Cyclization of Ylidenemalonodinitriles

The utility of malonodinitrile in annelation reactions, via the intermediate ylidenemalonodinitriles, is reviewed. The method is shown to present a convenient pathway to the synthesis of a variety of cyclic compounds, including indenones, indanones, naphthalenes, lactones, and coumarins. Annelation may occur on heterocyclic rings which are susceptible to electrophilic substitution, and hetero atoms may be incorporated in more complex fused-ring systems. The cyclization is quite susceptible to differences in the character of the acidic cyclizing media, which allows selectivity in the kinds of products obtained. 1. Preparation of Ylidenemalonodinitriles 2. Cyclization to Form Five-membered Carbocyclic Rings 2.1. Steric and Bulk Effects on Cyclization 2.2. Influence of Ring Size and Heteroatoms on Cyclization 2.3. Electronic Effects of Substituents on the Aromatic Ring on Cyclization 2.4. Cyclization to Aromatic Heterocyclic Systems 2.5. Indenone-Indanone Equilibrium and Prototropy 3. Cyclization of Saturated Malonodinitrile Derivatives 4. Cyclization to Form Six-membered Carbocyclic Rings 5. Formation of Lactones 5.1. Synthesis of γ-Butyrolactones 5.2. Synthesis of Coumarins 6. Conclusion