8‐Aza‐7‐deaza‐2′‐de­oxy‐2‐(methyl­sulfan­yl)adenosine

In the title compound, 4‐amino‐1‐(2‐de­oxy‐β‐d ‐erythro‐pentofuranos­yl)‐6‐methyl­sulfanyl‐1H‐pyrazolo[3,4‐d]pyrimidine, C₁₁H₁₆N₅O₃S, the conformation of the glycosidic bond is between anti and high anti. The 2′‐deoxy­ribofuranosyl moiety adopts the C3′‐exo–C4′‐endo conformation (₃T⁴, S‐type sugar pucker), and the conformation at the exocyclic C—C bond is +sc (+gauche). The exocyclic 6‐amine group and the 2‐methyl­sulfanyl group lie on different sides of the heterocyclic ring system. The mol­ecules form a three‐dimensional hydrogen‐bonded network that is stabilized by O—H⋯N, N—H⋯O and C—H⋯O hydrogen bonds.     

1‐(β‐d‐Erythrofuranos­yl)cytidine (β‐erythrocytidine)

1‐(β‐d ‐Erythrofuranosyl)cytidine, C₈H₁₁N₃O₄, (I), a derivative of β‐cytidine, (II), lacks an exocyclic hydroxy­methyl (–CH₂OH) substituent at C4′ and crystallizes in a global conformation different from that observed for (II). In (I), the β‐d ‐erythrofuranosyl ring assumes an E₃ conformation (C3′‐exo; S, i.e. south), and the N‐glycoside bond conformation is syn. In contrast, (II) contains a β‐d ‐ribofuranosyl ring in a ³T₂ conformation (N, i.e. north) and an anti‐N‐glycoside linkage. These crystallographic properties mimic those found in aqueous solution by NMR with respect to furan­ose conformation. Removal of the –CH₂OH group thus affects the global conformation of the aldofuranosyl ring. These results provide further support for S/syn–anti and N/anti correlations in pyrimidine nucleosides. The crystal structure of (I) was determined at 200 K.     

8‐Aza‐7‐deaza‐7‐propynyladenosine methanol solvate

In the title compound, 4‐amino‐3‐propynyl‐1‐(β‐d ‐ribofur­anosyl)‐1H‐pyrazolo[3,4‐d]pyrimidine methanol solvate, C₁₃H₁₅N₅O₄·CH₃OH, the torsion angle of the N‐glycosylic bond is between anti and high‐anti [χ = −101.8 (5)°]. The ribofuranose moiety adopts the C3′‐endo (³T₂) sugar conformation (N‐type) and the conformation at the exocyclic C—C bond is +sc (gauche, gauche). The propynyl group is out of the plane of the nucleobase and is bent. The compound forms a three‐dimensional network which is stabilized by several hydrogen bonds (O—H·O and O—H·N). The nucleobases are stacked head‐to‐tail. The methanol solvent mol­ecule forms hydrogen bonds with both the nucleobase and the sugar moiety.     

Triazaphospholenium Tetrafluoroborate: A Phosphorus Analogue of a 1,2,3‐Triazole‐Derived Carbene

3H ‐1,2,3,4‐Triazaphosphole derivatives can be selectively alkylated with Meerwein's reagent at the most nucleophilic nitrogen atom. According to the principle of valence isoelectronicity, the corresponding phosphorus heterocycle represents the first formal phosphorus analogue of the well‐known 1,2,3‐triazolylidenes (mesoionic carbenes). Theoretical calculations revealed that the cation in triazaphospholenium tetrafluoroborate is an aromatic system with a high degree of π‐conjugation. First investigations showed that the cationic phosphorus heterocycle can stabilize a [Cu₂Br₄]²⁻ dianion by formation of a neutral coordination compound with an unusual bonding situation between phosphorus and copper(I).     

2′‐Deoxy‐5‐propynyluridine: a nucleoside with two conformations in the asymmetric unit

The title compound, 1‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐5‐(prop‐1‐ynyl)pyrimidin‐2,4(1H,3H)‐dione, C₁₂H₁₄N₂O₅, shows two conformations in the crystalline state: conformer 1 adopts a C2′‐endo (close to ²E; S‐type) sugar pucker and an anti nucleobase orientation [χ = −134.04 (19)°], while conformer 2 shows an S sugar pucker (twisted C2′‐endo–C3′‐exo), which is accompanied by a different anti base orientation [χ = −162.79 (17)°]. Both molecules show a +sc (gauche, gauche) conformation at the exocyclic C4′—C5′ bond and a coplanar orientation of the propynyl group with respect to the pyrimidine ring. The extended structure is a three‐dimensional hydrogen‐bond network involving intermolecular N—H...O and O—H...O hydrogen bonds. Only O atoms function as H‐atom acceptor sites.     

1,N6‐Etheno derivative of 7‐de­aza‐2,8‐di­aza­adenosine

In the tricyclic nucleoside 7‐(β‐d ‐ribo­furan­osyl)‐7H‐imidazo­[1,2‐c]­pyrazolo­[4,3‐e][1,2,3]­triazine, C₁₁H₁₂N₆O₄, the con­formation of the N‐gly­cosyl bond is intermediate between anti and high anti [χ = −103.5 (3)°]. The ribo­furan­ose moiety adopts a ₃T² sugar pucker (S‐type sugar) and the conformation at the exocyclic C—C bond is ap (gauche–trans). Molecules of the title compound form a three‐dimensional network via three medium–strong intermolecular hydrogen bonds (one O—H⋯N and two O—H⋯O bonds).     

2′‐De­oxy‐2‐fluoro­tubercidin

In the title compound [systematic name: 7‐(2‐de­oxy‐β‐d ‐erythro‐pentofuranos­yl)‐2‐fluoro‐7H‐pyrrolo[2,3‐d]pyrimidin‐2‐amine], C₁₁H₁₃FN₄O₃, the conformation of the N‐glycosylic bond is between anti and high‐anti [χ = −110.2 (3)°]. The 2′‐deoxy­ribofuranosyl unit adopts the N‐type sugar pucker (₄T³), with P = 40.3° and τ_m = 39.2°. The orientation of the exocyclic C4′—C5′ bond is −ap (trans), with a torsion angle γ = −168.39 (18)°. The nucleobases are arranged head‐to‐head. The crystal structure is stabilized by four inter­molecular hydrogen bonds of types N—H⋯N, N—H⋯O and O—H⋯O.     

Binuclear trans‐Bis(β‐iminoaryloxy)palladium(II) Complexes Doubly Linked with Pentamethylene Spacers: Structure‐Dependent Flapping Motion and Heterochiral Association Behavior of the Clothespin‐Shaped Molecules

The synthesis, structure, and solution‐state behavior of clothespin‐shaped binuclear trans‐bis(β‐iminoaryloxy)palladium(II) complexes doubly linked with pentamethylene spacers are described. Achiral syn and racemic anti isomers of complexes 1 – 3 were prepared by treating Pd(OAc)₂ with the corresponding N,N′‐bis(β‐hydroxyarylmethylene)‐1,5‐pentanediamine and then subjecting the mixture to chromatographic separation. Optically pure (100 % ee) complexes, (+)‐anti‐ 1 , (+)‐anti‐ 2 , and (+)‐anti‐ 3 , were obtained from the racemic mixture by employing a preparative HPLC system with a chiral column. The trans coordination and clothespin‐shaped structures with syn and anti conformations of these complexes have been unequivocally established by X‐ray diffraction studies. ¹H NMR analysis showed that (±)‐anti‐ 1 , (±)‐anti‐ 2 , syn‐ 2 , and (±)‐anti‐ 3 display a flapping motion by consecutive stacking association/dissociation between cofacial coordination planes in [D₈]toluene, whereas syn‐ 1 and syn‐ 3 are static under the same conditions. The activation parameters for the flapping motion (ΔH^≠ and ΔS^≠) were determined from variable‐temperature NMR analyses as 50.4 kJ mol^?1 and 60.1 J mol^?1 K^?1 for (±)‐anti‐ 1 , 31.0 kJ mol^?1 and ?22.7 J mol^?1 K^?1 for (±)‐anti‐ 2 , 29.6 kJ mol^?1 and ?57.7 J mol^?1 K^?1 for syn‐ 2 , and 35.0 kJ mol^?1 and 0.5 J mol^?1 K^?1 for (±)‐anti‐ 3 , respectively. The molecular structure and kinetic parameters demonstrate that all of the anti complexes flap with a twisting motion in [D₈]toluene, although (±)‐anti‐ 1 bearing dilated Z‐shaped blades moves more dynamically than I‐shaped (±)‐anti‐ 2 or the smaller (±)‐anti‐ 3 . Highly symmetrical syn‐ 2 displays a much more static flapping motion, that is, in a see‐saw‐like manner. In CDCl₃, (±)‐anti‐ 1 exhibits an extraordinary upfield shift of the ¹H NMR signals with increasing concentration, whereas solutions of (+)‐anti‐ 1 and the other syn/anti analogues 2 and 3 exhibit negligible or slight changes in the chemical shifts under the same conditions, which indicates that anti‐ 1 undergoes a specific heterochiral association in the solution state. Equilibrium constants for the dimerizations of (±)‐ and (+)‐anti‐ 1 in CDCl₃ at 293 K were estimated by curve‐fitting analysis of the ¹H NMR chemical shift dependences on concentration as 26 M ^?1 [K_D(racemic)] and 3.2 M ^?1 [K_D(homo)], respectively. The heterochiral association constant [K_D(hetero)] was estimated as 98 M ^?1, based on the relationship K_D(racemic)=1/2 K_D(homo)+1/4 K_D(hetero). An inward stacking motif of interpenetrative dimer association is postulated as the mechanistic rationale for this rare case of heterochiral association.     

Nichtplanare Doppelbindung in einem kristallinen Derivat von anti-Sesquinorbornen (anti-Tetracyclo[6.2.1.13,6.02,7]dodec-2(7)-en)

Non-Planar Double Bond in a Crystalline Derivative of anti-Sesquinorbornene (anti-Tetracyclo[6.2.1.1^3,6.0^2,7]dodec-2(7)-ene) The olefinic double bond of the title compound 2 has been observed by X-ray techniques to deviate from planarity by 13.2 (0.3)° (symmetric out-of-plane bending). Empirical force field calculations for the parent hydrocarbon 1 (anti-sesquinorbornene) give a much larger corresponding deviation of 35.6°. The calculated nonplanar double bond deformations in anti-sesquinorbornene 1 as well as in the syn-isomer 4 and other related olefins are a consequence of substantial widenings of the exocyclic C(sp³)–C(sp²–Csp³) angles when keeping the double bond planar. The considerable computational exaggeration of this novel out-of-plane bending mechanism is attributed to various possible force field deficiencies.     

7‐Cyclopropyl‐2′‐deoxytubercidin: a carbocyclic side‐chain derivative of 7‐deaza‐2′‐deoxyadenosine

The title compound {systematic name: 4‐amino‐5‐cyclopropyl‐7‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl)‐7H‐pyrrolo[2,3‐d]pyrimidine}, C₁₄H₁₈N₄O₃, exhibits an anti glycosylic bond conformation, with the torsion angle χ = −108.7 (2)°. The furanose group shows a twisted C1′‐exo sugar pucker (S‐type), with P = 120.0 (2)° and τ_m = 40.4 (1)°. The orientation of the exocyclic C4′—C5′ bond is ‐ap (trans), with the torsion angle γ = −167.1 (2)°. The cyclopropyl substituent points away from the nucleobase (anti orientation). Within the three‐dimensional extended crystal structure, the individual molecules are stacked and arranged into layers, which are highly ordered and stabilized by hydrogen bonding. The O atom of the exocyclic 5′‐hydroxy group of the sugar residue acts as an acceptor, forming a bifurcated hydrogen bond to the amino groups of two different neighbouring molecules. By this means, four neighbouring molecules form a rhomboidal arrangement of two bifurcated hydrogen bonds involving two amino groups and two O5′ atoms of the sugar residues.     

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

Metalloradical species [Co₂Fv(CO)₄]^.+ ( 1 ^.+, Fv=fulvalenediyl) and [Co₂Cp₂(CO)₄]^.+ ( 2 ^.+, Cp=η⁵‐C₅H₅), formed by one‐electron oxidations of piano‐stool cobalt carbonyl complexes, can be stabilized with weakly coordinating polyfluoroaluminate anions in the solid state. They feature a supported and an unsupported (i.e. unbridged) cobalt–cobalt three‐electron σ bond, respectively, each with a formal bond order of 0.5 (hemi‐bond). When Cp is replaced by bulkier Cp* (Cp*=η⁵‐C₅Me₅), an interchange between an unsupported radical [Co₂Cp*₂(CO)₄]^.+ (anti‐ 3 ^.+) and a supported radical [Co₂Cp*₂(μ‐CO)₂(CO)₂]^.+ (trans‐ 3 ^.+) is observed in solution, which cocrystallize and exist in the crystal phase. 2 ^.+ and anti‐ 3 ^.+ are the first stable thus isolable examples that feature an unsupported metal–metal hemi‐bond, and the coexistence of anti‐ 3 ^.+ and trans‐ 3 ^.+ in one crystal is unprecedented in the field of dinuclear metalloradical chemistry. The work suggests that more stable metalloradicals of metal–metal hemi‐bonds may be accessible by using metal carbonyls together with large and weakly coordinating polyfluoroaluminate anions.     

Germacyclobutenes: Generation by 1,1‐Carbalumination or 1,1‐Carbagallation and Their Photophysical Properties

Aluminium‐ and gallium‐functionalised alkenylalkynylgermanes, R¹₂Ge(C?C?R²)[C{E(CMe₃)₂}?C(H)?R²] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α‐C atoms of the intact ethynyl groups. These interactions activate the Ge?C(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the E?C(vinyl) bonds by means of 1,1‐carbalumination or 1,1‐carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes. Germacyclobutenes with an unsaturated GeC₃ heterocycle and endo‐ and exocyclic C?C bonds resulted from concomitant Ge?C bond formation to the β‐C atoms of the alkynyl groups. These heterocyclic compounds show an interesting photoluminescence behaviour with Stokes shifts of >110 nm. The fascinating properties are based on extended π‐delocalisation including σ*‐orbitals localised at Ge and Al. High‐level quantum chemical DFT and TD‐DFT calculations for an Al compound were applied to elucidate their absorption and emission properties. They revealed a biradical excited state with the transfer of a π‐electron into the empty p‐orbital at Al and a pyramidalisation of the metal atom.     

Formation,Structural Characterization,and Calculated NMR Chemical Shifts of Selenium‐Nitrogen Compounds from SeCl4 and ArNHLi (Ar = supermesityl,mesityl)

Supermesityl selenium diimide [Se{N(C₆H₂^tBu₃‐2, 4, 6)}₂; Se{N(mes*)}₂] can be prepared in a good yield from the reaction of SeCl₄ and (mes*)NHLi. The molecule adopts an unprecedented anti, anti‐conformation, as deduced by DFT calculations at PBE0/TZVP level of theory and supported by ⁷⁷Se NMR spectroscopy and a crystal structure determination. An analogous reaction involving (C₆H₂Me₃‐2, 4, 6)NHLi [(mes)NHLi] unexpectedly lead to the reduction of selenium and afforded the selenium diamide Se{NH(mes)}₂ that was characterized by X‐ray crystallography and ⁷⁷Se NMR spectroscopy. The Se‐N bonds of 1.847(3) and 1.852(3) Å show normal single bond lengths. The <NSeN bond angle of 109.9(1)° also indicates a tetrahedral AX₂E₂ bonding arrangement around selenium. Two N‐H···N hydrogen bonds link the Se{NH(mes)}₂ molecule with two discrete (mes)NH₂ molecules. In the solid state selenium diamide adopts the anti‐conformation, whereas in solution the presence of both syn‐ and anti‐isomers could be observed. PBE0/TZVP calculations of the shielding tensors of 28 different types of selenium‐containing molecules, for which the ⁷⁷Se chemical shifts are unambiguously known, were carried out to assist the spectral assignment of Se{N(mes*)}₂ and Se{NH(mes)}₂.     

5‐Methyl‐6‐aza‐2′‐deoxy­isocytidine

In the title compound, 3‐amino‐2‐(2‐deoxy‐β‐d ‐erythro‐pento­furan­osyl)‐6‐methyl‐1,2,4‐triazin‐5(2H)‐one, C₉H₁₄N₄O₄, the conformation of the N‐glycosidic bond is high‐anti and the 2‐deoxy­ribo­furan­osyl moiety adopts a North sugar pucker (²T₃). The orientation of the exocyclic C—C bond between the –CH₂OH group and the five‐membered ring is ap (gauche, trans). The crystal packing is such that the nucleobases lie parallel to the ac plane; the planes are connected via hydrogen bonds involving the five‐membered ring.     

Factors that regulate the conformation of m-benziporphodimethene complexes: agostic metal-arene interaction, hydrogen bonding, and η2,π coordination

Group 12 and silver(I) tetramethyl‐m‐benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η²,π coordination from the β‐pyrrolic C?C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)? H(22) on the m‐phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd^II and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)–H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m‐benziporphyrin and tetraphenyl‐m‐benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen‐bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid‐state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent‐ and temperature‐dependent syn/anti ratio changes. The observation of dynamic ¹H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen‐bonding interactions between the axial chloride and CHCl₃ stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.     

7‐De­aza‐2′‐deoxy‐7‐propynyl­guanosine

The title compound, C₁₄H₁₆N₄O₄, adopts the anti conformation at the gly­cosylic bond [χ−117.1 (5)°]. The sugar pucker of the 2′‐deoxy­ribo­furan­osyl moiety is C2′‐endo–C3′‐exo, ²T₃ (S‐type). The orientation of the exocyclic C4′—C5′ bond is +sc (gauche). The propynyl group is linear and coplanar with the nucleobase moiety. The structure of the compound is stabilized by several hydrogen bonds (N—H⋯O and O—H⋯O), leading to the formation of a multi‐layered network. The nucleobases, as well as the propynyl groups, are stacked. This stacking might cause the extraordinary stability of DNA duplexes containing this compound.     

4‐Amino‐7‐(2‐de­oxy‐2‐fluoro‐β‐d‐arabinofuranos­yl)‐5‐fluoro‐7H‐pyrrolo[2,3‐d]pyrimidine: a bis‐fluorinated analogue of 2′‐deoxy­tubercidin

The title compound, C₁₁H₁₂F₂N₄O₃, exhibits an anti glycosylic bond conformation, with a torsion angle χ = −117.8 (2)°. The sugar pucker is N‐type (C4′‐exo, between ³T₄ and E₄, with P = 45.3° and τ_m = 41.3°). The conformation around the exocyclic C—C bond is −ap (trans), with a torsion angle γ = −177.46 (15)°. The nucleobases are stacked head‐to‐head. The crystal structure is characterized by a three‐dimensional hydrogen‐bond network involving N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds.     

Synthesis and Crystal Structures of O‐2′,3′‐Cyclic Cyclopentanone and Cyclohexanone Ketals of the Cytostatic 5‐Fluorouridine

The synthesis of two O‐2′,3′‐cyclic ketals, i.e., 5 and 6 , of the cytostatic 5‐fluorouridine ( 2 ), carrying a cyclopentane and/or a cyclohexane ring, respectively, is described. The novel compounds were characterized by ¹H‐, ¹⁹F‐, and ¹³C‐NMR, and UV spectroscopy, as well as by elemental analyses. Their crystal structures were determined by X‐ray analysis. Both compounds 5 and 6 show an anti‐conformation at the N‐glycosidic bond which is biased from +ac to +ap compared to the parent nucleoside 2 . The sugar puckering is changed from ^2′E to _3′E going along with a reduction of the puckering amplitude τ_m by ca. 10–13° due to the ketalization. The conformation about the sugar exocyclic bond C(4′)? C(5′) of 5 and 6 remains unchanged, i.e., g⁺, compared with compound 2 .     

Mechanism of Silver(I)‐Catalyzed Enantioselective Synthesis of Axially Chiral Allenes Based on Propargylamines

The silver(I)‐catalyzed synthesis picture of axially chiral allenes based on propargylamines has been outlined using density functional theory (DFT) method for the first time. Our calculations find that, the coordination of silver(I) into triple bond of propargylamines at anti‐position of nitrogen shows a stronger activation on the triple bond than that at cis‐position, which is favorable for the subsequent hydrogen transfer. The NBO charge analysis for the hydrogen transfer affirms the experimental speculation that this step is a hydride transfer process. The energy barrier of the anti‐periplanar elimination of vinyl‐silver is 26.9 kJ·mol^?1 lower than that of the syn‐periplanar elimination, supporting that (?)‐allene is the main product of this reaction. In a word, the most possible route for this reaction is that the silver(I) coordinates into the triple bond of propargylamines at anti‐position of nitrogen, then the formed silver(I) complex undergoes a hydride transfer to give a vinyl‐silver, finally the vinyl‐silver goes through an anti‐periplanar elimination to give (?)‐allene. The hydride transfer with the energy barrier of 44.8 kJ·mol^?1 is the rate‐limiting step in whole catalytic process. This work provides insight into why this reaction has a very high enantioselectivity.     

1‐(2‐Deoxy‐β‐d‐erythro‐pento­furan­osyl)‐4‐nitro‐1H‐indazole

In the title compound, C₁₂H₁₃N₃O₅, the conformation of the gly­cosyl­ic bond is anti [torsion angle = −105.3 (2)°]. The 2′‐deoxy­ribo­furan­ose moiety adopts an S‐type sugar pucker and the orientation of the exocyclic C—C bond is −sc (trans).