Parallel kinetic resolution of propargyl ketols: formal synthesis of (+)-bakkenolide A

We describe an intriguing new example of a parallel kinetic resolution; an asymmetric cyclization-carbonylation of propargyl ketols catalyzed by palladium(II) with chiral bisoxazoline (box) ligands. The 2S,3S enantiomer of (±)-6 was preferentially converted to 13 (45-49% yields, 37-46% ee), and the 2R,3R enantiomer of (±)-6 was preferentially converted to 14 (21-23% yields, 92-97% ee). As an application of this reaction, formal synthesis of (+)-bakkenolide A was achieved.     

First enantioselective synthesis of 4-aminoalcohol quinoline derivatives through a regioselective SN2 epoxide opening mechanism

Mefloquine derivatives, contrary to chloroquine derivatives have not been widely studied to date. Consequently, mefloquine and its derivatives still remain very attractive synthetic targets. Although mefloquine is usually used clinically as a racemic mixture, some studies have shown that its (+)-enantiomer is more potent than the (−)-enantiomer. Moreover, the (−)-enantiomer is responsible for side effects due to reaction with the central nervous system adenosine receptors, while the (+)-enantiomer does no bind at this binding site. Recently, different libraries of racemic 4-aminoalcohol quinolines showed interesting antimalarial activities. Herein, we describe an enantiopure synthetic and straightforward route to prepare pure enantiomer 4-aminoalcohol quinoline derivatives through a 4-oxirane key-intermediate. A regioselective S_N2 ring opening of this epoxide, by diverse amines, allows us to obtain the corresponding (R) or (S) 4-aminoquinolines with good yields and enantiomeric excesses generally superior to 92%. The reported methodology appears suitable for the synthesis of a large number of pure enantiomer 4-aminoalcohol quinoline derivatives.     

Catalytic asymmetric synthesis of the Colorado potato beetle pheromone and its enantiomer

An efficient catalytic asymmetric synthesis of the CPB pheromone [(S)-1,3-dihydroxy-3,7-dimethyl-6-octen-2-one] and its enantiomer was accomplished with 99% ee and in gram quantities from geraniol. The key steps in this procedure involve Sharpless asymmetric epoxidation and recrystallization of the 4-bromobenzoate from the CPB pheromone to improve its enantiomeric purity. Furthermore, the absolute configuration of the CPB pheromone enantiomer was confirmed as (R) for the first time by the X-ray crystallographic structure of its benzoate.     

Concise asymmetric synthesis of (R)-(+)-tanikolide

A highly stereoselective total synthesis of (R)-(+)-tanikolide, a δ-lactonic marine natural product, was accomplished in seven steps from easily available starting materials with a 51% overall yield. An asymmetric synthesis of an α-hydroxy aldehyde having a stereogenic quaternary center, by the use of (S)-2-(anilinomethyl)pyrrolidine as a chiral auxiliary, was employed in a key step.     

Stereoselective synthesis of (+)-sordidin, the male-produced aggregation pheromone of the banana weevil Cosmopolites sordidus

Stereoselective synthesis of (1S,3R,5R,7S)-(+)-sordidin, the natural male-produced aggregation pheromone of the banana weevil Cosmopolites sordidus (Germar) starting from 5-benzyloxy-(2E)-pentene-1-ol is described. The key transformations employed in the synthesis are Sharpless asymmetric epoxidation, Ueno-Stork cyclization, and Jacobsen kinetic resolution.     

An enantioselective total synthesis of the stilbenolignan (−)-aiphanol and the determination of its absolute stereochemistry

The title natural product (−)-aiphanol has been prepared by total synthesis. A key step involved the asymmetric dihydroxylation of (E)-3,5-dimethoxy-4-(methoxymethoxy)cinnamyl alcohol with the AD-mix-β to give triol (1R,2R)-1-(3′,5′-dimethoxy-4′-methoxymethoxyphenyl)-2,3-dihydroxypropanol, the absolute stereochemistry of which was confirmed by single-crystal X-ray analysis of a readily available bromo-derivative. These studies have established that the naturally occurring enantiomer of aiphanol possesses the (S)-configuration at each of C-2′ and C-3′.     

Diastereoselective α-iminoamine rearrangement: asymmetric synthesis of (R)-(−)- and (S)-(+)-2-benzyl-2-hydroxycyclohexanone

A convenient asymmetric synthesis of both (R)-(−)- and (S)-(+)-2-benzyl-2-hydroxycyclohexanones starting from racemic 2-benzyloxycyclohexanone and the chiral auxiliary 1-phenylethylamine is reported. The route involves a [1,3]-sigmatropic shift and a new diastereoselective α-iminoamine rearrangement of a 2-benzyl-2-iminocyclohexanamine substrate.     

First asymmetric total synthesis of (+)-curcutetraol

The first asymmetric total synthesis of (+)-curcutetraol, a marine phenolic bisabolane-type sesquiterpene, was achieved in eight steps in ca. 50% overall yield. The chiral tertiary benzylic alcohol moiety in the o-position of a phenol was constructed in high optical yield (99% ee) by an asymmetric synthesis using a chiral aminal, (2R,5S)-2-methoxycarbonyl-3-phenyl-1,3-diazabicyclo[3.3.0]octane.     

Asymmetric total synthesis of (+)-curcutetraol and (+)-sydonol

The asymmetric total syntheses of (+)-curcutetraol and (+)-sydonol, phenolic bisabolane-type sesquiterpenoids having chiral tertiary alcohol moiety in the o-position of a phenol, were achieved in high enantiomeric excesses (99% ee). The chiral tertiary benzylic alcohol moiety of these compounds was constructed by an asymmetric synthesis using an easily available chiral aminal, (−)-(2R,5S)-2-methoxycarbonyl-3-phenyl-1,3-diazabicyclo[3.3.0]octane. The absolute configurations of both (+)-curcutetraol and (+)-sydonol have been assumed to be S-configuration based on the stereochemical course of the well established asymmetric synthesis used in the syntheses.     

A novel building block for the synthesis of isofagomin analogues

The de novo synthesis of the novel potent building blocks phenyl (R)-3-acetoxymethyl-3,6-dihydro-2H-pyridine-1-carboxylate (−)-7 and phenyl (S)-3-butyryl-oxymethyl-3,6-dihydro-2H-pyridine-1-carboxylate (+)-8 starting from commercially available methyl-1-benzyl-4-oxo-3-piperidinecarboxylate is described. The key steps are the enantioselective esterification of the racemic alcohol 6 and the enantioselective hydrolysis of the racemic acetate 5, respectively, using lipase P from Pseudomonas caepacia.     

A Short and Elegant Synthesis of (±)‐Streptopyrrolidine

A brief and efficient approach for the synthesis of (±)‐5‐benzyl‐4‐hydroxy‐2‐pyrrolidine ( 1 ) from phenylalanine racemate is described. The key step is the stereocontrolled reduction of the keto functionality of benzylated pyrrolidinone intermediate ( 6 ) via sodium borohydride in carboxylic acid medium furnishing both (R,R)‐ and (S,S)‐configured diastereomers. The natural (R,R) enantiomer ( 2 ), however, crystallized out from its racemic mixture. Structure of 2 was confirmed by NMR, IR, elemental analyzer, and single crystal X‐ray crystallographic techniques.     

Asymmetric synthesis of (−)-chicanine using a highly regioselective intramolecular Mitsunobu reaction and revision of its absolute configuration

First asymmetric synthesis of (−)-chicanine has been accomplished in 14 steps by employing the Evans asymmetric syn-selective aldol reaction, diastereoselective hydroboration and an regioselective, intramolecular Mitsunobu etherification. The absolute configuration of (+)- and (−)-chicanine has been revised to 2R,3S,4R,5R and 2S,3R,4S,5S, respectively, through CD analysis.     

Determination of the absolute structure of albaconol

The concise synthesis of (8aR)-(−)-albaconol (1) from (8aR)-albicanol (2) obtained from the lipase-assisted asymmetric acetylation of rac-2, was achieved in 45% overall yield (eight steps). By comparison of the sign of specific rotation of between synthetic (8aR)-(−)-albaconol (1) and natural (+)-albaconol (1), the absolute structure of natural (+)-1 was determined to be 1R,2R,4aS,8aS configuration.     

Efficient and practical synthesis of both enantiomers of 6-silyloxy-3-pyranone derivatives

Lipase catalyzed kinetic resolution of racemic cis-6-(tert-butyldimethylsilyloxy)-3,6-dihydro-2H-pyran-3-ol (rac)-1 was achieved in high enantiomeric excess. Transesterification of (rac)-1 with vinylacetate in ^tBuOMe yielded the alcohol (3S,6R)-1 in 99.0% ee, whereas (3R,6S)-1 was obtained, in 99.0% ee, by the lipase catalyzed ester hydrolysis of acetate (3R,6S)-2, which was obtained along with the transesterification. Both (3S,6R)-1 and (3R,6S)-1 were subjected to oxidation to provide the corresponding 6-silyloxy-3-pyranone (6R)-3 and (6S)-3, respectively. Application to the synthesis of 7, which is the key intermediate of asymmetric synthesis of pseudomonic acid A 9 is also described.     

Temporary thio-derivatization in the synthesis of (+)-4-acetylbromoxone

A stereocontrolled synthesis of the marine natural products (+)-bromoxone (1) and (+)-4-acetylbromoxone (2) is reported. The sequence features the enzymatic kinetic resolution of 4-hydroxycyclohexenone (6) via its S-benzyl adduct. Thereafter, a base-mediated elimination-silylation generated an optically active (−)-4S-4-tert-butyldimethylsilyoxycyclohexenone (5), which then underwent diastereoselective epoxidation. Saegusa-Ito oxidation enabled formation of the corresponding α,β-unsaturated ketone 13. Bromination-elimination and subsequent removal of the silicon protecting group afforded (+)-bromoxone (1) which was converted into (+)-(4S,5R,6R)-4-acetoxy-2-bromo-5,6-epoxycyclohex-2-enone (2) [(+)-4-acetylbromoxone]. Using a luciferase gene reporter assay ED₅₀ for NFκB inhibition of 9 μM was determined.     

New asymmetric strategy for the total synthesis of naturally occurring (+)-alexine and (−)-7-epi-alexine

A novel and highly convenient process is described for the asymmetric synthesis of polyhydroxylated pyrrolizidine alkaloids, (+)-alexine [(1R,2R,3R,7S,7aS)-3-hydroxymethyl-1,2,7-trihydroxypyrrolizidine] and (−)-7-epi-alexine [(1R,2R,3R,7R,7aS)-3-hydroxymethyl-1,2,7-trihydroxypyrrolizidine], as the potent glycosidase inhibitors by featuring the efficient and stereodefined elaboration of the functionalized pyrrolidine derivatives, which were, in turn, prepared via stereoselective manipulation of the homochiral allyl alcohol precursors derived from l-xylose.     

An efficient preparative scale resolution of 3-phenylbutyric acid by lipase from Burkholderia cepacia (Chirazyme L1)

Lipase from Burkholderia cepacia (Chirazyme L1) catalysed the highly enantioselective hydrolysis of racemic methyl 3-phenylbutyrate to afford (R)-(−)-methyl 3-phenylbutyrate of >98% ee (E>50). The resolution was performed at 150 g scale yielding 68.7 g of (R)-(−)-methyl 3-phenylbutyrate (>98% ee, 92% yield on enantiomer) and (S)-(+)-3-phenylbutyric acid of 89% ee.     

Asymmetric reductions of propargyl ketones : An effective approach to the synthesis of optically-active compounds

Propargyl ketones are readily reduced by the asymmetric reducing agent B-3-pinanyl-9- borabicyclo[3.3.1]-nonane (Alpine-borane). The reagent prepared from (+)-α-pinene and 9-BBN provides the R enantiomer while the S enantiomer can be obtained from (-)-α-pinene. Alternatively the S enantiomer can be prepared from the reagent derived from 9-BBN and the benzyl ether of nopol (6,6-dimethyl-bicyclo[3.11.]hept-2-ene-2-ethanol). The limiting factor in obtaining high enantiomeric induction is often the enantiomeric purity of the α-pinene. With 100% enantiomerically pure α-pinene, propargyl alcohols of essentially 100% ee can be obtained. A predictive rationalization of the transition state leading to this remarkable selection is presented. The acetylene unit of the propargyl alcohol provides a convenient handle for transformations to other useful, optically-active products. The use of propargyl alcohols for the synthesis of optically-active α- and β-substituted γ-lactones, and δ-lactones is illustrated     

A highly stereocontrolled formal total synthesis of (±)- and of (−)-grandisol by 1,4-conjugated addition of organocopper reagents to cyclobutylidene derivatives

Starting from suitable cyclopropanes, a formal total synthesis of racemic grandisol and of the enantiopure (−)-grandisol is presented. The racemic synthesis of the grandisol precursor was accomplished in five steps. The synthesis of the chiral non-racemic precursor (1S,2S,2′R)-cis of this pheromone was realized in 10 steps, with an overall yield of 45%, using the enantiopure cyclobutanone (R,S), previously obtained by ring expansion of an optically pure oxaspiropentane. The key stereodefining step was the addition of lithium dimethylcuprate to a chiral α,β-unsaturated cyclobutylidene carbonyl derivative.     

Stereoselective synthesis of 3-heteroaromatic-substituted alanines

An asymmetric synthesis of (R)-3-heterocyclic-substituted alanines starting from (2S)-(+)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine (Schöllkopf reagent) and heteroaromatic halogenomethyl derivatives via hydrolysis of intermediate adducts is reported. The diastereocontrolled addition gives mainly compounds with the (2S,5R) configuration whose formation is explained on the basis of the accepted model for the alkylation reaction of the Schöllkopf reagent, and structure confirmed by spectroscopic data.