Synthesis,thermal reactivity and structure of 1,2-bis(4-hydroxy-3-methoxyphenylethynyl)benzene

The crystal structure of the new enediyne 3 was determined by X-ray diffraction. 3 crystallizes in the orthorhombic space group Pbca (No. 61) with a = 12.7148(9) Å, b = 7.4872(8) Å, c = 38.890(4) Å, = = = 90 and Z = 8. The three-dimensional structure is characterized by an 8-membered cyclic array linking four molecules of 3 by hydrogen bonding between phenolic and methoxy groups. Furthermore, stabilization by –-stacking and CH s interactions is present. The important cd distance (between the two terminal acetylene carbons of the enediyne) which is relevant for the Bergman cyclization, was determined to 401 pm, being in good agreement with the high cyclization barrier ( G = 147.9 kJ mol^–1 at 220C) that was determined by differential scanning calorimetry (DSC).