Thermal [2+2] Intramolecular Cycloadditions of Fuller-1,6-enynes

On the Origins of Stereo- and Regio-Selectivities in the Formation of Fullerene-Fluorene Dyads

Recently, a novel [2+2] cycloaddition between the classical I_h-C₆₀ and a fluorenylideneallene complex has been achieved experimentally. In the fullerene-fluorene dyad product, stereo- and regio-selectivities were found in the experiment, but the reasons are still unknown. Our theoretical studies suggest that, based on a diradical pathway, the structural selectivity of the product strongly depends on the structural/electronic features of the fluorenylideneallene and C₆₀ complexes. When the R¹ group in fluorenylideneallene denotes the H atom, the E-type product is more stable than the Z-type one, whereas other bulkier R¹ groups lead to the reverse due to their steric hindrance. The π orbital conjugation between the fluorenyl group and the C^β═C^γ bond in fluorenylideneallene is the main reason for the high selectivity of β,γ-cycloaddition. Analyses of both frontier orbitals and spin density for the intermediate structure suggest a diradical pathway of the reaction between fluorenylideneallene and C₆₀ and uncover a decisive role of the LUMO of C₆₀ toward regio-selectivity, which conduces to a high selectivity of the (6,6)-addition product.

Mechanistic Insights into the Ru(II)-Catalyzed Intramolecular Formal [3 + 2] Cycloaddition of (E)-1,6-Enynes

Design of a unique reaction pathway in transition-metal-catalyzed 1,6-enynes cyclization to construct valuable synthetic motifs is a significant challenge in organic chemistry. Herein, we report a Ru(II)-catalyzed formal [3 + 2] cycloaddition as an efficient method to prepare unprecedented bicyclo[3.3.0]octenes from readily available (E)-1,6-enynes. Mechanistic studies based on the deuterium labeling experiments and the DFT calculation disclose a reasonable mechanistic pathway, where a ruthenacyclopentene generated by an ene-yne oxidative cyclization undergoes a sequential ß-hydride elimination and intramolecular hydroruthenation to form a ruthenacyclohexene, producing the desirable bicyclo[3.3.0]octenes.

Cycloaddition reactions of pristine and endohedral fullerene molecules: possible anticancer activity

Epoxide of oestradiol is one of the main risk factors for the genesis and evolution of breast cancer; hence, in recent years there has been considerable interest in the investigation of new inhibitors capable of reducing its carcinogenic activity. The aim of this article is to study the [2 + 2] cycloaddition reaction of epoxide of oestradiol in different pristine (C₇₆ and D_5h-C₈₀) and endohedral metallofullerene (C₇₂@Sc₂C₂, C₇₆@Sc₂ and C₈₀@Sc₂) by means of molecular electrostatic potential (MEP) topological analysis. Different from other molecular scalar fields, MEP topology enables to find minima related to lone pairs and π electrons, therefore, this molecular scalar field is appropriate to identify the most reactive sites. In consonance with our results, it was found that C₈₀ was the best candidate to carry out the epoxide of oestradiol cycloaddition since more stable adducts were obtained. Furthermore, it is expected that more than one oestradiol epoxide molecule will be added to C₈₀, forasmuch as C₈₀ reactivity is enhanced once the adduct is formed. The study was carried through DFT framework included in the Gaussian 09 package (MPWB95/6-31G(d,p)).

Unexpected isomerism in cis-2 bis(pyrrolidino)[60]fullerene diastereomers

Similar yet different: A one-step regio- and diastereoselective synthesis of three new bis(pyrrolidine)[60]fullerenes, one cis-1 and two unprecedented cis-2 diastereoisomers, is reported. The compounds are easily purified using simple chromatographic techniques, and were fully characterized by spectroscopic techniques and X-ray crystallography. A mechanism for the isomeric conversion observed is proposed.

Buckyballs

Buckyballs represent a new and fascinating molecular allotropic form of carbon that has received a lot of attention by the chemical community during the last two decades. The unabating interest on this singular family of highly strained carbon spheres has allowed the establishing of the fundamental chemical reactivity of these carbon cages and, therefore, a huge variety of fullerene derivatives involving [60] and [70]fullerenes, higher fullerenes, and endohedral fullerenes have been prepared. Much less is known, however, of the chemistry of the uncommon non-IPR fullerenes which currently represent a scientific curiosity and which could pave the way to a range of new fullerenes. In this review on buckyballs we have mainly focused on the most recent and novel covalent chemistry of fullerenes involving metal catalysis and asymmetric synthesis, as well as on some of the most significant advances in supramolecular chemistry, namely H-bonded fullerene assemblies and the search for efficient concave receptors for the convex surface of fullerenes. Furthermore, we have also described the recent advances in the macromolecular chemistry of fullerenes, that is, those polymer molecules endowed with fullerenes which have been classified according to their chemical structures. This review is completed with the study of endohedral fullerenes, a new family of fullerenes in which the carbon cage of the fullerene contains a metal, molecule, or metal complex in the inner cavity. The presence of these species affords new fullerenes with completely different properties and chemical reactivity, thus opening a new avenue in which a more precise control of the photophysical and redox properties of fullerenes is possible. The use of fullerenes for organic electronics, namely in photovoltaic applications and molecular wires, complements the study and highlights the interest in these carbon allotropes for realistic practical applications. We have pointed out the so-called non-IPR fullerenes - those that do not follow the isolated pentagon rule - as the most intriguing class of fullerenes which, up to now, have only shown the tip of the huge iceberg behind the examples reported in the literature. The number of possible non-IPR carbon cages is almost infinite and the near future will show us whether they will become a reality.

Mass spectrometry studies of the retro-cycloaddition reaction of pyrrolidino and 2-pyrazolinofullerene derivatives under negative ESI conditions

Substituted pyrrolidino- and 3-alkyl-2-pyrazolinofullerenes ionize under ESI and MALDI mass spectrometry conditions and negative mode of detection undergoing mass spectral fragmentations, which can be easily correlated with the reported results for the thermal and electrochemical retro-cycloaddition reactions of these compounds. 2-Pyrazolinofullerenes lead directly to a [60]fullerene product ion formed through a retro-cycloaddition process regardless of the substituents attached at the carbon and nitrogen atoms of the heterocyclic ring. These results are different from whose reported for the thermal and electrochemical processes. In contrast, pyrrolidinofullerenes undergo different fragmentative reactions depending upon the substituents (hydrogen, alkyl, or acyl) attached at the nitrogen atom of the heterocyclic ring leading eventually to the pristine C(60) in the last step of the fragmentation pathway.

Controlled regio- and chemoselective addition of isothiocyanate to the dione moiety of a cage-opened fullerene-mixed peroxide derivative

Addition of the ambident SCN group to a fullerendione derivative follows two different pathways in the presence and absence of Lewis acid.

Minimal pharmacophoric elements and fragment hopping, an approach directed at molecular diversity and isozyme selectivity. Design of selective neuronal nitric oxide synthase inhibitors

Fragment hopping, a new fragment-based approach for de novo inhibitor design focusing on ligand diversity and isozyme selectivity, is described. The core of this approach is the derivation of the minimal pharmacophoric element for each pharmacophore. Sites for both ligand binding and isozyme selectivity are considered in deriving the minimal pharmacophoric elements. Five general-purpose libraries are established: the basic fragment library, the bioisostere library, the rules for metabolic stability, the toxicophore library, and the side chain library. These libraries are employed to generate focused fragment libraries to match the minimal pharmacophoric elements for each pharmacophore and then to link the fragment to the desired molecule. This method was successfully applied to neuronal nitric oxide synthase (nNOS), which is implicated in stroke and neurodegenerative diseases. Starting with the nitroarginine-containing dipeptide inhibitors we developed previously, a small organic molecule with a totally different chemical structure was designed, which showed nanomolar nNOS inhibitory potency and more than 1000-fold nNOS selectivity. The crystallographic analysis confirms that the small organic molecule with a constrained conformation can exactly mimic the mode of action of the dipeptide nNOS inhibitors. Therefore, a new peptidomimetic strategy, referred to as fragment hopping, which creates small organic molecules that mimic the biological function of peptides by a pharmacophore-driven strategy for fragment-based de novo design, has been established as a new type of fragment-based inhibitor design. As an open system, the newly established approach efficiently incorporates the concept of early "ADME/Tox" considerations and provides a basic platform for medicinal chemistry-driven efforts.

Microwave-Assisted One-Pot Tandem Reactions for Direct Conversion of Primary Alcohols and Aldehydes to Triazines and Tetrazoles in Aqueous Media

A series of primary alcohols and aldehydes were treated with iodine in ammonia water under microwave irradiation to give the intermediate nitriles, which without isolation underwent [2+3] cycloadditions with dicyandiamide and sodium azide to afford high yields of the corresponding triazines and tetrazoles, including the alpha-amino- and dipeptidyl tetrazoles in high optical purity.

Intramolecular Ene Reaction of 1,6-Fullerenynes: A New Synthesis of Allenes

[Structure: see text] Thermal treatment of 1,6-fullerenynes bearing an alkyl group on the terminal carbon of the alkyne moiety leads quantitatively to new allenes through a reaction mechanism involving an intramolecular ene process. This reaction outcome is in contrast to that recently found for free terminal alkynes which form cyclobutene derivatives through a [2+2] cyclization mechanism.

The Mechanistic Puzzle of Transition-Metal-Catalyzed Skeletal Rearrangements of Enynes

Three pathways actually compete in metal-catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo-cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition-metal-catalyzed cyclization of some enynes, particularly, 1,7-enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn-cyclopropyl metal-carbenes formed in the syn pathway.

Rapid Assembly of Matrix Metalloprotease Inhibitors Using Click Chemistry

[reaction: see text] A panel of 96 metalloprotease inhibitors was assembled using "click chemistry" by reacting eight zinc-binding hydroxamate warheads with 12 azide building blocks. Screens of the bidentate compounds against representative metalloproteases provided discerning inhibition fingerprints, revealing compounds with low micromolar potency against MMP-7. The relative ease and convenience of the strategy in constructing focused chemical libraries for rapid in situ screening of MMPs is thereby demonstrated.

New challenges in fullerene chemistry

The aim of this focus article is to emphasize those aspects of fullerene chemistry which currently represent important challenges for the development and future applications of these new carbon allotropes, as well as to celebrate the tenth anniversary of the fullerenes Nobel Prize.