Advances in Total Synthesis of Some 2,3,5-Trisubstituted Tetrahydrofuran Natural Products

2,3,5-Trisubstituted tetrahydrofuran moiety is ubiquitous in natural products. These have served as appealing candidates for total synthesis due to their varied bio- and pharmaceutical activities. This tutorial review delineates the ingenious efforts by many researchers in the total synthesis of selected natural products based on a common 2,3,5-trisubstituted tetrahydrofuran core structure. Many of the syntheses display nuanced interplay between new methods and the ingenuity of planned strategies achieved through catalysis or cascade chemistry. In some cases, the chiron approach has come quite handy, wherein the structural features and the stereochemistry in select molecules could map well with naturally available starting materials. This compilation also aims to enhance the diversity space based on these natural products and further interest in sustainable total synthesis.

Hydrogels with multiple clickable anchor points: synthesis and characterization of poly(furfuryl glycidyl ether)-block-poly(ethylene glycol) macromonomers

Functional polyacrylamide hydrogels containing multiple furfuryl anchor points for Diels–Alder reactions were prepared employing new macromonomers.

Irreversible endo-selective diels-alder reactions of substituted alkoxyfurans: a general synthesis of endo-cantharimides

The [4+2] cycloaddition of 3-alkoxyfurans with N-substituted maleimides provides the first general route for preparing endo-cantharimides. Unlike the corresponding reaction with 3H furans, the reaction can tolerate a broad range of 2-substitued furans including alkyl, aromatic, and heteroaromatic groups. The cycloaddition products were converted into a range of cantharimide products with promising lead-like properties for medicinal chemistry programs. Furthermore, the electron-rich furans are shown to react with a variety of alternative dienophiles to generate 7-oxabicyclo[2.2.1]heptane derivatives under mild conditions. DFT calculations have been performed to rationalize the activation effect of the 3-alkoxy group on a furan Diels-Alder reaction.

Concerted and stepwise mechanisms in the Diels–Alder and Michael reactions of furans with methyl 3-nitroacrylate — Experimental and theoretical studies

The Diels–Alder reactions of the furans (1a–1c) and methyl 3-nitroacrylate (2) produce the cycloadducts via a concerted mechanism. Then, the products subsequently undergo retro-Diels–Alder reactions to give the Michael adducts via a stepwise mechanism. We have used both experimental and theoretical methods to investigate the mechanism for, and the regio- and stereo-selectivity in, the Diels–Alder and Michael reactions. To account for the selectivity observed in these additions, we examined the frontier molecular orbitals involved, and to locate the transition states (TS), we employed a B3LYP/6-31G* level density functional calculation. We were able to explain the regio- and stereo-selectivity of the four possible Diels–Alder adducts observed in the experiments by a comparison of their calculated activation energies, which ranged from 11 to 18 kcal/mol (1 cal = 4.184 J). The s-cis forms of the dienophile (2) were found to be more stable than the s-trans forms, both in the ground state and in the transition state. In addition, the endo transition structures for the ester group exhibited a lower energy barrier by 0.3 kcal/mol than their exo counterparts, a value that is in disagreement with the experimental results. The presence of the nitro group in 2 may play an important role in determining the selectivity. Our attempts to find a stepwise mechanism leading to the Diels–Alder adduct via a zwitterion were unsuccessful. We did find that two stepwise processes lead to the formation of Michael adducts generated via an electrophilic attack by 2 at the α position of 1b and 1c, and an intramolecular proton-transfer mediated by the formation of a four-membered structure. The potential energies for these reactions showed values in the following ranges: 17 kcal/mol for 5a and 5b, and 11 to 12 kcal/mol for 7a and 7b for the first step, and, for the proton-transfer step, 48–51 kcal/mol for 5a and 5b, and 41–45 kcal/mol for 7a and 7b. Solvent effects in chloroform on the Michael addition did not appear in the electrophilic attack step (less than 4 kcal/mol), but the transition state of the later process was stabilized by 6–13 kcal/mol.Key words: Diels–Alder reactions, Michael reactions, density functional method, furan, retro-Diels–Alder reactions, solvent effect.

A Total Synthesis of (±)-trans-Kumausyne

A short total synthesis of (+/-)-trans-kumausyne is reported. Key steps include a tandem ring-opening-ring-closing metathesis and the effective introduction of the pentenyl side chain by allylation-cross metathesis. [reaction: see text]

Total Synthesis of Epoxyquinols A, B, and C and Epoxytwinol A and the Reactivity of a 2H-Pyran Derivative as the Diene Component in the Diels−Alder Reaction

Full details of two versions of the total synthesis of epoxyquinols A, B, and C and epoxytwinol A (RKB-3564D) are described. In the first-generation synthesis, the HfCl(4)-mediated diastereoselective Diels-Alder reaction of furan with Corey's chiral auxiliary has been developed. In the second-generation synthesis, a chromatography-free preparation of an iodolactone, by using acryloyl chloride as the dienophile in the Diels-Alder reaction of furan, and the lipase-mediated kinetic resolution of a cyclohexenol derivative have been developed. This second-generation synthesis is suitable for large-scale preparation. A biomimetic cascade reaction involving oxidation, 6pi-electrocyclization, and then Diels-Alder dimerization is the key reaction in the formation of the complex heptacyclic structure of epoxyquinols A, B, and C. Epoxytwinol A is synthesized by the cascade reaction composed of oxidation, 6pi-electrocyclization, and formal [4 + 4] cycloaddition reactions. A 2H-pyran, generated by oxidation/6pi-electrocyclization, acts as a good diene, reacting with several dienophiles to afford polycyclic compounds in one step. An azapentacyclic compound is synthesized by a similar cascade reaction composed of the four successive steps: oxidation, imine formation, 6pi-azaelectrocyclization, and Diels-Alder dimerization.