A HIGHLY EFFECTIVE ONE-POT BICYCLOANNULATION METHODOLOGY FOR THE SYNTHESIS OF BERBAN AND YOHIMBAN SYSTEMS BASED ON ORGANOTIN-MEDIATED 3-COMPONENT COUPLING (N-ACYLATIVE PENTADIENYLATION OF C=N BONDS)

A highly effective bicycloannulation methodology for the synthesis of berban and yohimban alkaloid systems is described. Three-component coupling reactions of 2,4-pentadienyltin reagents with C=N bonds and alpha,beta-unsaturated acyl chlorides furnish bicycloannulated products in a one-pot operation. For example, the reactions of 2,4-pentadienyltributyltin (1) with isoquinoline derivatives activated by acryloyl chloride afford the tetracyclic (+/-)-allo-berban systems stereoselectively. Similarly, the reaction of 1 with 3,4-dihydro-beta-carboline (11) gives the pentacyclic (+/-)-allo-yohimban system. The reaction is not affected by the stereochemistry of the 2,4-pentadienyltin reagent. A new substituted 2,4-pentadienyltin reagent, 3-(hydroxymethyl)-2,4-pentadienyltrimethyltin (19), is prepared via 3-(hydroxymethyl)pentadienyl dianion. The three-component coupling reaction of 19 with 11 and acryloyl chloride affords the (+/-)-allo-16-(hydroxymethyl)yohimban system, from which (+/-)-nitraraine is readily synthesized. In addition, 1,3-asymmetric induction leads to the high diastereoselectivity realized in bicycloannulation (up to 94% de) when (S)-3-[(tert-butyldimethylsiloxy)methyl]-3,4-dihydroisoquinoline (27), which is readily derived from L-phenylalanine, is used in the three-component coupling reaction.