A new synthetic entry to pentacyclic Strychnos alkaloids. Total synthesis of (.+-.)-tubifolidine, (.+-.)-tubifoline, and (.+-.)-19,20-dihydroakuammicine
                                
                                    
                                        作者:Mercedes Amat、Ana Linares、Joan Bosch                                    
                                    
                                        DOI:10.1021/jo00313a017
                                    
                                    
                                        日期:1990.12
                                    
                                    A new strategy for the synthesis of pentacyclic Strychnos alkaloids has been developed.  It consists in the closure of the five-membered E ring by cyclization upon the indole 3-position from a suitably N-substituted tetracyclic system embodying rings ABCD of the alkaloids.  Attempts to effect the key cyclization either by Pummerer rearrangement of sulfinylacetamides 4 and 6, from chloroacetamide 12, or from bis(methylthio)acetamide 10b (exocyclic amide carbonyl group) resulted in failure.  In the first case dithioacetals 9 and 10, respectively, were formed in good yields.  Cyclization from alcohol 13 or from the indole-deactivated acetal 15 and dithiocetal 18 were also unsuccessful: noncyclized products coming from the initially formed oxonium or thionium intermediates 16 were obtained.  Cyclization was satisfactorily accomplished in 49% yield by treatment of the N-unsubstituted indole dithioacetal 23 with DMTSF.  The resulting pentacycle 25 was converted to 20-deethyltubifolidine (27).  A similar treatment from dithioacetal 41a, prepared from the secondary amine 32a, afforded pentacycle 42a, from which the alkaloids tubifoline, tubifolidine, and 19,20-dihydroakuammicine were synthesized.