Laurencia natural products. Part I. Crystal structure and absolute stereochemistry of laurencin

Abstract

The crystal structure and the absolute stereochemistry of laurencin have been determined by a single-crystal X-ray analysis. The crystals are orthorhombic, space group P2₁2₁2₁ with four molecules of C₁₇H₂₃O₃Br in a unit cell of dimensions a= 7·70, b= 9·70, c= 22·93 Å. The structure was solved by the heavy-atom method and refined by least-squares calculations. R is 0·103 for 1152 reflexions. The absolute stereochemistry was determined by consideration of anomalous dispersion effects. The molecule is unusual in that it has an eight-membered ether-ring containing a cis-double bond. Laurencin provides an example of strong intermolecular–C⋮CH ⋯ O hydrogen-bonding in the solid state as revealed by i.r. absorption data. In the crystal, the terminal ethynyl group of a trans-hex-3-ene-5-yne side-chain is involved in a bifurcated hydrogen bond with ether and acetate oxygen atoms of a neighbouring molecule.