trans-2-Tritylcyclohexanol as a chiral auxiliary in permanganate-mediated oxidative cyclization of 2-methylenehept-5-enoates: application to the synthesis of trans-(+)-linalool oxide

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.

Asymmetric Synthesis of Cyclopentene Compounds Containing All-Carbon Quaternary Stereocenters by (3 + 2) Cycloaddition and Its Application in the Formal Synthesis of (R)-(-)-Puraquinonic Acid

A highly stereoselective (3 + 2) cycloaddition for the asymmetric synthesis of versatile cyclopentene compounds containing all-carbon quaternary stereocenters was developed. The phosphine-catalyzed reactions of alkynoates with α-alkylated electron-deficient alkenes bearing Oppolzer's camphorsultam showed high to excellent diastereoselectivities and perfect regioselectivities. The usefulness of this reaction was demonstrated in the concise formal synthesis of (R)-(-)-puraquinonic acid.

A Practical Laboratory-Scale Synthesis of All Eight Stereoisomeric Forms of Terpene Linalool Oxide

In this work, we describe a user-friendly procedure for the preparation of all the isomeric forms of the terpene linalool oxide. The presented method is based on the transformation of the linalool enantiomers into the corresponding diastereoisomeric mixtures of the two furanoid oxides and two pyranoid oxides. Taking advantage of the different steric hindrance of the hydroxyl functional groups, the pyranoid forms were separated as a diastereoisomeric mixtures of their benzoate esters. Conversely, the cis- and trans-furanoid isomers were transformed in the corresponding acetates, which were directly separated by chromatography. The hydrolysis of the latter esters afforded cis- and trans-furanoid linalool oxides whereas the same reaction performed on the benzoates mixture afforded a separable mixture of cis- and trans-pyranoid linalool oxide. Overall, the method features, as a unique mandatory requirement, the availability of both linalool enantiomers, and can be conveniently performed from a milligram to a multigram scale.

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.

Trityl-Containing Alcohols-An Efficient Chirality Transmission Process from Inductor to the Stereodynamic Propeller and their Solid-State Structural Diversity

The cascade process of a dynamic chirality transmission from the permanent chirality center to the stereodynamic triphenylmethyl group has been studied for series of optically active trityl derivatives. The structural analysis, carried out with the use of complementary methods, enabled us to determine the mechanism of chirality transfer. The process of chirality transmission involves a set of weak but complementary electrostatic interactions. The induction of helicity in a trityl propeller is revealed by rising non-zero cotton effects in the area of trityl UV-absorption. The presence of an additional stereogenic center in close proximity to the trityl-containing stereogenic center significantly affects the sign and, to a lesser extent, magnitude of the respective cotton effects. Despite the bulkiness of the trityl, in the crystalline phase, the molecules under study strictly fill the space. In the crystal, molecules form aggregates stabilized by OH•••O hydrogen bonds. However, the presence of two trityl groups precludes formation of OH•••O hydrogen bonding. Additionally, the trityl group seems to be responsible for the formation of the solid solutions by e.g., racemates of trans- and cis-2-tritylcyclohexanol. Therefore, the trityl group acts as a supramolecular protective group, which in turn can be used in the crystal engineering.

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.

Applications of chiral naphthyloxycyclohexanols in deracemization of α-substituted carboxylic acids by dynamic thermodynamic resolution

Synthesis of chiral naphthyloxycyclohexanols and their applications in the preparation of optically pure α-substituted carboxylic acids by dynamic thermodynamic resolution.

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

The direct oxidative cyclization of 1,5-dienes is a valuable synthetic method for the (dia)stereoselective preparation of substituted tetrahydrofurans. Closely related reactions start from 5,6-dihydroxy or 5-hydroxyalkenes to generate similar products in a mechanistically analogous manner. After a brief overview on the history of this group of transformations and a survey on mechanistic and stereochemical aspects, this review article provides a summary on applications in natural product synthesis. Moreover, current limitations and future directions in this area of chemistry are discussed.

Metal catalyst-free substitution of allylic and propargylic phosphates with diarylmethyl anions

Substitution of secondary allylic and propargylic phosphates with the anions derived from Ar₂CH₂ with BuLi or LDA proceeded regioselectively and stereoselectively without a metal catalyst, affording inversion products in good yields.

Stereoselective synthesis of trans-THF rings using an oxidative cyclisation-radical deoxygenation sequence: application to the formal synthesis of trans-(2R,5R)-linalool oxide

Permanganate-mediated oxidative cyclisation-Barton–McCombie radical deoxygenation sequence is applied to 1,5-dienes to achieve stereoselective synthesis of trans-2,5-disubstituted THF rings.

Comparative perspective and synthetic applications of transition metal mediated oxidative cyclisation of 1,5-dienes towards cis-2,5-disubstituted tetrahydrofurans

Synthesis ofcis-2,5-disubstituted tetrahydrofuran rings from 1,5-diene precursors using metal-oxo species along with their mechanistic proposals and synthetic applications is concisely reviewed.