Conformation of the strained meta,meta′-bridged biphenyl system in the lythraceae alkaloids lythrancine-II and lythrancine-IV: X-ray analyses of lythrancine-II O-p-bromobenzenesulphonate and lythrancine-IV

Abstract

X-Ray structural studies of lythrancine-II O-p-bromobenzenesulphonate (2c) and lythrancine-IV (2b) have provided details of the conformation of the macrocycle which is common to a number of lythraceae alkaloids. The biphenyl system has a seven-atom chain bridging the meta,meta′-positions and the constraint results in a marked asymmetry in the valency angles around the inter-aryl bond, angles endocyclic with respect to the macrocycle being <120° and exocyclic >120°. The dihedral angle between the benzene planes is 49° in (2b) and 47° in (2c). The C–C–C valency angles at the carbon atoms of the CH₂·CH₂·CHR·CH₂ portion of the bridging chain are generally enlarged beyond the tetrahedral value, mean 115 (2b) and 114°(2c). The six-membered rings of the cis-quinolizidine system have chair conformations. The p-bromobenzenesulphonate group in (2c) adopts a conformation in which the S–OC bond is steeply inclined to the benzene plane and the relative orientation of phenyl and quinolizidine substituents about this bond is gauche.

Crystals of both compounds are orthorhombic, space group P2₁2₁2₁, and Z= 4: for (2b)a= 8·769(7), b= 15·368(8), c= 22·927(11)Å; for (2c)a= 21·435(10), b= 13·819(14), c= 11·537(9)Å. The atomic co-ordinates were determined by direct and heavy atom methods and refined by Fourier and least-squares calculations, and the analyses terminated at R 4·3%[2745 reflexions, (2b)] and 10·8%[1903 reflexions, (2c)]. The absolute configuration was established for (2c) by the anomalous-dispersion effect.