(I) For an excellent discussion of this and related phenomenon, me: Ah", N.-T. Ton. C ur . Chem. 1980.88. 145. 18) The new componds cited in this manuscript gave sstiofactary ' H IRM and 400 MHzi and I'C NMR IR, and mas3 spectra. Those inter- mediates that were stable gave cormt elrmental analyses. (91 J acokn . R. MI Ahhaspur. A,; Williams, DR Tetmhedmn Lett. 19HI. 2?, 3565. Hiyama. T.; Shinrrla. M.; Saimoto. H ; Komki. H. Hpt- erorycles 1981. 1.5, ...