Dynamic Refolding of Ion-Pair Catalysts in Response to Different Anions.

Four distinct folding patterns are identified in two foldamer-type urea-thiourea catalysts bearing a basic dimethylamino unit by a combination of X-ray crystallography, solution NMR studies, and computational studies (DFT). These patterns are characterized by different intramolecular hydrogen bonding schemes that arise largely from different thiourea conformers. The free base forms of the catalysts are characterized by folds where the intramolecular hydrogen bonds between the urea and the thiourea units remain intact. In contrast, the catalytically relevant salt forms of the catalyst, where the catalyst forms an ion pair with the substrate or substrate analogues, appear in two entirely different folding patterns. With larger anions that mimic the dialkyl malonate substrates, the catalysts maintain their native fold both in the solid state and in solution, but with smaller halide anions (fluoride, chloride, and bromide), the catalysts fold around the halide anion (anion receptor fold), and the intramolecular hydrogen bonds are disrupted. Titration of catalyst hexafluoroacetylacetonate salt with tetra-n-butylammonium chloride results in dynamic refolding of the catalyst from the native fold to the anion receptor fold.

Photooxygenation of Furylalkylamines: Easy Access to Pyrrolizidine and Indolizidine Scaffolds.

A highly adaptable method targeting the ubiquitous and very important pyrrolizidine and indolizidine scaffolds is presented. The general synthetic utility of the method is underscored by its application to the rapid and easy synthesis of five natural products starting from readily accessible alkylfuran precursors. These unprotected primary furylalkylamines are subjected to photooxygenation conditions, which initiate a complex cascade reaction sequence concluding with the production of high value motifs. This sequence can be tailored to need by varying the choice of both photosensitizer and base additive.

First Total Synthesis of Pandamarine.

The first total synthesis of pandamarine, an alkaloid isolated from Pandanus amaryllifolius is reported. The key step of this extremely short (six steps in total) and protecting group-free synthesis is a highly efficient cascade reaction sequence initiated by the photooxidation of an easily accessible and symmetric difuran precursor.

Methylene Blue as a Photosensitizer and Redox Agent: Synthesis of 5-Hydroxy-1H-pyrrol-2(5H)-ones from Furans.

A highly efficient and general singlet-oxygen-initiated one-pot transformation of readily accessible furans into 5-hydroxy-1H-pyrrol-2(5H)-ones has been developed. The methodology was extended to the synthesis of other high-value α,ß-unsaturated γ-lactams. This useful set of transformations relies not only on the photosensitizing ability of methylene blue, but also on its redox properties: properties that have until now been virtually ignored in a synthetic context.

First total synthesis of paracaseolide A.

The first total synthesis of the paracaseolide A, a very unusual tetraquinane oxa-cage bislactone recently isolated from the mangrove Sonneratia paracaseolaris, has been achieved. The final step and culmination of the eight-step synthetic sequence is a [4 + 2] dimerization of a 4-hydroxybutenolide, generated by singlet oxygen-mediated oxidation of a furan precursor.

Green oxidations of furans--initiated by molecular oxygen--that give key natural product motifs.

In this article, we explore how changes in the positioning of pendant hydroxyl functionalities in the photooxygenation substrate dramatically alter the course of furan oxidations that are initiated by singlet oxygen; and, how these different reactivities can be harnessed through cascade reaction sequences to access, rapidly and effectively, a broad range of important natural product motifs.

Scope and limitations of the photooxidations of 2-(α-hydroxyalkyl)furans: synthesis of 2-hydroxy-exo-brevicomin.

Photooxygenation of 2-(α-hydroxyalkyl)furans at 5 °C in MeOH followed by in situ reduction affords, in one synthetic operation, 6-hydroxy-3(2H)-pyranones and/or 5-hydroxy-2(5H)-furanones. The relative ratio of the final products is highly dependent on the substitution of the starting furan substrate. Photooxygenation of 2-(α,ß-dihydroxyalkyl)furans followed by in situ reduction and ketalization with acid rapidly provides the 6,8-dioxabicyclo[3.2.1]oct-3-en-2-one framework. This new methodology was successfully applied to the synthesis of 2-hydroxy-exo-brevicomin.