Aspects of the intramolecular diels-alder reaction of a furan diene (IMDAF) leading to the formation of 1,4-epoxydecalin systems

Synthesis and ethylene-promoted metathesis of adducts of tandem [4+2]/[4+2] cycloaddition between bis-furyl dienes and maleic acid derivatives

A series of 1,4:5,8-diepoxynaphthalenes, annellated with carbo- and heterocycles, was synthesized based on the tandem intermolecular/intramolecular [4+2] cycloaddition of bis-furyl dienes with moderately to highly reactive cyclic dienophiles (maleic anhydride and maleinimides).

Synthetic Study toward the Total Synthesis of Taxezopidines A and B

A concise synthetic approach to construct the [6,8,6]-tricyclic core of taxezopidines A and B, which contains a synthetically challenging bridged bicyclo[5.3.1]undecane ring system bearing most of the desired functionalized groups and stereocenters, has been established. This approach features a diastereoselective type II intramolecular Diels-Alder furan reaction. The stereochemistry of the acetoxy group at the allylic position of the dienophile alkene group, such as in 6a, was found to be critical for achieving the desired highly diastereoselective outcome.

Synthesis of hydronaphthalenes through coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes

The coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes to afford furans that contain pendant alkene groups is described. Subsequent intramolecular Diels-Alder reactions are effective in select cases, resulting in hydronaphthalene systems after dehydration. Although the Diels-Alder event is thermodynamically unfavorable, the overall transformation of alkene-furans to dihydronaphthalenes is a favorable process.

Chemical and biological studies of nakiterpiosin and nakiterpiosinone

Nakiterpiosin and nakiterpiosinone are two related C-nor-D-homosteroids isolated from the sponge Terpios hoshinota that show promise as anticancer agents. We have previously described the asymmetric synthesis and revision of the relative configuration of nakiterpiosin. We now provide detailed information on the stereochemical analysis that supports our structure revision and the synthesis of the originally proposed and revised nakiterpiosin. In addition, we herein describe a refined approach for the synthesis of nakiterpiosin, the first synthesis of nakiterpiosinone, and preliminary mechanistic studies of nakiterpiosin's action in mammalian cells. Cells treated with nakiterpiosin exhibit compromised formation of the primary cilium, an organelle that functions as an assembly point for components of the Hedgehog signal transduction pathway. We provide evidence that the biological effects exhibited by nakiterpiosin are mechanistically distinct from those of well-established antimitotic agents such as taxol. Nakiterpiosin may be useful as an anticancer agent in those tumors resistant to existing antimitotic agents and those dependent on Hedgehog pathway responses for growth.

Chameleon Reactivity of the Allene Bond of 4-Vinylidene-2-oxazolidinone: Novel Through-Space Conjugative Nucleophilic Addition of Electron-Rich Alkenes and Hetero-Nucleophiles

The Calpha==Cbeta double bond of allene carbamates 1 serves as an electron acceptor similar to the double bond of conjugated enones by means of a through-space interaction with the N--SO2 bond; the carbamate double bond is thus subject to nucleophilic addition for a wide variety of nucleophiles, which proceeds under mild conditions by heating at 70-100 degrees C. Depending on the kind of nucleophiles, 1 displays three different reaction modes: 1) Typically enol ethers and allylsilanes promote 1,3-sulfonyl migration of 1 and undergo the inverse electron demand Diels-Alder reaction with the 1-aza-1,3-butadiene intermediates II thus formed to furnish bicyclic 2-alkoxy-5-sulfonyltetrahydropyridines 2 and 2-silylmethyl-5-sulfonyltetrahydropyridines 3, respectively, with high regio- and stereoselectivity and retention of configuration of the double bonds of these electron-rich alkenes; 2) silanes (RnSiH4-n, n=1-3) and thiols deliver the hydride and the thiolate at the Cbeta carbon and promote the 1,3-sulfonyl migration, followed by protonation of the thus-formed carbamate anion (Z)-III to provide, for example, (Z)-4 a and (Z)-4 j, respectively; 3) alcohols simply add to the Calpha==Cbeta double bond and provide (E)-6. Usually, the reaction with alcohols is accompanied by the second pathway, giving rise to, for example, (Z)-4 b in addition to (E)-6 b. Phenol engages in the third pathway and provides (E)-6 g exclusively. Heteroaromatics, such as furans and benzofurans follow the first pathway, however, in a different regioselectivity from enol ethers and allylsilanes, delivering the oxygen atom at the 3-position of 5-sulfonyltetrahydropyridines (2 g and 2 h). Indoles, on the other hand, show a dichotomy, equally enjoying the first and the third pathways and provide mixtures of (E)-7 and (E)-8, respectively.

Rhodium(I)-catalyzed nucleophilic ring-opening reactions of oxabicyclo adducts derived from the [4 + 2]-cycloaddition of 2-imido-substituted furans

A series of 2-imido-substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-ylcarbamic acid tert-butyl ester to a solution of the mixed anhydride of an appropriately substituted 3-butenoic acid. The initially formed imido furans undergo a rapid intramolecular [4 + 2]-cycloaddition at room temperature to deliver the Diels-Alder cycloadducts in good to excellent yield. Isolation of the highly labile oxabicyclic adduct is believed to be a consequence of the lower reaction temperatures employed as well as the presence of the extra carbonyl group, which diminishes the basicity of the nitrogen atom, thereby retarding the ring cleavage/rearrangement reaction generally encountered with related systems. By using a Rh(I)-catalyzed ring opening of the oxabicyclic adduct with various nucleophilic reagents, it was possible to prepare highly functionalized hexahydro-1H-indol-2(3H)-one derivatives in good yield. The major stereoisomer obtained possesses a cis-relationship between the nucleophile and hydroxyl group in the ring-opened product. The stereochemistry was unequivocally established by X-ray crystallographic analysis. Coordination of Rh(I) to the alkenyl pi-bond followed by a nitrogen-assisted cleavage of the carbon-oxygen bond occurs to furnish a pi-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting pi-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads to the cis-diastereomer.

Chiral Template Mediated Diastereoselective Intramolecular Diels−Alder Reaction Using Furan as a Diene. Toward the Synthesis of Enantiopure Trisubstituted Tetrahydroepoxyisoindolones

[reaction: see text] Chiral precursors to the intramolecular Diels-Alder reaction using furan as a diene (IMDAF) have been prepared by diastereoselective addition of furyllithium to imines attached to perhydro-1,3-benzoxazines derived from (-)-8-aminomenthol. The acylation of the furylamines with alpha,beta-unsaturated acyl chlorides and subsequent IMDAF reaction under thermal conditions provided the corresponding cycloadducts as single diastereoisomers. The hydrolytic elimination of the chiral template yielded enantiopure perhydroepoxyisoindolones with up to five stereocenters.

A Stereoselective and Atom-Efficient Approach to Multifunctionalized Five- and Six-Membered Rings via a Novel Michael-Initiated Intramolecular Diels−Alder Furan Reaction

[reaction: see text] A variety of key precursors to the intramolecular Diels-Alder reaction of furan diene (IMDAF) have been prepared via a very facile 1,4-addition of O-, S-, N-, and C-centered nucleophiles possessing unsaturated tether to beta-furyl nitroethylene. Subsequent IMDAF reaction of the 1,4-adducts, carried out under thermal conditions, provided five- and six-membered carbocycles and heterocycles fused to an easily cleavable oxabicycloheptene moiety. The structure and stereochemistry of the cycloadducts were determined by 2D-NMR experiments and further confirmed by X-ray crystallography. The salient features of the strategy include high degree of stereoselectivity (>80:20) in the cycloaddition, atom and step economy, and generation of multiple chiral centers and functionalities. The feasibility of the cleavage of the oxa bridge in the cycloadducts to afford novel multifunctional molecules has also been demonstrated.