CN a COOEt a COCH, a CONH, b CONHBU-t a, b, c CONMe, C a and b encountered in the earlier examples. Compound 20 results from cyclization mode c followed by elimination of tert-butylamine from the tetrahedral intermediate. Finally, when the group Rz is a tertiary amide group, ie, dimethylamide, the chemistry reverts to the simple situation in which a single product is formed. Cyclization mode c prevails, and the hydroxyfuran 20 is obtained ...