FURAN-TERMINATED N-ACYLIMINIUM ION INITIATED CYCLIZATIONS IN ALKALOIDSYNTHESIS

A study of the utility of furan-terminated N-acyliminium ion initiated cyclizations for the synthesis of linearly fused alkaloid precursors (Figure 2) is presented. The outcome of the cyclization event depends on the position of furan tether attachment (2 vs 3), tether length, an d furan 5-substituent (R = H, CH3, Ar). 3-Substituted furans cyclized to form 6- and 7-membered ring containing furans 35-38, 50, and 51 in good to excellent yields. 2-Substituted furans closed to form only 6-m embered rings; however, the products obtained were a function of the f uran Ei-substituent. The 5-H furans 17 and 18 led exclusively to the c orresponding furans 21 and 22, while the 5-CH3-furans 42 and 43 gave o nly diketone containing compounds 44 and 45. 5-Arylfurans 66-71 provid ed mixtures of furan- and diketone-containing products 72-83, with the ratio related to the substitution on the phenyl moiety. A preparation of epilupinine 10 is also discussed.