A General Synthesis of Bis-α-acyloxy-1,4- and -1,5-diketones Through Catalytic Oxidative Opening of Acylated THF and THP Diols

Synthesis of 5α,6-Dihydroveragranines A and B

The 5α,6-dihydro congeners of veragranines A and B, two steroidal alkaloids with an unprecedented hexacyclic skeleton and potent analgesic effects, were synthesized from hecogenin acetate within six steps. This work enables quick access to the hexacyclic skeleton and is amendable to prepare other D-ring-modified congeners.

(±)-((2S,5R)-5-(Acetoxymethyl)tetrahydrofuran-2-yl)methyl Benzoate

In this note we report the synthesis of a doubly acylated cis-THF-diol product synthesised in three steps by the stereoselective RuO4-catalysed oxidative cyclisation of 1,5-hexadiene, followed by benzoylation and acetylation. This substance is one of the substrates chosen to probe a new developed oxidative procedure to transform bis-acylated THF-diols into bis-acylated 1,4-diketones. This new derivative was fully characterised by spectroscopic methods.

Improved synthesis of 2,4,6-trialkylpyridines from 1,5-diketoalkanes: the total synthesis of Anibamine

Many pyridine syntheses have been developed to date. In this study, we focused on pyridine synthesis with 1,5-diketone derivatives and hydroxylamine. Treatment of simple 1,5-diketoalkanes and hydroxylamine in basic or acidic conditions gave aldol adducts without any pyridine compounds. However, by screening the reaction conditions, we found that acidic conditions produced via the formation of oxime intermediates derived from 1,5-diketoalkanes allowed the formation of the corresponding pyridine derivatives. This is the first example of 2,4,6-trialkylpyridine synthesis from quite simple 1,5-diketoalkanes. In order to demonstrate the utility of the reaction, we demonstrated the synthesis of pyridine derivatives and the total synthesis of a 6-substituted pyridyl-natural product, anibamine. This was achieved by following the above methodology using a reported compound as the starting material to give the product in 12% yield.

Cobalt-Catalyzed Stereoselective Synthesis of 2,5- trans-THF Nitrile Derivatives as a Platform for Diversification: Development and Mechanistic Studies

A straightforward protocol integrating a sustainable approach for the synthesis of new 2,5- trans-THF nitrile derivatives enabling an easy diversification of its side chain scaffolds is described. The reaction tolerated different aromatic and alkyl substituents, affording the corresponding 2,5- trans-THFs in high diastereoselectivity. A detailed mechanistic study using DFT calculation reveals details of the ligand-exchange step, suggesting an inner-sphere syn attack to form the 2,5- trans stereochemistry as the most likely pathway, excluding the previous cation radical intermediate. The formation of a Co-C intermediate is suggested on the basis of the homolytic cleavage to give the previously proposed free carbon radical intermediate.

Synthesis and Biological Evaluation of a New Structural Simplified Analogue of cADPR, a Calcium-Mobilizing Secondary Messenger Firstly Isolated from Sea Urchin Eggs

Herein, we reported on the synthesis of cpIPP, which is a new structurally-reduced analogue of cyclic ADP-ribose (cADPR), a potent Ca2+-releasing secondary messenger that was firstly isolated from sea urchin eggs extracts. To obtain cpIPP the "northern" ribose of cADPR was replaced by a pentyl chain and the pyrophosphate moiety by a phophono-phosphate anhydride. The effect of the presence of the new phosphono-phosphate bridge on the intracellular Ca2+ release induced by cpIPP was assessed in PC12 neuronal cells in comparison with the effect of the pyrophosphate bridge of the structurally related cyclic N1-butylinosine diphosphate analogue (cbIDP), which was previously synthesized in our laboratories, and with that of the linear precursor of cpIPP, which, unexpectedly, revealed to be the only one provided with Ca2+ release properties.

Crystal structure of an ep-oxy-sterol: 9α,11α-ep-oxy-5α-cholest-7-ene-3β,5,6α-triol 3,6-di-acetate

The title compound, C31H48O6, is a polyoxygenated ep-oxy steroid obtained by a multi-step synthesis involving oxidation of 7-de-hydro-cholesterol. It crystallizes in the P212121 space group; however, the absolute structure of the molecule in the crystal could not be determined by resonant scattering. The configuration at the C5 and C6 positions is in both cases of the α-type, as is that of the C atoms of the ep-oxy ring. Mol-ecules in the crystal form chains parallel to the b axis by hydrogen bonding between O-H donors and carbonyl O-atom acceptors. Some atoms of the alkyl chain are disordered over two orientations, with a refined occupancy ratio of 0.511 (10):0.489 (10).

Pseudosymmetry and high Z' structures: the case of rac-(2R,2'R,5'S)-2-methyl-5'-[(1R,2R,5S,5'S)-1,4,4,5'-tetra-methyl-dihydro-3'H-3,8-dioxa-spiro-[bi-cyclo-[3.2.1]octane-2,2'-furan]-5'-yl]-3,4,1',2',3',4'-hexa-hydro-[2,2'-bi-furan]-5(2H)-one

The title compound, C22H34O6, is one of the products obtained by oxidation of squalene with the catalytic system RuO4(cat.)/NaIO4. It crystallizes in the P-1 space group, with four crystallographically independent mol-ecules related by a pseudo-C2 symmetry axis. The structural analysis also shows that the title compound is isomeric with two products previously reported in the literature and that are obtained by the same reaction procedure. In particular, out of the seven chiral C atoms present in the mol-ecule, the title compound shows the opposite configuration at, respectively, four and two chiral centres with respect to the isomeric compounds.

Crystal structure of a new spiro-polytetra-hydro-furan compound with translational pseudosymmetry: rac-(2S,2'S,5'R)-2-methyl-5'-[(1R,2R,5S,5'R)-1,4,4,5'-tetra-methyl-dihydro-3'H-3,8-dioxa-spiro[bi-cyclo-[3.2.1]octane-2,2'-furan]-5'-yl]hexa-hydro[2,2'-bi-furan]-5(2H)-one

The title compound, C22H34O6, is a product of oxidation of squalene with the catalytic system RuO4(cat.)/NaIO4. The asymmetric unit contains two crystallographically independent mol-ecules of very similar geometry approximately related by the non-crystallographic translation vector c/2. As a consequence, the average diffracted intensity in the hkl layers with odd l is systematically lower than in the layers with even l. In one mol-ecule, the lactone ring and part of the adjacent tetra-hydro-furan ring are disordered over two orientations with refined occupancy ratio of 0.831 (10):0.169 (10). The crystal structure is mainly governed by van der Waals forces.

5-Amino-1-(2',3'-O-iso-propyl-idene-d-ribit-yl)-1H-imidazole-4-carboxamide: a crystal structure with Z' = 4

The title compound, C12H20N4O5, crystallizes in the monoclinic space group P21, with four crystallographically independent mol-ecules in the asymmetric unit. The four mol-ecules have a very similar conformation that is basically determined by the formation of two intra-molecular hydrogen bonds between the amino NH2 donors and the carbonyl and ring O-atom acceptors, forming, respectively, R(6) and R(7) ring motifs.. In the crystal, inter-molecular hydrogen bonding leads to the formation of R22(10) ring patterns, involving one amide CONH2 donor and an imidazole N-atom acceptor. The cluster of the four independent mol-ecules has approximate non-crystallographic C2 point symmetry. The structural analysis also shows that during the synthesis of the title compound, the reductive cleavage of the d-ribose ring of the inosine precursor proceeds stereoselectively, with retention of configuration.

Ruthenium tetroxide and perruthenate chemistry. Recent advances and related transformations mediated by other transition metal oxo-species

In the last years ruthenium tetroxide is increasingly being used in organic synthesis. Thanks to the fine tuning of the reaction conditions, including pH control of the medium and the use of a wider range of co-oxidants, this species has proven to be a reagent able to catalyse useful synthetic transformations which are either a valuable alternative to established methods or even, in some cases, the method of choice. Protocols for oxidation of hydrocarbons, oxidative cleavage of C-C double bonds, even stopping the process at the aldehyde stage, oxidative cleavage of terminal and internal alkynes, oxidation of alcohols to carboxylic acids, dihydroxylation of alkenes, oxidative degradation of phenyl and other heteroaromatic nuclei, oxidative cyclization of dienes, have now reached a good level of improvement and are more and more included into complex synthetic sequences. The perruthenate ion is a ruthenium (VII) oxo-species. Since its introduction in the mid-eighties, tetrapropylammonium perruthenate (TPAP) has reached a great popularity among organic chemists and it is mostly employed in catalytic amounts in conjunction with N-methylmorpholine N-oxide (NMO) for the mild oxidation of primary and secondary alcohols to carbonyl compounds. Its use in the oxidation of other functionalities is known and recently, its utility in new synthetic transformations has been demonstrated. New processes, synthetic applications, theoretical studies and unusual transformations, published in the last eight years (2006-2013), in the chemistry of these two oxo-species, will be covered in this review with the aim of offering a clear picture of their reactivity. When appropriate, related oxidative transformations mediated by other metal oxo-species will be presented to highlight similarities and differences. An historical overview of some aspects of the ruthenium tetroxide chemistry will be presented as well.

(3R,3aR,6R,6aR)-Hexa-hydro-furo[3,2-b]furan-3,6-diyl dibenzoate

The title compound, C₂₀H₁₈O₆, prepared from d-mannitol by a two-step procedure, is a functionalized fused bis-tetra­hydro­furan. In the central fragment, consisting of two fused tetra­hydro­furan rings, one O atom and its two adjacent C atoms, a methyl­ene and a bridgehead C atom, are disordered over two sets of sites with an occupancy ratio of 0.735 (9):0.265 (9). In the major component, the ring containing the disordered O atom is a half-chair conformation with twisted methylene and benzoate-substituted C atoms, whereas the other ring has a half-chair or T-form conformation. In the minor component, the ring with the disordered O atom has an envelope conformation, with the O atom as the flap, and the other ring has a half-chair conformation, with the O atom and the other bridgehead CH atom being twisted. The two aromatic rings are inclined to one another by 20.00 (12)°. In the crystal, adjacent molecules are linked via C—H⋯π interactions, forming chains propagating along [010].

(3S,3aS,6R,6aR)-2-Oxohexa-hydro-furo[3,2-b]furan-3,6-diyl dibenzoate

The title compound, C₂₀H₁₆O₇, contains a cis-fused γ-lactone tetra­hydro­furan ring system functionalized with two benzo­yloxy groups. Both rings adopt an envelope conformation. The mol­ecule assumes an elongated shape and exibits non-crystallographic C₂ symmetry. The benzo­yloxy groups are almost planar [maximum deviations of 0.0491 (15) and 0.0336 (17) Å for the O atoms] and their mean planes are inclined to one another by 16.51 (4)°. The crystal packing features weak C—H⋯O inter­actions. The aryl groups of adjacent mol­ecules are parallel shifted with face-to-face contacts and a shortest inter­molecular C⋯C distance of 3.482 (4) Å.

3β,6α-Diacet-oxy-5,9α-dihy-droxy-5α-cholest-7-en-11-one

The title compound, C₃₁H₄₈O₇, a polyoxygenated steroid, was obtained by chemical oxidation of 7-de­hydro­cholesteryl acetate. The mol­ecular geometry features trans A/B and C/D junctions at the steroid core with the acetyl groups in the equatorial position and a fully extended conformation for the alkyl side chain. A chair conformation is observed for rings A and C while ring B adopts a half-chair conformation. The five-membered ring D has an envelope conformation, with the C atom bearing the methyl group at the flap. The terminal isopropyl group and one acetyl group are disordered over two sets of sites with 0.774 (8):0.226 (8) and 0.843 (7):0.157 (7) ratios, respectively. An intra­molecular S(6) O—H⋯O hydrogen-bonding motif involving a hy­droxy donor and acceptor is observed. In the crystal, chains of mol­ecules running along the b axis are formed via O—H⋯O hydrogen bonds between hy­droxy donors and carbonyl acceptors of the ordered acetyl group, giving rise to a C(14) motif. The chains are wrapped around the 2₁ screw axes.

3β,6β-Diacet-oxy-5,9α-dihy-droxy-5α-cholest-7-en-11-one acetic acid 0.04-solvate

The title compound, C31H48O7·0.04CH3COOH, is a polyoxy-genated steroid obtained by selective chemical oxidation of 7-de-hydro-cholesteryl acetate. The asymmetric unit comprises three mol-ecules of the steroid (Z' = 3) and a mol-ecule of acetic acid which has occupancy factor 0.131 (5). The geometric parameters of the independent mol-ecules do not reveal significant differences. In one mol-ecule, the terminal isopropyl group is disordered over two sets of sites with occupancy ratio 0.869 (5):0.131 (5). The three mol-ecules reveal different hydrogen-bonding patterns. Each of them is involved in an intra-molecular S(6) hydrogen-bonding motif, involving hy-droxy groups as donor and acceptor. In the crystal, two independent mol-ecules form dimers through hydrogen bonding between an OH donor and an acetate carbonyl acceptor, giving rise to R 2 (2)(16) ring patterns. A single hydrogen bond between the OH group and a ketone carbonyl group is observed between two symmetry-independent mol-ecules.

2-(4-Methyl-phen-yl)-6-nitro-1,3-benzoxazole

The title compound, C14H10N2O3, is a π-conjugated mol-ecule containing a benzoxazole aromatic fused heterobicycle. The benzoxazole ring system is planar within 0.01 Å. The mol-ecule assumes an approximately flat conformation, the benzoxazole ring system forming dihedral angles of 6.52 (12) and 7.4 (3)° with the benzene ring and the nitro group, respectively. In the crystal, mol-ecules are connected by very weak C-H⋯O hydrogen inter-actions, forming chains running parallel to the a or c axes. The methyl H atoms are disordered over two sets of sites of equal occupancy rotated by 60°.

3,3'-({4-[(4,5-Di-cyano-1H-imidazol-2-yl)diazen-yl]phen-yl}imino)-dipropionic acid

The title compound, C₁₇H₁₅N₇O₄, is a push–pull non-linear optical chromophore containing a di­alkyl­amino donor group and the di­cyano­imidazolyl acceptor separated by a π-conjugated path. The benzene and imidazole rings are not coplanar, making a dihedral angle of 10.0 (2)°. In the crystal, mol­ecules are linked by an extended set of hydrogen bonds and several motifs are recognized. Pairs of mol­ecules are held together by hydrogen bonding between carb­oxy O—H donor groups and diazenyl N-atom acceptors, forming R₂²(24) ring patterns across inversion centres. Four-mol­ecule R₄⁴(28) ring motifs are formed, again across inversion centres, through hydrogen bonding involving carb­oxy O—H donor groups and diazenyl and imidazole N-atom acceptors. Four-mol­ecule R₄⁴(42) patterns are formed among mol­ecules related by translation and involve carb­oxy O—H and imidazole N—H donor groups with carbonyl O-atom and imidazole N-atom acceptors.