Synthesis of Functionalised 3-Isochromanones by Silylcarbocyclisation/Desilylation Reactions

A new protocol for the synthesis of 3-isochromanone derivatives based on rhodium-promoted silylcarbocyclisation reactions of ethynylbenzyl alcohol with different arylsilanes is described. The structure of the isochromanone depends upon the reaction conditions used: when the reaction is carried out without base, (Z)-4-(aryldimethylsilyl)methylene]isochroman-3-ones are obtained as major products. These compounds can be submitted to a desilylation/aryl migration reaction to give 4-(arylmethyl)isochroman-3-ones in high yields. In contrast, in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), hydrogenation of methyleneisochroman-3-ones takes place, and the corresponding -(silylmethyl)-3-isochromanones are formed. Moreover, when internal alkynes are treated with hydrosilanes under silylcarbocyclisation reaction conditions, alcoholysis of the hydrosilanes occurs exclusively.