Synthesis of (Z)-alkene-containing cis-proline dipeptide mimetics using samarium(II) diiodide (SmI2)-mediated reductive alkylation reaction

The amide bond between an amino acid and proline can take the trans- or cis-conformation. The conformation influences both the structure and function of peptides and proteins. Therefore, constrained mimetic, which corresponds to Pro-dipeptides whose amide bond is replaced with an (E)- or (Z)-alkene, is a useful bioprobe for elucidating the structure-function relationship of peptides and proteins. Herein, we report the synthesis of cis-(Z)-alkene-containing Pro-dipeptide mimetics via a samarium(II) diiodide (SmI₂)-mediated reductive alkylation reaction.     

A concise and efficient route to the Alzheimer's therapeutic agent (R)-arundic acid

A short and efficient procedure for the preparation of (R)-arundic acid, a therapeutic agent for the treatment of Alzheimer's disease, has been developed. Based on cheap and commercially available (1R)-(+)-camphor as the source of chiral information, (R)-arundic acid is synthesized in a four-step cyclic sequence with 55% overall yield and high optical purity, ≥98% ee. Alkyl halide and acetylene constitute the only consumable carbon sources of this method, which allows obtaining of both enantiomers and recycling of chiral auxiliary.     

Intramolecular reductive amination strategy to the synthesis of (R)-N-Boc-2-hydroxymethylmorpholine, N-(3,4-dichlorobenzyl)(R)-2-hydroxymethylmorpholine, and (R)-2-benzylmorpholine

A concise high yielding enantioselective synthesis of (R)-N-Boc-2-hydroxymethylmorpholine, N-(3,4-dichlorobenzyl)(R)-2-hydroxymethylmorpholine, and (R)-benzylmorpholine has been achieved by employing proline-catalyzed asymmetric α-aminooxylation of aldehyde and palladium-catalyzed intramolecular reductive amination of azido aldehyde as the key steps.     

An expedient synthesis of 7(S)-ethyl-8(R or S)-indolizidinols based on a thiophene reductive desulfurization

Chiral hexahydrothieno[2,3-f]indolizine-4,7-dione (S)-12 and the ancillary alcohol 13 were generated from thiophene-2-carboxaldehyde and (S)-glutamic acid in three and four steps, respectively, in good overall yields and both high enantio- and diastereomeric purities. Applying a thiophene reductive desulfurization, compound 12 was readily converted into 7(S)-ethyl-8(S)-indolizidinol 9. The 8(R)-epimer of 9 was advantageously obtained using the Mitsunobu alcohol inversion or, starting from 13, by chemical separation after O-benzylation and lactam reduction. During these studies, the reduction of regioisomers of 12 and 13, namely 17 and 18, was investigated and the results obtained are also discussed.     

Enantioselective synthesis of (L)-Fmoc-α-Me-Lys(Boc)-OH via diastereoselective alkylation of oxazinone as a chiral auxiliary

Benzyl (2R,3S)-(−)-6-oxo-2,3-diphenyl-4-morpholinecarboxylate (4) was successively alkylated with methyl iodide and 1,4-diiodobutane using a base. In each alkylation step anti-alkylated product formed exclusively. The iodo group was displaced with azide, which served as a precursor for the side-chain amino function. Catalytic hydrogenation with concomitant cleavage of the chiral auxiliary afforded (L)-α-Me-Lys-OH (9) in a total of four steps in good yield. (L)-Fmoc-α-Me-Lys(Boc)-OH (16) was obtained from 9 via regioselective benzyloxycarbonylation. Alternately, (L)-Fmoc-α-Me-Lys(Boc)-OH (16) was obtained via Staudinger reduction of azide (8) in a total of six steps in good yield.     

Chromium(II)-catalyzed diastereoselective pinacol type cross coupling: studies of substrate-controlled effects

Using 20 mol % of CrCl₂ as catalyst, manganese powder as reducing agent, and TMS-Cl as scavenger, various acroleins and aldehydes were coupled with moderate to high yields and diastereomeric excesses. Alkyl aldehydes usually favor syn configuration while aldehydes with functional groups containing chelating hetero atoms promote the formation of anti configuration. Using sterically demanding alkyl residues on the acrolein substrate, the syn configuration is definitely preferred.     

Efficient synthesis of (R)- and (S)-3-octanol, (R)-2-dodecanol, (R)-2-methyl-4-heptanol and (R)-2-methyl-4-octanol: the pheromones of Myrmica scabrinodis, Crematogaster castanea, C. liengmei, C. auberti and Metamasius hemipterus

A simple and efficient synthesis of optically active insect pheromones, such as (R)- and (S)-3-octanol, (R)-2-dodecanol, (R)-2-methyl-4-heptanol and (R)-2-methyl-4-octanol starting from non-racemic β-hydroxy sulfides has been established.     

Catalytic asymmetric synthesis of ethyl (1R,2S)-dehydrocoronamate

A synthesis of (1R,2S)-dehydrocoronamic acid ethyl ester was developed employing a regio- and enantioselective palladium-catalysed nucleophilic ring-opening of 3,4-epoxy-1-butene with a glycine anion equivalent as the key enantiodifferentiating step. The desired selectivity was achieved using Trost’s naphthyl ligand. The subsequent activation of the free hydroxyl group and ring-closure by intramolecular S_N2 reaction gave the desired amino acid ethyl ester.     

B(C6F5)3: an efficient catalyst for reductive alkylation of alkoxy benzenes and for synthesis of triarylmethanes using aldehydes

Tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been used as an efficient catalyst for reductive alkylation of alkoxy benzenes using aldehydes as an alkylating agent in the presence of polymethylhydrosiloxane (PMHS). Various alkylated trimethoxybenzene derivatives have been prepared in good to high yields. In addition, B(C₆F₅)₃ was also used as a catalyst for the reaction of electron-rich arenes with aldehydes to obtain triarylmethanes. The use of reductive alkylation protocol for the synthesis of an isochroman and tetrahydroisoquinoline derivatives has also been demonstrated.     

Highly diastereoselective addition of organometallic reagents to a trifluoroacetaldehyde hydrazone derived from (R)-N-benzylphenylglycinol

Organolithium and Grignard reagents add efficiently with very high stereoselectivity (>98% de) to a trifluoroacetaldehyde hydrazone derived from (R)-N-benzylphenylglycinol. The addition proceeds on the re face of the chelated hydroxyhydrazone providing the (R)-α-trifluoromethylated amine after hydrogenolysis.     

A facile and efficient method for the synthesis of N-substituted isoindolin-1-one derivatives under Pd(OAc)2/HCOOH system

A facile and efficient method for the synthesis of N-substituted isoindolin-1-one derivatives from 2-formylbenzoic acid and amine under Pd(OAc)₂/HCOOH system has been described. The whole process is carried out in ligand-free conditions and furnished the desired products by reductive intramolecular cyclization. Furthermore, this procedure is applied successfully for the modification of natural products, such as vindoline and estrone.     

Pd-catalyzed asymmetric allylic alkylation with nitromethane using a chiral diaminophosphine oxide: (S,RP)-Ph-DIAPHOX. Enantioselective synthesis of (R)-preclamol and (R)-baclofen

A Pd-catalyzed asymmetric allylic alkylation with nitromethane using an aspartic acid-derived P-chirogenic diaminophosphine oxide [(S,R_P)-Ph-DIAPHOX] is described. This method was successfully applied to enantioselective synthesis of (R)-preclamol and (R)-baclofen.     

Racemic N-sulfonyloxaziridines as highly diastereoselective enolate hydroxylating agents: enantioselective synthesis of (2S,3S)-3-amino-N-cyclopropyl-2-hydroxyhexanamide

A new, highly enantioselective synthesis of (2S,3S)-3-amino-N-cyclopropyl-2-hydroxyhexanamide, a synthetic fragment of the experimental hepatitis C drug Telaprevir, has been described. Conjugate addition of the enantiomerically pure Davies lithium amide followed by hydroxylation of the in situ generated β-amino enolate was employed for the formation of the required stereogenic centres. Importantly, very high diastereoselectivities can still be achieved in the key-step when the relatively expensive and enantiopure (camphorsulfonyl)oxaziridine hydroxylating agent is replaced by racemic trans-N-sulfonyloxaziridines. Among the tested N-sulfonyloxaziridines the iso-propyl substituted analogue proved to be the ideal choice from an economic viewpoint.     

The first reductive cyclization by lithium aluminum hydride: stereospecific reductive cyclization of 1-(methoxycarbonylmethyl)imidazolidin-4-ones

The first LiAlH₄-driven reductive cyclization of the bifunctional 1-(methoxycarbonylmethyl)imidazolidin-4-ones, (−)-1a-c or(+)-1a-c, stereospecifically generated the corresponding 1,6-diaza-4-oxa-bicyclo[3.2.1]octanes, (−)-3a-c or(+)-3a-c, with a novel bicyclic system.     

Stereoselective addition of dialkyl phosphites to di-salicylaldimines bearing the (R,R)-1,2-diaminocyclohexane moiety

The addition of dialkyl phosphites to the azomethine bond of N,N′-disalicylidene-1,2-diaminocyclohexane imines, catalyzed by sodium hydride led to bis-aminophosphonates in a high diastereoselectivity. One of the bis-aminophosphonates was analyzed by X-ray diffraction. Another bis-aminophosphonate was converted to a bis-aminophosphonic acid and the structure was studied by X-ray diffraction. Addition of phosphites to a diimine resulted in an aminophosphonate, which was also studied by X-ray diffraction. An explanation of the diastereoselectivity is suggested.     

4-Formylazetidin-2-ones, synthon for the synthesis of (2R,3S) and (2S,3R)-3-amino-2-hydroxydecanoic acid (AHDA)

An efficient synthesis of 3-amino-2-hydroxydecanoic acid (AHDA), a nonproteinogenic amino acid, using enantiopure 3-benzyloxy-4-formylazetidin-2-one as a building block is described. Both the enantiomers of AHDA have been synthesized from the corresponding enantiomer of 3-benzyloxy-4-formylazetidin-2-one in good yield and optical purity.     

Highly diastereoselective alkylation of vicinal dianions of chiral succinic acid derivatives: a new general strategy to (R)-β-arylmethyl-γ-butyrolactones

The vicinal dianions derived from chiral succinic acid derivatives, 1,4-bis[(4R,5S)-3,4-dimethyl-2-oxo-5-phenylimidazolidin-1-yl]butane-1,4-dione and 1,4-bis[(4S,5R)-3,4-dimethyl-2-oxo-5-phenylimidazolidin-1-yl]butane-1,4-dione react with arylmethyl bromides with high diastereo- and regio-selectivity to provide the corresponding chiral α-arylmethylated succinic acid derivatives; the (R)-products are converted into (R)-β-arylmethyl-γ-butyrolactones and (R)-α-arylmethyl-γ-butyrolactones.     

Stereoselective synthesis of methyl branched chiral deoxypropionate units: a new route for synthesis of insect pheromone (−)-lardolure and (2R,4R,6R,8R) 2,4,6,8-tetramethylundecanoic acid

(−)-Lardolure and (2R,4R,6R,8R)-2,4,6,8-tetramethylundecanoic acid have been synthesized via lipase catalyzed desymmetrization strategy to create two methyl chiral centers. Other key steps involved in the synthesis are Wittig reaction, Evan’s asymmetric alkylation, Grignard reaction, Pd-catalyzed isomerization of primary allylic alcohol to corresponding saturated aldehyde, and PhNO/proline catalyzed MacMillan α-hydroxylation.     

A process for the rapid removal of dialkylamino-substituents from aromatic rings. Application to the expedient synthesis of (R)-tolterodine

A range of N,N-dialkylanilines have been successfully converted to the parent substituted benzenes by a novel two-step pathway. The products are obtained in good yields and optical purity of adjacent stereocenters is maintained. This technology has been applied toward the synthesis of (R)-tolterodine.     

Stereoselective synthesis of (−)-allosedamine and (1R,3R)-HPA-12 from β-p-toluenesulfonamido-γ,δ-unsaturated sulfoxide

A stereoselective synthesis of (−)-allosedamine and HPA-12 is disclosed. The key steps of the synthesis include the diastereoselective synthesis of a β-sulfonamido unsaturated sulfoxide, elaboration of a bromohydrin via intramolecular sulfinyl group participation and a ring-closing metathesis reaction for the construction of the piperidine ring of allosedamine.