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|Type:||Artigo de periódico|
|Title:||Synthesis Of Indolizidines And Pyrrolizidines Through The [2 + 2]cycloaddition Of Five-membered Endocyclic Enecarbamates To Alkyl Ketenes. Unusual Regioselectivity Of Baeyer-villiger Ring Expansions Of Alkyl Aza-bicyclic Cyclobutanones|
|Author:||De Faria A.R.|
|Abstract:||The total systheses of two indolizidine skeletons and of the necine base (±)-platynecine were accomplished in a concise manner with good overall yields starting from a common five-membered endocyclic enecarbamate. These syntheses feature a (2 + 2)cycloaddition of the five-membered endocyclic enecarbamate 5 to alkykletenes that proceed in high yields and with high stereo-selectivity to provide an endo alkyl cycloadduct as the major or only product. The minor exo alkyl cycloadducts, which can be observed in some [2 +2]cycloadditions, seem to derive from the endo cycloadduct, the putative kinetic product, by epimerization. An unusual regioselectivity was observed for the Baeyer-Villiger oxidation is drastically reduced, leading to mixtures of regioselectivity of the Baeyer-Villager oxidation is drastically reduced, leading to mixtures of regioisomeric lactones in a ratio of ∼1.5 to 1. It is hypothesized that the steric strain built into the Criegee cyclobutane intermediate is the regioselectivity controlling factor in these oxidationas, overriding any stereoelectronic bias for ring expansion. A rationale for the mechanism of the [2 + 2]cycloaddition involving enecarbamates and ketenes is presented, which seems to involved the participation of an N-acyliminium-enolate intermediate.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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