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 beta,beta-disubstituted 3-alkylidene oxindoles 6-12 respectively via [2+2] cycloaddition of (IS)-I-3* 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 (IS)-I-3* with the alkynes to give spirooxetenes Va and Vb, which upon spontaneous ring opening gave the alpha,beta-unsaturated aldehydes IVa and IVb. It is proposed that hydrogen abstraction of (IS)-I-3* 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 (ISA) radical Q 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 a-phenyl-substituted vinyl radical where the phenyl provides spin delocalization of the radical center at the sp carbon atom.