17 ワンポット不斉アザ電子環状反応の開発と立体化学を制御したピペリジンアルカロイド合成への展開(口頭発表の部)

抄録

Although there are a number of examples of 6π-azaelectrocyclization reactions, studies on the reactivity of 1-azatrienes toward azaelectrocyclization are limited, and applications of this reaction to natural product synthesis are also limited. Recently, we found that (E)-3-carbonyltrienal compound A irre-versibly reacted with the specific lysine residue of phospholipase A_2 (PLA_2) to produce 1,2-dihydropyridine via facile 6π-azaelectrocyclization leading to inactivation of the enzyme (Figure 1). Moreover, we found that the presence of both the C4-carbonyl group and the C6-alkenyl group in 1-azatrienes significantly contributed to the acceleration of the azaelectrocyclization (Figure 2). Next, we successfully realized the asymmetric 6π-azaelectrocyclization by using a chiral cis-aminoindanol (-)-a, which play not only as a chiral auxiliary but also as a nitrogen source (Figure 3). We also found 7-substituted aminoindanol (-)-b was more effective in the reaction with more general aldehydes 2 and 3 (Table 1). The aminoalcohol (-)-b made possible to control the direction of the π orbital rotation in concerted disroratory pericyclization of the azatrienes (Figure 4). In order to achieve further efficient synthesis of poly-substituted piperidine alkaloids, we investigated one-pot asymmetric 6π-aza-electrocyclization. Heating a mixture of vinyl stannane, vinyl iodide, and cis-aminoindanol derivative (-)-b in the presence of Pd (0) catalyst successfully provided the expected compound 3b in good yield and high stereoselectivity. This one-pot asymmetric 6π-azaelectrocyclization protocol can be applied into wide range of compounds (Figure 6). We also established stereocontrol synthesis of chiral 2,4,6-substituted piperidine derivatives (Scheme 1, 2). We actually applied these methods to the formal asymmetric synthesis of indole alkaloid, 20-epiureine and to the asymmetric synthesis of indolizidine alkaloid, dendroprimine (Scheme 3, 4).