The structure of hallerol, a boat-boat germacrane lactol

Abstract

The crystal and molecular structure of the trans,trans-germacradiene hallerol has been determined by direct methods from 1 118 unique intensities collected on a diffractometer and refined by least-squares methods to R= 0.038. The crystals are orthorhombic, space group P2₁2₁2₁, with a= 6.010(1), b= 11.846(3), c= 20.035(5)Å. The cyclodeca-trans-trans-1(10),4-diene ring shows the boat-boat conformation with the double bonds in the parallel orientation; the methyl groups C(14) and C(15) are in the anti-β and α orientation respectively. This is the first case of such a configuration being found in the solid state. The α-methyl-γ-lactol is trans-fused at C(7)–C(8) and adopts an envelope conformation with C(12) above the plane of the other four atoms. The hydroxy lactol group acts as donor in an intramolecular hydrogen bond [with the hydroxy group at C(6) as acceptor] which plays an important role in stabilizing the energetically less favoured conformation of the cyclodecadiene ring of hallerol.