma-aminobutyric acid (A) receptor subtype-selective benzodiazepine analogues in mice and rat models

The antiseizure activity of benzodiazepines (BDZs) 1-5 in mice and rats as animal models is described. These BDZs have selective efficacy for alpha2beta3gamma2 and alpha3beta3gamma2 GABA(A)-receptors. Significant anticonvulsant activity with little or no motor impairment and therapeutic indexes (TI) of 2.8-44 (mice, ip) were observed for compounds 2-4 in the subcutaneous metrazole seizure (scMET) test. In rats, orally (po) the TI was >5 to 105. These compounds represent novel leads in the search for anticonvulsants devoid of sedative, ataxic, and amnestic side effects.

General Approach to the Total Synthesis of Macroline-Related Sarpagine and Ajmaline Alkaloids1

Described in this review is a general and efficient strategy for the synthesis of macroline-related sarpagine and ajmaline alkaloids. The tetracyclic ketone in the parent system, as well as the alkoxy substituted series served as templates for the synthesis of these complex molecules. The palladium-mediated enolate cross coupling process, regiospecific hydroboration, and Tollens reaction are some of the key transformations that have been employed for further functionalization of these templates. Synthetic routes that have been improved, in order to obtain gram quantities of these alkaloids form a part of this review.