Synthetic Applications of the Pyrolysis of Meldrum's Acid Derivatives

The pyrolysis of Meldrum's acid (2,2-dimethyl-1,3-dioxane-4,6-dione) 1 derivatives in solution and in the gas-phase takes place by loss of acetone and carbon dioxide to provide ketene intermediates. In particular, methylene Meldrum's acid derivatives 7 often provide methyleneketenes 8, which act as substrates for internal hydrogen transfer leading to cyclisation reactions. The availability of versatile synthetic routes to 7 (in particular R = heteroatom) has led to the efficient preparation of a diverse range of cyclic compounds such as quinolinones, 3-hydroxythiophenes, naphthols, azepin-3(2H)-ones or pyrrolizin-3-ones initiated respectively by 1,3- 1,4- 1,5- 1,6- or 1,7-prototropic shifts. These reactions are discussed in the context of a rigorous understanding of the chemistry of the ketene intermediates involved. Most of the work was published in the period 1980-2000 but important references to earlier literature are also included.