Reactions of Aminopentadienal Derivatives with 5,6-Dihydropyridinium Salts as an Approach to Manzamine Alkaloids Based upon Biogenetic Considerations

Recent results from our laboratory have prompted us to introduce a modification of the original Baldwin proposal for the biosynthesis of manzamine alkaloids. In this new model, aminopentadienal derivatives of general structures 2 or 3 can not only cyclize to give pyridinium salts such as cyclostellettamines but also react with 5,6-dihydropyridinium salts 1 leading to species such as 6 or 7, which would be key intermediates in the syntheses of manzamine A and halicyclamine B. On the basis of the chemistry depicted in Scheme 2, model reactions of dienes 9, 10, 12, and 13 with salts 1 have been investigated. Diene 9 gave an interesting rearrangement product 15a, while dienes 10 and 12 gave halicyclamine- and manzamine-type adducts 19 and 23, respectively. In addition, glutaconaldehyde derivative 13 gave adduct 32 possessing some characteristic features of sarain A.