A Novel, Expeditious Synthesis of Racemic Camptothecin

Diastereoselective Cascade Cyclization of Diazoimides with Alkylidene Pyrazolones for Preparation of Pyrazole-Fused Oxa-Bridged Oxazocines

Under the catalysis of Rh2(OAc)4 (10 mol%) and binapbisphosphine ligand (±)-L3 (20 mol%) in DCE at 80 °C, the cascade cyclization of diazoimides with alkylidenepyrazolones underwent stereoselectively (dr > 20:1), affording pyrazole-fused oxa-bridged oxazocines in reasonable chemical yields. The chemical structure and relative configuration of title products were firmly identified by X-ray diffraction analysis.

Total Synthesis of Camptothecin and Related Natural Products by a Flexible Strategy

A flexible strategy for constructing natural products containing indolizinone or quinolizinone scaffolds and their analogues was developed, which was based on a cascade exo hydroamination followed by spontaneous lactamization. This method was applied in the total synthesis of camptothecin in nine steps in a new ring-forming approach. It was also used to efficiently prepare five biogenetically or structurally related natural alkaloids, including 22-hydroxyacuminatine, oxypalmatine, norketoyobyrine, naucleficine, and nauclefine, as well as 35 natural-product-like molecules. We believe that this method and the small-molecule library prepared with it can open new avenues for studying the bioactivity of camptothecin and Nauclea natural products.

Synthesis of a 2(1H)-pyridone library via rhodium-catalyzed formation of isomunchones

Through the use of the dipolar cycloaddition of isomunchones with olefins the 2(1H)-pyridone ring system has been synthesized. (1) The use of different cyclization partners followed by diversification of the initial scaffold has provided libraries of 4-hydroxy-2(1H)-pyridones. There are no examples of this ring system in either PubChem or the MLSMR.

Domino reactions of rhodium(II) carbenoids for alkaloid synthesis

In this tutorial review, the rhodium(II)-catalyzed domino reactions of alpha-diazo carbonyl compounds are summarized. The article will emphasize some of the more recent synthetic applications of the rhodium carbenoid cyclization/cycloaddition domino cascade for alkaloid synthesis. The many structurally diverse and highly successful examples of both oxa and azapolycyclic ring formation cited in this tutorial review clearly demonstrate that the domino cyclization/cycloaddition cascade of metallo carbenoids has evolved as an important strategy for the synthesis of nitrogen containing natural products.

Tandem Michael Addition/Amino-Nitrile Cyclization from 2-Formyl-1,4-DHP in the Synthesis of Novel Dihydroindolizine-Based Compounds

A simple and efficient methodology for the synthesis of a small library of substituted indolizines with different degrees of saturation starting from the racemic 2-formyl-1,4-DHP reagent was described. The large synthetic possibilities of this reagent as well as of its Knoevenagel corresponding 2-dicyanovinyl-1,4-DHP reagent were investigated using four kinds of activated methylenes as nucleophiles. The key step of the sequential reaction was based on the highly diastereoselective tandem Michael addition/intramolecular amino-nitrile cyclization catalyzed by an organic base, which resulted in the formation of 1,7-dihydroindolizines in a diastereoselective manner. The process seems to be a straightforward one and can be extended to numerous active methylenes such as malononitrile, 1,3-diketones, and alkyl acetoacetates. The 1,3-hydrogen shift of partially hydrogenated indolizines was accomplished easily with a base at room temperature, giving rise to the corresponding 7,8-dihydroindolizines in very good yields. Interestingly, when the active methylene bears a leaving group, the latter process could not be accomplished because a rare cis-elimination of phenylsulfinic acid and nitrous acid preceded the hydrogen shift. The resulting 1,7-dihydroindolizines bearing an exo-methylene group at C1 were not isolated in all cases, as they turned rapidly to indolizines as the thermodynamically more stable products. During these investigations, oxidization of 1,7-dihydroindolizines with CuCl(2) resulted in the formation of polysubstituted pyridines. Also, the epimerization of certain 1,7-dihydroindolizines was evidenced in the solution studied by NMR spectroscopy, whereas in the solid state, they existed only in a unique form as shown by X-ray diffraction analysis of a representative structure. Finally, all products reported herein bear a primary amine and a nitrile function crucial for further transformations. These include the introduction of various pharmacophore groups at either NH(2) or CN groups as well as at both groups at the same time to access the more elaborated indolizines fused to N- or N,N-heterocycles.

Novel, efficient total synthesis of natural 20(S)-camptothecin

Enantiopure 20(S)-camptothecin has been prepared from a known hydroxypyridone through a novel approach that involves a Claisen rearrangement, an asymmetric nucleophilic ethylation, a Heck coupling and a Friedländer condensation as the key transformations.

Concise synthesis of 22-hydroxyacuminatine, cytotoxic camptothecinoid from Camptotheca acuminata, by pyridone benzannulation

A short, efficient synthesis of 22-hydroxyacuminatine, starting from a readily accessible hydroxy pyridone, is presented; key steps include a Heck coupling with methyl pentadienoate, a flash vacuum pyrolytic cyclization, and a Friedländer condensation.

1,3-Dipolar Cycloaddition Chemistry for the Preparation of Novel Indolizinone-Based Compounds

[Chemical reaction: See text] Starting from methyl 5-oxo-6-trifluoromethanesulfonyloxy-1,2,3,5-tetrahydroindolizine-8-carboxylate, obtained by a Rh(II)-catalyzed 1,3-dipolar cycloaddition reaction of 1-(2-benzenesulfonyl-2-diazoacetyl)pyrrolidin-2-one and methyl acrylate, several indolo- and furano-fused indolizinones were efficiently prepared. In the first case, a palladium-mediated C-N coupling of the triflate with a variety of substituted anilines provided the desired methyl 5-oxo-6-(arylamino)-1,2,3,5-tetrahydroindolizine-8-carboxylates in high yield. Methyl 6-(2-bromophenylamino)-5-oxo-1,2,3,5-tetrahydroindolizine-8-carboxylate as well as its decarboxylated analogue, 6-(2-bromophenylamino)-2,3-dihydro-1H-indolizin-5-one, were synthesized in excellent yield and were found to undergo an intramolecular Heck cyclization to give 1,2,3,6-tetrahydroindolizino[6,7-b]indol-5-ones. To prepare furano-fused indolizinones, methyl 6-hydroxy-5-oxo-1,2,3,5-tetrahydroindolizine-8-carboxylate was etherified with different allyl halides, and the resultant allyl ethers were subjected to a thermal Claisen rearrangement to give the corresponding methyl 7-allyl-6-hydroxy-5-oxo-1,2,3,4-tetrahydroindolizine-8-carboxylates. Cyclization under Wacker oxidation conditions afforded methyl 2-methyl-8-oxo-5,6,7,8-tetrahydro-1-oxa-7a-aza-s-indacene-4-carboxylates in near-quantitative yield.