Total synthesis of (±)-anatoxin-a via N-acyliminium intermediates

(±)Anatoxin-a has been synthesized in 8 steps, starting from succinimide, 4-bromo-1-butene and dimethyl (2-oxopropyl)phosphonate, by employing as the key step an intramolecular reaction of an N-acyliminium precursor with an α,β-unsaturated ketone moiety, induced by saturated HCl in MeOH at −50°C.     

Palladium-catalyzed synthesis of 4-oxaspiro[2.4]heptanes via central attack of oxygen nucleophiles to π-allylpalladium intermediates

A palladium-catalyzed decarboxylative cyclopropanation of γ-methylidene-δ-valerolactones with aromatic aldehydes has been developed to give 4-oxaspiro[2.4]heptanes with high selectivity. The site of nucleophilic attack to a π-allylpalladium intermediate has been controlled with a sterically demanding phosphine ligand. The course of the reaction is highly dependent on ligands and solvents, and selective formation of methylenetetrahydropyrans has also been realized.     

Reversed-phase high performance liquid chromatographic separation of diastereomers of β-amino alcohols and microwave assisted synthesis of Marfey's reagent,its chiral variants and diastereomers

Ten chiral derivatizing reagents (CDRs) were synthesized by replacing the l-Ala–NH₂ moiety in Marfey's reagent (MR) by seven l-amino acid amides and three l-amino acids employing microwave irradiation (MW) and were characterized. Ten racemic amino alcohols were derivatized with these CDRs under MW. The diastereomers were separated on a reversed-phase C₁₈ column using binary mixtures of acetonitrile with aqueous trifluoroacetic acid (TFA) and triethylammonium phosphate buffer (TEAP). In general, amino acid variants of MR provided better separation of diastereomers in comparison to amino acid amide variants. The method was also found successful for the separation of 20 diastereomers from a mixture.     

Asymmetric synthesis of heteroorganic analogs of natural products 5. Synthesis of pure diastereomers and enantiomers of fluoro-substituted (2S,3R)- and (2R,3R)-β-phenylserines

A convenient method is proposed for the preparative synthesis of pure diastereomers and enantiomers of fluoro-substituted (2S,3R)- and (2R,3R)--phenylserines by the hydroxyalkylation of the Ni(II) — complex of the Schiff base of glycine with (S)-2-N-(N¹-benzylpropyl)aminobenzophenone using the corresponding fluoro-substituted benzaldehydes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya. No. 8, pp. 1906–1913, August, 1991.     

Total synthesis of Yingzhaosu B and its three diastereoisomers

Yingzhaosu B (1), coexisting with Yingzhaosu A (2) which is an antimalarialprinciple in the traditional Chinese herbal medicine Yingzhao (Artabotrys uncina-tus (L.) Merr.), is a polyhydroxy bisabolene compound. Although its structure wasproposed by Liang and coworkers in 1979, the stereochemistry remained unsettled.From biogenetic viewpoint, it is reasonable to assume that the configuration of 4-Cin Yingzhaosu B is S as that in Yingzhaosu A. Recently, the synthesis of degrada     

A Highly Asymmetric Gold(III) η3-Allyl Complex

A highly asymmetric Au^III η³-allyl complex has been generated by treating Au(η¹-allyl)Br(tpy) (tpy=2-(p-tolyl)pyridine) with AgNTf₂. The resulting η³-allyl complex has been characterized by NMR spectroscopy and X-ray crystallography. DFT calculations and variable temperature ¹H NMR suggest that the allyl ligand is highly fluxional.     

Asymmetric synthesis of β,β-difluoroamino acids via cross-coupling and Strecker reactions

β,β-Difluoroamino acids were synthesized from commercially available ethyl bromodifluoroacetate using cross-coupling and Strecker reactions as key steps. The coupling reaction of aryl iodides with ethyl bromodifluoroacetate gave the corresponding coupling products, which were transformed to 2-difluoromethyl-1,3-oxazolidines in two steps. Boron trifluoride etherate promoted Strecker reaction of 2-difluoromethyl-1,3-oxazolidines gave α-amino nitriles in good yields and diastereoselectivities. After removal of chiral auxiliary and hydrolysis of the nitrile group, β,β-difluorophenylalanine was obtained with 73% ee. Partial racemization occurred during the hydrolysis of nitrile group.     

Asymmetric synthesis of (−)-(R)-sitagliptin

The asymmetric synthesis of (?)-(R)-sitagliptin was achieved in seven steps from commercially available starting materials using the highly diastereoselective conjugate additions of either lithium (R)-N-benzyl-N-(α-methylbenzyl)amide or lithium (R)-N-benzyl-N-(α-methyl-p-methoxybenzyl)amide to tert-butyl 4-(2′,4′,5′-trifluorophenyl)but-2-enoate to install the correct stereochemistry. Subsequent sequential acid-catalysed hydrolysis of the resultant β-amino esters, HOBt/EDC mediated coupling with the triazolopyrazine fragment, and hydrogenolysis gave (?)-(R)-sitagliptin in 43% and 42% overall yields, respectively.     

One-pot synthesis of β-2,6-dideoxyglycosides via glycosyl iodide intermediates

An efficient and convenient approach for stereoselective synthesis of β-linked 2,6-dideoxyglycosides has been developed through one-pot glycosylation strategy using glycosyl iodide intermediates. The glycosidation reaction, promoted by easily available silver nitrate, provided the corresponding 2,6-dideoxyglycosides and oligosaccharides with preponderant β-configuration (β/α = 2.3:1 to 11:1) and good to excellent yields (52–84%).     

Contemporaneous dual catalysis: chemoselective cross-coupling of catalytic vanadium-allenoate and π-allylpalladium intermediates

This paper presents a detailed investigation of a dual catalytic system that combines a vanadium-catalyzed Meyer-Schuster rearrangement and a palladium-catalyzed allylic alkylation. The implementation of this novel reaction relies on matching the formation rates of vanadium-allenoate and π-allylpalladium intermediates with their bimolecular coupling rate in order to minimize the undesired protonation or O-alkylation of the catalytically generated intermediates. Chemoselectivity in this dual catalytic process was successfully achieved by adjusting ligand structure and catalyst loading ratios of the vanadium and palladium catalysts. A great range of coupling partners for both the propargyl alcohol and allyl carbonate components are readily accommodated in this new transformation, which in turn provides a novel avenue to a variety of α-allylated α,β-unsaturated ketones, esters, and amides in moderate to excellent isolated yields.     

A convenient and general approach to the synthesis of properly protected d-nucleoside-3′-hydrogenphosphonates via phosphite intermediates

Evidence will be presented to show that the monofunctional phosphitylating reagents bis(N,N-di-ethylamino)chlorophosphine and salicylchlorophosphine are very effective for the preparation of 5′-0,N-protected d-nucleoside-3′-hydrogenphosphonates.     

Asymmetric synthesis of 3-substituted pyrrolidones via α-alkylation of a chiral non-racemic γ-lactam

3-Alkyl pyrrolidones 9 were synthesized in good yield and high diastereoselectivity by α-alkylation of the new chiral non-racemic lactam 8 derived from (R)-(−)-phenylglycinol. After debenzylation and introduction of an electron-withdrawing group, 3-methylpyrrolidone 10 is easily hydrolyzed in a basic medium to produce γ-aminobutyric acid (GABA) analogue 13.     

Asymmetric synthesis of β-substituted γ-lactams via rhodium/diene-catalyzed 1,4-additions: application to the synthesis of (R)-baclofen and (R)-rolipram

An efficient rhodium/diene-catalyzed asymmetric addition of arylboronic acids to α,β-unsaturated γ-lactams has been developed. The power of this methodology is further demonstrated by the concise synthesis of (R)-baclofen and (R)-rolipram.     

An approach to the synthesis of α-l-fucopyranosyl phosphoric mono- and diesters via phosphite intermediates

The reagent chloro-β-cyanoethyl-N,N-diisopropylamino-phosphoramidite reacts smoothly with the anomeric hydroxyl group of a properly protected (benzyl) α-L-fucopyranose to afford a relatively stable phosphite intermediate in high yield. The latter can easily be converted into valuable α-L-fucopyranosyl phosphoric mono- and diesters.     

Asymmetric synthesis of (−)-bissetone via a highly enantioselective hetero-Diels–Alder reaction

We demonstrate a new approach for the asymmetric synthesis of bissetone. The key reaction is the highly enantioselective hetero-Diels–Alder cycloaddition of triene 3 with ethyl glyoxylate catalyzed by readily available BINOL–Ti complexes. The HDA cycloadduct 4 was then transformed in five steps into O-protected bissetone (8) and its C5-epimer in good yield.     

A novel and stereoselective synthesis of (±)-cephalotaxine and its analogue

Cephalotaxine (1) and its analogue (3) were stereoselectively synthesized from proline via the pyrrolobenzazepine (4) through Claisen rearrangement, cationic cyclization, and regio- and stereo-selective hydroxylation.     

Asymmetric synthesis of aspartic and α-methylaspartic acids via Ni(II) complexes with schiff bases of glycine and alanine and chiral carbonyl-containing reagents

We propose a method for the synthesis of aspartic and -methylaspartic acids by alkylation with ethyl bromoacetate of the -carbon atom of the amino acid moiety in Ni(II) complexes of Schiff bases of glycine with (S)-2-[(N-benzylprolyl)amino]benzophenone and alanine with (S)-2-[(N-benzylprolyl)amino]-benzaldehyde, respectively. Attempts to synthesize -methylaspartic acid by oxidative cleavage of the C=C bond to a COOH group in the complex of the Schiff base of -allylalanine with (S)-2-[(N-benzylprolyl)amino]benzophenone were unsuccessful.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1542–1547, July, 1991.     

Asymmetric total synthesis of (+)-isocryptotanshinone and formal synthesis of (−)-cryptotanshinone

The first asymmetric total synthesis of (+)-isocryptotanshinone was achieved in 12 linear steps with 12% overall yield from commercially available dihydrobenzopyrone. The key step was a base-mediated cyclization reaction. In addition, the synthetic strategy included the formal synthesis of (−)-cryptotanshinone.     

Asymmetric synthesis of (S)-(−)-acromelobinic acid

A total synthesis of (S)-(−)-acromelobinic acid 2, which was isolated from clitocybe acromelalga, was achieved via an asymmetric hydrogenation protocol. Dehydroamino acid derivative 12 was prepared from 2,5-lutidine 5 and subjected to asymmetric hydrogenation using (S,S)-[Rh(Et-DuPHOS)(COD)]BF₄ to give the (S)-(+)-pyridylalanine derivative 13 in 93% yield and >96% e.e. Removal of the protecting groups in (S)-(+)-13 afforded (S)-(−)-acromelobinic acid 2.     

Asymmetric synthesis of (S)-(−)-tetrahydropalmatine and (S)-(−)-canadine via a sulfinyl-directed Pictet–Spengler cyclization

(S)-(?)-Tetrahydropalmatine 2 and (S)-(?)-canadine 4 were synthesized in three steps from (S)-6, in 33% and 34% overall yield, respectively. Thus, condensation of the (S)-(E)-sulfinylimines 10 and 11 with the carbanion derived from (S)-6 gave the tetrahydroisoquinolines 12 and 13, respectively, which upon TFA induced N-desulfinylation, and subsequent microwave assisted Pictet–Spengler cyclization effected both cyclization and C-desulfinylation producing (S)-(?)-tetrahydropalmatine 2 and (S)-(?)-canadine 4 in optically pure form.