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.

1-乙酰靛红（1-acetylisatin，简称IS）与二苯乙炔类1a~c、1-(对甲氧基苯基)丙炔2以及1,4-二苯基-1,3-丁二炔3发生光诱导反应，经由三重态激发态IS（3IS*）与炔烃的[2+2]环加成（[2+2] cycloaddition）以及后续氧杂环丁烯开环步骤，分别得到β,β-二取代3-亚烷基羟吲哚（β,β-disubstituted 3-alkylidene oxindoles）6~12。

IS与苯乙炔类4a~d及环丙基乙炔5的光反应则合成得到二螺吲哚[3,2‘]呋喃[3‘,3‘ ‘]吲哚（dispiroindole[3,2‘]furan[3‘,3‘ ‘]indoles）13和14。

化合物13与14通过串联反应生成：首先3IS*与炔烃发生[2+2]环加成得到螺氧杂环丁烯（spirooxetenes）中间体Va和Vb，随后自发开环生成α,β-不饱和醛IVa和IVb。研究提出的反应机理为：3IS*从IV的C(O)-H官能团中夺取氢原子，随后三重态靛红酮基（A）-醛酰基（B）自由基对发生解离，且酰基自由基B对中性IS的C3位羰基氧发生亲氧进攻，最终得到物质的量比为2:1（IS:4）的自由基中间体C；该中间体经分子内自由基环化（intramolecular radical cyclization），最终生成二螺吲哚[3,2‘]呋喃[3‘,3‘ ‘]吲哚13和14。

IS与4发生[2+2]光环加成生成氧杂环丁烯Va的区域选择性，取决于更稳定的1,4-双自由基中间体（1,4-diradical intermediates）VI的形成：该中间体含有线型α-苯基取代乙烯基自由基，其中苯基可对sp杂化碳原子上的自由基中心起到自旋离域作用。