Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem-Hydrogenation or through Carbophilic Activation of Alkynes

Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized ("gem-hydrogenation"), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X-ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi-Dunitz trajectory.

Site-Selective and Enantioselective α,β,γ-Functionalization of 5-Alkylidenefuran-2(5 H)-ones: A Route to Polycyclic γ-Lactones

A new strategy for a direct α,β,γ-functionalization of the γ-lactone framework in the corresponding 5-alkylidenefuran-2(5 H)-ones is reported. The developed approach is based on a stereocontrolled cascade reaction with 2-mercaptocarbonyl compounds proceeding in a sequence of thia-Michael/aldol/oxa-Michael reactions. Such a synthetic strategy allows for a construction of a unique polycyclic architecture containing γ-lactone, tetrahydrofuran, and tetrahydrothiophene ring systems. Excellent enantioselectivities and diastereoselectivities have been obtained in the presence of bifunctional catalyst derived from cinchona alkaloids.

Regiodivergent Lewis base-promoted O- to C-carboxyl transfer of furanyl carbonates

Triazolinylidenes promote γ-selective C-carboxylation (up to 99 : 1 regioselectivity) in the O- to C-carboxyl transfer of furanyl carbonates in contrast to DMAP that promotes preferential α-C-carboxylation with moderate regiocontrol (typically 60 : 40 regioselectivity).

Selective syntheses of Δ(α,β) and Δ(β,γ) butenolides from allylic cyclopropenecarboxylates via tandem ring expansion/[3,3]-sigmatropic rearrangements

Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δ(β,γ) butenolides via a Claisen rearrangement or to the corresponding Δ(α,β) butenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters.