A general, flexible, ring closing metathesis (RCM) based strategy for accessing the fused furo[3,2-b]furanone moiety present in diverse bioactive natural products

Palladium-Catalyzed Intermolecular Dearomatization Annulation Cascade Reaction of Furans for Stereoselective Access to 2,5-Dihydrofurans

A novel palladium-catalyzed intermolecular dearomatization of furans with alkynes via a three-component formal [3 + 2] spiroannulation/allylic substitution cascade reaction has been successfully developed for the stereoselective assembly of spiro 2,5-dihydrofuran frameworks. High step economy and efficacy as well as excellent stereoselectivity were achieved for a broad substrate scope. Two new C-C bonds and one new C-O bond were generated sequentially in a one-pot manipulation. The yielded spiro 2,5-dihydrofuran skeleton bearing a tetrasubstituted carbon center constitutes the core structure for plenty of useful natural products or corresponding analogues. This work represents a significant advancement in the dearomatization strategy for furan heterocycles and provides a practical methodology for expedited access to complex spiro dihydrofuran scaffolds.

Towards a qualitative understanding of the carbonyl reactivity of α-substituted ethyl 5-oxohomoadamantyl-4-carboxylates

Substituted ethyl 5-oxohomoadamantane-4-carboxylates were subjected to reactions with several nucleophiles to establish some aspects of the carbonyl reactivity. However, only one example of the desired Claisen retro-reaction was observed as 3,7-disubstituted bicyclo[3.3.1]nonane formation. Most of the reactions yielded α-substituted homoadamantan-5-ones or some products of their further transformations. Reductive amination of substituted homoadamantane-5-ones gave several homoadamantane-fused nitrogen heterocycles, which could be considered as GABA- and/or ∂-aminovaleric acid analogues. Reduction products of several substituted ketones were observed as single products when treated with organomagnesium reagents. These deviations from the general chemical reactivity patterns are associated with steric factors and the geometry of the cage unit and could be a particular case, which is reflecting the overall specificity of the chemistry of cage carbonyl compounds.

Dual Lewis Acid/Photoredox-Catalyzed Addition of Ketyl Radicals to Vinylogous Carbonates in the Synthesis of 2,6-Dioxabicyclo[3.3.0]octan-3-ones

A combined Lewis acid/photoredox catalyst system enabled the intramolecular umpolung addition of ketyl radicals to vinylogous carbonates in the synthesis of 2,6-dioxabicyclo[3.3.0]octan-3-ones. This reaction proceeded on a variety of aromatic ketones to provide THF rings in good yield (up to 95%). Although diastereoselectivity was found to be modest (1.4-5:1) for the C-C bond forming reaction, the minor diastereomers were converted to 2,6-dioxabicyclo[3.3.0]octan-3-ones by an efficient Lewis acid-mediated epimerization cascade in up to 90% yield.

Recent applications in natural product synthesis of dihydrofuran and -pyran formation by ring-closing alkene metathesis

Examples from 2003 to 2015 of ring-closing alkene metathesis (RCM) in the generation of dihydro-furans or -pyrans for natural product synthesis, are reviewed.

Thermal and Lewis acid promoted intramolecular Diels–Alder reaction of furanose tethered 1,3,9-decatriene systems: a synthetic and computational investigation

The intramolecular Diels–Alder (IMDA) reaction of furanose tethered 1,3,9-decatrienes (4a–4r) was investigated under thermal conditions and in the presence of a Lewis acid.

Regenerative γ-Lactone Annulations: A Modular, Iterative Approach to Oligo-tetrahydrofuran Molecular Stairs and Related Frameworks

A unified, stereocontrolled, regenerative γ-butyrolactone annulation approach has been conceptualized and validated through syntheses of a range of oligo-THFs. The new protocol is short (four steps), simple (table-top reagents), and efficient (50-61% overall yields). Although the scope of this approach is unlimited, it has been demonstrated up to five iterations on commercial γ-butyrolactone to assemble six fused tetrahydrofuran moieties in a staircase-like architecture. A selection of exploratory transformations is presented to exemplify the potential applications of this protocol.