Benzylamine as Hydrogen Transfer Agent: Cobalt‐Catalyzed Chemoselective C=C Bond Reduction of β‐Trifluoromethylated α,β‐Unsaturated Ketones via 1,5‐Hydrogen Transfer

An efficient cobalt‐catalyzed chemoselective reduction of β‐CF₃‐α,β‐unsaturated ketones using benzylamine as hydrogen transfer agent involving intramolecular 1,5‐hydrogen transfer is reported. The reaction proceeded smoothly with a relatively wide range of substrates including those bearing aromatic heterocycles such as a furyl ring system in high yields (74–92 %). This provides an efficient method for the synthesis of β‐CF₃ saturated ketones in one‐pot. This methodology was also applied to the selective C=C reduction of other enone substrates bearing no β‐CF₃‐substituent, of which β‐substituted or β,β‐disubstituted enones are tolerated, giving the desired products in good yields (72–75 %). Mechanistic studies indicate that the reaction involves 1,5‐hydrogen transfer.     

Ethyl 1,4‐Dihydro‐4‐oxo‐1,8‐naphthyridine‐3‐carboxylates by a Tandem SNAr‐Addition‐Elimination Reaction

A series of N‐substituted 1,4‐dihydro‐4‐oxo‐1,8‐naphthyridine‐3‐carboxylate esters has been prepared in two steps from ethyl 2‐(2‐chloronicotinoyl)acetate. Treatment of the β‐ketoester with N,N‐dimethylformamide dimethyl acetal in N,N‐dimethylformamide (DMF) gave a 95% yield of the 2‐dimethylaminomethylene derivative. Subsequent reaction of this β‐enaminone with primary amines in DMF at 120^oC for 24 h then afforded the target compounds in 47–82% yields by a tandem S_NAr‐addition‐elimination reaction. Synthetic and procedural details as well as a mechanistic rationale are presented.     

Ethyl 1,4‐dihydro‐4‐oxo‐3‐quinolinecarboxylates by a tandem addition‐elimination‐SNAr reaction

The ethyl 1,4‐dihydro‐4‐oxo‐3‐quinolinecarboxylate ring structure, important in several drug compounds, has been prepared in two steps from ethyl 2‐(2‐fluorobenzoyl)acetate. Treatment of this β‐ketoester with N,N‐dimethylformamide dimethyl acetal gives a 97% yield of the 2‐dimethylaminomethylene derivative. Reaction of this β‐enaminone with primary amines in N,N‐dimethylformamide at 140°C for 48 h then affords the 1,4‐dihydro‐4‐oxo‐3‐quinolinecarboxylate esters in 60–74% yields by a tandem addition‐elimination‐S_NAr reaction. The synthesis of the starting material as well as procedural details and a mechanistic scenario are presented. J. Heterocyclic Chem., (2011).     

Disorder and conformational analysis of methyl β‐d‐galactopyranosyl‐(1→4)‐β‐d‐xylopyranoside

Methyl β‐d ‐galactopyranosyl‐(1→4)‐β‐d ‐xylopyranoside, C₁₂H₂₂O₁₀, (II), crystallizes as colorless needles from water with positional disorder in the xylopyranosyl (Xyl) ring and no water molecules in the unit cell. The internal glycosidic linkage conformation in (II) is characterized by a ϕ′ torsion angle (C2′_Gal—C1′_Gal—O1′_Gal—C4_Xyl) of 156.4 (5)° and a ψ′ torsion angle (C1′_Gal—O1′_Gal—C4_Xyl—C3_Xyl) of 94.0 (11)°, where the ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (–CH₂OH) C atoms in the β‐Xyl and β‐Gal residues, respectively. By comparison, the internal linkage conformation in the crystal structure of the structurally related disaccharide, methyl β‐lactoside [methyl β‐d ‐galactopyranosyl‐(1→4)‐β‐d ‐glucopyranoside], (III) [Stenutz, Shang & Serianni (1999). Acta Cryst. C 55 , 1719–1721], is characterized by ϕ′ = 153.8 (2)° and ψ′ = 78.4 (2)°. A comparison of β‐(1→4)‐linked disaccharides shows considerable variability in both ϕ′ and ψ′, with the range in the latter (∼38°) greater than that in the former (∼28°). Inter‐residue hydrogen bonding is observed between atoms O3_Xyl and O5′_Gal in the crystal structure of (II), analogous to the inter‐residue hydrogen bond detected between atoms O3_Glc and O5′_Gal in (III). The exocyclic hydroxymethyl conformations in the Gal residues of (II) and (III) are identical (gauche–trans conformer).     

Methyl 4‐O‐β‐D‐mannopyranosyl β‐D‐xylopyranoside

Methyl β‐D‐mannopyranosyl‐(1→4)‐β‐D‐xylopyranoside, C₁₂H₂₂O₁₀, (I), crystallizes as colorless needles from water, with two crystallographically independent molecules, (IA) and (IB), comprising the asymmetric unit. The internal glycosidic linkage conformation in molecule (IA) is characterized by a ϕ′ torsion angle (O5′_Man—C1′_Man—O1′_Man—C4_Xyl; Man is mannose and Xyl is xylose) of −88.38 (17)° and a ψ′ torsion angle (C1′_Man—O1′_Man—C4_Xyl—C5_Xyl) of −149.22 (15)°, whereas the corresponding torsion angles in molecule (IB) are −89.82 (17) and −159.98 (14)°, respectively. Ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (–CH₂OH) C atom in the β‐Xylp and β‐Manp residues, respectively. By comparison, the internal glycosidic linkage in the major disorder component of the structurally related disaccharide, methyl β‐D‐galactopyranosyl‐(1→4)‐β‐D‐xylopyranoside), (II) [Zhang, Oliver & Serriani (2012). Acta Cryst. C 68 , o7–o11], is characterized by ϕ′ = −85.7 (6)° and ψ′ = −141.6 (8)°. Inter‐residue hydrogen bonding is observed between atoms O3_Xyl and O5′_Man in both (IA) and (IB) [O3_Xyl...O5′_Man internuclear distances = 2.7268 (16) and 2.6920 (17) Å, respectively], analogous to the inter‐residue hydrogen bond detected between atoms O3_Xyl and O5′_Gal in (II). Exocyclic hydroxymethyl group conformation in the β‐Manp residue of (IA) is gauche–gauche, whereas that in the β‐Manp residue of (IB) is gauche–trans.     

Low‐Concentration 1,2‐trans β‐Selective Glycosylation Strategy and Its Applications in Oligosaccharide Synthesis

This study develops an operationally easy, efficient, and general 1,2‐trans β‐selective glycosylation reaction that proceeds in the absence of a C2 acyl function. This process employs chemically stable thioglycosyl donors and low substrate concentrations to achieve excellent β‐selectivities in glycosylation reactions. This method is widely applicable to a range of glycosyl substrates irrespective of their structures and hydroxyl‐protecting functions. This low‐concentration 1,2‐trans β‐selective glycosylation in carbohydrate chemistry removes the restriction of using highly reactive thioglycosides to construct 1,2‐trans β‐glycosidic bonds. This is beneficial to the design of new strategies for oligosaccharide synthesis, as illustrated in the preparation of the biologically relevant β‐(1→6)‐glucan trisaccharide, β‐linked Gb₃ and isoGb₃ derivatives.     

Glycosidic linkage,N‐acetyl side‐chain,and other structural properties of methyl 2‐acetamido‐2‐deoxy‐β‐d‐glucopyranosyl‐(1→4)‐β‐d‐mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis–trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.     

4‐Ethoxycarbonyl‐3‐hydroxy‐3‐phenylcyclohexanone

The title compound, ethyl 2‐hydroxy‐4‐oxo‐2‐phenyl­cyclo­hexane­carboxyl­ate, C₁₅H₁₈O₄, was obtained by a Michael–Aldol condensation and has the cyclo­hexanone in a chair conformation. The attached hydroxy, ethoxy­carbonyl and phenyl groups are disposed in β‐axial, β‐equatorial and α‐­equatorial configurations, respectively. An intermolecular hydrogen bond, with an O?O distance of 2.874 (2) Å, links the OH group and the ring carbonyl. Weak intermolecular C—H?O=C (ester and ketone), O—H?O=C (ketone) and C—H?OH hydrogen bonds exist.     

3‐Deoxy‐β‐d‐ribo‐hexopyranose (3‐deoxy‐β‐d‐glucopyranose)

The β‐pyranose form, (III), of 3‐deoxy‐d ‐ribo‐hexose (3‐deoxy‐d ‐glucose), C₆H₁₂O₅, crystallizes from water at 298 K in a slightly distorted ⁴C₁ chair conformation. Structural analyses of (III), β‐d ‐glucopyranose, (IV), and 2‐deoxy‐β‐d ‐arabino‐hexopyranose (2‐deoxy‐β‐d ‐glucopyranose), (V), show significantly different C—O bond torsions involving the anomeric carbon, with the H—C—O—H torsion angle approaching an eclipsed conformation in (III) (−10.9°) compared with 32.8 and 32.5° in (IV) and (V), respectively. Ring carbon deoxygenation significantly affects the endo‐ and exocyclic C—C and C—O bond lengths throughout the pyranose ring, with longer bonds generally observed in the monodeoxygenated species (III) and (V) compared with (IV). These structural changes are attributed to differences in exocyclic C—O bond conformations and/or hydrogen‐bonding patterns superimposed on the direct (intrinsic) effect of monodeoxygenation. The exocyclic hydroxymethyl conformation in (III) (gt) differs from that observed in (IV) and (V) (gg).     

Methyl β‐allolactoside [methyl β‐d‐galactopyranosyl‐(1→6)‐β‐d‐glucopyranoside] monohydrate

Methyl β‐allolactoside [methyl β‐d ‐galactopyranosyl‐(1→6)‐β‐d ‐glucopyranoside], (II), was crystallized from water as a monohydrate, C₁₃H₂₄O₁₁·H₂O. The βGalp and βGlcp residues in (II) assume distorted ⁴C₁ chair conformations, with the former more distorted than the latter. Linkage conformation is characterized by ϕ′ (C2_Gal—C1_Gal—O1_Gal—C6_Glc), ψ′ (C1_Gal—O1_Gal—C6_Glc—C5_Glc) and ω (C4_Glc—C5_Glc—C6_Glc—O1_Gal) torsion angles of 172.9 (2), −117.9 (3) and −176.2 (2)°, respectively. The ψ′ and ω values differ significantly from those found in the crystal structure of β‐gentiobiose, (III) [Rohrer et al. (1980). Acta Cryst. B 36 , 650–654]. Structural comparisons of (II) with related disaccharides bound to a mutant β‐galactosidase reveal significant differences in hydroxymethyl conformation and in the degree of ring distortion of the βGlcp residue. Structural comparisons of (II) with a DFT‐optimized structure, (II_C), suggest a link between hydrogen bonding, pyranosyl ring deformation and linkage conformation.     

Methyl 4‐O‐β‐d‐galactopyranosyl α‐d‐mannopyranoside methanol 0.375‐solvate

Methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐mannopyranoside methanol 0.375‐solvate, C₁₃H₂₄O₁₁·0.375CH₃OH, (I), was crystallized from a methanol–ethanol solvent system in a glycosidic linkage conformation, with ϕ′ (O5_Gal—C1_Gal—O1_Gal—C4_Man) = −68.2 (3)° and ψ′ (C1_Gal—O1_Gal—C4_Man—C5_Man) = −123.9 (2)°, where the ring is defined by atoms O5/C1–C5 (monosaccharide numbering); C1 denotes the anomeric C atom and C6 the exocyclic hydroxymethyl C atom in the βGalp and αManp residues, respectively. The linkage conformation in (I) differs from that in crystalline methyl α‐lactoside [methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐glucopyranoside], (II) [Pan, Noll & Serianni (2005). Acta Cryst. C 61 , o674–o677], where ϕ′ is −93.6° and ψ′ is −144.8°. An intermolecular hydrogen bond exists between O3_Man and O5_Gal in (I), similar to that between O3_Glc and O5_Gal in (II). The structures of (I) and (II) are also compared with those of their constituent residues, viz. methyl α‐d ‐mannopyranoside, methyl α‐d ‐glucopyranoside and methyl β‐d ‐galactopyranoside, revealing significant differences in the Cremer–Pople puckering parameters, exocyclic hydroxymethyl group conformations and intermolecular hydrogen‐bonding patterns.     

Methyl β‐d‐galactopyranosyl‐(1→4)‐β‐d‐allopyranoside tetrahydrate

The title compound, C₁₃H₂₄O₁₁·4H₂O, (I), crystallized from water, has an internal glycosidic linkage conformation having ϕ′ (O5_Gal—C1_Gal—O1_Gal—C4_All) = −96.40 (12)° and ψ′ (C1_Gal—O1_Gal—C4_All—C5_All) = −160.93 (10)°, where ring‐atom numbering conforms to the convention in which C1 denotes the anomeric C atom, C5 the ring atom bearing the exocyclic hydroxymethyl group, and C6 the exocyclic hydroxymethyl (CH₂OH) C atom in the βGalp and βAllp residues. Internal linkage conformations in the crystal structures of the structurally related disaccharides methyl β‐lactoside [methyl β‐d ‐galactopyranosyl‐(1→4)‐β‐d ‐glucopyranoside] methanol solvate [Stenutz, Shang & Serianni (1999). Acta Cryst. C 55 , 1719–1721], (II), and methyl β‐cellobioside [methyl β‐d ‐glucopyranosyl‐(1→4)‐β‐d ‐glucopyranoside] methanol solvate [Ham & Williams (1970). Acta Cryst. B 26 , 1373–1383], (III), are characterized by ϕ′ = −88.4 (2)° and ψ′ = −161.3 (2)°, and ϕ′ = −91.1° and ψ′ = −160.7°, respectively. Inter‐residue hydrogen bonding is observed between O3_Glc and O5_Gal/Glc in the crystal structures of (II) and (III), suggesting a role in determining their preferred linkage conformations. An analogous inter‐residue hydrogen bond does not exist in (I) due to the axial orientation of O3_All, yet its internal linkage conformation is very similar to those of (II) and (III).     

3,4‐Di­hydro‐6‐methyl‐3‐β‐d‐ribo­furan­osyl‐4,5′‐cyclo‐9H‐imidazo[1,2‐a]­purin‐9‐one hydrate

In the title compound, C₁₃H₁₃N₅O₄·H₂O (4,5′‐cyclo­wyosine·H₂O), the cyclization forces a syn arrangement of the aglycon with respect to the sugar moiety. The ribo­furan­ose part of the mol­ecule displays a β‐d configuration with an envelope C1′‐endo pucker. The mol­ecules are arranged in columns along the short a axis and are linked to water mol­ecules through O—H?O and O—H?N hydrogen bonds.     

Form II of monoclinic methyl β‐carboline‐3‐carboxyl­ate

The crystal structure of the second monoclinic P2₁/c form of the β‐carboline‐3‐carboxyl­ate, C₁₃H₁₀N₂O₂, has been determined. Very small changes in the packing scheme lead to a different unit cell; the role of weak C—H?O hydrogen bonds seems to be crucial.     

Asymmetric Conjugate Reduction of α,β‐Unsaturated Ketones and Esters with Chiral Rhodium(2,6‐bisoxazolinylphenyl) Catalysts

New asymmetric conjugate reduction of β,β‐disubstituted α,β‐unsaturated ketones and esters was accomplished with alkoxylhydrosilanes in the presence of chiral rhodium(2,6‐bisoxazolinylphenyl) complexes in high yields and high enantioselectivity. (E)‐4‐Phenyl‐3‐penten‐2‐one and (E)‐4‐phenyl‐4‐isopropyl‐3‐penten‐2‐one were readily reduced at 60 °C in 95 % ee and 98 % ee, respectively, by 1 mol % of catalyst loading. (EtO)₂MeSiH proved to be the best hydrogen donor of choice. tert‐Butyl (E)‐β‐methylcinnamate and β‐isopropylcinnamate could also be reduced to the corresponding dihydrocinnamate derivatives up to 98 % ee.     

Palladium‐Catalyzed Intermolecular Oxidative Coupling Reactions of (Z)‐Enamines with Isocyanides through Selective β‐C(sp2)‐H and/or C=C Bond Cleavage

Herein, two efficient palladium‐catalyzed intermolecular oxidative coupling reactions of (Z)‐enamines with isocyanides via selective β‐C(sp²)‐H and/or C=C bond cleavage have been developed, leading to controllable chemodivergent and stereoselective construction of a wide range of (E)‐β‐carbamoylenamine derivatives containing strong intramolecular hydrogen bonds. Furthermore, possible reaction pathways for these transformations are proposed on the basis of preliminary mechanism studies.     

4‐Ethynyl‐4‐hydroxycyclohexan‐1‐one and 4‐ethynyl‐4‐hydroxy‐2,3,5,6‐tetramethylcyclohexa‐2,5‐dien‐1‐one

The title compounds, C₈H₁₀O₂, (I), and C₁₂H₁₄O₂, (II), occurred as by‐products in the controlled synthesis of a series of bis­(gem‐alkynols), prepared as part of an extensive study of synthon formation in simple gem‐alkynol derivatives. The two 4‐(gem‐alkynol)‐1‐ones crystallize in space group P2₁/c, (I) with Z′ = 1 and (II) with Z′ = 2. Both structures are dominated by O—H?O=C hydrogen bonds, which form simple chains in the cyclo­hexane derivative, (I), and centrosymmetric dimers, of both symmetry‐independent mol­ecules, in the cyclo­hexa‐2,5‐diene, (II). These strong synthons are further stabilized by C[triple‐bond]C—H?O=C, C_methylene—H?O(H) and C_methyl—H?O(H) interactions. The direct intermolecular interactions between donors and acceptors in the gem‐alkynol group, which characterize the bis­(gem‐alkynol) analogues of (I) and (II), are not present in the ketone derivatives studied here.     

Highly Stereoselective β‐Mannopyranosylation via the 1‐α‐Glycosyloxy‐isochromenylium‐4‐gold(I) Intermediates

While the gold(I)‐catalyzed glycosylation reaction with 4,6‐O‐benzylidene tethered mannosyl ortho‐alkynylbenzoates as donors falls squarely into the category of the Crich‐type β‐selective mannosylation when Ph₃PAuOTf is used as the catalyst, in that the mannosyl α‐triflates are invoked, replacement of the ^?OTf in the gold(I) complex with less nucleophilic counter anions (i.e., ^?NTf₂, ^?SbF₆, ^?BF₄, and ^?BAr₄^F) leads to complete loss of β‐selectivity with the mannosyl ortho‐alkynylbenzoate β‐donors. Nevertheless, with the α‐donors, the mannosylation reactions under the catalysis of Ph₃PAuBAr₄^F (BAr₄^F=tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate) are especially highly β‐selective and accommodate a broad scope of substrates; these include glycosylation with mannosyl donors installed with a bulky TBS group at O3, donors bearing 4,6‐di‐O‐benzoyl groups, and acceptors known as sterically unmatched or hindered. For the ortho‐alkynylbenzoate β‐donors, an anomerization and glycosylation sequence can also ensure the highly β‐selective mannosylation. The 1‐α‐mannosyloxy‐isochromenylium‐4‐gold(I) complex ( Cα ), readily generated upon activation of the α‐mannosyl ortho‐alkynylbenzoate ( 1 α ) with Ph₃PAuBAr₄^F at ?35 °C, was well characterized by NMR spectroscopy; the occurrence of this species accounts for the high β‐selectivity in the present mannosylation.     

Methyl β‐d‐fructopyranoside

In methyl β‐d ‐fructopyranoside, C₇H₁₄O₆, the thermodynamically most stable methyl glycoside of the ketose d ‐fructose, the pyranose ring is close to being an ideal ²C₅ chair. The compound forms bilayers involving a complex hydrogen‐bonding pattern of five independent hydrogen bonds. Graph‐set analysis was applied to distinguish the hydrogen‐bond patterns at unary and higher level graph sets.     

Precursor of a β‐lactamase inhibitor: allyl (4S,8S,9R)‐10‐[(E)‐ethyl­idene]‐4‐methoxy‐11‐oxo‐1‐aza­tri­cyclo­[7.2.0.03,8]­undec‐2‐ene‐2‐carboxyl­ate

The molecular structure of the title tricyclic compound, C₁₇H₂₁NO₄, which is the immediate precursor of a potent synthetic inhibitor {Lek157: sodium (8S,9R)‐10‐[(E)‐ethyl­idene]‐4‐methoxy‐11‐oxo‐1‐aza­tri­cyclo­[7.2.0.0^3,8]­undec‐2‐ene‐2‐carboxyl­ate} with remarkable potency, provides experimental evidence for the previously modelled relative position of the fused cyclo­hexyl ring and the carbonyl group of the β‐lactam ring, which takes part in the formation of the initial tetrahedral acyl–enzyme complex. In this hydro­phobic mol­ecule, the overall geometry is influenced by C—H?O intramolecular hydrogen bonds [3.046 (4) and 3.538 (6) Å, with corresponding normalized H?O distances of 2.30 and 2.46 Å], whereas the mol­ecules are interconnected through intermolecular C—H?O hydrogen bonds [3.335 (4)–3.575 (5) Å].