Asymmetric domino reactions. Part B: Reactions based on the use of chiral catalysts and biocatalysts

Sequential Processes in Palladium-Catalyzed Silicon-Based Cross-Coupling

Although developed somewhat later, silicon-based cross-coupling has become a viable alternative to the more conventional Suzuki-Miyaura, Stille-Kosugi-Migita, and Negishi cross-coupling reactions because of its broad substrate scope, high stability of silicon-containing reagents, and low toxicity of waste streams. An empowering and yet underappreciated feature unique to silicon-based cross-coupling is the wide range of sequential processes available. In these processes, simple precursors are first converted to complex silicon-containing cross-coupling substrates, and the subsequent silicon-based cross-coupling reaction affords an even more highly functionalized product in a stereoselective fashion. In so doing, structurally simple and inexpensive starting materials are quickly transformed into value-added and densely substituted products. Therefore, sequential processes are often useful in constructing the carbon backbones of natural products. In this review, studies of sequential processes involving silicon-based cross-coupling are discussed. Additionally, the total syntheses that utilize these sequential processes are also presented.

Sequential one-pot combination of multi-component and multi-catalysis cascade reactions: an emerging technology in organic synthesis

Creating sequential one-pot combinations of multi-component reactions (MCRs) and multi-catalysis cascade (MCC) reactions is a challenging task that has already emerged as a new technology in synthetic organic chemistry. Through one-pot sequential combination of MCRs/MCC reactions, the chemical products (fine chemicals, agrochemicals and pharmaceuticals) that add value to our lives can be produced with less waste and greater economic benefits. Within this Emerging Area, we describe our recent developments and designs for sequential one-pot MCRs/MCC reactions to facilitate their realization as biomimetics in organic chemistry.

Total synthesis of diptoindonesin G via a highly efficient domino cyclodehydration/intramolecular Friedel-Crafts acylation/regioselective demethylation sequence

A highly efficient total synthesis of diptoindonesin G is described employing a domino dehydrative cyclization/intramolecular Friedel-Crafts acylation/regioselective demethylation reaction of aryloxyketone 7 by the action of BCl(3) wherein the tetracyclic 6H-anthra[1,9-bc]furan-6-one skeleton was constructed via the 3-arylbenzofuran in a one-pot manner. This is the first example of the strategic combination of these three reactions in a cascade fashion. The routes presented here allow for direct access to diptoindonesin G and its analogues.

New one-pot methodologies for the modification or synthesis of alkaloid scaffolds

There are several avenues by which promising bioactive natural products can be produced in sufficient quantities to enable lead optimization and medicinal chemistry studies. The total synthesis of natural products is an important, but sometimes difficult, approach and requires the development of innovative synthetic methodologies to simplify the synthesis of complex molecules. Various classes of natural product alkaloids are both common and widely distributed in plants, bacteria, fungi, insects and marine organisms. This mini-review will discuss the scope, mechanistic insights and enantioselectivity aspects of selected examples of recently developed one-pot methods that have been published in 2009 for the synthesis of substituted piperidines, quinolizidines, pyrrolidines, hexahydropyrrolizines, octahydroindolizines and γ-lactams. In addition, progress on the synthesis of β-carboline (manzamine) alkaloids will also be discussed.

Concise and very efficient synthesis of the N-methylwelwistatin tetracyclic core based on an anionic domino process

An efficient synthesis of the N-methylwelwistatin tetracyclic core in only two steps from Kornfeld's ketone is described, whose key transformation involves the generation of a fused bicyclo[4.3.1]decane ring system through a one-pot sequence comprising a Michael-intramolecular aldolization anionic domino process and a DBU-promoted hydrolysis of the N-pivaloyl protecting group. Besides providing the most efficient synthesis of the welwistatin core to date, this method has the advantage of installing an oxygenated function at the welwistatin D ring.

Synthesis of function-oriented 2-phenyl-2H-chromene derivatives using L-pipecolinic acid and substituted guanidine organocatalysts

Organocatalytic domino oxa-Michael/aldol reactions between salicylaldehyde with electron deficient olefins are presented. We screened guanidine, 1,1,3,3-tetramethylguanidine (TMG) and L-pipecolinic acid as organocatalysts for this transformation. 3-Substituted 2-phenyl-2H-chromene derivatives are synthesized with high yields and with poor enantioselectivity (5-17% ee) using L-pipecolinic acid while TMG works well with cinnamaldehyde without using co-catalyst. These 3-substituted-2-phenyl-2H-chromene derivatives are further derivatized to synthesize triazole and biotin-containing chromene derivatives, to facilitate purification of protein targets.

Origin of stereocontrol in the construction of the 12-oxatricyclo[6.3.1.0(2,7)]dodecane ring system by Prins-pinacol reactions

Polycyclic products containing the 12-oxatricyclo[6.3.1.0(2,7)]dodecane moiety having either the trans (8a-e) or cis (9a-e) relative configuration of the oxacyclic bridge and the cis angular substituents are formed stereospecifically by Prins-pinacol cyclizations of unsaturated alpha-dithianyl acetals 14a-e or 15a-e. These results show that the topography (boat or chair) of the Prins cyclization of the sulfur-stabilized oxocarbenium ions generated from acetals 14a-e or 15a-e is controlled by the stereoelectronic influence of the allylic substituents, with steric effects playing a minor role. A complex molecular rearrangement that is terminated by a thio-Prins-pinacol reaction is also identified.

Dynamic kinetic asymmetric transformation (DYKAT) by combined amine- and transition-metal-catalyzed enantioselective cycloisomerization

The first examples of one-pot highly chemo- and enantioselective dynamic kinetic asymmetric transformations (DYKATs) involving alpha,beta-unsaturated aldehydes and propargylated carbon acids are presented. These DYKATs, which proceed by a combination of catalytic iminium activation, enamine activation, and Pd(0)-catalyzed enyne cycloisomerization, give access to functionalized cyclopentenes with up to 99 % ee and can be used for the generation of all-carbon quaternary stereocenters.

A general approach to chiral building blocks via direct amino acid-catalyzed cascade three-component reductive alkylations: formal total synthesis of HIV-1 protease inhibitors, antibiotic agglomerins, brefeldin A, and (R)-gamma-hexanolide

Multicatalysis cascade (MCC) process for the synthesis of highly substituted chiral building blocks (2-alkyl-CH-acids, 2-alkylcyclohexane-1,3-diones, 2-alkylcyclopentane-1,3-diones, and H-P ketone analogues) is presented based on the cascade three-component reductive alkylation's (TCRA) platform. Herein, we developed the high-yielding alkylation of a variety of CH-acids with (R)-glyceraldehyde acetonide/(S)-Garner aldehyde and Hantzsch ester through amino acid-catalyzed TCRA reaction without racemization at the alpha-position to carbonyl. Direct sequential combination of the L-proline-catalyzed TCRA reaction with other reactions like cascade alkylation/ketenization/esterification (A/K/E), alkylation/ketenization/esterification/alkylation (A/K/E/A), Brønsted acid-catalyzed cascade hydrolysis/lactonization/esterification (H/L/E), hydrolysis/esterification (H/E), hydrolysis/oxy-Michael/dehydration (H/OM/DH), and Robinson annulation (RA) of CH-acids, chiral aldehydes, Hantzsch ester, diazomethane, methyl vinyl ketone, various active olefins, and acetylenes furnished the highly functionalized chiral building blocks in good to high yields with excellent diastereoselectivities. In this context, many of the pharmaceutically applicable chiral building blocks were prepared via MCC reactions.

8H-Chromeno[2',3':4,5]imidazo[2,1-a]isoquinoline

The title compound, C(18)H(12)N(2)O, comprises two aromatic fragments, viz., imidazo[2,1-a]isoquinoline and benzene, linked by oxygen and methyl-ene bridges. Despite the absence of a common conjugative system within the mol-ecule, it adopts an essentially planar conformation with an r.m.s. deviation of 0. 036 Å. In the crystal, due to this structure, mol-ecules form stacks along the b axis by π⋯π stacking inter-actions, with shortest C⋯C distances in the range 3.340 (4)-3.510 (4) Å. The mol-ecules are bound by inter-molecular C-H⋯O inter-actions within the stacks and C-H⋯π inter-actions between the stacks.

Organocatalytic asymmetric Friedel-Crafts alkylation/cyclization cascade reaction of 1-naphthols and alpha,beta-unsaturated aldehydes: an enantioselective synthesis of chromanes and dihydrobenzopyranes

An enantioselective Friedel-Crafts alkylation/cyclization cascade reaction of 1-naphthols and alpha,beta-unsaturated aldehydes promoted by diphenylprolinol ether has been developed. The method affords one-pot access to chiral and synthetically useful chromanes and dihydrobenzopyranes in high yields and enantioselectivities from readily available compounds. In addition, the addition/cyclization products could be afterward transformed to various natural products and biologically active derivatives. On the basis of the experimental results and the observed absolute configurations of the products, a plausible mechanism has been proposed to explain the origin of the activation and the asymmetric induction.

Photoinitiated domino reactions: N-(adamantyl)phthalimides and N-(adamantylalkyl)phthalimides

Phthalimides 1-6 undergo photochemical reactions upon direct irradiation as well as triplet sensitization and give rise to new products. Besides formation of the primary photoproducts, the first photochemical step initiates a subsequent thermal domino reaction or a domino sequence of a thermal and a photochemical reaction. The latter, involving two photochemical intramolecular gamma-H abstractions, was observed with phthalimides 1, 3, and 6 and delivered stereospecifically the hexacyclic benzazepine products 12, 19, and 27, respectively. The lowest triplet states of 1-6 were characterized in several solvents upon direct and acetone-sensitized excitation. The intermolecular electron transfer from triethylamine and DABCO was studied, and the radical anions were observed. Electrochemical measurements showed that intramolecular electron transfer from the adamantyl group of 1-6 to the lowest triplet state of the phthalimides is not feasible. The formation of products can be explained by intramolecular H-abstraction from the (alkyl)adamantane to the excited phthalimide, either from the excited singlet state or a hidden upper excited triplet state.

N-heterocyclic carbene-catalyzed domino ring-opening/redox amidation/cyclization reactions of formylcyclopropane 1,1-diesters: direct construction of a 6-5-6 tricyclic hydropyrido[1,2-a]indole skeleton

Catalyzed by N-heterocyclic carbenes (NHCs), domino ring-opening/redox amidation/cyclization reactions of the readily available formylcyclopropane 1,1-diesters with 2-chloro-1H-indole-3-carboaldehydes were reported. This methodology provides an efficient and direct construction of a 6-5-6 tricyclic hydropyrido[1,2-a]indole skeleton, which can be potentially applied for the synthesis of several types of polycyclic indole alkaloids.