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[reaction: see text] Cobalt catalysis enables a new method for the generation of zinc enolates using diethylzinc to reduce alpha,beta-unsaturated amides. This method has been applied to a high-yielding diastereoselective reductive aldol cyclization.
Eight bromoalkynes were prepared from substituted benzoic acids and treated with n-Bu3SnH to provide trans-fused perhydroindans or cis- and trans-fused perhydronaphthalenes. Atom-transfer reactions that accompany the free radical reactions resulted in several tandem radical cyclizations with formation of up to three carbon-carbon bonds in a single reaction. The relationship between these reactions and an approach to the quassinoid family of natural products is also described.
[reaction: see text] 3-(2-Aminophenylthio)prop-1-yne 8 reacted with aryl iodides 9-18 under palladium-copper catalysis leading to the disubstituted alkynes 19-28 which after tosylation underwent a novel cyclization under copper catalysis to 2-substituted benzothiazolines 29-38. The expected 3-(arylidine)-2,3-dihydrobenzothiazines were not obtained.
[formula: see text] The diastereoselective synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans via Pd(0)-catalyzed cyclization of 2-methylene-1,4-diols is described. Investigations into the scope of the reaction and its application toward the synthesis of amphidinolide K is reported.
[reaction: see text] Angularly substituted, optically active bicyclic ketones of up to 94% ee were prepared by the Ti(II) alkoxide-mediated tandem cyclization of open-chain substrates, that is, 8-phenylmenthyl enynoates having a trisubstituted double bond.
[formula: see text] Cyclizations of substituted (E)-1-nitro-1,7,9-decatrienes under thermal and Lewis acid conditions have led to the formation of decalin ring systems with excellent endo selectivity. This strategy has been applied to the synthesis of the AB ring system of norzoanthamine.
[reaction: see text] To provide experimental evidence on the relationship between stucture and reactivity in 1,4-hydroxybiradical intermediates, the Norrish type II photochemistry of a homologous series of spirobenzoyladamantane derivatives was investigated in the crystalline state and the outcome correlated with the structures of the compounds as determined by X-ray crystallography. The results provide an unusually detailed and compelling picture of the geometric factors responsible for the partitioning of these reactive intermediates among cleavage, cyclization, and reverse hydrogen transfer.
[reaction: see text] The effect of chiral substituents on the enamide nitrogen atom upon the diastereoselection of the Mn(III)-mediated 4-exo-trig cyclization to beta-lactams was studied. A significant level of diastereoselectivity was achieved when an amino acid ester moiety was included into the enamidic skeleton. The structure of the major diastereoisomer was suggested by semiempirical calculations.
This work reveals a novel mechanism for triplet carbonyl formation (and hence chemiluminescence) during lipid peroxidation, whose chemiluminescence has been attributed to both triplet carbonyls and singlet oxygen. As a model for polyunsaturated fatty acid hydroperoxides, we have synthesized 3-hydroperoxy-2,3-dimethyl-1-butene by photooxygenation of tetramethylethylene. One-electron oxidation of this hydroperoxide with heme proteins and peroxynitrite to the corresponding alkylperoxyl radical results in chemiluminescence, both direct and 9,10-dibromoanthracene-2-sulfonate-sensitized, the latter attributed to the formation of triplet acetone. It is postulated that triplet acetone results from the cyclization of the alkylperoxyl radical to a dioxetane radical intermediate followed by its thermolysis. This is supported by EPR spin-trapping experiments in which discrimination between carbon-centered radicals derived from the alkyloxyl and alkylperoxyl radicals is achieved through the use of one-electron oxidants and reductants, e.g., FeII- and TiIII.
A novel strategy for solid-phase synthesis of hydantoins with high optical purity is described using a thermal pH-neutral cyclization and simultaneous release from resin. Hereby even hydantoins bearing a pH-sensitive side chain (protection) are available. The reaction conditions are well screened applying the parallel organic synthesizer APOS 1200.
1,5-Difluoro-2,4-dinitrobenzene starting material was treated via primary and/or secondary substitution with a variety of amino acids or amines and the aromatic m-dinitro groups were then reductively cyclized provide the 2-quinoxalinol analogs. The conditions for 1,5-dialkylamino-2,4-dinitrobenzene reduction have been systematically studied and optimized in solution. Three effective methods are described for the high-throughout generation of 2-quinoxalinol analogs.
The peptide, EMTPVNPG, derived from alpha-fetoprotein, inhibits estrogen-stimulated growth of immature mouse uterus and estrogen-dependent proliferation of human breast cancer cells. However, the biological activities of the peptide diminish over time in storage, even when in the lyophilized state, probably because of peptide aggregation through hydrophobic interaction among monomers. Two analogs of EMTPVNPG were designed with the intent of minimizing aggregation and retaining biological activity during prolonged storage. EMTOVNOG, where O is 4-hydroxyproline, is a linear peptide generated by substituting 4-hydroxyproline for the two prolines, thereby increasing peptide hydrophilicity. This analog exhibited a dose-dependent inhibition of estrogen-stimulated growth of immature mouse uterus similar to that of EMTPVNPG (maximal activity at 1 microg/mouse). A second analog, cyclo-(EMTOVNOGQ), a hydrophilic, cyclic analog with increased conformational constraint, was as potent as the other peptides in its inhibition of estrogen-dependent growth of immature mouse uterus, and had an expanded effective dose range. Both linear and cyclized hydroxyproline-substituted analogs exhibited indefinite shelf-life. Furthermore, both analogs inhibited the estrogen-dependent growth of MCF-7 human breast cancer growing as a xenograft in SCID mice. These analogs may become significant, novel agents for the treatment of breast cancer.
The combination of anionic Michael addition of lithium ester enolates with radical 5-exo cyclizations through SET oxidation gives highly functionalized cyclopentanes.
An efficient methodology provides an easy access to highly optically active polysubstituted butenolides starting from aryl halides and 1:1:salts of optically active 2,3-allenoic acid-base via an oxidative addition-coordinative cyclization-reductive elimination mechanism, which led to the high efficiency of this chirality isomerization reaction.
PdCl2(CH3CN)2 catalyzed the cyclization of alkenyl beta-keto esters in the presence of a stoichiometric amount of SiMe3Cl to form 2-carboalkoxycyclohexanones in good yield with excellent regioselectivity.
Molecular orbital calculations predict that the 5-azahexenoyl radical ring closes via nucleophilic addition to the acyl carbon to afford the 5-exo product; CCSD(T)/cc-pVDZ/ /BHLYP/cc-pVDZ calculations predict energy barriers of 36.1 and 46.9 kJ mol-1 for the exo and endo cyclization modes of the 5-azahexenoyl radical, respectively.
Unusual Pd-catalyzed cyclizations of unsymmetrical allylic acetates, where the nucleophile is tethered to the central allyl carbon, are shown for the first time to be remarkably controlled by heteroatom-mediated preassociation of the reacting partners.
A highly efficient asymmetric alkaloid synthesis via ene-type cyclizations catalyzed by cationic chiral palladium(II) complexes of PN-ligands with an achiral gem-dimethyl oxazoline unit is shown to give 5-membered products with a quaternary carbon center, including a spiro-amide, almost quantitatively with extremely high enantiomeric excesses.