Robert E. Gawley

The relationship of brevetoxin ‘length’ and A-ring functionality to binding and activity in neuronal sodium channels

BACKGROUND Brevetoxins are polyether ladder toxins that are ichthyotoxic at nanomolar concentrations. They bind to voltage-gated sodium channels, causing four distinct electrophysiological effects: (i) a shift of activation potential; (ii) occurrence of subconductance states; (iii) induction of longer mean open times of the channel; and (iv) inhibition of channel inactivation. We set out to determine whether these functions all require the same structural elements within the brevetoxin molecules. RESULTS Several synthetically prepared structural analogs of brevetoxin B were examined in synaptosome receptor binding assays and by functional electrophysiological measurements. A truncated analog is not ichthyotoxic at micromolar concentrations, shows decreased receptor-binding affinity, and causes only a shift of activation potential without affecting mean open times or channel inactivation. An analog with the A-ring carbonyl removed binds to the receptor with nanomolar affinity, produces a shift of activation potential and inhibits inactivation, but does not induce longer mean open times. An analog in which the A-ring diol is reduced shows low binding affinity, yet populates five subconductance states. CONCLUSIONS Our data are consistent with the hypothesis that binding to sodium channels requires an elongated cigar-shaped molecule, approximately 30 A long. The four electrophysiological effects of the brevetoxins are not produced by a single structural feature, however, since they can be decoupled by using modified ligands, which are shown here to be partial sodium channel agonists. We propose a detailed model for the binding of brevetoxins to the channel which explains the differences in the effects of the brevetoxin analogs. These studies also offer the potential for developing brevetoxin antagonists.

Highly Enantioselective Catalytic Dynamic Resolution of N-Boc-2-lithiopiperidine: Synthesis of (R)-(+)-N-Boc-Pipecolic Acid, (S)-(―)-Coniine, (S)-(+)-Pelletierine, (+)-β-Conhydrine, and (S)-(―)-Ropivacaine and Formal Synthesis of (―)-Lasubine II and (+)-Cermizine C

The catalytic dynamic resolution (CDR) of rac-2-lithio-N-Boc-piperidine using chiral ligand 8 or its diastereomer 9 in the presence of TMEDA has led to the highly enantioselective syntheses of both enantiomers of 2-substituted piperidines using a wide range of electrophiles. The CDR has been applied to the synthesis of (R)- and (S)-pipecolic acid derivatives, (+)-beta-conhydrine, (S)-(+)-pelletierine, and (S)-(-)-ropivacaine and the formal synthesis of (-)-lasubine II and (+)-cermizine C.

Application of Catalytic Dynamic Resolution of N-Boc-2-lithiopiperidine to the Asymmetric Synthesis of 2-Aryl and 2-Vinyl Piperidines

The highly enantioselective synthesis of 2-aryl- and 2-vinyl-piperidines has been accomplished through a catalytic dynamic resolution (CDR) of N-Boc-2-lithiopiperidine. The method has been applied to the synthesis of both enantiomers of the tobacco alkaloid anabasine.

Binding of brevetoxins and ciguatoxin to the voltage-sensitive sodium channel and conformational analysis of brevetoxin B

The marine toxins known generically as brevetoxins, as well as their structural relative ciguatoxin, are known as polyether ladder toxins, and bind uniquely to site 5 of the voltage-sensitive sodium channel. Rat brain synaptosome binding data show similarities in binding affinity for brevetoxins having the same structural (ladder) backbone, but different affinities between brevetoxins having different backbones. Ciguatoxin has a different backbone from the brevetoxins, but binds even more strongly to the same site. Could the flexibility of the backbone be related to their relative toxicities? As part of an effort to identify the common pharmacophore for the toxins, Monte Carlo methods were used to generate conformational models of the polyether ladder toxin brevetoxin B (PbTx-2) which shows significant flexibility at the juncture of the two 7-membered rings.

Search for configurationally stable, aracemic α-amino organolithiums

Abstract The search for configurationally stable α-amino carbanions has led to an interesting observation of differing reactivity of diastereomeric organolithiums and to the characterization of aracemic 2-lithio-N-methylpiperidine and 2-lithio-N-methylpyrrolidine as configurationally stable α-aminoorganolithiums. Details for the preparation of these and related α-lithioheterocycles, evaluation of their chemical and configurational stability, and a preliminary evaluation of the stereoselectivity of their reactions with electrophiles is presented.

Application of a C2-Symmetric Copper Carbenoid in the Enantioselective Hydrosilylation of Dialkyl and Aryl–Alkyl Ketones

We report excellent reactivity and enantioselectivity of a C(2)-symmetric copper-bound N-heterocyclic carbene (NHC) in the hydrosilylation of a variety of structurally diverse ketones. This catalyst exhibits extraordinary enantioselctivity in the reduction of such challenging substrates as 2-butanone and 3-hexanone. Even at low catalyst loading (2.0 mol %), the reactions occur in under an hour at room temperature and often do not require purification beyond catalyst and solvent removal. The scope of this transformation was investigated in the reduction of 10 aryl-alkyl and alkyl-alkyl ketones.

Coumaryl crown ether based chemosensors: Selective detection of saxitoxin in the presence of sodium and potassium ions

Two novel fluorescent chemosensors in which an aza-crown is linked to 4-coumaryl fluorophores by a methylene spacer have been synthesized for sensing saxitoxin. Fluorescence enhancement was observed upon binding of the dicationic toxin molecule, whereas several metal ions produced no effect.

A PROPOSAL FOR (SLIGHT) MODIFICATION OF THE HUGHES-INGOLD MECHANISTIC DESCRIPTORS FOR SUBSTITUTION REACTIONS

Abstract The term “S E Ar” is proposed to represent electrophilic aromatic substitution, leaving the term “S E 2” to refer exclusively to electrophilic substitutions where a steric course is possible. To describe the steric course of an aliphatic substitution reaction, the suffixes “ret” and “inv” are proposed, referring to retention and inversion of configuration, respectively. The rationale for these proposals is discussed.

Lithiated camphor-derived oxazolidinone S,N-acetals as chiral formyl anion synthons in additions to aldehydes. Asymmetric synthesis of α-hydroxy aldehydes and α-hydroxy acids

N-(Phenylthiomethyl)oxazolidinones derived from camphor can be lithiated and added to aldehydes in good yields and stereoselectivities. The adducts are crystalline, which simplifies isolation of the major diastereomer from the product mixture. Hydrolysis affords enantiopure α-hydroxy aldehydes, which can be oxidized to α-hydroxy acids in good yields. The steric course of the reaction is analyzed in detail and a mechanistic model is presented.

Synthetic applications and inversion dynamics of configurationally stable 2-lithio-2-arylpyrrolidines and -piperidines.

In diethyl ether, N-Boc-2-lithio-2-arylpiperidines have been found to be configurationally stable at -80 °C, whereas N-Boc-2-lithio-2-arylpyrrolidines are configurationally stable at -60 °C. Several tertiary benzylic carbanions derived from enantioenriched 2-aryl heterocycles have been successfully alkylated or acylated with little to no loss of enantiopurity. The scope of the reactions has been explored. The enantiomerization dynamics of N-Boc-2-lithio-2-phenylpyrrolidine and N-Boc-2-lithio-2-phenylpiperidine have been studied in the presence of different solvents and achiral ligands.