P-17 Manzamine生合成前駆体と考えられる海綿由来の新規アルカロイドの構造(ポスター発表の部)

抄録

The manzamine alkaloids from some genera of marine sponges are a series of β-carboline compounds having unique heterocyclic systems. In our search for biogenetic precursors of the manzamines, five novel alkaloids, keramaphidins B (6) and C (9), ircinols A (7) and B (8), and keramamine C (10), together with known manzamines have been isolated from an Okinawan marine sponge Amphimedon sp. The strucutures have been elucidated on the basis of NMR data and X-ray analysis. Interestingly ircinols A (7) and B (8) possessed opposite absolute configurations to those of ircinals A (4) and B (5), respectively, while a small amount of crystals of keramaphidin B (6) employed for X-ray analysis was revealed to be racemic. We propose a plausible biogenetic path of manzamines A〜C (1〜3) on the basis of the structures and the ratios of all the manzamine-related alkaloids isolated from this sponge and describe some differences from the biogenetic path proposed by Baldwin and Whitehead as follows; i) the presence of (-)-keramaphidin B ((-)-6) and ircinols A (7) and B (8) corresponding to antipodes of the manzamines, suggesting an incomplete stereoselectivity in intramolecular Diels-Alder type [4+2] cycloaddition of the bis-3-alkyldihydropyridine (a) to afford (+)-6 and (-)-6 as a major and a minor products, respectively. ii) 2,3-iminium form of major (+)-6 may be hydrolyzed to generate (+)-ircinals A (4) or B (5), which are probably converted into manzamines A (1) and B (2), respectively, through Pictet-Spengler cyclization with tryptamine, although Baldwin and Whitehead have proposed that manzamine A (1) could be derived from manzamine B (2). iii) on the other hand, minor (-)-6 may be converted into ircinols A (7) and B (8).