The racemic form, (±)-1, of the structure originally assigned to the crinine alkaloid haemultine has been prepared for the first time. A key step involved the conversion of compound (±)-4 into the isomeric cis-C3a-arylhexahydroindole (±)-3 using a Pd0-catalysed intramolecular Alder-ene reaction. The amino-alcohol (±)-2 derived from the latter compound reacted with paraformaldehyde in the presence of trifluoroacetic acid to give, via a Pictet–Spengler reaction, the target (±)-1. The diastereoisomeric Mosher esters 15 and 16 obtained by coupling the racemate (±)-1 with the R-form, 14, of the Mosher acid could be separated chromatographically and then reductively cleaved to give the enantiomerically pure compounds (+)-1 and (–)-1, respectively. The physical and spectroscopic data derived from the former enantiomer are consistent with the proposition that the title natural product is, in fact, a mixture of (+)-1 and its Δ2,3-double bond isomer.
我们首次制备出了最初归属于辣木碱的外消旋形式 (±)-1。其中一个关键步骤是利用 Pd0 催化的分子内 Alder-ene 反应将化合物 (±)-4 转化为同分异构的顺式-C3a-芳基六氢吲哚 (±)-3。由后一种化合物衍生的氨基醇 (±)-2 在三氟乙酸存在下与多聚甲醛反应,通过 Pictet-Spengler 反应得到目标化合物 (±)-1。通过将外消旋体 (±)-1 与莫舍酸的 R 型 14 偶联得到的非对映异构体莫舍酯 15 和 16 可以通过色谱分离,然后进行还原裂解,分别得到对映体纯度为 (+)-1 和 (-)-1 的化合物。从前一种对映体得到的物理和光谱数据与标题天然产物实际上是 (+)-1 及其 Δ2,3-双键异构体的混合物这一命题相一致。