Abstract
The ortho-nitro effect was discovered in sulfa-Staudinger cycloadditions of ethoxycarbonylsulfene with linear imines. When an ortho-nitro group is present at the C-aryl substituents of linear imines, the sulfa-Staudinger cycloadditions deliver cis-β-sultams in considerable amounts, together with the predominant trans-β-sultams. In other cases, the above sulfa-Staudinger cycloadditions give rise to trans-β-sultams exclusively. Further mechanistic rationalization discloses that the ortho-nitro effect is attributed to its strong electron-withdrawing inductive effect. Similarly, the ortho-nitro effect also exists in Staudinger cycloadditions of ethoxycarbonyl ketene with the imines. The current research provides further insights into the diastereoselective control in sulfa-Staudinger and Staudinger cycloadditions.
Highlights
Β-sultams have received much attention in both synthetic and medicinal chemistry [1,2,3,4,5,6,7,8], mainly because of their outstanding biological activities as, for example, antibiotics and enzyme inhibitors [9,10,11,12,13]
C-3- and revealing that the ortho-nitro effect does not exist in the sulfa-Staudinger cycloadditions involving
The results indicate that the ortho-nitro effect on the cis-selectivity is its strong electron-withdrawing inductive effect
Summary
Β-sultams have received much attention in both synthetic and medicinal chemistry [1,2,3,4,5,6,7,8], mainly because of their outstanding biological activities as, for example, antibiotics and enzyme inhibitors [9,10,11,12,13]. The most promising is the sulfa-Staudinger cycloaddition [18,19,20,21,22,23,24], which is referred to as the [2s + 2i ] annulation between sulfenes (or their equivalents) with imines, because of the rich and diverse sources of the starting materials. We successfully disclosed the stepwise mechanism of the sulfa-Staudinger cycloaddition [25,26], and the substituent-controlled annuloselectivity [27,28] and diastereoselectivity [29]. Annulation of the active ethoxycarbonylmethanesulfonyl chloride (1a) and ethyl malonyl chloride (1b) with various linear imines 2 gave trans-β-sultams 3 or trans-β-lactams 4 exclusively, regardless of the imine substituents (Scheme 1) [27,28,29,30,31]. In our continuing studies, we further found that when C-2-nitroaryl imines 2 were applied in the reactions, besides trans-β-sultams 3, cis-β-sultams
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