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A Prins cascade process was developed for the synthesis of tetrahydro‐3H‐spiro[isobenzofuran‐pyran] derivatives in good yields and selectivity by the condensation of 3‐[2‐(hydroxymethyl)phenyl]but‐3‐en‐1‐ols with aldehydes or ketones. The reaction proceeds smoothly in the presence of indium(III) trifluoromethanesulfonate (30 mol‐%) in dichloroethane at 80 °C. This is the first report on the synthesis of tetrahydro‐3H‐spiro[isobenzofuran‐pyran] scaffolds through a Prins cascade reaction.

2‐Arylethylbut‐3‐en‐1‐ol is found to undergo smooth Prins cascade reactions with various aldehydes in the presence of Sc(OTf)3 (10 mol‐%) and a stoichiometric amount of TsOH to afford the corresponding trans‐fused hexahydro‐1H‐benzo[f]isochromenes in good yields with excellent selectivity. Likewise, N‐tosyl‐2‐phenethylbut‐3‐en‐1‐amine gives trans‐fused octahydrobenzo[f]isoquinoline derivatives under similar conditions. This is the first example of the synthesis of hexahydro‐1H‐benzo[f]isochromene and octahydrobenzo‐[f]isoquinoline from 2‐arylethylbut‐3‐en‐1‐ol and N‐tosyl‐2‐phenethylbut‐3‐en‐1‐amine, respectively.

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This novel Prins spirocyclization gives a variety of the title compounds with high diastereoselectivities.