Photocrosslinked hydrogels from coumarin derivatives of hyaluronic acid for tissue engineering applications.

Hydrogels are an increasingly attractive choice in the fields of regenerative medicine, wound care and tissue engineering as important forms of bio-scaffolds. For many clinical needs, injectable in situ crosslinkable hydrogels are strongly preferred, due to treatment effectiveness and ease of use. In this study, hyaluronic acid (HA), containing side-arms linked to photo-active coumarin moieties, was used for the preparation of wall-to-wall hydrogels. This photocrosslinkable HA, hereafter called HA-TEG-coumarin, produces colourless aqueous solutions that solidify upon near-UV irradiation (at a specific wavelength of 365 nm) via a clean [2 + 2] photocycloaddition reaction, without by-products formation. The crosslinking event, a robust and non-cytotoxic process, does not require catalysts or radical initiators: in the field of hyaluronan photocrosslinking, this innovative feature is significant to ensure the whole biocompatibility and to avoid collateral reactions. Mechanical and rheological tests showed that hyaluronan derivatives became hydrogels after 3-5 min of irradiation, with average values for bulk and surface elastic moduli of about 32 kPa and 193 kPa, respectively. Fluorescence recovery after photobleaching (FRAP) assay showed that the hydrogels are porous and allow a good permeation for nutrients and growth factors. Cell metabolism and proliferation assays revealed that hydrogel-encapsulated fibroblasts maintained their viability and that HA-TEG-coumarin sustained the proliferation of non-adherent myoblasts. For all of these reasons and thanks to a safe free-radical approach, this novel hyaluronan coumarin derivative could be a good candidate for tissue engineering and regenerative medicine applications.

Reductive opening of carbohydrate phenylsulfonylethylidene (PSE) acetals.

The phenylsulfonylethylidene (PSE) acetal is a relatively new protecting group in carbohydrate chemistry. However, carbohydrate-derived phenylsulfonylethylidene (PSE) acetals show a different behavior in reductive desulfonylation than simple symmetrical acetals. Here we have investigated various SET-type reaction conditions in order to open PSE acetals regioselectively and to produce chiral ω-hydroxyethenyl ethers. Whereas sodium amalgam leads to a mixture of regioisomeric vinyl ethers besides the ethylidene acetal, samarium iodide is suited for regioselective ring opening. This is shown with seven different carbohydrate PSE acetals, both of the 1,3-dioxane and the 1,3-dioxolane type.

Practical synthesis of aryl-2-methyl-3-butyn-2-ols from aryl bromides via conventional and decarboxylative copper-free Sonogashira coupling reactions.

Two efficient protocols for the palladium-catalyzed synthesis of aryl-2-methyl-3-butyn-2-ols from aryl bromides in the absence of copper were developed. A simple catalytic system consisting of Pd(OAc)2 and P(p-tol)3 using DBU as the base and THF as the solvent was found to be highly effective for the coupling reaction of 2-methyl-3-butyn-2-ol (4) with a wide range of aryl bromides in good to excellent yields. Analogously, the synthesis of aryl-2-methyl-3-butyn-2-ols was performed also through the decarboxylative coupling reaction of 4-hydroxy-4-methyl-2-pentynoic acid with aryl bromides, using a catalyst containing Pd(OAc)2 in combination with SPhos or XPhos in the presence of tetra-n-butylammonium fluoride (TBAF) as the base and THF as the solvent. Therefore, new efficient approaches to the synthesis of terminal acetylenes from widely available aryl bromides rather than expensive iodides and using 4 or propiolic acid rather than TMS-acetylene as inexpensive alkyne sources are described.

Chiral M3L2 self-assembled capsules through metal coordination of enantiopure ligating benzocyclotrimers: NMR spectroscopic and ESI mass spectrometric investigation.

The synthesis of enantiopure (+)-benzotricamphor syn-5, an important chiral C3-symmetric rigid building block for supramolecular applications, was studied in detail to reduce the number of steps and to increase the diastereoselectivity and overall yield. The new synthetic procedure allowed larger amounts of syn-5 to be obtained and used for the preparation of new derivatives, such as the corresponding tris-trifluoromethanesulfonate syn-12, which was efficiently transformed into (+)-benzotribornenetrinitrile syn-1 and (+)-benzotribornenetris(ethynyl-4-pyridine) syn-2. The previously reported (+)-benzotricamphortrioxime syn-6 was transformed into tris-nitrile syn-3 by Beckman reaction. Compounds syn-1-3 were employed as multidentate ligands for silver(I) and platinum(II) centres in apolar solvents. The linear coordination geometry of Ag(I) and square-planar geometry of cis-chelated Pt(II) in combination with the chiral tripodal ligands syn-1-3 led to the formation of chiral enantiopure capsules with M3 L2 stoichiometry, as confirmed by 2D NMR NOESY and DOSY experiments as well as ESI mass spectrometry.

(+)-syn-Benzotriborneol an enantiopure C3-symmetric receptor for water.

(+)-syn-Benzotriborneol forms stable complexes with one molecule of water. This is due to the ability of the host to form three hydrogen bonds with water, to act simultaneously as a hydrogen-bond acceptor and donor, and to a perfect geometrical match between the pair. We report experimental (X-ray and neutron diffraction, VT NMR, DSC, TGA) and stereochemical studies carried out to elucidate and quantify the molecular and thermodynamic aspects of this supramolecular complex.

Benzocyclotrimers: from the Mills-Nixon effect to gas hosting.

The formal annulation of three bicylic olefins yields a class of molecules termed benzocyclotrimers (BCTs), which have unusual electronic properties. The bonds in the central aromatic ring, for example, alternate in length: rather than resembling a substituted benzene, a BCT instead evokes comparison to a cyclohexatriene. Forty years have passed since the synthesis of heptiptycene, the first BCT, was reported. In the interim, many methods have been developed for preparing tris-bicycloannulated benzenes. More than thirty different BCTs have so far been reported, with a variety of morphological features and properties. Over the same period, yields have increased from just a few percent to almost quantitative conversion. This improvement in synthetic access has expanded interest beyond the original theoretical considerations (bond-length fixation in aromatics) to functional applications (supramolecular scaffolds). In this Account, we describe the evolution of synthetic approaches to BCTs and their derivatives, as well as the applications that are now being explored for these compounds. Early syntheses of BCTs involved chloroolefins treated with butyl lithium. A strained alkyne intermediate was postulated early on, and was indeed trapped in 1981. Subsequent efforts have focused on improving chemoselectivity by mitigating the drastic conditions required for the generation of the alkyne intermediate. Our introduction of Cu(I) to induce lithium-copper exchange was successful in this regard. Further improvement resulted from the use of bicylic bromo(trimethylstannyl)olefins. In an effort to avoid the toxicity of the tin reagents, the Heck reaction and Pd catalysis have been pursued for cyclotrimerizing bicylic bromo- and iodoolefins. Depending on the symmetry of the starting bicylic olefin, two diastereomers can be obtained in the preparation of a BCT: a syn compound with C(3) symmetry and an anti compound with C(s) symmetry. Studying the diastereomeric outcome in a variety of synthetic approaches has yielded valuable insight into the cyclotrimerization reaction. Moreover, highly symmetric compounds, such as a D(3)-symmetric trindane and C(3v)-symmetric sumanene, have been prepared as BCT derivatives. The structure of BCTs offers a versatile three-dimensional scaffold for studying molecular recognition. Like calixarenes, BCTs form complexes with a variety of guest molecules. Recent developments include the trapping of gases in a hydrogen-bonded dimer and the encapsulation of larger molecules within a covalently linked condensation derivative. Future innovations in this fertile research area will likely include highly functionalized curved aromatics, receptors, and sensors.

Synthesis and structure of D(3h)-symmetric triptycene trimaleimide.

A new D(3h) symmetric triptycene derivative has been synthesized with the aim of obtaining molecules that are able to assemble into porous structures, and can be used in the development of new ligands. The synthesis involves a Diels-Alder reaction as the key step, followed by an oxidation and the formation of a maleimide ring. Triptycene trimaleimide furnished single crystals which have been analyzed by means of X-ray diffraction.

Synthesis, chiroptical properties, and their theoretical simulation of some highly rotating benzotricamphor derivatives.

The large molecules 1-3 (69, 90, and 102 atoms, respectively), prepared by cyclotrimerization of enantiomerically pure derivatives of (-)-bornyl acetate, show intense ECD spectra, high optical rotation (OR) values (200-1300, in absolute value) dominated in sign and order of magnitude by the lowest-energy Cotton effects, that is, they are the ideal candidates to test the reliability of our "approximate" (TDDFT/B3LYP/6-31G* or smaller basis set) approach to the calculation of chiroptical properties. As a matter of fact, a correct simulation of the OR values and ECD spectra of 1 and 2 can be obtained even using STO-3G basis set and semiempirical or molecular mechanics input geometries: for 1, at the TDDFT/B3LYP/STO-3G level, the OR values are of the order of 500-550, versus an experimental value ranging between 660 and 690, depending on the solvent. On the contrary, the case of 3 (exp. OR between -1330 and -1500) is really complex (for instance, the OR values range between -3216 and -729 (TDDFT/B3LYP/6-31G* calculations) or -1824 and -444 (TDDFT/B3LYP/STO-3G calculations)), making the comparison between calculated and experimental values more difficult. The behavior of 3 is due to its molecular flexibility, whereas 1 is a really rigid molecules and 2 behaves (vide infra) as it were a rigid system. These observations strongly indicate that the conformational freedom constitutes one of the major difficulties for a correct but simple simulation of the chiroptical properties.

Gases as guests in benzocyclotrimer cage hosts.

An imine-linked (+)-syn-benzotricamphor derivative gives access to chiral unimolecular cages exhibiting internal cavities of new shapes and volumes. One of these hosts hydrocarbon gases at low temperatures in solution through CH-pi attractive interactions. No encapsulation is observed when the cage structure is too narrow or too large for the guest.

Thione-thiol rearrangement: miyazaki-newman-kwart rearrangement and others.

Thione-to-thiol rearrangements represent a general route for the synthesis of sulfur compoundsfrom hydroxyl functionalities. In particular, the Miyazaki-Newman-Kwart rearrangementhas been widely used for the synthesis of aromatic thiols from the corresponding phenols.

Benzotriazole complexes with amines and phenol: cooperativity mediated by induction effects in the crystal state.

[structure: see text] Benzotriazole forms complexes of different stoichiometries with amines and phenols. Four of them have been characterized by single-crystal X-ray diffraction. The trends of donor-acceptor hydrogen-bond distances between corresponding molecular entities in the different complexes are related to induction-mediated cooperativity effects.

The pyrrole approach toward the synthesis of fully functionalized cup-shaped molecules.

[reaction: see text] A novel method for the synthesis of new highly functionalized cyclotrimers is described. The method consists of an original synthesis of beta-dibromosubstituted pyrroles, metalation, cycloaddition, and cyclotrimerization. The sequence is highly compatible with common functional groups and allows the construction of cup-shaped molecules functionalized both at the upper and bottom rim. This feature makes the newly formed structures useful scaffolds for the development of supramolecular receptors.

Efficient cyclotrimerization of bicyclic vic-bromostannylalkenes promoted by copper(I) thiophen-2-carboxylate.

Copper(I) thiophen-2-carboxylate was successfully employed in the trimerization of [2.2.1] bicyclic vic-bromotrimethyltin olefins (in their racemic composition), bearing different functionalities, to invariably obtain almost quantitative yields of the syn and anti tris-annelated benzenes. The two isomers come in different ratios, smaller than or equal to the statistical 1:3 ratio, depending on the steric hindrance opposed by the functionalities. In the case of enantiopure (3-bromo-4,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-yl)trimethylstannane, the 1:9 ratio found with Cu(NO(3))(2).3H(2)O increases to 1:6.

Enantiomeric Discrimination in a Reiterative Domino Coupling Process: CuI -mediated Syn Cyclotrimerization of Racemic Polycyclic Trimethylstannyl Bromonorbornadienes.

The challenge to generate all-syn architecture has prompted the synthesis of syn cyclotrimers 2 from racemic bromotrimethylstannyl polycyclic alkenes 1. The syn-selective CuI -catalyzed cyclotrimerization process is both regio- and chemoselective.

Central to Axial Transfer of Chirality in Menthone or Camphor-Derived 2,2'-Biphenols.

A study aimed at defining a molecular arrangement where a chiral fragment derived from menthone or camphor transfers its central chirality to a 2,2'-biphenol residue, inducing an axial chirality, is reported. The menthol or isoborneol groups are attached at the two benzylic positions at 3,3' in order to maximize efficiency in practical applications. A reliable and high-yielding procedure for the synthesis of such C(2)-symmetric molecules substituted at the 3,3'-positions has been developed. The procedure entails Mannich condensation with paraformaldehyde and morpholine, protection of the hydroxylic functions, chlorination, metalation, and addition to (-)-menthone and (+)-camphor. The use of samarium diiodide is essential in the latter step for optimum selectivity and efficiency. The tetrols exhibit intramolecular hydrogen bonding between phenolic and alcoholic hydroxy functions within each monomeric unity, so that they retain their rotational freedom. NOEDS and COSY experiments show that the tetrols are present in more than one rotamer. The tetrols react with tetrachlorosilane to afford siloxanes as pure diastereoisomers, showing that the metal is able to induce preferential helicity at the biphenyl residue; i.e., the central chirality of menthol or isoborneol auxiliary is totally transfered to the axial chirality of the biphenyl. The configurations could be determined by NOEDS and heterocorrelated HMQC experiments. Remarkably, while the menthol derivative induces total M helicity, the camphor induces complementary P helicity. These results suggest that these tetrols may be useful as ligands in catalysts for asymmetric synthesis.