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a variety of N-acyl-O -ethyl- N, O-acetals was concisely prepared under mild conditions mediated by titanium ethoxide (Ti(OEt)4). The method also o ffers a new strategy to make other O-alkyl- N, O-acetals. Furthermore, this strategy was extended to the synthesis of an analogue of the natural product turtschamide. The presence of these unique N-acyl-N, O-acetals in intricate natural product structures and their value as synthons for reactive N -acylimine species has stimulated interest in the construction of t his functio nal group . Asym metric N , O -acetalization has been investigated in the construction of more stable as well as easily accessible cyclic N,O-acetals that are also found in pharmaceuticals.These stable cyclic N, O-acetals were reportedly employed as reactive chemical reagents,undergoing ring-opening with allylsilane mediated by strong Lewis acids. In this protocol, two steps are required. In the first step, the reaction was performed in refluxing toluene, and then a sodium alkoxide was employed to install the alkoxyl group of N, O-acetals (path a). A s imilar strategy with an α -amido sulfone as an intermediate was broadly employed to prepare N-acylimines in situ under basic conditions,and N-acyl-N, O-acetals were reportedly obtained if sodium methoxide was used (path b).However, the conditions also led to the formation of N-acylenamides exclusively in another report. In their method, three successive steps, i.e., nitrile hydromet alation, acylat ion, and nucl eophi le additi on were carried out in one pot (path c). Other methods to prepare N-acyl-N, O -acetals are often limited to a certain substrate, for example , resin - supported aromatic amides and chloral.Recently, Antilla and co-workers reported the synthesis of enantiomeric N -acyl-N , O -acetals but started w ith freshly prepared unstable N-benzoylimines.Some strategies including Curtius rearrangement to form these unique motifs in total synthesis of several natural products were also reported.Here,we report a general concise method to prepare these unique species with a broad range of substrates under mild conditions mediated by Ti(OEt)4. |
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