Asymmetric addition of alkylzinc reagents to cyclic alpha,beta-unsaturated ketones and a tandem enantioselective addition/diastereoselective epoxidation with dioxygen

Although over 100 catalysts have been reported to catalyze the asymmetric addition of alkyl groups to aldehydes, these catalysts fail to promote additions to ketones with >90% enantioselectivity. This paper describes the asymmetric 1,2-addition of alkyl groups to conjugated cyclic enones to give allylic alcohols with chiral quaternary centers. The resultant allylic alcohols are converted into epoxy alcohols with excellent diastereoselectivities. Treatment of the epoxy alcohols with BF3.OEt2 induces a semipinacol rearrangement to provide alpha,alpha-dialkyl-beta-hydroxy ketones with all-carbon chiral quaternary centers. We also report a one-pot procedure for the asymmetric addition/diastereoselective epoxidation reaction. Simply exposing the reaction mixture to dioxygen after the asymmetric addition reaction is complete results in epoxidation of the allylic alcohol with high diastereoselectivity.     

Synthesis and applications of chiral organoboranes generated from sulfonium ylides

The reactions of aryl-stabilized sulfonium ylides with trialkyl/triarylboranes have been investigated. Clean monohomologation of the boranes with only a small amount of the higher homologation products (<10%) was observed. The homologation products were isolated as the alcohols (treatment with H2O2/NaOH) and amines (treatment with NH2OSO3H). Although the reactions were conveniently conducted at 5 degrees C, the ylide reaction with tributylborane was very fast even at -78 degrees C (complete after 15 min). Use of chiral sulfides rendered the reactions asymmetric, and high enantioselectivity (>95% ee) was observed in all cases. The ylide-borane reaction was applied to short syntheses of the anti-inflammatory agents neobenodine and cetirizine, both of which contain a chiral diarylmethylalkoxy and diarylmethylamino moiety, respectively.     

Chirality control in the enantioselective arylation of aromatic aldehydes catalyzed by cis-(1R,2S)-2-benzamidocyclohexanecarboxylic acid derived 1,3-aminoalcohols

A series of chiral 1,3-aminoalcohols derived from cis-(1R,2S)-2-benzamidocyclohexanecarboxylic acid were synthesized and applied to the enantioselective arylation of aromatic aldehydes. The reactions exhibited good yields (up to 90%) and moderate to high enantioselectivities (up to 99%). Not only the enantioselectivity but also the stereochemistry of the product were controlled by the substituent effect of the chiral ligands.     

Sparteine-mediated asymmetric nucleophilic substitution at prochiral, sp(3)-hybridized carbon

[reaction: see text] ++-Phenyl-1,3-dioxolanes (1) react with organolithium reagents (2), associated with (-)-sparteine, in the presence of BF(3).OEt(2) to afford chiral monosubstitution products 3. Enantioselectivity is highest if both 1 and 2 carry alkyl substituents in the ortho position. However, the enantioselectivity decreases in the case of very bulky substituents such as tert-butyl or phenyl.     

Regulated-stereoselective construction of thirteen stereogenic centers necessary for the frame of (+)-discodermolide, based on iterative Lewis acid-promoted aldol reactions

The segments C(1)-C(13) and C(15)-C(21) containing the 13 stereogenic centers required for the frame of (+)-discodermolide were synthesized in good to excellent enantio- and diastereoselectivities from a common racemic aldehyde, derived from 2-methyl-1,3-propanediol. The enantioselective aldol reactions of the racemic aldehyde with a silylketene acetal, derived from ethyl 2-bromopropionate, in the presence of chiral oxazaborolidinones, prepared in situ with N-p-toluenesulfonyl-(R)- and -(S)-valine and BH(3).THF, proceeded under kinetic control to give the stereotriads with a high degree of enantioselectivity. Enantioselective (chiral borane) and diastereoselective (BF(3).OEt(2) and TiCl(4)) aldol reactions with the silylketene acetal, coupled with diastereoselective radical debrominations (Bu(3)SnH, Et(3)B, with or without MgBr(2)), were used iteratively. This aldol reaction strategy for the construction of the polypropionate frame dramatically shortened the steps needed for the construction of the final segments.     

Enantioselective organocatalytic aminomethylation of aldehydes: a role for ionic interactions and efficient access to beta2-amino acids

Organocatalytic Mannich addition of aldehydes to a formaldehyde-derived iminium species catalyzed by proline-derived chiral pyrrolidines provides beta-amino aldehydes with >/=90% ee. Mechanistic analysis of the proline-catalyzed reactions suggests that non-hydrogen-bonded ionic interactions at the Mannich reaction transition state can influence stereochemical outcome. The beta-amino aldehydes from our process bear a substituent adjacent to the carbonyl and can be efficiently converted to protected beta2-amino acids, which are important building blocks for beta-peptide foldamers that display useful biological activities.     

(R)- and (S)-4-TIPS-3-butyn-2-ol. Useful precursors of chiral allenylzinc and indium reagents

A convenient route to the enantiomers of 4-TIPS-3-butyn-2-ol of >95% enantiomeric purity by reduction of the ynone precursor 4 with the Noyori N-tosyl-1,2-diphenylethylenediamineruthenium cymene catalyst is described. The mesylate derivative of the (S) enantiomer (1c) is converted in situ to an allenylzinc or indium reagent in the presence of a catalyst derived from Pd(OAc)2 and Ph3P and either Et2Zn or InI. A second in situ addition of these reagents to aldehydes leads to anti homopropargylic alcohol adducts. The additions proceed in generally high (60-90%) yield with modest to excellent diastereoselectivity and high enantioselectivity. Only slight mismatching (<5%) is observed with chiral alpha-methyl and alpha-silyloxy aldehydes. Additions to alpha-substituted enals are highly diastereoselective, while beta,beta-disubstituted enals afford ca. 2:1 mixtures of anti and syn adducts.     

Asymmetric intermolecular C-H functionalization of benzyl silyl ethers mediated by chiral auxiliary-based aryldiazoacetates and chiral dirhodium catalysts

[reaction: see text] C-H functionalization of benzyl silyl ethers by means of rhodium-catalyzed insertions of aryldiazoacetates can be achieved in a highly diastereoselective and enantioselective manner by judicious choice of chiral catalyst or auxiliary. The dirhodium tetraprolinates such as Rh2((S)-DOSP)4 have been widely successful as chiral catalysts in the C-H functionalization chemistry of aryldiazoacetates, but give poor enantioselectivity in the reactions of aryldiazoacetates with benzyl silyl ether derivatives. The use of (S)-lactate as a chiral auxiliary resulted in C-H functionalization with moderately high diastereoselectivity (79-88% de) and enantioselectivity (68-85% ee). The best results (91-95% de, 95-98% ee), however, were achieved using Hashimoto's Rh2((S)-PTTL)4 catalyst.     

Regioselective rhodium-catalyzed hydroformylation of 1,3-dienes to highly enantioenriched β,γ-unsaturated aldehyes with diazaphospholane ligands

Regioselective and enantioselective rhodium-catalyzed hydroformylation of 1,3-dienes with chiral bisdiazaphospholane ligands yields β,γ-unsaturated aldehydes that retain a C═C functionality for further conversion. The reaction conditions are mild, featuring low catalyst loadings (0.5 mol %), pressures readily obtained in glass bottles, and convenient reaction times (1.5-12 h). Optimized reaction conditions produce high enantioselectivity (>90% ee), regioselectivity (88-99%), and conversion to β,γ-unsaturated aldehydes (99%) for ten 1,3-dienes encompassing a variety of substitution patterns.     

N-Salicyl-β-aminoalcohols as a new class of ligand for catalytic asymmetric Strecker reactions

An enantioselective Strecker synthesis employing novel chiral titanium complex catalysts derived from structurally simple chiral N-salicyl-β-amino alcohols is described. Reactions of N-benzylidenebenzylamine with trimethylsilyl cyanide in the presence of the catalyst (10 mol%) gave the corresponding α-aminonitrile in good to excellent yields, along with relatively high enantioselectivity (up to 86% ee). Similar reactions with various imines derived from aromatic aldehydes resulted in moderate to good enantioselectivity (44-81% ee).     

Direct asymmetric aldol reaction of aryl ketones with aryl aldehydes catalyzed by chiral BINOL-derived zincate catalyst

Direct asymmetric aldol reaction of aryl ketones with aryl aldehydes catalyzed by chiral metal complex is reported for the first time herein. Two novel semicrown chiral ligands 1a and 1b were synthesized from (S)- and (R)-BINOL, respectively, and then employed to catalyze the direct asymmetric aldol addition of aryl ketones to aryl aldehydes. Introduced with 2.0 equiv of diethylzinc, 1b had higher enantioselectivity than 1a. Up to 97% yield and up to 80% enantioselectivity were achieved.     

Bidentate NHC-based chiral ligands for efficient Cu-catalyzed enantioselective allylic alkylations: structure and activity of an air-stable chiral Cu complex

Cu-catalyzed addition of alkylzinc reagents to a range of allylic phosphates is promoted efficiently and with high enantioselectivity to afford tertiary as well as quaternary carbon centers (up to 98% ee). Reactions proceed to completion with 0.5-5 mol % catalyst loading and are best promoted by commercially available CuCl2.2H2O. The X-ray structure of the chiral NHC-Ag(I) complex used in the study as well as that of a catalytically active NHC-Cu(II) complex are also reported; both complexes are air-stable and are formed in >/=95% isolated yield. The isolated Cu complex, which can be handled in air, is catalytically active. The present report provides the first precedent for efficient Cu-catalyzed allylic alkylations with chiral NHC ligands.     

Reactivity studies of 3,3-bis(trimethylsilyl)-2-methyl-1-propene in Lewis acid-catalyzed allylation reactions

Allylation reagents, which possess geminal bis-trimethylsilyl substitution, are readily prepared from E- or Z-alkenyl bromides. The reactivity of 3,3-bis(trimethylsilyl)-2-methyl-1-propene (1) is described and predominantly provides ene reactions with aldehydes to give alcohol 2 in the presence of BF3.OEt2. Alternatively, Sakurai allylation reactions of 1 are observed by using stronger Lewis acids in methylene chloride to exclusively yield E-trisubstituted alkenylsilanes 3.     

A chiral primary-tertiary-1,2-diamine as an efficient catalyst in asymmetric aldehyde–ketone or ketone–ketone aldol reactions

A novel chiral 1,2-diaminocyclohexane derivative, (1R,2R)-N¹-n-pentyl, N¹-benzyl-1,2-cyclohexanediamine, was designed, synthesized and applied as a catalyst in a number of aldol reactions between ketones and aryl aldehydes. Reactions between acetone and aryl aldehydes gave aldol products with moderate to good yields and with excellent enantioselectivity (up to yield 85%, ee 98%), while reactions between cyclohexanone and aryl aldehydes provided anti-β-hydroxyketone products with excellent yields, diastereoselectivity and with enantioselectivity (up to 82% yield, anti/syn ratio 99:1, ee 99%). The aldol reactions between acetone and isatins were investigated, which afforded excellent yields and enantioselectivity (up to 95% yield, 98% ee). The (R)- and (S)-isomers of convolutamydine A were obtained with 95% yield and 96% ee, and 95% yield and 94% ee, respectively.     

An efficient catalytic asymmetric addition of trimethylsilyl cyanide to aldehydes at room temperature

The BINOL-salen compound (S)-5c in combination with Ti(OⁱPr)₄ is found to catalyze the addition of TMSCN to aldehydes to form chiral cyanohydrins with very good enantioselectivity (75-85% ee). The reactions are carried out in one-pot at room temperature without the need to isolate the chiral Lewis acid catalyst.     

Reaction of optically active alpha-aminoallenylstannanes with aldehydes formed in situ from the Lewis-acid-catalyzed rearrangement of epoxides

Reaction of optically active alpha-oxazolidinonylallenylstannanes with oxiranes in the presence of BF3.OEt2 produced beta-hydroxypropargylamines with high syn diastereoselectivity and high enantioselectivity through an initial Lewis-acid-catalyzed rearrangement of the oxirane to the corresponding aldehyde via an alkyl, aryl, or hydride shift. This permits the use of readily available oxiranes as alternatives to aldehydes that are difficult to prepare and/or unstable.     

Remarkably efficient enantioselective titanium(IV)-(R)-H8-BINOLate catalyst for arylations to aldehydes by triaryl(tetrahydrofuran)aluminum reagents

Novel asymmetric triarylaluminum AlAr3(THF) additions to aldehydes catalyzed by 10 mol % of the titanium(IV) complex of (R)-H8-BINOL ligand are reported. The catalytic system is extremely efficient with reactions completing within 10 min. The system applies to the most diversified aldehydes to date, and more than 20 aldehydes were examined to afford diarylmethanols having an electron-donating or an electron-withdrawing group at the 2-, 3-, or 4-position on the aryl moiety, linear or branched 1-aryl aliphatic alcohols, aryl furyl methanols, 1-aryl allylic alcohols, and, especially, 1-aryl propargylic alcohols in excellent enantioselectivities of >/=90% ee, except for the case of 1-naphthyl addition to benzaldehyde. Noteworthily, diarylmethanols in both R- and S-configurations can be obtained.     

Catalysis of 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives in enantioselective anti-Mannich-type reactions: importance of the 3-acid group on pyrrolidine for stereocontrol

The development of enantioselective anti-selective Mannich-type reactions of aldehydes and ketones with imines catalyzed by 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives is reported in detail. Both (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid and (R)-3-pyrrolidinecarboxylic acid efficiently catalyzed the reactions of aldehydes with alpha-imino esters under mild conditions and afforded anti-Mannich products with high diastereo- and enantioselectivities (anti/syn up to 99:1, up to >99% ee). For the reactions of ketones with alpha-imino esters, (R)-3-pyrrolidinecarboxylic acid was an efficient catalyst (anti/syn up to >99:1, up to 99% ee). Evaluation of a series of pyrrolidine-based catalysts indicated that the acid group at the beta-position of the pyrrolidine ring of the catalyst played an important role in forwarding the carbon-carbon bond formation and in directing anti-selectivity and enantioselectivity.     

Lewis base activation of lewis acids. Addition of silyl ketene acetals to aldehydes

The weak Lewis acid silicon tetrachloride can be activated by catalytic amounts of the chiral bisphosphoramide (R,R)-3 to form a highly reactive, chiral trichlorosilyl cation which is an extremely effective promoter of aldol addition reactions between aldehydes and silyl ketene acetals. The tert-butyldimethylsilyl ketene acetal of methyl acetate adds nearly instantaneously to aromatic and olefinic aldehydes as well as aliphatic aldehydes (albeit more slowly) with excellent enantioselectivity. The homologous tert-butyldimethylsilyl ketene acetal of tert-butyl propanoate adds with nearly exclusive anti diastereoselectivity to a similar range of aldehydes also with excellent enantioselectivity. The origin of the slower reaction rate with aliphatic aldehydes is revealed to be the formation of chlorosilyl ether adducts.     

Enol ethers as substrates for efficient Z- and enantioselective ring-opening/cross-metathesis reactions promoted by stereogenic-at-Mo complexes: utility in chemical synthesis and mechanistic attributes

The first examples of catalytic enantioselective ring-opening/cross-metathesis (EROCM) reactions that involve enol ethers are reported. Specifically, we demonstrate that catalytic EROCM of several oxa- and azabicycles, cyclobutenes and a cyclopropene with an alkyl- or aryl-substituted enol ether proceed readily in the presence of a stereogenic-at-Mo monopyrrolide-monoaryloxide. In some instances, as little as 0.15 mol % of the catalytically active alkylidene is sufficient to promote complete conversion within 10 min. The desired products are formed in up to 90% yield and >99:1 enantiomeric ratio (er) with the disubstituted enol ether generated in >90% Z selectivity. The enol ether of the enantiomerically enriched products can be easily differentiated from the terminal alkene through a number of functionalization procedures that lead to the formation of useful intermediates for chemical synthesis (e.g., efficient acid hydrolysis to afford the enantiomerically enriched carboxaldehyde). In certain cases, enantioselectivity is strongly dependent on enol ether concentration: larger equivalents of the cross partner leads to the formation of products of high enantiomeric purity (versus near racemic products with one equivalent). The length of reaction time can be critical to product enantiomeric purity; high enantioselectivity in reactions that proceed to >98% conversion in as brief a reaction time as 30 s can be nearly entirely eroded within 30 min. Mechanistic rationale that accounts for the above characteristics of the catalytic process is provided.