Abstract
Abstractα‐Oxoketenes generated in situ by a microwave‐assisted Wolff rearrangement at 170°C were found to react with stabilized sulfur ylides in (4 + 1) annulation processes to afford functionalized 3‐hydroxyfurans. Theoretical mechanistic investigations revealed that the reaction proceeds in two steps via a short‐lived betaine intermediate, a general feature of the reactions of α‐oxoketenes with stable phosphorous, nitrogen, and sulfur ylides.
Highlights
The energy of the transition state leading to D was recomputed at this level of theory and found at +122.7 kJ mol −1, indicating a kinetically disfavored (4 + 1) annulation when compared with the Wittig and abnormal Wittig olefinations that are observed experimentally. It seems that the model employed in our previous study on the Wittig‐type reac- this part of the study, we concluded that betaine C is a tions with α‐oxoketenes overestimated the relative ener- somewhat stable intermediate possibly observable gies of most stationary points.[5]
The 2‐diazo‐1,3‐dicarbonyl compounds 1 were prepared by diazo transfer reactions with tosyl azide using known procedures[1.5] The stable sulfur ylides 3 were prepared by known procedures as follows: Sulfonium salts were obtained from the corresponding α‐bromoketones treated with excess dimethyl sulfur, and the salts were further reacted with aqueous sodium hydroxide to give the ylides after liquid/liquid extraction.[16]
Carbonyl‐stabilized sulfur ylides were found to react with α‐oxoketenes generated in situ by a Wolff rearrangement of the corresponding diazo precursors to afford functionalized 3‐hydroxyfuran products through a (4 + 1) annulation
Summary
Nitrogen ylide cycloaddition partner, for example using ethyl diazoacetate.This prompted us to investigate the α‐Oxoketenes (1) are very reactive short‐lived synthetic reactivity of stable sulfur ylides 3 with α‐oxoketenes in intermediates that, with few exceptions, cannot be iso- order to prepare 3‐hydroxyfurans of type 4 through a lated and must be generated in situ with a rich chemis- related (4 + 1) annulation reaction (Scheme 1).[7]. Overall, it seems that the model employed in our previous study on the Wittig‐type reac- this part of the study, we concluded that betaine C is a tions with α‐oxoketenes overestimated the relative ener- somewhat stable intermediate possibly observable gies of most stationary points.[5] The differences are and/or isolable under some specific conditions and that believed to result essentially from the inclusion of stabi- phosphorous ylides are not suitable to trigger (4 + 1). The 2‐diazo‐1,3‐dicarbonyl compounds 1 were prepared by diazo transfer reactions with tosyl azide using known procedures[1.5] The stable sulfur ylides 3 were prepared by known procedures as follows: Sulfonium salts were obtained from the corresponding α‐bromoketones treated with excess dimethyl sulfur, and the salts were further reacted with aqueous sodium hydroxide to give the ylides after liquid/liquid extraction.[16]
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