Synthetic Strategies for the Preparation of Butenolides and Their Transformation into Other Derivatives

Butenolides, five membered, alpha,beta-unsaturated gamma-lactones, are ubiquitous moieties present in the structures of many natural products and synthetic compounds endowed with a large array of biological activities. Butenolides with an gamma-alkylidene moiety are termed as gamma-alkylidenebutenolides. Because of their chemical structures as well as their biological activities, butenolides have attracted the attention of synthetic organic chemistry community. As a consequence, several methodologies have been developed to synthesize these classes of compounds. The potential use of such substances is expanded due to their easy conversion into lactams and pyridazine derivatives which are also of potential biological interest. Moreover, the chemistry of such compounds is very rich in large extension associated also with the nature of their substituents. Herein, we present some classical and recent methodologies for the synthesis of butenolides and gamma-alkylidene butenolides. These methods involve the use of several transition metal reagents, condensation and conjugated addition reactions. Some methods used to convert butenolides and gamma-alkylidene congeners into lactams and pyridazines are described. Methodologies for the preparation of oxazolones namely azlactones, compounds which are structurally similar to butenolides and have attracted the attention of synthetic organic chemists due to their versatility as building blocks, are also highlighted. Examples of applications of the methodologies in the synthesis of natural products as well as in the preparation of analogues endowed with biological activities, including some contributions from the author's groups, are discussed.