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St. John's wort is a common plant that has been used medicinally for over 20 centuries. This herb is currently used by millions of people, primarily as natural antidepressant; yet, its efficacy is still under constant debate. St. John's wort contains a large aromatic molecule, hypericin, twisted by steric interactions into the shape of a propeller. For use as antidepressant, St. John's wort is standardized to the content of hypericin, but this molecule was recently found not to be the active ingredient. A totally different bicyclic molecule with complex substitution pattern, hyperforin, was then studied as the causative agent. Both molecules are strongly active in biological systems. Hypericin has shown antiviral activity and is a potent natural photosensitizer that has been used in photodynamic therapy against cancer and against HIV in stored blood. Hyperforin was found to activate a particular receptor in the liver that induces the production of an enzyme used for the metabolism of medications. This effect causes more rapid breakdown of many prescription medications and can interfere with their effectiveness. This finding should prompt a reevaluation of regular use of St. John's wort.

This article will present a brief introduction to the WLN system.

Woodward-Hoffmann Rules:Cycloaddition Reactions A major innovation in organic chemistry has been the development of a set of symmetry-based selection rules for concerted chemical reactions (1). These rules, named Woodward-Hoffmann rules in honor of their authors, have proved to be a powerful tool for predicting the preferred pathway and stereochemical course of certain reactions. In a recent review article in this Journal we have attempted to elucidate the derivation and application of the Woodward-Hoffmann rules for electro cyclic reactions (2). Another -class of reactions which can be understood in a similar maimer are those called cycloaddition reactions (i3). A few examples in this reaction class are (4-9)The preceding reactions are called cycloaddition reactions because they occur with ring formation between two conjugated ir-eleetron systems, involving

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