Photoreactions of 1-Acetylisatin with Alkynes: Regioselectivity in Oxetene Formation and Easy Access to 3-Alkylideneoxindoles and Dispiro[oxindole[3,2‘]furan[3‘,3‘ ‘]oxindole]s

Photoinduced reactions of 1-acetylisatin (IS) with diphenylacetylenes 1a−c, 1-(p-methoxyphenyl)propyne 2, and 1,4-diphenyl-1,3-butadiyne 3 gave β,β-disubstituted 3-alkylidene oxindoles 6−12
respectively via [2+2] cycloaddition of 3IS* with the alkyne and subsequent oxetene ring opening.
Photoreactions of IS with phenylacetylenes 4a−d and cyclopropylacetylene 5 furnished the
dispiroindole[3,2‘]furan[3‘,3‘ ‘]indoles 13 and 14. Compounds 13 and 14 are formed in tandem
reactions initiated by [2+2] cycloaddition of 3IS* with the alkynes to give spirooxetenes Va and
Vb, which upon spontaneous ring opening gave the α,β-unsaturated aldehydes IVa and IVb. It is
proposed that hydrogen abstraction of 3IS* from the C(O)−H functionality in IV followed by
dissociation of the triplet isatin ketyl (A)−aldehyde acyl (B) radical pair and an oxygenphilic attack
of the acyl radical B at the C3 carbonyl oxygen atom of a neutral IS gave the 2:1 (IS:4) radical C,
which took part in an intramolecular radical cyclization to give the dispiroindole[3,2‘]furan[3‘,3‘ ‘]indoles 13 and 14. The regioselectivity in the [2+2] photocycloadditions of IS with 4 to afford the
oxetene Va depends on the intervening of the more stable 1,4-diradical intermediates VI, which
have a linear α-phenyl-substituted vinyl radical where the phenyl provides spin delocalization of
the radical center at the sp carbon atom.