Stereoselective formation of quaternary carbon centers and related functions

The Enantioselective Birch−Cope Sequence for the Synthesis of Carbocyclic Quaternary Stereocenters. Application to the Synthesis of (+)-Mesembrine

A synthetic technique for generating carbocyclic quaternary stereocenters with exceptionally high levels of enantioselectivity is described. A sequence of three reactions, enantioselective Birch reduction-allylation, enol ether hydrolysis, and Cope rearrangement, is used to stereoselectively generate chiral quaternary centers on a 2-cyclohexen-1-one ring. The products of the sequence are 4,4-disubstituted-2-carboxamide-2-cyclohexen-1-one structures which are versatile intermediates in complex natural product synthesis. An application of the sequence to the synthesis of (+)-mesembrine illustrates the utility of these intermediates.

Catalytic Enantioselective Aldol Reaction to Ketones

An enantioselective aldol reaction between ketones and ketene silyl acetals is described using CuF-chiral phosphine as a catalyst. The key for high enantioselectivity was the development of a novel ligand derived from Taniaphos combined with the unique accelerative effect of PhBF3K. These conditions are applicable to various substrates such as aromatic, aliphatic, and heteroaromatic ketones. In the case of substituted nucleophiles, the reaction proceeds well. The diastereoselectivity is independent of ketene silyl acetal geometry. This is the first example of a catalytic enantio- and diastereoselective aldol reaction to ketones using ketene silyl acetals.

Rhodium-Catalyzed Asymmetric Construction of Quaternary Carbon Stereocenters: Ligand-Dependent Regiocontrol in the 1,4-Addition to Substituted Maleimides

A rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to substituted maleimides has been described. The regioselectivity in this reaction is controlled by the choice of ligand (dienes or bisphosphines), and 1,4-adducts with a quaternary stereocenter can be obtained with high regio- and enantioselectivity by the use of (R)-H8-binap.

Synthesis of Strained Tricyclic β-Lactams by Intramolecular [2+2] Cycloaddition Reactions of 2-Azetidinone-Tethered Enallenols: Control of Regioselectivity by Selective Alkene Substitution

A convenient metal-free methodology for the preparation of structurally novel, strained tricyclic beta-lactams containing a cyclobutane ring has been developed. The first examples accounting for the intramolecular [2+2] cycloaddition reactions in beta-lactams have been achieved by the thermolysis of 2-azetidinone-tethered enallenols, which have been prepared in aqueous media by regio- and diastereoselective indium-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes. Notably, the regioselectivity of the cycloaddition can be tuned in the allene component just by a subtle variation in the substitution pattern of the alkene component.

Acid-Base Catalysis of Chiral Pd Complexes: Development of Novel Catalytic Asymmetric Reactions and Their Application to Synthesis of Drug Candidates

Using the unique character of the chiral Pd complexes 1 and 2, highly efficient catalytic asymmetric reactions have been developed. In contrast to conventional Pd(0)-catalyzed reactions, these complexes function as an acid-base catalyst. Thus active methine and methylene compounds were activated to form chiral palladium enolates, which underwent enantioselective carbon-carbon bond-forming reactions such as Michael reaction and Mannich-type reaction with up to 99% ee. Interestingly, these palladium enolates acted cooperatively with a strong protic acid, formed concomitantly during the formation of the enolates to activate electrophiles, thereby promoting the C-C bond-forming reaction. This palladium enolate chemistry was also applicable to electrophilic enantioselective fluorination reactions, and various carbonyl compounds including beta-ketoesters, beta-ketophosphonates, tert-butoxycarbonyl lactone/lactams, cyanoesters, and oxindole derivatives could be fluorinated in a highly enantioselective manner (up to 99% ee). Using this method, the catalytic enantioselective synthesis of BMS-204352, a promising anti-stroke agent, was achieved. In addition, the direct enantioselective conjugate addition of aromatic and aliphatic amines to alpha,beta-unsaturated carbonyl compound was successfully demonstrated. In this reaction, combined use of the Pd complex 2 having basic character and the amine salt was the key to success, allowing controlled generation of the nucleophilic free amine. This aza-Michael reaction was successfully applied to asymmetric synthesis of the CETP inhibitor torcetrapib.

Enantioselective rhodium-catalyzed addition of arylboronic acids to trifluoromethyl ketones

The catalytic asymmetric 1,2-addition of a series of arylboronic acids to 2,2,2-trifluoroacetophenones is described with high isolated yields (up to 96%) and good enantioselectivities (up to 83% ee) using a rhodium(I)/phosphoramidite catalyst.

Metal enolates ofα-CF3 ketones: theoretical guideline, direct generation, and synthetic use

The difficulty as well as the significance of the direct generation of metal enolates of alpha-CF(3) ketones cannot be easily understood by chemists unfamiliar with F. In sharp contrast to the sunny side of non-F, hydrocarbon chemistry, F chemistry has long been overshadowed. Metal enolates of carbonyl compounds are synthetically important in C-C bond-forming reactions. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to alpha-F metal enolates. Alpha-CF(3) metal enolates have generally been recognized as unstable and difficult to prepare because of the facile beta-M-F elimination. However, we have developed a direct generation and synthetic application of alpha-CF(3) metal enolates. Therefore, the present results regarding the direct generation and synthetic use of metal enolates of alpha-CF(3) ketones might be recognized as a real breakthrough for the general use of metal enolates in F chemistry.

Design and application of linked-BINOL chiral ligands in bifunctional asymmetric catalysis

The design and application of linked-BINOLs investigated in our group are reviewed. Linked-BINOLs are a kind of semi-crown ether, thus they are flexible and applicable to metals having various ionic radii (Ga3+, Li+, Zn2+, In3+, La3+, and Y3+). The flexible linker segment, containing a coordinative heteroatom, has a crucial role in the construction of a unique and effective chiral environment that is not accessible from BINOL itself. Applications of linked-BINOLs to an epoxide opening reaction, Michael reactions, a direct aldol reaction, direct Michael reactions of a hydroxyketone, and direct Mannich-type reactions of hydroxyketones and N-acylpyrrole are described.

Catalytic Enantioselective Conia-Ene Reaction

An enantioselective intramolecular Conia-ene reaction of beta-dicarbonyl compounds and alkynes to afford methylene cyclopentanes is described. The reaction employs a DTBMSegphos-Pd(II)/Yb(III) dual catalyst system that allows for the asymmetric synthesis of all-carbon quaternary centers and generates a product containing an alkene that can be further manipulated.

Asymmetric Total Synthesis of Fredericamycin A: An Intramolecular Cycloaddition Pathway

The asymmetric total synthesis of the potent antitumor antibiotic fredericamycin A ((S)-1) was achieved by the intramolecular [4+2] cycloaddition of the silylene-protected styrene derivative (S)-7 followed by the aromatic Pummerer-type reaction of the sulfoxide (S)-5. Although we had already succeeded in the total synthesis of racemic 1 by the same approach, synthesis of its asymmetric version was more complicated than we had expected due to the difficulties involved in constructing the quaternary carbon center and the tendency of this center to undergo facile racemization. Racemization of this center during the installation of the acetylene moiety on the dione (R)-8 was the most serious aspect. Systematic studies of its DE-ring analogue (R)-25 revealed that racemization of the quaternary carbon center proceeded by a retro-aldol-aldol reaction of the initial adduct, (1R)-39 a-Li, and that the degree of racemization was dependent on the reaction temperature. The racemization process could be completely depressed by keeping the reaction temperature at -78 degrees C. The construction of the stereogenic quaternary carbon center was achieved by the lipase-catalyzed desymmetrization of the prochiral 1,3-diol 9 a bearing the DEF-ring moiety. These studies enabled us to attain the asymmetric total synthesis of (S)-1 while completely retaining the chiral integrity created by the enzymatic reactions.

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.

Catalytic and Asymmetric Vinylogous Mukaiyama Reactions on Aliphatic Ketones: Formal Asymmetric Synthesis of Taurospongin A

Catalytic asymmetric vinylogous Mukaiyama reactions on ketones, leading to the formation of alpha,beta-unsaturated lactones with tertiary alcohols, have been described (11 examples, up to 93% ee). This methodology has been applied in a formal enantioselective synthesis of taurospongin A (12 steps, 6% overall yield).

Generation of Quaternary Stereocenters by Asymmetric Michael Reactions: Enamine Regiochemistry as Configuration Switch

Regioselective enamine formation from cyclic beta-diketones 1 is obtained by the appropriate choice of activating agent: Brønsted acid catalyzed condensation gives endocyclic enamines 3 as the thermodynamically favored products. Activation with Lewis acid BF(3).OEt(2) affords betaines 8 as intermediate products, which can be reacted with L-valine diethylamide (2) to preferentially furnish exocyclic enamines 4 as kinetic products. Derivatives with quaternary stereocenters were accessible from both isomeric enamines by using asymmetric, copper(II)-catalyzed Michael reactions at ambient temperature. Both regioisomers afford the triketones 7 with the same constitution but bearing the opposite absolute configuration at the quaternary stereocenter. Thus, both enantiomers of the product are prepared by using the same chiral auxiliary derived from L-valine.

p-Menthane-3-carboxaldehyde: a useful chiral auxiliary for the synthesis of chiral quaternary carbons of high enantiomeric purity

(+)- or (-)-p-Menthane-3-carboxaldehyde is made in two easy steps from (+)- or (-)-menthone, respectively. This auxiliary allows for the synthesis of carbonyl compounds bearing a alpha-chiral quaternary carbon. The flexibility, efficiency, and ease of use of the method are demonstrated in a series of examples, which include the total synthesis of (+)-cuparenone as well as a partial synthesis of (-)-cassiol.

Synthesis of the Core Structure of Acutumine

[reaction: see text] The tricyclic core of the bioactive natural product acutumine has been synthesized. Key steps include an oxidative phenolic coupling to form a masked o-benzoquinone, an anionic oxy-Cope rearrangement to construct an all-carbon quaternary center, and a Michael-type cyclization to form an amine-bearing quaternary carbon. The target compound exists in solution as an enol, in contrast to related compounds that are ketones. A model explaining these observations is presented.

Silylene Transfer to Carbonyl Compounds and Subsequent Ireland−Claisen Rearrangements to Control Formation of Quaternary Carbon Stereocenters

In this communication, we demonstrate that silylene transfer to carbonyl compounds provides a new method for regio- and stereoselective enolate formation. Silylene transfer to a range of alpha,beta-unsaturated esters under silver-catalyzed conditions proved to be a general method for the formation of oxasilacyclopentenes containing a cyclic silyl ketene acetal functionality. These oxasilacyclopentenes are useful synthetic intermediates that can undergo facile and selective Ireland-Claisen rearrangements and aldol addition reactions to provide products with multiple contiguous stereocenters and quaternary carbon centers.

1-alkoxyvinyl esters: renaissance of half-century-old acyl donors with potential applicability

Among the various kinds of acyl donors, the 1-alkoxyvinyl esters have characteristic features, such as a high reactivity under nearly neutral conditions and the generation of neutral and volatile esters as single coproducts. Although their use in organic syntheses began in the middle of the 1950s, no significant progress has been seen. This is probably because the existing method of preparing alkoxyvinyl esters used toxic mercuric salts and was not totally applicable for those esters having functionalized acyl moieties. We have discovered that the use of a catalytic amount of the less toxic [RuCl2(p-cymene)]2 effectively accelerates the addition of carboxylic acids to ethoxyacetylene to give ethoxyvinyl esters bearing a variety of functionalized acyl groups in high yields. This discovery has opened a new avenue for developing new reactions and new synthetic methodologies based on the design and use of these acyl donors with suitable functional groups. Such examples include (i) the installation of hydrophilic acyl moieties on biologically active compounds, (ii) asymmetric Pummerer-type reactions, (iii) aromatic Pummerer-type reactions, (iv) the lipase-catalyzed desymmetrization of symmetrical 1,3-diols, and (v) lipase-catalyzed domino reactions. Future possibilities for these acyl donors are also discussed.

Enantioselective construction of stereogenic quaternary centres via Rh-catalyzed asymmetric addition of alkenylboronic acids to α,β-unsaturated pyridylsulfones

The highly enantioselective construction of all-carbon quaternary stereogenic centres via Rh-catalyzed Chiraphos-mediated conjugate addition of alkenylboronic acids to beta,beta-disubstituted alpha,beta-unsaturated 2-pyridylsulfones is described.

Synthesis of Tertiary and Quaternary Stereogenic Centers: A Diastereoselective Tandem Reaction Sequence Combining Mukaiyama and Free Radical-Based Allylation

Reported herein is a strategy employing a Mukaiyama reaction in tandem with a free radical-based allyl transfer reaction for the elaboration of functionalized tertiary and quaternary centers. The appropriate choice of alcohol-protecting group on the starting alpha-methyl-beta-hydroxyaldehyde and the nature of the Lewis acid used in the Mukaiyama reaction provided access to 3,4-anti and 3,4-syn aldolization products, precursors of the free-radical allylation reaction. After migration or exchange of the Lewis acid, the allyl transfer reaction with allyltributylstannane is then performed by taking advantage of the endocyclic effect, leading to the 2,3-anti relative stereochemistry. Importantly, (13)C NMR studies of the chelated intermediates are also reported and provide additional support for the endocyclic effect. In some cases, the remarkable reactivity of the aluminum-based Lewis acids allowed the use of allyltrimethylsilane, an interesting reagent from an ecological standpoint. The isolation of a key intermediate is also indicative of an atom transfer mechanism when the silicon-based reagent is employed.

Studies toward the Total Synthesis of Wiedemannic Acid

[reaction: see text] We report a novel and efficient diastereoselective synthesis of wiedemannic acid analogue 30 in 16 steps from 7 using a tandem oxy-Cope/Claisen/ene reaction as the key step. Comparison of NMR data between wiedemannic acid (1) and analogue 30 leads us to believe that the reported stereochemistry at the ring junction of 1 is incorrect.

Gold(I)-catalyzed Conia-ene reaction of beta-ketoesters with alkynes

The intramolecular addition of beta-ketoesters to unactivated alkynes under neutral conditions and at room temperature is described. The method employs triphenylphosphinegold(I) cation as a catalyst for the formation of exo-methylenecycloalkanes. Both monocyclic and bicyclic cyclopentanes and cyclohexanes can be formed in excellent yields and with good diastereoselectivity.

Lewis Acid Catalyzed Allylboration: Discovery, Optimization, and Application to the Formation of Stereogenic Quaternary Carbon Centers

A full account of the development of the first catalytic manifold for the additions of allylboronates to aldehydes is described. The thermal additions (both diastereospecific and enantioselective) of 2-carboxyester 3,3-disubstituted allylboronates 1 to both aromatic and aliphatic aldehydes give biologically and synthetically important exo-methylene butyrolactones 2 containing a beta-quaternary carbon center. Although the thermal reaction requires 14 d at room temperature to reach completion, the presence of certain metal salts allows for a 12-16 h reaction while preserving the diastereospecificity observed in the uncatalyzed process. Preliminary mechanistic studies on the origin of the catalytic effect are described as well as stereoselective transformations of lactones 2 into cyclic and acyclic stereotriads with potential usefulness as synthetic intermediates.