Ring size and substituent steric effects in cyclic ketene-N,O/S-acetal trifluoroacetylations
作者:Hondamuni I. De Silva、Yingquan Song、William P. Henry、Charles U. Pittman
DOI:10.1016/j.tetlet.2012.03.068
日期:2012.6
Trifluoroacetylations of cyclic ketene-N,O/S-acetals exhibit a ring size effect. The five-membered rings 2,4,4-trimethyl-2-oxazoline, 2-methyl-2-oxazoline, and 2-methyl-2-thiazoline, each form cyclic ketene-N,O/S-acetal intermediates which then react with trifluoroacetic anhydride to give β-monotrifluoroacetylation products. Conversely, the six-membered ring 2-methyl-2-oxazine gives its β,β-bistrifluoroacetylation
环状烯酮-N,O / S-乙缩醛的三氟乙酰化表现出环尺寸效应。五元环2,4,4-三甲基-2-恶唑啉,2-甲基-2-恶唑啉和2-甲基-2-噻唑啉分别形成环状烯酮-N,O / S-缩醛中间体,然后发生反应用三氟乙酸酐得到β-单三氟乙酰化产物。相反,六元环2-甲基-2-恶嗪给出其β,β-双三氟乙酰化产物。原位生成的五元环N -Me环状烯酮-N,O-乙缩醛,3,4,4-三甲基-2-亚甲基-恶唑烷,3-甲基-2-亚甲基-恶唑烷和六元环3-甲基-2-亚甲基-1,3-恶嗪烷分别为β, β-双三氟乙酰化。然而,3-甲基-2-亚甲基-恶唑烷也通过碘化物催化的其β,β-双三氟乙酰化衍生物的重排而提供了γ-内酰胺。原位-生成3-甲基-2-亚甲基噻唑烷给出两个β单-和β,β-bistrifluoroacetylation产品和没有内酰胺,而相比之下,其Ñ,ø -模拟3-甲基-2- methyleneoxazolidine。