The Reaction of 1-Tetralones with Thallium Trinitrate Supported on Clay: Ring Contraction vs -Oxidation

The indan system occurs in several natural products of biological relevance. The most common approaches for synthesizing such molecules are either the direct modification of substrates that already bear the indan skeleton or the cycloaddition reactions. In a previous paper, we have reported the preparation of the hydrindan system by thallium trinitrate (TTN) mediated ring contraction of 2-decalones. Similarly, functionalized indans were prepared from 3-alkenols or from 1,2-dihydronaphthalenes. The comparison between the prices of 1-tetralones and 1-indanones, as well as our long standing interest in the thallium(III) chemistry, prompted us to investigate the application of the methodology above mentioned for the transformation of 1-tetralones into indanic derivatives. There are only two papers regarding the reaction of tetralones with thallium(III) salts. Taylor et al., in a communication during the seventies, reported that 1tetralone (1) is converted into a complex mixture of more than 10 products, when treated with TTN in methanol. However, products of ring contraction 2 and of -oxidation 3 were isolated, in 1:1 ratio, using TTN adsorbed on Montmorillonite K-10. Unfortunately, the authors did not provide experimental information, such as time, molar ratio of TTN, temperature and solvent (there is only an indication that it was employed an inert one). The other work dealing with the reaction of tetralones with thallium(III) salt, published in 1990, reported that the treatment of 6-methoxy-1-tetralone (7 in Scheme 4) with TTN in acetic acid gives 2,2-dinitrato-6-methoxy-1tetralone, as the only isolated product, in 10% yield.