T. Sandmeyer, Helv. Chim. Acta 2, 234 (1919).
Formation of isonitrosoacetodiphenylamidine by condensation of chloral hydrate, hydroxylamine and aniline, cyclization with concentrated sulfuric acid, and quantitative hydrolysis to isatin on dilution:

J. Martinet, P. Cousset, Compt. Rend. 172, 1234 (1921); C. Hollins, The Synthesis of Nitrogen Ring Compounds (London, 1924) p 103; C. S. Marvel, G. S. Hiers, Org. Synth. coll. vol. I, 327 (1943); P. L. Julian et al., Heterocycl. Compd. 3, 208 (1952); F. E. Sheibley, J. S. McNulty, J. Org. Chem. 21, 171 (1956); O. Bayer, W. Eckert, Houben-Weyl 7/4, 14 (1968); S. J. Garden et al., Tetrahedron Lett. 38, 1501 (1997). Cf. Sandmeyer Diphenylurea Isatin Synthesis.