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The coumarin ring moiety of (+)-decursin is planar and the adjacent dihydropyran ring forms distorted half-chair conformation with axial 3-methyl-2-butenoyl group (Figure 1). Generally, bulky substituent like 3-methyl-2-butenoyl is favored at equatorial position of the ring structure, but it is found at axial position and almost rectangular to the pyranocoumarin ring. The O4-C11-C10 and O4-C11-C12 angles are found at 109.87(18) o and 106.06 (17) o , respectively. Although short interaction of 2.453 A between O5 and H9 is found in the packing diagram (data not shown), the major reason of the axial conformation by 3-methyl-2-butenoyl substituent appears to be the steric hindrance of 3-methyl-2butenoyl group with two methyl groups of C13 and C14. Space filling model of (+)-decursin showed no room for O5 of 3-methyl-2-butenoyl group close to the two methyl groups, when it forms equatorial 3-methyl-2-butenoyl conformation (data not shown). The distance of O4-C11 single bond is found at 1.450(3) A, and those of O3-C7 and O3C12 single bonds are found at 1.373(3) A and 1.464(3) A, respectively. It seems the resonance structure of coumarin ring expands to O3 atom, based on the short distance and partial double bond character of O3-C7 bond. For the acetylcholinesterase inhibitors, it is known that hydrophobic and electrostatic interactions are required at the active site. 18 The charged glutamate and polar serine and tyrosine residues form electrostatic site and the aromatic

Axial Conformation of 3-Methyl-2-butenoyl Group in Pyranocoumarin Ring Endows Biological Activity of (+)-Decursin