A versatile three-step procedure to annulate a γ-pyrone onto a methylene ketone was developed involving (i) aldol reaction with a dithiolane-protected β-ketoaldehyde, (ii) oxidation of the aldol adduct to a β-diketone, and (iii) treatment of the resulting dithiolane-protected 1,3,5-trione with 2-iodoxybenzoic acid (IBX) and trifluoromethanesulfonic acid (triflic acid; TfOH) in acetonitrile at ambient temperature to give the corresponding γ-pyrone. Cyclization proceeded with IBX alone, but significantly improved yields were obtained with added acid, particularly triflic acid. A dithiolane was more effective than a dithiane or acyclic dithioacetal protecting group. The method was amenable to the preparation of a variety of substituted γ-pyrones simply by altering the initial aldol reactants. Overall yields of 50%–60% were obtained in six examples. The method was applied to prepare a key fragment for the total synthesis of baconipyrones A and C and siphonarin B.
开发了一种多功能的三步程序,用于将γ-吡喃酮环化到亚甲基酮上,包括(i)与二硫杂环保护的β-酮醛进行醛缩反应,(ii)将醛缩加合物氧化为β-二酮,以及(iii)用2-碘代苯甲酸(IBX)和三氟甲磺酸(三氟甲磺酸;TfOH)在乙腈中在室温下处理所得的二硫杂环保护的1,3,5-三酮,以得到相应的γ-吡喃酮。环化反应可仅使用IBX进行,但添加酸,特别是三氟甲磺酸,可显著提高收率。二硫杂环保护基比二硫醚或无环二硫缩醛保护基更有效。该方法适用于通过改变初始醛缩试剂来制备各种取代的γ-吡喃酮。在六个实例中,总产率为50%–60%。该方法被应用于为合成baconipyrones A和C以及siphonarin B的关键片段做准备。