Nitrostyrenes as 1,4-CCNO-dipoles: diastereoselective formal [4+1] cycloaddition of indoles

An unusual reactivity of nitrostyrenes in phosphorous acid was discovered, which permits the employment of these readily available synthons as 1,4-CCNO-dipole surrogates in a highly diastereoselective (4+1)-cycloaddition of indoles to afford 4H-spiro[indole-3,5-isoxazoles] in a diastereomerically pure form.