Polycyclic Ring Formation Using Bis-diazolactams for Cascade Stitching

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The metallo carbenoid derived from the D/A diazo group is preferentially formed and undergoes selective CH, NH, and OH insertion reactions, cyclopropanation, cyclopropenation, sulfur ylide formation/2,3-sigmatropic rearrangement, as well as nitrogen ylide formation followed by azetidine ring expansion. The initial reaction can be paired with a subsequent tandem cascade sequence involving dipole formation/cycloaddition in either an intra- or intermolecular sense to generate polycyclic N-heterocycles in one pot, with the formation up to three new rings in a single operation. Excellent diastereoselectivity was observed in the intramolecular cycloaddition reaction producing 5 to 7-membered rings.

Natural Product Synthesis via the Rhodium Carbenoid-Mediated Cyclization of α-Diazo Carbonyl Compounds

The chemistry of metal carbene complexes has provided chemists with exceptionally fertile ground for designing and developing new stereoselective bond construction for application towards the synthesis of various natural products. In particular, the RhII-catalyzed reaction of 2-diazo-3-oxobutanoates bearing tethered π-bonds represents a synthetically useful protocol for the construction of a variety of novel polycyclic skeletons. The related RhII-catalyzed reaction of the 2-diazo-2-(1H-indol-2-yl)acetate system has recently been examined as a potential route towards scandine, a member of the melodinus family of alkaloids.

Rh(II)-catalyzed reactions of differentially substituted bis(diazo) functionalities

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The D/A diazo group undergoes selective cyclopropanation as well as XH-insertion, leaving behind the second diazo group for a further intramolecular dipolar cycloaddition reaction.

Use of sulfur fragments for the synthesis of nitrogen heterocycles

This contribution contains a representative sampling from the Padwa laboratory of the conjugate addition of oximes with 2,3-bis(phenylsulfonyl)-1,3-butadiene followed by a subsequent dipolar cycloaddition cascade to produce a variety of alkaloids. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation of the target compounds.

Heteroaryl cross-coupling as an entry toward the synthesis of lavendamycin analogues: a model study

ABC analogues of the antitumor antibiotic lavendamycin, which contain the key metal chelation site and redox-active quinone unit essential for biological activity, were prepared via the palladium(0)-catalyzed cross-coupling reaction of various 2-haloheteroaromatics with 2-stannylated pyridines and quinolines. Using the Stille reaction, 2-bromo substituted quinolines and 1-bromoisoquinolines were found to undergo efficient coupling with 2-pyridinylstannanes to provide unsymmetrical heterobiaryl derivatives. While the Stille reaction using the reverse coupling partners (i.e., 2-quinolinylstannanes and haloheteroaromatics) had not received much attention in the literature, we found that this alternative coupling reaction efficiently provided several new heterobiaryl derivatives. The gold-catalyzed intramolecular cycloisomerization of N-(prop-2-ynyl)-1H-indole-2-carboxamide smoothly afforded a beta-carbolinone derivative that was subsequently used for a Pd(0)-catalyzed cross-coupling directed toward the synthesis of lavendamycin analogues.

Synthesis of 2,4-disubstituted pyrroles by rearrangements of 2-furanyl carbamates

2,4-Disubstituted pyrroles were synthesized by an oxidative rearrangement of a furanyl carbamate followed by sequential reaction of the resulting 5-methoxypyrrol-2(5H)-one with different alkyl lithiates. The final step of the procedure involves heating the ring opened 1-methoxy-5-oxopentylcarbamate with a primary amine.

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.

Cycloaddition across the benzofuran ring as an approach to the morphine alkaloids

The intramolecular Diels-Alder reaction of several amidofurans tethered onto a benzofuran ring was examined as a strategy for the synthesis of morphine. Bromo substitution on the furan ring did not provide sufficient activation to allow the cycloaddition to take place across the aromatic benzofuran. However, the presence of a large o-methylbenzyl group on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be highly populated, thereby facilitating its Diels-Alder cycloaddition across a tethered benzofuran.

A Rh(II)-catalyzed cycloaddition approach towards the synthesis of komaroviquinone

Using a rhodium(II)-catalyzed cyclization/cycloaddition sequence as the key reaction step, the icetexane core of komaroviquinone was constructed by an intramolecular dipolar-cycloaddition of a carbonyl ylide dipole across a tethered pi-bond. The ylide was arrived at by cyclization of a rhodium carbenoid intermediate onto a proximal ester group. Efforts towards the preparation of the required precursor for elaboration to the natural product are discussed.

A general synthetic entry to the pentacyclic strychnos alkaloid family, using a [4 + 2]-cycloaddition/rearrangement cascade sequence

The total synthesis of (+/-)-strychnopivotine, (+/-)-tubifolidine, (+/-)-strychnine, and (+/-)-valparicine is reported. The central step in the synthesis consists of an intramolecular [4 + 2]-cycloaddition/rearrangement cascade of an indolyl-substituted amidofuran that delivers an aza-tetracyclic substructure containing the ABCE-rings of the Strychnos alkaloid family. A large substituent group on the amide nitrogen atom causes the reactive s- trans conformation of the amidofuran to be more highly populated, thereby facilitating the Diels-Alder cycloaddition. The reaction also requires the presence of an electron-withdrawing substituent on the indole nitrogen for the cycloaddition to proceed. The cycloaddition/rearrangement cascade was remarkably efficient given that two heteroaromatic systems are compromised in the reaction. Closure to the remaining D-ring of the Strychnos skeleton was carried out from the aza-tetracyclic intermediate by an intramolecular palladium-catalyzed enolate-driven cross-coupling between the N-tethered vinyl iodide and the keto functionality. The cycloaddition/rearrangement approach was successfully applied to (+/-)-strychnopivotine (2), the only Strychnos alkaloid bearing a 2-acylindoline moiety in its pentacyclic framework. A variation of this tactic was then utilized for a synthesis of the heptacyclic framework of (+/-)-strychnine. The total synthesis of (+/-)-strychnine required only 13 steps from furanyl indole 18 and proceeded in an overall yield of 4.4%.

General access to the vinca and tacaman alkaloids using a Rh(II)-catalyzed cyclization/cycloaddition cascade

The total synthesis of several members of the vinca and tacaman classes of indole alkaloids has been accomplished. The central step in the synthesis consists of an intramolecular [3+2]-cycloaddition reaction of an alpha-diazo indoloamide which delivers the pentacyclic skeleton of the natural product in excellent yield. The acid lability of the oxabicyclic structure was exploited to establish the trans-D/E ring fusion of (+/-)-3H-epivincamine (3). Finally, a base induced keto-amide ring contraction was utilized to generate the E-ring of the natural product. A variation of the cascade sequence of reactions used to synthesize (+/-)-3H-epivincamine was also employed for the synthesis of the tacaman alkaloids (+/-)-tacamonine and (+/-)-apotacamine.

The Rhodium(II) Carbenoid Cyclization-Cycloaddition Cascade of alpha-Diazo Dihydroindolinones for the Synthesis of Novel Azapolycyclic Ring Systems

Tandem carbonyl ylide formation-1,3-dipolar cycloaddition of alpha-diazo N-acetyl-tetrahydro-beta-carbolin-1-one derivatives occur efficiently in the presence of a dirhodium catalyst to afford bimolecular cycloadducts in high yield. The Rh(II)-catalyzed reaction also takes place intramolecularly to give products derived from trapping of the carbonyl ylide dipole with a tethered alkene. The power of the intramolecular cascade sequence is that it rapidly assembles a pentacyclic ring system containing three new stereocenters and two adjacent quaternary centers stereospecifically in a single step and in high yield.