Organocatalytic enantioselective Diels-Alder reaction of dienes with alpha-(N,N-Diacylamino)acroleins

Catalytic and highly enantioselective Diels-Alder reaction of cyclic and acyclic dienes with alpha-phthalimidoacroleins provides cyclic alpha-quaternary alpha-amino acid precursors. The conformationally flexible chiral ammonium salt of H-L-Phe-L-Leu-N(CH(2)CH(2)) 2-reduced triamine with pentafluorobenzensulfonic acid is very effective as an asymmetric catalyst for the Diels-Alder reaction.     

Design of a small-molecule catalyst using intramolecular cation-pi interactions for enantioselective Diels-Alder and Mukaiyama-Michael reactions: L-DOPA-derived monopeptide.Cu(II) complex

[reaction: see text] We have designed a small-molecule artificial metalloenzyme that is prepared in situ from Cu(OTf)(2) or Cu(NTf(2))(2) (1.0 equiv) and l-DOPA-derived monopeptide (1.1 equiv). This catalyst (2-10 mol %) is highly effective for the enantioselective Diels-Alder (DA) and Mukaiyama-Michael (MM) reactions with alpha,beta-unsaturated 1-acyl-3,5-dimethylpyrazoles. The present results demonstrate that cation-pi interactions may be available for controlling the conformation of sidearms of chiral ligands, and monopeptides are readily tunable ligands that include only one chiral center.     

Enantioselective Diels-Alder reaction of α-(acylthio)acroleins: a new entry to sulfur-containing chiral quaternary carbons

A catalytic and enantioselective Diels-Alder reaction of α-(carbamoylthio)acroleins induced by an organoammonium salt of chiral triamine is described. α-(Carbamoylthio)acroleins are designed and synthesized as new sulfur-containing dienophiles for the first time. The Diels-Alder reaction affords chiral tertiary thiol precursors with up to 91% ee.     

Enantioselective Diels-Alder reactions with N-hydroxy-N-phenylacrylamide

[reaction: see text] The use of hydroxamic acids as templates for Lewis acid catalyzed enantioselective Diels-Alder reactions has been examined. A very simple chiral Lewis acid, prepared by mixing optically pure binaphthol with 3 equiv of trimethylaluminum, catalyzes the [4 + 2] cycloaddition of N-hydroxy-N-phenylacrylamide with cyclopentadiene at 0 degrees C in high yield (>96%) and with a fairly good level of enantioselectivity (91% ee). Facile conversion of the products to the corresponding alcohols or aldehydes makes the hydroxamic acid intermediates particularly useful.     

Enantioselective intramolecular [2 + 2]-photocycloaddition reactions of 4-substituted quinolones catalyzed by a chiral sensitizer with a hydrogen-bonding motif

Six 2-quinolones, which bear a terminal alkene linked by a three- or four-membered tether to carbon atom C4 of the quinolone, were synthesized and subjected to an intramolecular [2 + 2]-photocycloaddition. The reaction delivered the respective products in high yields (78-99%) and with good regioselectivity in favor of the straight isomer. If conducted in the presence of a chiral hydrogen-bonding template (2.5 equiv) at low temperature in toluene as the solvent, the reaction proceeded enantioselectively (83-94% ee). An organocatalytic reaction was achieved when employing a chiral hydrogen-bonding template with an attached sensitizing unit (benzophenone or xanthone). The xanthone-based organocatalyst proved to be superior as compared to the respective benzophenone. Closer inspection revealed that the reaction of 4-(pent-4-enyloxy)quinolone leading to a six-membered ring, annelated to the cyclobutane, was less enantioselective (up to 41% ee with 30 mol % catalyst) than the reaction of 4-(but-3-enyloxy)quinolone leading to a five-membered ring (90% ee with 5 mol % and 94% ee with 20 mol % catalyst). Photophysical data (emission spectra, laser flash photolysis experiments) proved that the latter photocycloaddition was significantly faster, supporting the idea that the dissociation of the substrate from the catalyst prior to the photocycloaddition is responsible for the decreased enantioselectivity. Under optimized conditions, employing 10 mol % of the xanthone-based organocatalyst at -25 °C in trifluorotoluene as the solvent, three of the other four substrates gave the intramolecular [2 + 2]-photocycloaddition products with high enantioselectivities (72-87% ee). In all catalyzed reactions, the yields based on conversion were moderate to good (40-93%).     

From highly enantioselective catalytic reaction of 1,3-diynes with aldehydes to facile asymmetric synthesis of polycyclic compounds

(S)-1,1'-Binaphth-2-ol (BINOL) in combination with ZnEt(2), Ti(O(i)Pr)(4), and biscyclohexylamine was found to catalyze the highly enantioselective (83-95% ee) addition of various 1,3-diynes to aldehydes of diverse structures. This method provides a convenient pathway to generate a number of optically active dienediynes as the acyclic precursors to polycyclic compounds. The chiral dienediynes undergo highly chemoselective Pauson-Khand (PK) cycloaddition in benzaldehyde by using [Rh(cod)Cl](2) as the catalyst in the presence of rac-BINAP. High diastereoselectivity (up to >20:1) has also been achieved with the chiral dienediyne substrates containing a bulky substituent adjacent to the chiral center. In the presence of the Grubbs II catalyst, ring-closing enyne metathesis of the PK cycloaddition products led to the formation of the desired 5,5,7- and 5,5,8-fused tricyclic compounds. Further highly diastereoselective Diels-Alder reaction of a 5,5,7-tricyclic compound with maleic anhydride produced a 5,5,7,6-polycyclic product. The asymmetric synthesis of polycyclic compounds from optically active dienediynes has established a novel and efficient synthetic route to the structural framework of many biologically significant molecules.     

An efficient synthetic methodology of chiral isoquinuclidines by the enantioselective Diels-Alder reaction of 1,2-dihydropyridines using chiral cationic palladium-phosphinooxazolidine catalyst

High purity chiral isoquinuclidines (97% ee) were obtained from the enantioselective Diels-Alder reaction of 1-phenoxycarbonyl-1,2-dihydropyridine with 1-benzyl-2-acryloylpyrazolidin-3-one using chiral cationic palladium-phosphinooxazolidine (Pd-POZ) catalyst. The obtained DA adduct was easily converted to the chiral piperidine derivative bearing three stereogenetic centers in the structure.     

Enantioselective total synthesis of eunicenone A.

An enantioselective, stereocontrolled total synthesis of eunicenone A (1) is described starting from geranylgeranylacetylene (9) in 14 steps via intermediates 10-20. The most critical construction in the synthesis is the highly effective Diels-Alder combination of the achiral components 2-bromoacrolein and diene 13 in the presence of the chiral Lewis acid catalyst 14 to form 15 (85% yield, 97% ee, >98:2 endo-exo ratio). The synthesis utilizes a novel reagent (12) for introduction of silicon, which serves to activate and direct the diene 13 for Diels-Alder reaction and to provide for eventual oxygen functionality of homoallylic alcohol 17 under mild conditions. Other noteworthy steps include the position selective and diastereoselective epoxidation 17 --> 18, the methoxycarbonylation with allylic transposition 19 --> 20, and the alpha,beta-enone unmasking 20 --> 1.     

Investigation of the asymmetric ionic Diels-Alder reaction for the synthesis of cis-decalins

The ionic Diels-Alder reaction, whereby an alpha,beta-unsaturated acetal in combination with a Lewis or Bronsted acid forms an equilibrium concentration of an activated dienophile, has been developed to provide an enantioselective synthesis of cis-decalins. Cyclohex-2-enone type chiral acetals of (2R,3R)-butane-2,3-diol have been screened against Lewis and Br?nsted acids with a variety of dienes and are efficient for the synthesis of a limited subset of cis-decalin structures. Diastereoselectivities of 73% and 82% have been found for the asymmetric ionic Diels-Alder reaction between the chiral acetal derivatives of cyclohex-2-enone (6) and 2-methylcyclohex-2-enone (18) with 2,3-dimethyl-1,3-butadiene (7). Terminal substituents on the diene partner in general render the system unreactive. However a synthetically useful cis-decalin 31, derived from the reaction of 2-methylcyclohex-2-enone and Z-3-t-butyldimethyl-silyloxypenta-1,3-diene has been prepared in enantiomerically pure form in 74% isolated yield using this asymmetric ionic Diels-Alder protocol.     

Highly enantioselective Diels-Alder reactions of Danishefsky type dienes with electron-deficient alkenes catalyzed by Yb(III)-BINAMIDE complexes

1-Methoxy-3-trimethylsiloxy-1,3-butadiene (Danishefsky's diene) is recognized as a synthetically useful diene due to its high reactivity in the Diels-Alder reaction with electron-deficient alkenes to give oxygen-functionalyzed cyclohexenes and substituted cyclohexenones, which are important building blocks for the total synthesis of natural products. However, the development of catalytic enantioselective versions of Diels-Alder reactions using Danishefsky type dienes with electron-deficient alkenes has been difficult because of the instability of the dienes under Lewis acidic conditions. Only highly reactive CO and CN double bonds are employed in a hetero-Diels-Alder reaction which proceeds under catalysis of chiral Lewis acids. We have developed a new chiral ligand, BINAMIDE, which is easily prepared from 1,1'-binaphtyl-2,2'-diamine by acylation. The highly diastereo- and enantioselective Diels-Alder reaction of Danishefsky type dienes with electron-deficient alkenes in the presence of an Yb(III)-BINAMIDE complex has been developed. The reaction proceeded in an exoselective mode and gave chiral highly functionalized cyclohexene derivatives in good yields.     

Highly enantioselective mukaiyama aldol reaction of alpha,alpha-dichloro ketene silyl acetal: an efficient synthesis of a key intermediate for diltiazem

An efficient synthesis of methyl (2R,3S)-3-(4-methoxyphenyl)glycidate (-)-2, a key intermediate for diltiazem (1), has been developed on the basis of the highly enantioselective Mukaiyama aldol reaction of p-anisaldehyde (4a) with alpha,alpha-dichloro ketene silyl acetal 5. Thus, the reaction using a stoichiometric amount of chiral oxazaborolidinone catalyst 12a proceeded to excellent yield (83%) and high enantioselectivity (96% ee), together with the chiral ligand 13a in nearly quantitative recovery. The reaction using a substoichiometric amount of 12e (20 mol %) also proceeded to excellent yield (88%), with somewhat lower enantioselectivity (77% ee). The aldol product 3a thus obtained was easily converted to (-)-2 in excellent yield (80%) and high optical purity (>99% ee). The highly enantioselective Mukaiyama aldol reaction with 5 catalyzed by 12a proved to be applicable to various aldehydes. An efficient preparation of 5 from inexpensive starting materials was also described.     

Chiral N-heterocyclic carbene catalyzed, enantioselective oxodiene Diels-Alder reactions with low catalyst loadings

Chiral N-heterocyclic carbene (NHC) catalyzed redox reactions of racemic alpha-chloroaldehydes lead to the generation of chiral enolates suitable for highly enantioselective inverse-electron-demand 1-oxodiene Diels-Alder reactions. Significantly, these reactions proceed under mild, operationally friendly reaction conditions (EtOAc, room temperature, 2-8 h) using less than 1 mol % of a chiral N-mesityl triazolium salt as the precatalyst. A broad array of densely functionalized dihydropyran-2-ones bearing either aliphatic or aromatic substiutents are formed in excellent yields and exceptional enantioselectivities from readily available reactants. Stereochemical mismatching between the racemic starting materials and the chiral catalyst is avoided by rapid epimerization of the chloroaldehydes under the reaction conditions.     

Bis(((S)-binaphthoxy)(isopropoxy)titanium) oxide as a mu-oxo-type chiral Lewis acid: application to catalytic asymmetric allylation of aldehydes

A new, chiral bis-Ti(IV) oxide of type 1 was successfully designed and can be utilized for strong activation of aldehyde carbonyls, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin. The chiral bis-Ti(IV) catalyst (S,S)-1 can be readily prepared either by treatment of bis(triisopropoxy)titanium oxide with (S)-binaphthol or by reaction of ((S)-binaphthoxy)isopropoxytitanium chloride with silver(I) oxide. Reaction of hydrocinnamaldehyde with allyltributyltin (1.1 equiv) under the influence of in situ generated chiral bis-Ti(IV) oxide (S,S)-1 (10 mol %) in CH2Cl2 at 0 degrees C for 4 h afforded 1-phenyl-5-hexen-3-ol in 84% yield with 99% ee. The present asymmetric allylation using nonracemic bis-Ti(IV) oxide 1 with partially resolved (S)-binaphthol exhibits a positive nonlinear effect in correlating the enantiopurity of allylation product with the ee of (S)-binaphthol. This asymmetric approach provides a very useful way for obtaining high reactivity and selectivity by the simple introduction of the M-O-M unit in the design of chiral Lewis acid catalysts.     

Hydroxy-amide functionalized azolium salts for Cu-catalyzed asymmetric conjugate addition: stereocontrol based on ligand structure and copper precatalyst

A series of hydroxy-amide functionalized azolium salts have been designed and synthesized for Cu-catalyzed asymmetric conjugate addition reaction. The (CH(2))(2)-bridged hydroxy-amide functionalized azolium ligand precursors 2, in addition to the previously reported CH(2)-bridged azolium salts 1, have been prepared from readily available enantiopure β-amino alcohols. The combination of a Cu species with 1 or 2 efficiently promoted the 1,4-addition reaction of cyclic enones with dialkylzincs. For example, the reaction of 2-cyclohepten-1-one (17) with Bu(2)Zn in the presence of catalytic amounts of Cu(OTf)(2) and 1 gave (S)-3-butylcycloheptanone (20) in 99% yield and 96% ee. On the other hand, when the reaction was carried out under the influence of Cu(OTf)(2) combined with 2, (R)-20 in preference to (S)-20 was obtained in 98% yield and 80% ee. In this manner, the enantioselecvity was switched by controlling the structure of chiral ligand. Additionally, the reversal of enantioselectivity was also achieved by changing the Cu precatalyst from Cu(OTf)(2) to Cu(acac)(2) with the same ligand. The combination of Cu(acac)(2) with CH(2)-bridged azolium salt 1 in the reaction of 17 with Bu(2)Zn led to formation of (R)-20 as a major product in 55% yield and 80% ee. This result was in contrast to the Cu(OTf)(2)/1 catalytic system, where the 1,4-adduct with opposite configuration was obtained. Moreover, use of the Cu(acac)(2)/2 catalytic system produced (S)-20, while (R)-20 was formed by the Cu(OTf)(2)/2 catalytic system. Thus, it was found that either varying the linker of the chiral ligands or changing the counterion of Cu species between a OTf and acac ligand initially on the metal led to dual enantioselective control in the 1,4-addition reaction.     

cis,cis-Spiro[4.4]nonane-1,6-diol: A new chiral auxiliary for the asymmetric Diels-Alder reaction

The use of a mono-pivalate mono-acrylate bis-ester of (+)-1S,5S,6S-spiro[4.4]nonane-1,6-diol in an asymmetric Diels-Alder reaction with cyclopentadiene (2 equiv. BCl₃, −85°C, CH₂Cl₂) provided the expected endo bicyclo adduct in >97% de. Iodolactonization of the bicyclo adduct provided the (+)-lactone (5) with a 1S,4S,6S,8R,9S configuration (97% ee). The de's obtained from using various types and amounts of Lewis acids, and both chiral and racemic bis-esters in the Diels-Alder reaction with cyclopentadiene are also reported.     

The influence of ligand side chain on the enantioselectivity of Lewis acid catalyzed Diels-Alder reactions

The enantioselective Lewis acid-catalyzed Diels-Alder reaction of 3-(2-propenoyl)-1,3-oxazolidin-2-one 8 with cyclopentadiene was examined using a series of chiral mox ligands 2-6, deferring in the side chain at 2-position of the chiral oxazoline and in the nature of the substituent at the chiral center (4-position) of the oxazoline ring, and a combination of N-[(1R)-2-chloro-1-phenylethyl]-2-[(4R)-4-phenyl-4,5-dihydrooxazol-2-yl]butyramide 2-MgI(2)-I(2) was the most efficient catalyst.     

Asymmetric Morita-Baylis-Hillman reactions catalyzed by chiral Brønsted acids

Chiral BINOL-derived Br?nsted acids catalyze the enantioselective asymmetric Morita-Baylis-Hillman (MBH) reaction of cyclohexenone with aldehydes. The asymmetric MBH reaction requires 2-20 mol % of the chiral Br?nsted acid 2e or 2f and triethylphosphine as the nucleophilic promoter. The reaction products are obtained in good yields (39-88%) and high enantioselectivities (67-96% ee). The Br?nsted-acid-catalyzed reaction is the first example of a highly enantioselective asymmetric MBH reaction of cyclohexenone with aldehydes.     

Asymmetric PTC C-alkylation catalyzed by chiral derivatives of tartaric acid and aminophenols. Synthesis Of (R)- and (S)-alpha-methyl amino acids

A new type of efficient chiral catalyst has been elaborated for asymmetric C-alkylation of CH acids under PTC conditions. Sodium alkoxides formed from chiral derivatives of tartaric acid and aminophenols (TADDOL's 2a-e and NOBIN's 3a-h) can be used as chiral catalysts in the enantioselective alkylation, as exemplified by the reaction of Schiff's bases 1a-e derived from alanine esters and benzaldehydes with active alkyl halides. Acid-catalyzed hydrolysis of the products formed in the reaction afforded (R)-alpha-methylphenylalanine, (R)-alpha-naphthylmethylalanine, and (R)-alpha-allylalanine in 61-93% yields and with ee 69-93%. The procedure could be successfully scaled up to 6 g of substrate 1b. When (S,S)-TADDOL or (R)-NOBIN are used, the (S)-amino acids are formed. A mechanism rationalizing the observed features of the reaction has been suggested.     

Asymmetric synthesis of (+)-cis-nemorensic acid from a chiral Diels-Alder adduct of 2,5-dimethylfuran

(+)-cis-Nemorensic acid was synthesized from a chiral Diels-Alder adduct prepared by a catalytic enantioselective Diels-Alder reaction with 2,5-dimethylfuran and 2,2,2-trifluoroethyl acrylate.     

Catalytic enantioselective Diels-Alder reaction via a chiral indium(III) complex

[reaction: see text] A chiral indium complex has been developed to effect high enantioselectivities in catalyzing enantioselective Diels-Alder reaction. The cycloaddition of a variety of cyclic and open-chained dienes to 2-methacrolein and 2-bromoacrolein resulted in good yields and excellent enantioselectivities (up to 98% ee).