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Conformational analysis of a model for the trans-fused FGH ether rings in brevetoxin A.

We have applied the Low Mode:Monte Carlo (LM:MC) conformational search method to an investigation of the low energy structures for 3R,4aS,7aR,11aS,12aR)-11a-methyl-3-phenyldecahydro-1H-[1,3]dioxino[5,4-b]pyrano[2,3-g]oxocine, 3, a model for the FGH rings in brevetoxin A, a potent marine toxin. Searches performed using a variety of classical force field-solvent combinations yielded ensembles containing between 30 and 60 structures with the central G-ring adopting the crown, twist-crown, boat-chair, and boat conformations. The lowest energy structure depended on the force field-solvent treatment, and the twist-crown and boat-chair conformers were typically lowest in energy. B3LYP/6-31G** geometry optimizations using the SCRF continuum solvent model confirmed these structures but indicated that the crown was the energetically preferred conformer with the boat-chair lying within 1.4 kcal/mol.