Construction of functionalized indans by thallium(III) promoted ring contraction of 3-alkenols

An efficient four step protocol for the construction of functionalized indans was developed. 1-Tetralones were transformed into unsaturated esters by Reformatsky reaction with bromo-esters (ethyl 2-bromopropionate, ethyl bromoacetate or ethyl 2-bromobutyrate), followed by dehydration of the β-hydroxy-ester formed. Reduction of the ester moiety led to homoallylic alcohols bearing an endocyclic double bond. The oxidative rearrangement of these 3-alkenols was performed by thallium trinitrate (TTN) in a 2:1 mixture of AcOH/H2O, leading to the ring contraction products in moderate to very good yield. Fourteen examples of this sequence were studied. The oxidation of 3-alkenols prepared from 2-tetralones with TTN was also investigated. With these substrates the ring contraction reaction was not observed.