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).     

Highly stereocontrolled synthesis of fluorinated 2,6-trans dihydropyrans via Prins cyclization

A highly efficient method for the synthesis of fluorinated 2,6-trans dihydropyrans via BF₃·Et₂O-promoted Prins cyclization of allenic alcohols and aldehydes is developed. Various 2,6-trans fluorodihydropyrans are obtained in moderate to good yields with excellent diastereoselectivities.     

Synthesis of chiral 3,3′-disubstituted 1,1′-binaphthyl-2,2′-disulfonic acids

A convenient synthesis of chiral 3,3′-disubstituted 1,1′-binaphthyl-2,2′-disulfonic acids (BINSA, 1) was developed. The key was directed ortho-lithiation of BINSA methyl ester 2 with n-BuLi and subsequent reaction with an electrophile. Electrophiles such as Br₂, I₂, Me₃SiOTf, and i-PrOB(Pin) reacted smoothly with 3,3′-dilithiated BINSA methyl ester, and the corresponding 3,3′-dihalo-, 3,3′-bis(trimethylsilyl)-, and 3,3′-diboryl-BINSA derivatives were obtained in yields of 21–78%. This simple synthetic method is highly attractive since the ability to prepare 3,3′-disubstituted BINOLs in advance can be useful.     

Asymmetric Diels-Alder Reactions of Neopentyl-Ether-Shielded Acrylates and Allenic Esters: Syntheses of (−)-Norbornenone and (−)-β-Santalene

Starting from (+)- or (?)-camphor, the antipodal alcohols 14 and 18 , respectively, have been prepared; the corresponding acrylates 15 and 19 underwent TiCl₂(i-PrO)₂-mediated Diels-Alder additions to cyclopentadiene to give adducts 20a and 22a respectively, with 95 % endo- and 99.2% πpH-face selectivities. Adduct 22a was converted to enantiomerically pure norbornenone 26 . Addition of 1,3-butadiene to acrylate 15 in the presence of TiCl₄ afforded 3-cyclohexenyl carboxylate 29 with > 95.6 % stereodifferentiation. The TiCl₂(i-PrO)₂-promoted [4 + 2] cycloaddition of cyclopentadiene to allenic ester 43 proceeding with 99 % face differentiation served as the key step for an efficient enantioselective synthesis of (?)-β-santalene ((?)- 41 ) with concomitant recovery of the chiral control alcohol 14 .     

One-Pot Synthesis of 3-(Trimethylsilyl)propynamides

A highly efficient one-pot procedure was developed for the synthesis of 3-trimethylsilylpropynamides by acylation of amines with trimethylsilylpropynoyl chloride generated in situ. Selective N-acylation and N,O-bis-acylation of ß-amino alcohols with formation of, respectively, N-(hydroxyalkyl)propynamides and 2-methyl-2-[3-(trimethylsilyl)propynoylamino]propyl 3-(trimethylsilyl)propynoate were accomplished with high yields.     

Eine einfache Allencarbonsäureester-Synthese mittels der Wittig-Reaktion

A Simple Synthesis of Alkyl Allenecarboxylates (Allenic Esters) by the Wittig-Reaction A simple one-pot synthesis of variously substituted α-allenic esters ( 7–23 , cf. Scheme 4) is described. Either of the phosphonium salts 1 and 2 or the phosphoranes 3–6 in methylene chloride (or acetonitrile) when treated with acid chlorides at room temperature in the presence of triethylamine give the corresponding allenic compounds in fairly good yields (cf. Tables 3a and 3b). This procedure shows for the first time, that the Wittig-reaction with acid chlorides as starting material can also be used for the synthesis of 2-H-substituted allenic esters ( 7–13 ; for numbering see Scheme 7). The deuterium labeled compounds 26–31 (cf. Scheme 7) are prepared in the same manner. The course of the reaction involves an in situ generation of ketenes which are trapped by the resonance stabilized phosphonium ylides. The thus formed betains readily decompose, already at room temperature, to the anticipated allenes by the well known Wittig-reaction.     

Synthesis of chiral ferrocenyl aminoalcohols involving a diastereospecific oxidation: use in asymmetric catalysis

The synthesis of six new chiral ferrocenic aminoalcohols has been performed. One step of this synthesis was a diastereospecific oxidation of diastereomeric mixtures of 1-[2-(N,N-dimethylaminomethyl)ferrocenyl]alcohols by manganese dioxide: one diastereomer was oxidized, while the other stayed inert. The six new chiral compounds in association with ZnEt₂ proved to be good catalysts in the asymmetric ethylation of benzaldehyde (77 to 88% ee).     

Syntheses of Cyclopropyl Silyl Ketones

The synthesis of the cyclopropyl silyl ketones 1 – 4 is described. The trimethylsilyl ketone 1 was prepared from geraniol ((E)- 5 ) in ca. 10% overall yield by cyclopropanation leading to 6 , CrO₃ oxidation to the aldehyde 8 , reaction of the latter with trimethylsilyl anion to 14A + B , and CrO₃ oxidation to 1 . Also for the (t-butyl)dimethylsilyl ketones 2 – 4 , an efficient four-step synthesis with overall yields of 48%, 85%, and 13%, respectively, was elaborated, starting from the allylic alcohols (E)- 5 , and 23 . The method of preparation involves as the key step a Wittig rearrangement of the silylallyl ethers ((E/Z)- 20 , 24 ) to the silyl alcohols ((E/Z)- 21 , 25 ), subsequent cyclopropanation ( 19A + B , 22A + B , 26 ), and oxidation to the cyclopropyl silyl ketones 2 – 4 .     

First enantioselective synthesis of a 1-(trimethylsilyl)alkylamine

Reduction of n-propanoyltrimethylsilane NH-imine with the complex hydride LiBH₄/diethyltartrate in THF yielding enantiomerically enriched 1-(trimethylsilyl)butylamine with a 60% enantiomeric excess is the first example of the enantioselective synthesis of such an amine. Other combinations of solvent (ether or THF), hydride (NaBH₄ or LiAlH₄) and chiral inductor (mono- or diol) failed.     

Synthesis of chiral norbornane derivatives as γ-amino alcohol catalysts: the effect of the functional group positions on the chirality transfer

Starting from (1S,4R) chiral ketone (+)-6, we developed a synthetic route to the synthesis of new chiral γ-amino alcohols (+)- and (−)-syn-2-amino-7-hydroxy norbornane derivatives with excellent yields and enantiomeric excesses (up to 99%). These compounds were tested as chiral catalysts in the enantioselective addition of diethylzinc to benzaldehyde presenting moderate results. The results obtained, compared with others previously reported, showed that the relative disposition of the amino and hydroxyl groups on C(2) and C(7) positions, play an important role in the catalytic activity.     

Rhodium‐catalyzed Asymmetric Hydrogenation of α‐Dehydroamino Ketones: A General Approach to Chiral α‐amino Ketones

Rhodium/DuanPhos‐catalyzed asymmetric hydrogenation of aliphatic α‐dehydroamino ketones has been achieved and afforded chiral α‐amino ketones in high yields and excellent enantioselectives (up to 99 % ee), which could be reduced further to chiral β‐amino alcohols by LiAlH(tBuO)₃ with good yields_. This protocol provides a readily accessible route for the synthesis of chiral α‐amino ketones and chiral β‐amino alcohols.     

Rapid identification of enantioselective ketone reductions using targeted microbial libraries

A collection of about 300 microbes was surveyed for the ability to generate chiral secondary alcohols by enantioselective reduction of a series of alkyl aryl ketones. Microbial cultures demonstrating utility in reducing model ketones were arrayed in multi-well plates and used to rapidly identify specific organisms capable of producing chiral alcohols used as intermediates in the synthesis of several drug candidates. Approximately 60 cultures were shown to selectively reduce various ketones providing both the R and S enantiomers of the corresponding alcohols in 92-99% ee with yields up to 95% at 1-4 g/L. An alternative approach to chiral alcohols based on selective microbial oxidation of racemic alcohols is also reported. This study provides a useful reference for generating chiral alcohols by selective microbial bioconversion.     

Convenient preparation of (Z)-α-halo-α,β-unsaturated aldehydes: synthesis of a Laurencia flexilis toxin

The CrCl₂-mediated two-carbon halo-homologation of aryl, alkenyl, and aliphatic aldehydes with chloral ethyl hemiacetal or bromal affords (Z)-α-chloro- and (Z)-α-bromo-α,β-unsaturated aldehydes, respectively, in good to excellent yields and high stereoselectivity. The utility of this methodology was illustrated by a synthesis of 2-chloropentadec-2(Z)-enal, a toxin isolated from the marine red alga Laurencia flexilis.     

Chemo- and diastereoselective Bi(OTf)3-catalyzed benzylation of silyl nucleophiles

The direct alkylation of silyl enol ethers with para-methoxybenzylic alcohols or their corresponding acetates was efficiently catalyzed by Bi(OTf)₃ in CH₃NO₂ as the solvent. The reaction provided the α-benzylated carbonyl compounds in high yields after short reaction times using 1-2.5 mol % of the catalyst. Benzylic acetates other than para-methoxybenzylic acetates also underwent the reaction. High facial diastereoselectivities were observed with acetates derived from chiral α-branched para-methoxybenzylic alcohols. In addition, a catalytic reduction with Et₃SiH as the reducing agent is reported.     

Studies on ZnBr2-mediated synthesis of axially chiral aryl-substituted allenes from terminal alkynes,aromatic aldehydes and (S)-α,α-diphenylprolinol

The studies on the ZnBr₂-mediated synthesis of axially chiral aryl-substituted allenes from terminal alkynes, aromatic aldehydes, and commercially available chiral amine (S)-α,α-diphenylprolinol were conducted. Axially chiral aryl-substituted allenes can be obtained in moderate yields with up to 96% ee.     

Copper-catalyzed enantioselective addition of alcohols to isatin-derived ketimines

The first copper-catalyzed enantioselective addition of alcohols to imines was described. In the presence of 1?mol% Cu(OAc)₂ complexed with chiral cyclohexane-based N,P-ligand L4, chiral isatin-derived N,O-aminals were obtained in excellent yields (93–99%) and good-to-excellent enantioselectivities (up to 93% ee) under mild conditions.     

Synthesis of furan from allenic sulfide derivatives

In this paper, we report the synthesis of furan derivatives from allenic sulfides. By the reaction with NaH, β-Hydroxyl allenic sulfides were found to generate furan products in excellent yields with the removal of phenylthio group. β-Aldehyde allenic sulfides were found to give similar furan products with one more substituent when treated with additional nucleophilic reagents. β-ketone allenic sulfides can also cyclize to give furan derivatives with the promotion of P₂O₅.     

Synthesis of new C2-symmetric bis(β-hydroxy amide) ligands and their applications in the enantioselective addition of alkynylzinc to aldehydes

A series of chiral C₂-symmetric bis(β-hydroxy amide) ligands was synthesized via the reaction of isophthaloyl dichloride and amino alcohols derived from l-amino acid. The titanium(IV) complex of C₂-symmetric chiral ligand 3b was effective for the asymmetric alkynylation of aldehydes and the propargyl alcohols were obtained in high yields (up to 94%) and high enantiomeric excesses (up to 98%) under optimized conditions. The results obtained using ligand 3h support that the two β-hydroxy amide moieties in these ligands behave as two independent ligands in the catalytic system.     

Ru-Catalyzed Hydrogenolysis of Chiral Allylic and Propargylic Cyclic Carbonates: Synthesis of Optically Active (E)-Allylic and Allenic Alcohols

RuH₂(PPh₃)₄-catalyzed reductive cleavage reactions of chiral allylic cyclic carbonates with ammonium formate afforded optically active (E)-allylic alcohols with excellent regioselectivity. Alternatively, hydrogenolysis of propargylic cyclic carbonates in the presence of RuH₂(PPh₃)₄ catalyst afforded allenic alcohols as a sole products.     

Reactions of α-imino ketones derived from arylglyoxals with (trifluoromethyl)trimethylsilane; a new route to β-amino-α-trifluoromethyl alcohols

The reactions of α-imino ketones, derived from arylglyoxals, with (trifluoromethyl)trimethylsilane (CF₃SiMe₃) in DME solution, in the presence of catalytic amount of CsF, at room temperature, yield O-silylated β-imino alcohols in the chemoselective manner. Subsequent reduction of these products with NaBH₄ in ethanolic solution leads to the corresponding β-(N-alkyl)amino-α-trifluoromethyl alcohols in good to excellent yields. Trifluoromethylation of enantiomerically pure α-imino ketones (with Ar = Ph or p-MeOC₆H₄), bearing as a chiral auxiliary the PhCH(Me) group attached to the nitrogen atom, yields mixtures of diastereomeric products in the ratio of ca. 3:2.