An adventurous synthetic journey with MNBA from its reaction chemistry to the total synthesis of natural products

Abstract

After initially establishing a novel cyclization reaction of ω-hydroxycarboxylic acids (seco-acids) using 4-trifluoromethylbenzoic anhydride (TFBA) and Lewis acid catalysts to form the corresponding lactones, we discovered a more advanced method for this transformation using 2-methyl-6-nitrobenzoic anhydride (MNBA) as a coupling reagent with nucleophilic catalysts. The latter lactonization is promoted by acyl-transfer catalysts, such as 4-(dimethylamino)pyridine (DMAP), 4-pyrrolidinylpyridine (PPY), and 4-(dimethylamino)pyridine N-oxide (DMAPO). (+)-Ricinelaidic acid lactone ((+)-2) was first synthesized by the TFBA-mediated cyclization with Lewis acid catalysts, while the threoaleuritic acid lactone (20) was alternatively synthesized by the MNBA-mediated cyclization with acyl-transfer catalysts. Using this effective lactonization technology to form the ester linkage under mild conditions, we then demonstrated the preparation of various large-, medium-, and small-sized natural and unnatural lactones including (-)-cephalosporolide D ((-)-3), (-)-octalactin A ((-)-4), (-)-octalactin B ((-)-25), 2-epibotcinolide (49), (-)- and (+)-2-hydroxytetracosanolides ((-)-77 and (+)-77), (-)- and (+)-2-hydroxy-24-oxooctacosanolides ((-)-78 and (+)-78), (-)-tetrahydrolipstatin ((-)-THL, (-)-117), and the erythromycin A skeletons 103b, 103c, 104c, and 105b. The transition structures involved in the formation of the β-lactones from the corresponding 3-hydroxycarboxylic acids were then determined using DFT calculations at the B3LYP/6-31G*//B3LYP/6-31G* level, and the reactivity of several seco-acids was successfully predicted on the basis of the calculated thermodynamic properties of the transition structures. © 2013 The Chemical Society of Japan.