Synthesis and Reactions of 4-Chromanones

Many natural products, such as the plant insecticides rotenone and ageratochromene (precocene), some fungitoxic phytoalexins, hashish constituents, vitamin E, and plant pigments, contain the chroman ring system as common structural unit. Some years ago we discovered a particularly simple entry to this class of heterocyclic compounds: The condensation of o‐hydroxyacetophenones with aliphatic aldehydes and ketones in the presence of pyrrolidine leads to 4‐chromanones in good yields. The scope of application and limitations as well as modification of this versatile synthesis are outlined; the accompanying tables give a good indication of the wealth and variety of possible substituents. The reaction of o‐hydroxyacetophenone, e. g. with glyoxylic acid or with α,β‐unsaturated ketones, proceeds atypically. The majority of the new chromanones can be used, inter alia, for the preparation of chromones, chromones and chromons, and for the synthesis of tricyclic and larger ring systems.