By Gould R.F. (ed.)
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In an analogous manner, the cycloaddition chemistry of amidofurans 334 provided the azatricyclic products 335. Apparently, the rate of the 1,2-thio shift of the initially formed cycloadduct is much faster than the deprotonation/dehydration pathway previously described in Scheme 21. K. BUR AND A. PADWA 44 R H2C O O N Me SMe Me OPiv N SMe DMTSF Me SMe ∆ O O SMe [Sec. C O N CH2 Me O N R O R SMe O R 331 R = CO2Me; 332 R = CO2Me; 333 R = CO2Me; R = Me; R = Ph; R=H R = Me; R = Ph; R=H R = Me; R = Ph; R=H ( )n N O S Me O CH 2 ( ) m R ( )n ∆ S Me N O ( )m O R 334 R = CO2Me, m = 1, n = 1 335 R = CO2Me, m = 1, n = 1 R = Me, m = 1, n = 1 R = Ph, m = 1, n = 1 R = H; m = 1, 2; n = 1, 2 R = Me, m = 1, n = 1 R = Ph, m = 1, n = 1 R = H; m = 1, 2; n = 1, 2 Scheme 62 An interesting example of a Wagner–Merwein-type rearrangement that triggers a subsequent Pummerer cyclization has recently been reported (02OL2565).
The [3,3]-rearrangement resulted in the formation of a second iminium ion 234 that was intercepted by enol 232 to give the Mannich adduct 235. Finally, elimination of the a-methylene 1,3-dione afforded amine 236. For ease of isolation, the crude reaction mixtures were subjected to the action of benzyl chloroformate. Several examples demonstrated the versatility of this sequence in that the original ring size could be varied (m ¼ 1–3) as well as the annulated ring size (n ¼ 1, 2) to produce predominantly cis-fused bicycles 237 in ca.
B O SPh ( )n O ( )n ZnI2 O SPh BF3•2AcOH N ( )n O N OTBS NH OMe 316 315 318 317 O ( ) O ( )n SPh n NH 319 Bn N ZnI2 O N O SPh Bn N BF3 • 2AcOH ( ) n Bn N N 316 320 321 Scheme 59 a-Thiophenylamides were also employed as precursors for the formation of N-acyliminium ions, which were then used as intermediates for subsequent cyclization chemistry. For example, treatment of amido sulfoxide 315 with silylketene acetal 316 in the presence of ZnI2 gave lactam 317 in excellent yield (490%, Scheme 59) (00T10159, 98TL8585).