[57] R . R . Schmidt. J . Talbiwsky. and H . [‘on Bendu. unpublished experiments. [58] F . D. Popp, Advan. Heterocycl. Chem. 9, I (1968). [59] C. E. Cruwforrh and 0. Meth-Cohn, J. C. S. Chem. Comm. 1972, 865. [GO] K . Wallenfels, W Ertrl, and K . Friedrich, Liebigs Ann. Chem. 1973, 1663, and references cited therein. [61] P . Puck. R. Gleiter. and G. Kohrich, Chem. Ber. 103, 1431 (1970), and references cited therein. [62] L. Field and D. L. Tulem in A. Weissberger and E . C . Taylor: The Chemistry of Heterocyclic Compounds. W h y , New York 1972, Vol. 26, p. 574. [63] R. R . Schmidr and M . Dimmler, Chem. Ber. 107, 3800 (1974). [64] D. Lrdnicer and D. E. Emmerr, J. Heterocycl. Chem. 8, 903 (1971). [65] Owing to the pronounced acidifying effect of divalent sulfur, however, this has to be assumed in many cases: see D. Seebach, Angew. Chem. 81. 690 (1969); Angew. Chem. internat. Edit. 8, 639 (1969); Synthesis 1969, 17. Y. Yamadu, D. Diljbovic, P . Wehrli, B. Golding, P . LBliger, R . Keese, K . Mullen. and A . Eschenmoser. Angew. Chem. 81,301 (1969); Angew. Chem. internat. Edit. 8. 343 (1969). Pyrrole formation is promoted by nucleophilic attack of the base at the episulfide atom; see also B. M . T r i m and S . D . Ziman, J. Org. Chem. 38,933 (1973). Calculations by the extended Huckel method without 3d-orbital participation: R . Gleirer, Technische Hochschule Darmstadt. H . Beyer, Z. Chem. 9, 361 (1969), and references cited therein; H . Beyer. H . Honeck, and L. Reichelt, Liebigs Ann. Chem. 741, 45 (1970). R . R. Schmidt and H . Huth, Tetrahedron Lett., in press. C . Giordano, personal communication. R. R . Schmidt and U. Burkert, Tetrahedron Lett. 1973,4355. L’. Burkert, Dissertation, Universitit Stuttgart 1974. M . Polk, M . Siskin, and C. C . Price, J. Amer. Chem. SOC.Y I , 1206 (1969). and references cited therein. A . G. Horrmonn and R . L. Harris, J . Amer. Chem. Soc. 92, 1803 ( 1970). G . H . Senkler, J . Stackhouse, B. E. Maryanott, and K . Mi.rlow, J. Amer. Chem. SOC.96, 5648 (1974), and references cited therein. G. M a r k / and A. M e r z , Tetrahedron Lett. 1971, 1215. K . Dimroth, Fortschr. Chem. Forsch. 38, 1 (1973). I . Muruta, 7: Tarsuoka, and Y. Sugihara, Tetrahedron Lett. 1973, 4261; Angew. Chem. 86, 161 (1974); Angew. Chem. internat. Edit. 13. 142 ( 1974). M . J . S. D t w a r : The Molecular Orbital Theory of Organic Chemistry. McGraw-Hill, New York 1969, pp. 430-436.

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W G. Sufmond, Quart. Rev. (Chem. Soc.) 22, 253 (196X): R . Glriter and R . Hqfmann, Tetrahedron 24, 5899 (1968), and references cited therein. S. Wolf‘. A. Rank, and I . C. Csirmadiu, J. Amer. Chem. SOC. 91, 1567 (1969): N . D. Epioris, F . Bernardi, and S . Wolfe, VI. International Symposium on Sulphur Chemistry, Bangor, July 1974. S. Oae, W Taguki, and A . Ohno, Tetrahedron 20, 417 (1964); R . L. Slauqh and E. Bergman, J. Org. Chem. 26. 3158 (1961); R . Brrslo\