Daniel Solé

Total Synthesis of (−)‐Strychnine via the Wieland–Gumlich Aldehyde

Fifteen steps suffice for an enantioselective total synthesis of (-)-strychnine (1) from 1,3-cyclohexanedione. The key steps are the easy generation of the enantiopure intermediate 2, the closure of the piperidine ring by a reductive Heck reaction, and the elaboration of the indoline nucleus in an advanced synthetic stage. TBDMS=tert-butyldimethylsilyl.

Total Synthesis of the Bridged Indole Alkaloid Apparicine

An indole-templated ring-closing metathesis or a 2-indolylacyl radical cyclization constitute the central steps of two alternative approaches developed to assemble the tricyclic ABC substructure of the indole alkaloid apparicine. From this key intermediate, an intramolecular vinyl halide Heck reaction accomplished the closure of the strained 1-azabicyclo[4.2.2]decane framework of the alkaloid with concomitant incorporation of the exocyclic alkylidene substituents.

Intramolecular Pd(0)-Catalyzed Reactions of β-(2-Iodoanilino) Carboxamides: Enolate Arylation and Nucleophilic Substitution at the Carboxamide Group

Two different reaction pathways, the enolate arylation and the acylation of the aryl halide, can be promoted by a Pd(0) catalyst starting from beta-(2-iodoanilino) carboxamides. The intramolecular acylation of beta-(2-iodoanilino) carboxamides reported here is the first example of a nucleophilic attack of a sigma-arylpalladium species at the carboxamide group, a framework that is usually inert toward organopalladium reagents.

Enantioselective total synthesis of Wieland-Gumlich aldehyde and (-)-strychnine

A total synthesis of (-)-strychnine in 15 steps from 1,3-cyclohexanedione in 0.15% overall yield is described. The sequence followed in the assembling of rings is: E-->AE [2-(2-nitrophenyl)-1,3-cyclohexanedione]-->ACE (3a-aryloctahydroindol-4-one)-->ACDE (arylazatricyclic core)-->ABCDE (strychnan skeleton)-->ABCDEF (Wieland-Gumlich aldehyde)-->ABCDEFG (strychnine). The key steps of the synthesis are the enantioselective construction of the 3a-(2-nitrophenyl)-octahydroindol-4-one ring system and the closure of the piperidine ring by a reductive Heck cyclization to generate the pivotal intermediate (-)-14. In contrast, a Lewis acid promoted a-alkoxypropargylic silane-enone cyclization did not lead to synthetically useful azatricyclic ACDE intermediates. The introduction of C-17 and the closure of the indoline ring by reductive amination of the alpha-(2-nitrophenyl) ketone moiety complete the strychnan skeleton from which, via the Wieland-Gumlich aldehyde, the synthesis of (-)-strychnine is achieved.

Intramolecular Pd(0)-catalyzed reactions of (2-iodoanilino)-aldehydes: a joint experimental-computational study.

An extensive joint experimental-computational density functional theory (DFT) study has been carried out to gain insight into the factors that control the chemoselectivity (i.e., acylation vs α-arylation reaction) of palladium-catalyzed cyclizations of (2-iodoanilino)-aldehydes. To this end, the nature of the tethers joining the aniline nitrogen and the aldehyde moiety, different palladium precatalysts and reaction conditions (base and temperature), as well as different additives (mono- and bidendate ligands) has been explored. The adequate selection of these variables allows for the control of the selectivity of the process. Thus, (2-iodoanilino)-aldehydes generally lead to the formation of nucleophilic addition derived products when Cs(2)CO(3)/Et(3)N is used as base. In contrast, the use of stronger bases like K(t)OBu (in the presence of PhOH) mainly forms α-arylation reaction products. The different reaction pathways leading to the experimentally observed reaction products have been studied by means of computational tools.

Selective Synthesis of Either Isoindole- or Isoindoline-1-carboxylic Acid Esters by Pd(0)-Catalyzed Enolate Arylation

Two efficient palladium-catalyzed intramolecular α-arylation reactions of α-amino acid esters have been developed that allow either 1-isoindolecarboxylic acid esters or the corresponding isoindolines to be selectively synthesized simply by a slight change of reaction conditions.

3A-(O-NITROPHENYL)OCTAHYDROINDOL-4-ONES : SYNTHESIS AND SPECTROSCOPIC ANALYSIS

Abstract A short entry to 3a-( o -nitrophenyl)octahydroindol-4-ones employing ozonolysis and double reductive amination of 2-allyl-2-( o -nitrophenyl)-1,3-cyclohexanedione ( 9 ) is described. The symmetric dione 9 is synthesized in a 50% overall yield from 1,3-cyclohexanedione by means of o -nitroarylation followed by O -allylation and subsequent Claisen rearrangement. Configurational and conformational aspects of azabicyclic derivatives 1 (a-k) are discussed.

Palladium-catalysed synthesis of 1-isoindolecarboxylic acid esters and sequential Diels–Alder reactions: access to bridged- and fused-ring heterocycles

The Pd-catalysed intramolecular alpha-arylation of alpha-amino acid esters provides a useful methodology for the synthesis of substituted isoindole derivatives, which have been used in Diels-Alder reactions to access diverse skeletal frameworks.

A straightforward synthetic entry to cleavamine-type indole alkaloids by a ring-closing metathesis-vinyl halide Heck cyclization strategy.

An indole-templated ring-closing metathesis has been used to create the central nine-membered ring of the cleavamine-type alkaloids. A subsequent intramolecular vinyl halide Heck reaction upon the resulting azacyclononene ring completes the assembly of the strained 1-azabicyclo[6.3.1]dodecane framework of the alkaloids. The usefulness of the approach is illustrated with the synthesis of (±)-cleavamine and (±)-dihydrocleavamine.

A DFT Study of the Ambiphilic Nature of Arylpalladium Species in Intramolecular Cyclization Reactions

The remarkable structure-dependent reactivity observed in the cyclization of (2-haloanilino)-ketones with Pd-catalysts has been studied computationally within the density functional theory framework. The experimental reaction products ratio may be explained through the formation of a common palladaaminocyclobutane intermediate which can undergo a nucleophilic addition reaction and/or an enolate α-arilation process. The evolution of this metallacycle to the final products depends on two factors, the length of the tether joining the amino and the carbonyl groups, and the electronic nature of the substituent directly attached to the nitrogen atom. Thus, shorter chains (2 CH(2)) facilitate the nucleophic addition reaction by approximating the reactive aryl and Pd-coordinated carbonyl groups whereas longer chains (3 CH(2)) favor the enolate α-arylation proccess. For electron-withdrawing groups attached to the aniline nitrogen atom, the nucleophilic addition pathway becomes slightly disfavored, mainly due to the electron-withdrawing effect of the CO(2)Me group which avoids the delocation of the LP in the π-system, thus decreasing the nucleophilicity of the reactive arylic carbon atom. In contrast, the enolate α-arylation reaction is facilitated by the CO(2)Me group. This is translated into a small computed barrier energy difference of these competitive reaction pathways which should lead to a mixture of reaction products as experimentally found.