Application of donor/acceptor-carbenoids to the synthesis of natural products

Total synthesis of indoline alkaloids: A cyclopropanation strategy

Indoline alkaloids constitute a large class of natural products; their diverse and complex structures contribute to potent biological activities in a range of molecules. Designing an appropriate strategy for the total synthesis of indoline alkaloids is a difficult task that depends on being able to efficiently assemble the core architectures. The best strategies allow access to a variety of different indoline alkaloid structures in a minimum of steps. The cyclopropanation of simple olefins and the subsequent synthetic transformation of the resulting cyclopropyl intermediates has been intensively studied in recent decades. In contrast, the cyclopropanation of enamines, especially for the construction of complex nitrogen-containing ring systems, remained relatively unexplored. Previous success with the cyclopropanation of simple indoles to form stable indolylcyclopropanocarboxylates encouraged us to explore the assembly of indoline alkaloid skeletons with cyclopropanation as a key reaction. Theoretically, indolylcyclopropanocarboxylates are doubly activated by a vicinally substituted amino group and carboxyl group; that is, they are typical donor-acceptor cyclopropanes. Accordingly, they tend to yield an active iminium intermediate, which can undergo inter- and intramolecular nucleophilic reactions to form the core structure of indoline alkaloids with an expanded ring system. In this Account, we summarize our efforts to develop a cascade or stepwise reaction of cyclopropanation/ring-opening/iminium cyclization (the CRI reaction) on tryptamine derivatives for assembling indoline alkaloid skeletons. With the CRI approach, three types of indoline alkaloid skeletons have been efficiently constructed: (i) hexahydropyrrolo[2,3-b]indoline (type I), (ii) tetrahydro-9a,4a-iminoethano-9H-carbazole (type II), and (iii) tetrahydroquinolino[2,3-b]indoline (type III). The effects of substituents on tryptamine derivatives were carefully investigated for inter- and intramolecular CRI reactions during construction of type I and type II skeletons. These results provided a basis for the further design and synthesis of complex natural products containing nitrogen. The usefulness of the CRI reaction is well demonstrated by our total synthesis of structurally intriguing indoline alkaloids such as N-acetylardeemin, minfiensine, vincorine, and communesin F. In addition, we highlight advances by other groups in construction of the three types of skeletons as well as their total syntheses of these indoline alkaloids. Discussion of the total syntheses of these indoline alkaloids focuses on comparing the individual synthetic strategies for forming the ring systems embedded in the final products. We also describe the total synthesis of perophoramidine, which has the same type III skeleton as communesin F. The observation of a retro Diels-Alder reaction during our synthesis of communesin F inspired the hetero Diels-Alder reaction on which our total synthesis of perophoramidine was based.

Catalyst-controlled formal [4 + 3] cycloaddition applied to the total synthesis of (+)-barekoxide and (-)-barekol

The tandem cyclopropanation/Cope rearrangement between bicyclic dienes and siloxyvinyldiazoacetate, catalyzed by the dirhodium catalyst Rh(2)(R-PTAD)(4), effectively accomplishes enantiodivergent [4 + 3] cycloadditions. The reaction proceeds by a cyclopropanation followed by a Cope rearrangement of the resulting divinylcyclopropane. This methodology was applied to the synthesis of (+)-barekoxide (1) and (-)-barekol (2).

Computational study of cyclopropanation reactions with halodiazoacetates

The mechanism of rhodium(II)-catalyzed cyclopropanation reactions with ethyl bromo-, chloro-, and iododiazoacetate has been studied with density functional theory calculations. The halodiazoacetates were shown to be remarkably kinetically active compared to ethyl diazoacetate, as demonstrated experimentally in a study of reaction rates and supported by the calculated low potential energy barriers for the rate-determining loss of dinitrogen. In the rhodium carbenoids formed from the halodiazoacetates, pi-interactions between the halogen, the carbenoid carbon, and one rhodium atom were found. These interactions provide an explanation for the relatively high stability of these carbenoids and, consequently, the existence of small but significant potential energy barriers for the cyclopropanation step. The predicted diastereomeric ratios correspond well with the experimental results. In addition to transition states in which the alkene approaches the carbenoid in an end-on manner, as described in computational studies of cyclopropanations with other diazo compounds, side-on trajectory transition states were found to be of importance. The relative energies of the side-on trajectory transition states compared to the end-on trajectory transition states were shown to be affected by both the substrate alkene and the carbenoid substituents, a fact that should be taken into consideration when using models to explain and predict the stereochemical outcome of cyclopropanation reactions.