The intramolecular Diels−Alder reaction between an amidofuran moiety tethered onto an indole component was examined as a strategy for the synthesis of Aspidosperma alkaloids. Furanyl carbamate 23 was acylated using the mixed anhydride 26 to provide amidofuran 22 in 68% yield. Further N-acylation of this indole furnished 27 in 88% yield. Cyclization precursors were prepared by removing the carbamate
拴在
吲哚成分上的酰胺基
呋喃部分之间的分子内Diels-Alder反应已被检测为合成Asperdosperma
生物碱的策略。使用混合酸酐26将
氨基甲酸呋喃酯23酰化,以68%的产率提供酰胺基
呋喃22。该
吲哚的进一步N-酰化得到27,收率88%。环化前体通过除去
氨基甲酸酯部分,随后加入N-制备-用适当的卤代烷进行烷基化。酰胺基氮原子上的较大取代基会导致酰胺基
呋喃的反应性s-反式构型更高,从而促进Diels-Alder环加成反应。该反应需要在
吲哚氮上存在吸电子取代基,以便进行环加成反应。的治疗N-烯丙基bromoenamide 48与Ñ -Bu 3 SNH / AIBN优先引导到6-内TRIG环化产物50,具有较高的稀释条件下而获得的最好的产率(91%)。最初衍生自48的
环己烯基产生五环杂环50 通过直接的6-endo trig环化,或通过
乙烯基自由基重排途径。