Rearrangement of 2-azanorbornenes to tetrahydrocyclopenta[c]pyridines under the action of activated alkynes – A short pathway for construction of the altemicidin core

Total Synthesis of Altemicidin: A Surprise Ending for a Monoterpene Alkaloid

Monoterpene alkaloids encompass distinct chemical diversity and wide-ranging bioactivity. Their compact complexity has made them popular as synthetic targets and has inspired many distinct strategies and tactics in the field of heterocyclic chemistry. This article documents the evolution of a synthetic program aimed at accessing the unusual sulfonamide-containing natural product altemicidin, which was generally believed to be a monoterpene alkaloid throughout our entire synthetic investigations but has recently been found to originate through an unexpected and quite disparate biosynthetic pathway. By leveraging a pyridine dearomatization/cycloaddition strategy, we developed a concise pathway to the 5,6-fused bicyclic azaindane core and, after significant experimentation, an ultimate synthesis of altemicidin itself. Tactics to productively manipulate the multiple functional groups present on this highly polar scaffold proved challenging but were eventually realized via several carefully orchestrated and chemoselective transformations-investments that paid dividends in the form of significantly shorter chemical synthesis. Surprisingly, the bond-forming logic between our presumed abiotic synthetic strategy to this alkaloid class and its subsequently identified biosynthetic pathway is eerily similar.

Dearomative Synthetic Entry into the Altemicidin Alkaloids

Altemicidin and related Streptomyces-derived monoterpene alkaloids possess dense, highly polar azaindane cores as well as potent cytotoxic and tRNA synthetase inhibitory properties. The congested α-amino acid motif decorating their presumed iridoid-like core structure has proven to be both a synthetic challenge and a biosynthetic mystery to date. Herein, we report a distinct, abiotic strategy to these alkaloids resulting in a concise synthesis of altemicidin from simple chemical feedstocks. Key chemical findings include the exploitation of a dearomative pyridinium addition and dipolar cycloaddition sequence to stereospecifically install the quaternary amine moiety, and a chemoselective molybdenum-mediated double reduction to establish the fully functionalized azaindane nucleus with minimal redox manipulations.

Toward the Synthesis of SB-203207: Construction of Four Contiguous Nitrogen-Containing Stereogenic Centers

SB-203207 is an altemicidin-type alkaloid that potently inhibits isoleucyl tRNA synthetase activity. Its main structural feature is a hexahydro-6-azaindene framework containing a unique β-hydroxy α,α-disubstituted α-amino acid moiety on the cyclopentane portion. Herein we have established a method for constructing the four contiguous nitrogen-containing stereogenic centers of SB-203207 by using as key steps the stereoselective alkylation of bowl-shaped tricyclic lactone to construct a quaternary carbon at C1, the stereoselective hydroboration-oxidation reaction to install the C2 hydroxy group, and the Curtius rearrangement to introduce a nitrogen atom onto the C1 quaternary carbon.

Crystal structure of dimethyl (3aS,6R,6aS,7S)-2-pivaloyl-2,3-dihydro-1H,6H,7H-3a,6:7,9a-diepoxybenzo[de]isoquinoline-3a1,6a-dicarboxylate, C21H25NO8

C21H25NO8 , P21/n, (no. 14), a = 12.905(3) Å, b = 13.370(3) Å, c = 13.090(3) Å, β = 118.19(3)°, V = 1990.5(8) Å3, Z = 4, R gt(F) = 0.0629, wR ref(F 2) = 0.1966, T = 100(2) K.

Arylhydrazone ligands as Cu-protectors and -catalysis promoters in the azide-alkyne cycloaddition reaction

A series of water soluble copper(ii) complexes, [Cu(κO¹O²N-H₂L¹)(H₂O)₂]·2H₂O (2), [Cu(κO-H₃L¹)₂(H₂O)₄] (3), [Cu(κO-H₄L²)₂(H₂O)₄] (5) and [Cu(H₂O)₆]·2H₂L³·2(CH₃)₂NCHO (7), were prepared by the reaction of Cu(NO₃)₂·3H₂O with sodium (Z)-2-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)benzenesulfonate, [Na(μ₄-1:2κO¹,2κO²,3κO³,4κO⁴-H₃L¹)]_n (1; for 2 and 3), sodium (Z)-3-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)-4-hydroxybenzene-sulfonate, [Na(μ-1κO¹,2κO²-H₄L²)]₂ (4; for 5) or sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazineyl)naphthalene-1-sulfonate, [Na(μ-1κO¹O²,2κO³-H₂L³)(CH₃OH)₂]₂ (6; for 7). Compounds 1-7 were fully characterized, also by single-crystal X-ray diffraction analysis, and applied as homogeneous catalysts for the azide-alkyne cycloaddition (AAC) reaction to afford 1,4-disubstituted 1,2,3-triazoles. A structure-catalytic activity relationship has been recognized for the first time on the basis of the occurrence of resonance- and charge-assisted hydrogen bond interactions (RAHB and CAHB), in charge and ligand binding modes, enabling the catalytic activity of the compounds to be ordered as follows: Cu(NO₃)₂≪7 (complex salt with RAHB and CAHB) < 3 (with RAHB and CAHB) < 5 (with RAHB) < 2 (neither RAHB nor CAHB). Complex 2, without such non-covalent interactions, was found to be the most efficient catalyst for the AAC reaction, affording up to 98% product yield after being placed for 15 min, at 125 °C, in a water/acetonitrile mixture under low power (10 W) MW irradiation.

Crystal structure and Hirshfeld surface analysis of ethane-1,2-diaminium 3-[2-(1,3-dioxo-1,3-di-phenyl-propan-2-yl-idene)hydrazin-yl]-5-nitro-2-oxido-benzene-sulfonate dihydrate

In the anion of the title hydrated salt, C2H10N22+·C21H13N3O8S2-·2H2O, the planes of the phenyl rings and the benzene ring of the 5-nitro-2-oxido-benzene-sulfonate group are inclined to one another by 44.42 (11), 56.87 (11) and 77.70 (12)°. In the crystal, the anions are linked to the cations and the water mol-ecules by N-H⋯O and O-H⋯O hydrogen bonds, forming a three-dimensional network. Furthermore, there are face-to-face π-π stacking inter-actions between the centroids of one phenyl ring and the benzene ring of the 5-nitro-2-oxido-benzene-sulfonate group [centroid-centroid distance = 3.8382 (13) Å and slippage = 1.841 Å]. A Hirshfeld surface analysis was conducted to verify the contributions of the different inter-molecular inter-actions.

Crystal structure and Hirshfeld surface analysis of 3,3',3''-[(1,3,5-triazine-2,4,6-tri-yl)tris-(-oxy)]tris-(5,5-di-methyl-cyclo-hex-2-en-1-one)

The three cyclo-hexenone rings of the title compound, C27H33N3O6, adopt slightly distorted envelope conformations, with the C atom bearing two methyl groups as the flap atom in each case. These cyclo-hexenone mean planes form dihedral angles of 87.41 (11), 70.73 (11) and 70.47 (11)° with the 1,3,5-triazine ring, while the dihedral angle between the cyclo-hexenone mean planes are 57.52 (12), 23.75 (12) and 53.21 (12)°. In the crystal, mol-ecules are linked via C-H⋯O hydrogen bonds, forming a three-dimensional network.