Lipase catalyzed acylation of primary alcohols with remotely located stereogenic centres: the resolution of (±)-4,4-dimethyl-3-phenyl-1-pentanol

Enantioselective acylation of some (±)-3-alkyl-3-phenyl-1-propanols was performed with enzymes as catalysts. Moderate enantiomeric ratios (E), ranging up to E = 11.6, were obtained. In the resolution, some of the lipases selectively acylated the (+)-enantiomer while others acylated the (−)-enantiomer of the γ-substituted primary alcohols 1–4. Thus, it is possible to obtain both enantiomers of the alcohols as remaining substrate with high enantiomeric purity. The resolution of (±)-4,4-dimethyl-3-phenyl-1-pentanol 4 was extensively studied and screening experiments were conducted to select suitable lipase(s), reaction medium, acyl donor and appropriate temperature combinations to increase the enantiomeric ratio. Chirazyme^® L-6/chloroform/vinyl propionate/38 °C and Chirazyme^® L-7/di-iso-propyl ether/vinyl propionate/0 °C were chosen to obtain both enantiomers, (R)-(+)-4 and (S)-(−)-4, respectively, via sequential resolutions in excellent enantiomeric excess (>98%) and in 25% and 22% yield, respectively.     

Combining lipase-catalyzed enantiomer-selective acylation with fluorous phase labeling: a new method for the resolution of racemic alcohols

Lipase-catalyzed acylation of racemic alcohols with a highly fluorinated acyl donor allows their kinetic resolution accompanied by the simultaneous enantiomer-selective fluorous phase labeling. Both the tagged and the untagged enantiomer can be separated without chromatography by a very efficient partition between a fluorous and an organic phase. The method has been successfully applied to the resolution of typically racemic secondary alcohols of low molecular weight. The fluorous label can be recovered quantitatively.     

Kinetic resolution of primary alcohols having remote stereogenic centers: lipase mediated kinetic resolution of (±)-3-chloro-3-arylpropanols

Kinetic resolutions of (±)-3-chloro-3-arylpropanols by lipase mediated acetylation are described for the first time. Acetylation with CCL provided the best enantioselectivity amongst the enzymes used. Enantiomerically enriched products were obtained with up to 78% ee after two successive lipase-catalyzed acetylations. Different substituents on the aromatic ring and bromide, instead of chloride, on the substrates were found to have no drastic influence on the enantioselectivity of the reaction.     

Enzymatic resolution of a quaternary stereogenic centre as the key step in the synthesis of (S)-(+)-citalopram

The enzymatic resolution of 4-[(4-dimethylamino)-1-(4^′-fluorophenyl)-1-hydroxy-1-butyl]-3-(hydroxymethyl)-benzonitrile, a useful intermediate in the synthesis of enantiomerically pure citalopram, has been studied. Candida antarctica lipase B (CAL-B) catalyzes the enzymatic acetylation of the primary benzylic alcohol with high enantioselectivity at the quaternary stereogenic centre. The enzymatic enantioselective hydrolysis of the 3-acetyloxymethyl derivative catalyzed by CAL-B is also possible.     

Continuous-flow reactor-based enzymatic synthesis of phosphorylated compounds on a large scale

Acid phosphatase, an enzyme that is able to catalyze the transfer of a phosphate group from cheap pyrophosphate to alcoholic substrates, was covalently immobilized on polymethacrylate beads with an epoxy linker (Immobeads-150 or Sepabeads EC-EP). After immobilization 70% of the activity was retained and the immobilized enzyme was stable for many months. With the immobilized enzyme we were able to produce and prepare D-glucose-6-phosphate, N-acetyl-D-glucosamine-6-phosphate, allyl phosphate, dihydroxyacetone phosphate, glycerol-1-phosphate, and inosine-5'-monophosphate from the corresponding primary alcohol on gram scale using either a fed-batch reactor or a continuous-flow packed-bed reactor.     

Lipase-catalyzed asymmetric acylation in the chemoenzymatic synthesis of furan-based alcohols

Eight racemic 1-(furan-2-yl)ethanols were prepared from the corresponding carbonyl compounds for enantioselective acylation studies, and seven of them were used in preparative-scale kinetic resolutions with Candida antarctica lipase B (Novozym 435) and vinyl acetate in dried diisopropyl ether. Mechanism-based competition between the (R)-acetate (enzymatic acylation product), vinyl acetate (added acylating reagent), and acetic acid (enzymatic hydrolysis product) toward CAL-B, together with the residual water of the lipase were shown to be potential reasons for side reactions, which affected the course of the kinetic resolution of 1-[5-(2-chlorophenyl) and (4-bromophenyl)furan-2-yl]ethanols. Clear effects were not observed with the other alcoholic substrates. Alcoholysis of the enantiomerically enriched (R)-acetates with methanol and CAL-B in diisopropyl ether was shown to be a potential method for the deprotection of the (R)-acetates and the formation of (R)-alcohols.     

1-phenylethyl isocyanate is a powerful reagent for the chiral analysis of secondary alcohols and hydroxy fatty acids with remote stereogenic centres

1-phenylethyl isocyanate (1-PEIC), a chiral derivatisation reagent for the resolution of secondary alcohols is a powerful tool to determine the configuration and enantiomeric excess of medium- to long-chain secondary alcohols by capillary gas chromatography. The separation of 1-phenylethylcarbamates (1-PECs) of secondary alcohols was systematically evaluated depending on the position of the stereogenic centre in the molecule, namely in alkanols (C(15)-C(18)), alkenols (C(15)-C(18)) and hydroxy fatty acids (C(14)-C(18)). The successful separation of the diastereomeric carbamates of (+/-)-heptadecan-7-ol or (+/-)-12-hydroxyoctadecanoic acid methyl ester by gas liquid chromatography demonstrates the unique separation power for 1-PECs for analytes with remote stereogenic centres. Saturated derivatives showed consistently higher resolution factors than the corresponding unsaturated derivatives.     

Enantioselective synthesis of 2-alkylidenetetrahydrofurans based on a ‘cyclization/enzymatic resolution’ strategy

Enantiomerically pure 2-alkylidenetetrahydrofurans have been prepared by TiCl₄ mediated enantiospecific reactions of 1,3-bis-silyl enol ethers with enantiomerically pure epichlorohydrin. In addition, the enzymatic kinetic resolution of 2-alkylidenetetrahydrofurans, using Candida antarctica lipase B (CAL-B), was studied. Enzymatic kinetic resolution of monocyclic 5-vinyl-2-alkylidenetetrahydrofuran with CAL-B afforded the enantiomerically pure ester with 97% ee. For a bicyclic 2-alkylidenetetrahydrofuran, this proceeded with excellent enantioselectivity (E >100) affording the enantiomerically pure acid with 98% ee. 2-Alkylidenetetrahydrofurans were prepared by [3+2] cyclization reactions of 1,3-dicarbonyl dianions (‘free dianions’) or 1,3-bis-silyl enol ethers (‘masked dianions’).     

Nucleophilic acylation of α-haloketones with aldehydes: an umpolung strategy for the synthesis of 1,3-diketones

The first example of N-heterocyclic carbene (NHC)-promoted intermolecular acylation of α-haloketones with aldehydes and α,β-unsaturated aldehydes (enals) is reported. The protocol involves carbonyl umpolung reactivity of aldehydes and enals in which the carbonyl carbon attacks nucleophilically on electrophilic terminal of α-haloketones to afford 1,3-diketones and α,β-unsaturated 1,3-diketones, respectively. Short reaction time, ambient temperature, operational simplicity, and high yields are the salient features of the present procedure.     

Toward the enantioselective total synthesis of lyngbyatoxin A: on the stereocontrolled introduction of the quaternary stereogenic centre

This paper deals with an approach to the enantioselective total synthesis of Lyngbyatoxin A, with focus on the stereocontrolled introduction of the quaternary stereogenic centre. The key step in the synthesis involves an enantiospecific Lewis-acid mediated rearrangement of chiral vinyl epoxides carrying a 7-substituted indole moiety.     

An organocatalytic oxidative coupling strategy for the direct synthesis of arylated quaternary stereogenic centers

A broadly applicable oxidative coupling strategy of 3-substituted catechols and carbon-centered pro-nucleophiles for the construction of arylated quaternary stereogenic centers has been developed. Pivoting on a base-catalyzed addition of a carbon-centered acid to an in situ generated o-benzoquinone, the method is general and atom-economical and provides remarkably efficient access to one of the most challenging structural motifs. Furthermore, use of chiral bifunctional organocatalysts allows the process to be rendered asymmetric (up to 81% ee).     

Asymmetric trialkylaluminium addition to aldehydes catalyzed by titanium complexes of N-sulfonylated amino alcohols with two stereogenic centers

The asymmetric methylation, ethylation and allylation of aldehydes using trialkylaluminium reagents catalyzed by titanium(IV) complexes of N-sulfonylated amino alcohols gave excellent enantioselectivities of up to 99% ee.     

Kinetic resolution of alcohols using a 1,2-dihydroimidazo[1,2-a]quinoline enantioselective acylation catalyst

A new enantioselective acylation catalyst (Cl-PIQ), designed to provide enhanced pi-stacking with benzylic and cinnamyl alcohol substrates, was found to give considerably increased reaction rates and selectivities compared with the first-generation catalyst CF(3)-PIP. [reaction: see text]     

A green and expedient synthesis of enantiopure diketopiperazines via enzymatic resolution of unnatural amino acids

Dipeptides comprising a d-phenylglycyl moiety coupled to the l-enantiomer of 2-amino butyric acid, norvaline, norleucine, and homocysteine were successfully synthesized from d-phenylglycine amide and the racemate of the corresponding unnatural amino acid. The reaction is catalyzed by an immobilized form of penicillin G acylase in an aqueous medium. The dipeptides were subsequently converted into the corresponding enantiopure diketopiperazines in overall isolated yields of 22–33%.     

An enantiopure galactose oxidase model: synthesis of chiral amino alcohols through oxidative kinetic resolution catalyzed by a chiral copper complex

An enantiopure galactose oxidase (GO) enzyme model has been synthesized from readily available (R)-BINAM and Cu(OTf)₂, and the enantiopure GO model has been effectively used in situ as an efficient chiral catalyst for the synthesis of chiral amino alcohols through oxidative kinetic resolution (OKR), where molecular oxygen is used as the sole oxidant. Under the proposed catalytic conditions, both ortho- and para-substituted amino alcohols were resolved with good to excellent enantiomeric excesses through oxidative kinetic resolution.     

A stannylene strategy for regioselective acylation and phosphorylation of 1,2-cyclohexylidene-myo-inositol. Its convenient resolution and phosphatidylinositol synthesis

The bis(dibutylstannylene) derivative of 1,2-cyclohexylidene-myo-inositol reacted with (S)-O-acetylmandeloyl chloride and diphosphate tetraesters to give 3,6-dimandelate and 3-phosphate, respectively. Using the stannylene methodology for the optical resolution and regioselective phosphorylation of the ketal, a concise synthesis of phosphatidylinositol with the natural configuration was accomplished.     

On the use of succinic anhydride as acylating agent for practical resolution of aryl-alkyl alcohols through lipase-catalyzed acylation

A comparison is carried out of the E-values recorded in the lipase-catalyzed resolution of a series of secondary aryl-alkyl alcohols with enol esters versus succinic anhydride. Whereas all the substrates could be resolved by a proper choice of the lipase/enol ester couple with moderate (E=50) to good (E>100) enantioselectivities, only some of them showed satisfactory enantioselectivity (E>50) with the use of succinic acid as acylating agent. Notably, indanol and 1-quinolin-3-yl-ethanol were resolved in a practical way, with E>100 and E>80, respectively.     

N-tritylprolinal: an efficient building block for the stereoselective synthesis of proline-derived amino alcohols

N-Tritylprolinal (prepared in four steps from l-proline) shows a very high Felkin diastereoselectivity in its reaction with various nucleophiles, leading to a straightforward and highly stereoselective access to syn-proline-derived amino alcohols.