The direct oxidative diene cyclization and related reactions in natural product synthesis

Elucidating the Mechanism of Tetrahydrofuran-Diol Formation through Os(VI)-Catalyzed Oxidative Cyclization of 5,6-Dihydroxyalkenes Ligated by Citric Acid

A computational study is reported here on the mechanism of tetrahydrofuran (THF)-diol formation from the Os(VI)-catalyzed oxidative cyclization of 5,6-dihydroxyalkene ligated with citric acid and in the presence of Bro̷nsted acid. Initiated by Os(VI) dioxo citrate formation, coordination of co-oxidant pyridine-N-oxide (PNO) and protonation of its oxo group generate the active catalyst. The catalytic cycle commences through successive steps, including dihydroxyalkene addition to the active catalyst in a concerted mechanism to form hexacoordinated alkoxy-protonated PNO-complexed Os(VI) bisglycolate as a turnover-limiting step (TLS), cyclization to Os(IV) THF-diolate, reoxidation to Os(VI) THF-diolate, and hydrolysis via a dissociative mechanism to furnish the THF-diol and regenerate the active species, sustaining the catalytic cycle through an Os(VI)/Os(IV) cycle. Despite the overall exergonic nature of catalytic cycle (ΔGrcycle = -45.0 kcal/mol), the TLS is accelerated by the formation of an open-valence 16-electron Os(VI) intermediate but decelerated by the undesired formation of a saturated/hexacoordinate 18-electron Os(VI) intermediate. Bro̷nsted acid plays crucial roles in the formation of Os(VI) citrate and the active catalyst, impediment of the second cycle, and the cyclization step. Additionally, besides its role as a co-oxidant, and in the presence of acid, PNO is found to assist the insertion of dihydroxyalkene and, importantly, in releasing the THF-diol to regenerate the active intermediate.

Efficient synthesis of tetrahydrofurans with chiral tertiary allylic alcohols catalyzed by Ni/P-chiral ligand DI-BIDIME

Efficient nickel-catalyzed stereoselective asymmetric intramolecular reductive cyclization of O-alkynones with P-chiral bisphosphorus ligand DI-BIDIME is reported.

Ru-catalysed oxidative cyclisation of 1,5-dienes: an unprecedented role for the co-oxidant

The Ru-mediated oxidative cyclisation of 1,5-dienes to furnish 2,5-dihydroxyalkyl-substituted tetrahydrofuran-diols (THF-diols) represents a practical approach for the synthesis of many bioactive natural products. In the current study, we reported profound findings obtained by density functional theory (DFT) simulations, and they were consistent with the experimental conditions. The results set out a catalytic cycle within intermediacy of NaIO₄-complexed Ru(vi) species. Importantly, the co-oxidant played a critical role in the cyclisation step and subsequently the release of THF-diols. Following the formation of Ru(vi) glycolate, cyclisation and THF-diol release proceeded through NaIO₄-coordinated Ru(vi) intermediates, outpacing the Ru(viii) glycolate or THF-diolate intermediates and subsequently entering “second cycle” type pathways. The results indicated a cycle involving Ru(viii)/Ru(vi)/Ru(iv)/Ru(vi) rather than Ru(viii)/Ru(vi)/Ru(viii)/Ru(vi)/Ru(viii). Additionally, the existence of an electron-withdrawing group (EWG) on one of the double bonds of 1,5-dienes revealed that the regioselectivity of the Ru-catalysed oxidative cyclisation was predominantly initiated at the electron-rich alkene. Overall, this study offers new insights, which were ignored by earlier experimentalists and theoreticians, into the Ru-catalysed functionalizations of alkenes and 1,5-dienes.

The Ru-mediated oxidative cyclisation of 1,5-dienes to THF-diols proceeds with the intermediacy of NaIO₄-complexed Ru(vi) species and offers new insights into the Ru-catalysed functionalizations of alkenes and 1,5-dienes.

Ligand Substitution of Ru II –Alkylidenes to Ru(bpy) 3 2+ : Sequential Olefin Metathesis/Photoredox Catalysis

Ruthenium(II) alkylidene complexes such as the Grubbs’ 1st and 2nd generation catalysts undergo a ligand substitution with 2,2′‐bipyridine, which readily leads to the common photoredox catalyst Ru(bpy)₃²⁺. The application of this catalyst transformation in sequential olefin metathesis/photoredox catalysis is demonstrated by way of ring‐closing metathesis (RCM)/photoredox ATRA reactions.

Mechanism of Os-Catalyzed Oxidative Cyclization of 1,5-Dienes

The oxidative cyclization of 1,5-dienes by metal-oxo species is a powerful method for stereocontrolled synthesis of tetrahydrofuran diols (THF-diols), structural motifs present in many bioactive natural products. Oxidative cyclization of (2E,6E)-octa-2,6-diene catalyzed by OsO₄/NMO has been studied using density functional theory (DFT) calculations (M06-2X/aug-cc-pVDZ/Hay-Wadt VDZ (n+1) ECP), highlighting the remarkable effect of acid on the fate of the first intermediate, an Os(VI) dioxoglycolate. A strong acid promotes cyclization of the Os(VI) dioxoglycolate, or its NMO complex, through protonation of an oxo ligand to give more electrophilic species. By contrast, in the absence of acid, reoxidation may occur to afford the Os(VIII) trioxoglycolate, which is shown to favor conventional "second cycle" dihydroxylation reactivity rather than cyclization. The results of the calculations are consistent with experimental results for reactions of OsO₄/NMO with 1,5-dienes with acid (oxidative cyclization) and without acid (second cycle osmylation/dihydroxylation). Detailed evaluation of potential catalytic cycles supports oxidation of the cyclized Os(IV) THF-diolate intermediate to the corresponding Os(VI) species followed by slow hydrolysis and, finally, regeneration of OsO₄.

Oxovanadium(v)-catalyzed deoxygenative homocoupling reaction of alcohols

Catalytic direct hydrazination of allyl alcohol and deoxygenative homocoupling reaction of alcohols depending on hydrazine derivatives were performed by utilizing oxovanadium(v) catalysts.

Regioselective dehydration of α-hydroxymethyl tetrahydrofurans using Burgess' reagent under microwave irradiation

We here present a highly efficient and high yielding procedure for the preparation of 2-vinyl tetrahydrofurans starting from α-hydroxymethyl tetrahydrofurans. Best results for this dehydration were achieved using Burgess' reagent in dioxane under microwave irradiation. A range of functional groups as well as different cyclic and bicyclic frameworks were found to be compatible with the reaction conditions. The desired products were obtained within minutes in good to high yields and excellent regioselectivity.

Cobalt versus Osmium: Control of Both trans and cis Selectivity in Construction of the EFG Rings of Pectenotoxin 4

Catalytic oxidative cyclisation reactions have been employed for the synthesis of the E and F rings of the complex natural product target pectenotoxin 4. The choice of metal catalyst (cobalt- or osmium-based) allowed for the formation of THF rings with either trans or cis stereoselectivity. Fragment union using a modified Julia reaction then enabled the synthesis of an advanced synthetic intermediate containing the EF and G rings of the target.