Total synthesis of brevetoxin A.

Brevetoxin A is the most potent neurotoxin secreted by Gymnodinium breve Davis, a marine organism often associated with harmful algal blooms known as 'red tides'. The compound, whose mechanism of action involves binding to and opening of sodium channels, is sufficiently toxic to kill fish at concentrations of nanograms per ml and, after accumulation in filter-feeding shellfish, to poison human consumers. The precise pathway by which nature constructs brevetoxin A is at present unknown, but strategies for its total synthesis have been contemplated for some time. The synthetic challenge posed by brevetoxin A reflects the high complexity of its molecular structure: 10 oxygen atoms and a chain of 44 carbon atoms are woven into a polycyclic macromolecule that includes 10 rings (containing between 5 and 9 atoms) and 22 stereogenic centres. Particularly challenging are the 7-, 8- and 9-membered rings which allow the molecule to undergo slow conformational changes and force a 90 degrees twist at one of its rings. Here we describe the successful incorporation of methods that were specifically developed for the construction of these rings into an overall strategy for the total synthesis of brevetoxin A in its naturally occurring form. The convergent synthesis reported here renders this scarce neurotoxin synthetically available and, more importantly, allows the design and synthesis of analogues for further biochemical studies.

Effect of stereochemistry on the transport of Aca-linked beta-turn peptidomimetics across a human intestinal cell line.

Transcellular transport is one of the most important barriers facing the development of new therapeutic agents. However, little is known about the specific effects of structure and particularly stereochemistry on cell permeability. An attractive in vitro model has been developed for the direct assessment of cell transport, using the immortalized human epithelial cell line, Caco-2. The present study assesses the effects of stereochemistry on transport in a commonly used beta-turn model system. Thus, L,L- and L,D-Ala-Ala were cyclized with aminocaproic acid, resulting in macrocycles in which the dipeptides correspond to the i + 1 and i + 2 positions of a beta-turn. The transport of these dipeptides across a Caco-2 cell monolayer was determined, along with corresponding acyclic models (L,L- and L,D-CH3CH2C(O)-Ala-Ala-n-Pr). The transport studies were carried out in the presence and absence of verapamil, a known inhibitor of the apically polarized efflux system present in Caco-2 cells. Both apical-->basolateral and basolateral-->apical transport were measured. Measurements made in the presence of verapamil showed that the cyclic peptides experienced a ca. 4-5-fold difference in intrinsic flux depending on stereochemistry, with the L,D isomer being transported at a higher rate. These differences disappeared in the acyclic cases examined (permeability coefficient ratios of the L,D/L,L isomers were 1.04-1.13). These observations are discussed in terms of the conformations and hydrogen-bonding characteristics of the compounds as determined by NMR spectroscopy.