LACTONIZATION PRODUCTS OF 2-[(2,6-DIMETHOXYPHENYL)ETHYNYL]-3-METHOXYBENZOIC ACID

Structural analyses of 2-[(2,6-dimethoxyphenyl)ethynyl]3-methoxybenzoi c acid, (1), and the two products of lactonization, -dimethoxyphenyl)- 5-methoxy-1H-2-benzopyran-1-one, (2), and (Z)-3-[(2,6-dimethoxyphenyl) methylene] -4-methoxy-1(3H)-isobenzofuranone, (3), three isomers of C1 8H16O5, suggest an explanation for the favored pathway for lactonizati on. The two rings in (1) form a dihedral angle of 84.30 (4)degrees. Th e carboxy group is nearly coplanar with the benzoate ring with an O=C- C-C torsion angle of -3.7 (2)degrees. The triple-bond length is 1.191 (2) Angstrom and the bond angles at the ethynyl C atoms are 176.4 (1) and 171.0 (1)degrees, both significantly distorted from 180 degrees. T he benzopyran ring and the dimethoxyphenyl ring in (2) form a dihedral angle of 58.90 (5)degrees. The pyrano C=C double-bond length is 1.324 (2) Angstrom. Distortions appear in the two exocyclic angles [112.1 ( 1) and 127.3 (2)degrees] about the aryl substituted pyrano C atom. The isobenzofuran ring and the dimethoxyphenyl ring in (3) form a dihedra l angle of 120.83 (5)degrees. The exocyclic C=C double-bond length is 1.323 (2) Angstrom. Distortions appear in the benzenoid angles at the C atoms adjacent to the ring fusion [117.5 (1) and 116.7 (1)degrees] a nd in two of the three angles [121.2 (1), 131.00 (9) and 107.7 (1)degr ees] about the furano C atom of the exocyclic C=C double bond.