Efficient Cu-catalyzed asymmetric conjugate additions of alkylzinc reagents to aromatic and aliphatic acyclic nitroalkenes

An efficient and highly enantioselective (up to 95% ee) Cu-catalyzed method for asymmetric conjugate addition (ACA) of alkylzinc reagents to acyclic disubstituted nitroalkenes is presented. Reactions are typically effected at ambient temperature in the presence of 2 mol % chiral dipeptide phosphine and 1 mol % (CuOTf)(2).C(6)H(6). Nitroalkenes bearing aromatic as well as aliphatic substituents readily undergo asymmetric additions. [reaction: see text]     

Enantioselective total synthesis of erogorgiaene: applications of asymmetric Cu-catalyzed conjugate additions of alkylzincs to acyclic enones

The first enantioselective synthesis of erogorgiaene (1), an inhibitor of mycobacterium tuberculosis, is disclosed. The total synthesis highlights the utility of asymmetric conjugate additions (ACA) of alkylzincs to acyclic alpha,beta-unsaturated ketones catalyzed by peptidic phosphine ligands and (CuOTf)(2).C(6)H(6). Moreover, several critical attributes of this catalytic C-C bond-forming reaction are illustrated in the context of the total synthesis; these include the significance of various structural features of the amino acid-based chiral ligands and the chiral ligand's effectiveness in reactions involving achiral and chiral substrates. In addition, the total synthesis showcases some of the special properties of nonphosphine Ru complex 3 as a highly effective catalyst for olefin cross-metathesis.     

Enantioselective synthesis of alpha-alkyl-beta,gamma-unsaturated esters through efficient cu-catalyzed allylic alkylations

A method for enantioselective Cu-catalyzed allylic substitution between various alkylzincs and alpha,beta-unsaturated carboxylic esters that bear a gamma-phosphate is reported. These transformations afford alpha-alkyl-beta,gamma-unsaturated carbonyls with regioselectivities of 7:1 to >20:1 (S(N)2':S(N)2) and in 87-97% ee. The utility of the method is illustrated by a convergent total synthesis of topoisomerse II inhibitor (R)-elenic acid.     

Enantioselective synthesis of homoallylic amines through reactions of (pinacolato)allylborons with aryl-, heteroaryl-, alkyl-, or alkene-substituted aldimines catalyzed by chiral C1-symmetric NHC-Cu complexes

A catalytic method for enantioselective synthesis of homoallylamides through Cu-catalyzed reactions of stable and easily accessible (pinacolato)allylborons with aryl-, heteroaryl-, alkyl-, or alkenyl-substituted N-phosphinoylimines is disclosed. Transformations are promoted by 1-5 mol % of readily accessible NHC-Cu complexes, derived from C(1)-symmetric imidazolinium salts, which can be prepared in multigram quantities in four steps from commercially available materials. Allyl additions deliver the desired products in up to quantitative yield and 98.5:1.5 enantiomeric ratio and are amenable to gram-scale operations. A mechanistic model accounting for the observed selectivity levels and trends is proposed.     

Reversal of Enantioselectivity in the Conjugate Addition Reaction of Cyclic Enones with the CuOTf/Azolium Catalytic System

Hydroxyamide-functionalized azolium salt (NHC•HI 4) was evaluated for dual enantioselective control in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. This investigation was based on our previously reported ACA reaction catalyzed using CuOTf combined with NHC•AgI complex 1. It was revealed that the stereocontrol of the catalytic ACA reaction depended on the order of the addition of the substrates. Additionally, the chiral NHC ligand precursors, substrates, the relationship between the catalyst ee (ee_cat) and product ee (ee_pro), and halogen counter anion were completely evaluated. These results suggested that the catalytic performance of the CuOTf/4 system was comparable with that of the CuOTf/1 system. Furthermore, to gain knowledge of the Cu species generated using CuOTf and NHC ligand precursor, the reaction of CuOTf with 1 was investigated. Although obtaining the corresponding NHC•CuX species failed, the corresponding NHC•AuCl complex 11 could be synthesized by allowing 1 to react with AuCl•SMe₂.     

An enantioselective catalytic approach to syn deoxypropionate units combining asymmetric Cu-catalyzed 1,6- and 1,4-conjugate addition

A novel iterative approach to the synthesis of the naturally ubiquitous syn deoxypropionate motif is reported. The route comprises a new Horner–Wadsworth–Emmons reagent to prepare α,β,γ,δ-bisunsaturated thioesters. Next, two Me-substituents are introduced in high yield, regio- and enantioselectivity using sequential asymmetric Cu-catalyzed 1,6-conjugate addition, base-catalyzed olefin isomerization and Cu-catalyzed enantioselective 1,4-conjugate addition. After reduction to the aldehyde these transformations can be repeated to install three or more Me groups with a syn 1,3-relationship.     

Solid-phase development of chiral phosphoramidite ligands for enantioselective conjugate addition reactions

The development of a method for the optimization of chiral ligands for the steric steering of enantioselective Cu-catalyzed conjugate additions of Znalkyls to enones is described. The method is based on combinatorial principles and solid-phase techniques. It includes the combinatorial synthesis of chiral bispidine-derived ligands embodying a phosphoramidite group on the solid phase and their investigation in immobilized form in the conjugate addition of ZnEt2 to cyclohexenone as test reaction. The best identified ligands were also synthesized separately and investigated in its soluble form. The results obtained for the polymer-bound ligands correctly mirrored the performance of the soluble ligands. The library embodied members giving ee values varying between 3 and 67%. The "positional scanning" approach proved to be invalid for the study of the ligand system, indicating that this approach in general should be applied with care. Taken together, the method allowed for rapid and efficient optimization of the ligands and led to the development of the first enantioselective, Cu-catalyzed conjugate addition reaction with a polymer-bound ligand.     

Enantioselective synthesis of beta-aryl-gamma-amino acid derivatives via Cu-catalyzed asymmetric 1,4-reductions of gamma-phthalimido-substituted alpha,beta-unsaturated carboxylic acid esters

A series of chiral beta-aryl-substituted gamma-amino butyric acid derivatives were synthesized in good enantioselectivities via the Cu-catalyzed asymmetric conjugate reduction of gamma-phthalimido-alpha,beta-unsaturated carboxylic acid esters using Cu(OAc)2 x H2O as a catalyst precursor, (S)-BINAP as a ligand, PMHS as a hydride source, and t-BuOH as an additive. The methodology has been applied successfully to the enantioselective synthesis of a chiral pharmaceutical, (R)-baclofen.     

Copper-catalyzed intramolecular alkene carboetherification: synthesis of fused-ring and bridged-ring tetrahydrofurans

Fused-ring and bridged-ring tetrahydrofuran scaffolds are found in a number of natural products and biologically active compounds. A new copper-catalyzed intramolecular carboetherification of alkenes for the synthesis of bicyclic tetrahydrofurans is reported herein. The reaction involves Cu-catalyzed intramolecular addition of alcohols to unactivated alkenes and subsequent aryl C-H functionalization provides the C-C bond. Mechanistic studies indicate a primary carbon radical intermediate is involved and radical addition to the aryl ring is the likely C-C bond-forming mechanism. Preliminary catalytic enantioselective reactions are promising (up to 75% ee) and provide evidence that copper is involved in the alkene addition step, likely through a cis-oxycupration mechanism. Catalytic enantioselective alkene carboetherification reactions are rare and future development of this new method into a highly enantioselective process is promising. During the course of the mechanistic studies a protocol for alkene hydroetherification was also developed.     

Enantioselective Synthesis of N–C Axially Chiral Compounds by Cu-Catalyzed Atroposelective Aryl Amination

N−C axially chiral compounds have emerged recently as appealing motifs for drug design. However, the enantioselective synthesis of such molecules is still poorly developed and surprisingly no metal-catalyzed atroposelective N-arylations have been described. Herein, we disclose an unprecedented Cu-catalyzed atroposelective N−C coupling that proceeds at room temperature. Such mild reaction conditions, which are a crucial parameter for atropostability of the newly generated products, are operative thanks to the use of hypervalent iodine reagents as a highly reactive coupling partners. A large panel of the N−C axially chiral compounds was afforded with very high enantioselectivity (up to >99 % ee) and good yields (up to 76 %). Post-modifications of thus accessed atropisomeric compounds allows further expansion of the diversity of these appealing compounds.     

Cu-catalyzed asymmetric allylic alkylations of aromatic and aliphatic phosphates with alkylzinc reagents. An effective method for enantioselective synthesis of tertiary and quaternary carbons

Efficient enantioselective Cu-catalyzed allylic alkylations of aromatic and aliphatic allylic phosphates bearing di- and trisubstituted olefins are disclosed. Enantioselective C-C bond forming reactions are promoted in the presence of 10 mol % readily available chiral amino acid-based ligand (5 steps, 40% overall yield synthesis) and 5 mol % (CuOTf)2 x C6H6. Reactions deliver tertiary and quaternary stereogenic carbon centers regioselectively and in 78-96% ee. Data regarding the effect of variations in ligand structure on the efficiency and enantioselectivity of the alkylation process, as well as a mechanistic working model, are presented. The suggested model involves a dual role for the chiral Cu complex: association of the Cu(I) center to the olefin is facilitated by a two-point binding between the carbonyl of the ligand's amide terminus and the P=O of the substrate.     

Catalytic enantioselective alkylations of tetrasubstituted olefins. Synthesis of all-carbon quaternary stereogenic centers through Cu-catalyzed asymmetric conjugate additions of alkylzinc reagents to enones

A method for Cu-catalyzed asymmetric conjugate addition (ACA) of dialkylzinc reagents to tetrasubstituted five- and six-membered cyclic enones that afford quaternary all-carbon stereogenic centers in up to 95% ee is reported. Catalytic ACAs are practical and efficient. Reactions proceed to >98% conversion in undistilled commercial grade toluene in the presence of 2 mol % of an air-stable Cu salt (CuCN) and a readily available chiral ligand. Enantioselective ACA reactions deliver products that can be readily functionalized to afford a variety of synthetically versatile compounds in high optical purity.     

Cu-catalyzed enantioselective conjugate addition of alkylzincs to cyclic nitroalkenes: catalytic asymmetric synthesis of cyclic alpha-substituted ketones

An efficient and highly enantioselective (>/=92% ee) catalytic method for conjugate addition of alkylzinc reagents to cyclic nitroalkenes is reported. Reactions are promoted in the presence of 0.5-5 mol % (CuOTf)2.C6H6 and 1-10 mol % of chiral amino acid-based phosphine ligands at 0 degrees C in toluene. The Cu-catalyzed reactions can be effectively carried out with small-, medium-, and large-ring nitroalkenes. Depending on the reaction conditions used, either the nitro or the corresponding alpha-substituted ketone product can be readily accessed by the present protocol.     

Enantioselective synthesis of nitroalkanes bearing all-carbon quaternary stereogenic centers through Cu-catalyzed asymmetric conjugate additions

The first examples of catalytic asymmetric conjugate addition (ACA) of alkylzinc reagents to trisubstituted nitroalkenes, leading to the formation of nitroalkanes bearing a quaternary carbon stereogenic center, are reported. Reactions are promoted in the presence of 4 mol % of a readily available amino acid-based phosphine and 2 mol % (CuOTf).C6H6. Cu-catalyzed reactions proceed efficiently in up to 98% ee and can be carried out with a variety of dialkylzinc reagents and trisubstituted nitroolefins. We highlight the synthetic utility of the products obtained by efficient conversion of optically enriched nitroalkanes to the corresponding carboxylic acids.     

Enantioselective Pd‐Catalyzed Allylic Alkylation Reactions of Dihydropyrido[1,2‐a]indolone Substrates: Efficient Syntheses of (−)‐Goniomitine, (+)‐Aspidospermidine,and (−)‐Quebrachamine

The successful application of dihydropyrido[1,2‐a]indolone (DHPI) substrates in Pd‐catalyzed asymmetric allylic alkylation chemistry facilitates rapid access to multiple alkaloid frameworks in an enantioselective fashion. Strategic bromination at the indole C3 position greatly improved the allylic alkylation chemistry and enabled a highly efficient Negishi cross‐coupling downstream. The first catalytic enantioselective total synthesis of (−)‐goniomitine, along with divergent formal syntheses of (+)‐aspidospermidine and (−)‐quebrachamine, are reported herein.     

Enantioselective Pd‐Catalyzed Allylic Alkylation Reactions of Dihydropyrido[1,2‐a]indolone Substrates: Efficient Syntheses of (−)‐Goniomitine, (+)‐Aspidospermidine,and (−)‐Quebrachamine

The successful application of dihydropyrido[1,2‐a]indolone (DHPI) substrates in Pd‐catalyzed asymmetric allylic alkylation chemistry facilitates rapid access to multiple alkaloid frameworks in an enantioselective fashion. Strategic bromination at the indole C3 position greatly improved the allylic alkylation chemistry and enabled a highly efficient Negishi cross‐coupling downstream. The first catalytic enantioselective total synthesis of (?)‐goniomitine, along with divergent formal syntheses of (+)‐aspidospermidine and (?)‐quebrachamine, are reported herein.     

Enantioselective total synthesis of enokipodins A-D

The first enantioselective total synthesis of enokipodins A-D, highly oxidized α-cuparenone-type sesquiterpenoids with antimicrobial activity, was accomplished by using Meyers' diastereoselective alkylation protocol for the construction of the C7-quaternary asymmetric center. The present synthesis also constitutes a formal enantioselective synthesis of (S)-1,4-cuparenediol and (S)-cuparene-1,4-quinone.     

The catalytic enantioselective, protecting group-free total synthesis of (+)-dichroanone

Herein we report the first enantioselective total synthesis of (+)-dichroanone, confirming the absolute configuration of the natural product. This protecting group-free route features the first application of our enantioselective Tsuji allylation in the context of a natural product total synthesis. Additionally, this 11-step preparation of the molecule from commercial material features a novel Kumada-aromatization strategy and a rapid sequence for the conversion of a phenol to a hydroxy-p-benzoquinone.     

新型手性膦配体的合成及其在 1,4-共轭加成反应中的应用

 以联萘酚和大位阻的金刚烷酰氯为原料, 经两步合成了系列新型手性单齿亚磷酸酯配体, 并应用于铜催化的二乙基锌对环烯酮的不对称 1,4-共轭加成反应. 结果表明, 产物收率和对映选择性最高分别为 95% 和 61%. 配体 (S)-(2-(adamantane- 1-carboxylic acid)-1,1’-binapthalen-2’-yl)-((S)-1,1’-binaphthalen-2,2’-yl)phosphite [(S,S)-3a]的两个 (S)-联萘酚构型是匹配的组合, 产物的绝对构型主要由配体中 2,2’-二氧膦-1,1’-联萘酚的构型控制.     

Total synthesis of the tricyclic marine alkaloids (-)-lepadiformine, (+)-cylindricine c, and (-)-fasicularin via a common intermediate formed by formic acid-induced intramolecular conjugate azaspirocyclization

A very short and efficient enantioselective total synthesis of the tricyclic marine alkaloids (-)-lepadiformine (3), (+)-cylindricine C (1c), and (-)-fasicularin (4) has been developed utilizing the formyloxy 1-azaspiro[4.5]decane 5 as a common intermediate. The key strategic element for the synthesis was the formic acid-induced intramolecular conjugate azaspirocyclization, which proved to be a highly efficient and stereoselective way to rapid construction of the 1-azaspirocyclic substructure of these natural products in a single operation. Thus, the common intermediate 5, synthesized in two steps with 70% overall yield starting from the known (S)-N-Boc-2-pyrrolidinone 7 via the conjugate spirocyclization using an acyclic ketoamide 6, was utilized for the concise and stereoselective total synthesis of (-)-lepadiformine (3), which was accomplished in seven steps with 45% overall yield from 5 (31% yield from 7). The developed strategy based on the conjugate spirocyclization was also applied to the stereoselective total synthesis of (+)-cylindricine C (1c), which was achieved in 10 steps from 5 in 18% overall yield (12% yield from 7). Further application of this approach using 5 led to the synthesis of (-)-fasicularin (4), wherein an extremely efficient method for the introduction of the thiocyanato group via an aziridinium intermediate at the last step was developed. Thus, the highly efficient first enantioselective total synthesis of (-)-fasicularin was accomplished in nine steps with an overall yield of 41% from 5 (28% yield from 7).