Diastereo- and enantioselective synthesis of N,O-nucleosides

The diastereo- and enantioselective synthesis of α- and β-3′-hydroxymethyl-N,O-nucleosides is described, based on the 1,3-dipolar cycloaddition of a N-glycosyl nitrone. Two approaches have been evaluated: the one-step procedure, which uses vinyl nucleobases, showed a better stereoselectivity towards β-nucleosides.     

Diastereo- and enantioselective total synthesis of stigmatellin A

Stigmatellin A (1) isolated from the myxobacterium Stigmatella aurantiaca is a powerful inhibitor of electron transport in mitochondria and chloroplasts. The first highly diastereo- and enantioselective total synthesis of this important natural product is described. Key steps in the synthesis are the alkylation of the SAMP-hydrazone (S)-13, a titanium mediated syn-diastereoselective aldol reaction, the anti-diastereoselective triacetoxyborohydride reduction of the aldol adduct (R,R,S)-16, formation of the chromone system via Baker Venkataraman rearrangement and exclusive (E) C=C double bond formation via Horner-Wadsworth-Emmons reaction.     

Diastereo- and enantioselective synthesis of a conagenin skeletal amide moiety

A synthetic study was carried out to obtain epimers of a protected 2,4-dihydroxy-3-methylpentanoic ester 9, which is a central building block of the immunomodulator (+)-conagenin 1. The configuration of the three contiguous stereogenic centers was determined by NMR measurements and comparison with the stereogenic centers of lactone 10.     

Diastereo- and enantioselective aldol reaction of granatanone (pseudopelletierine)

Granatanone (granatan-3-one, 9-methyl-9-azabicyclo[3.3.1]nonan-3-one, pseudopelletierine or pseudopelletrierin) undergoes deprotonation with lithium amides giving a lithium enolate, which reacts with aldehydes diastereoselectively giving exclusively exo isomers and anti/syn selectivity up to 98:2. Granatanone can be enantioselectively lithiated by chiral lithium amides and the resulting non-racemic enolate can be reacted with aldehydes giving aldols with enantiomeric excess up to 93% (99% ee after recrystallization). The absolute and relative configuration of the aldol products was determined by NMR spectroscopy and X-ray analysis.Granatanone; aldol reaction; asymmetric synthesis; enantioselective deprotonation; chiral lithium amide.     

Synthesis of the azaphilones using copper-mediated enantioselective oxidative dearomatization

An approach to the asymmetric synthesis of the azaphilone natural products is reported involving copper-mediated enantioselective oxidative dearomatization of o-alkynylbenzaldehydes. The approach was successfully applied to the synthesis of (-)-S-15183a and several unnatural azaphilones.     

Diastereo- and enantioselective synthesis of alpha,beta-disubstituted gamma-nitro methyl sulfonates

A novel asymmetric synthesis of highly enantioenriched homotaurine precursors has been developed via diastereo-selective Michael addition of lithiated enantiopure sulfonates to nitroalkenes by using 1,2:5,6-di-O-isopropylidene-alpha-D-allofuranose as chiral auxiliary.     

Diastereo- and enantioselective catalytic carbometallative aldol cycloreduction: tandem conjugate addition-aldol cyclization

A catalytic diastereo- and enantioselective method for tandem conjugate addition-aldol cyclization is described. This methodology enables the formation of five- and six-membered ring products from aromatic and aliphatic mono-enone mono-ketone precursors. Notably, in a single manipulation, three contiguous stereogenic centers are created with high levels of relative and absolute stereocontrol.     

Diastereo- and enantioselective carbolithiation of allyl o-lithioaryl ethers. New chiral cyclopropane derivatives

[reaction: see text] Different allyl 2-lithioaryl ethers undergo a tandem carbolithiation/gamma-elimination in Et(2)O/TMEDA affording o-cyclopropyl phenol or naphthol derivatives in a diastereoselective manner. The use of (-)-sparteine as a chiral ligand instead of TMEDA allows the synthesis of cyclopropane derivatives with up 81% ee.     

Diastereo- and enantioselective conjugate addition of 3-substituted oxindoles to enones via bifunctional catalysts

This paper presents a synthetically challenging vicinal quaternary-tertiary C–C forming reaction of 3-substituted oxindoles and chalcones by a simple aryl substituted bifunctional thiourea catalyst. The reactions work with a broad range of chalcones, giving chiral 3,3′-substituted oxindoles type compounds with adjacent quaternary-tertiary stereocenters in high yields (up to 99%), very good dr (up to >98:2) and very good ee (up to 96%).     

Diastereo- and enantioselective catalytic vinylogous Mukaiyama-Mannich reactions of pyrrole-based silyl dienolates with alkyl-substituted aldehydes

A reliable, catalytic asymmetric vinylogous Mukaiyama-Mannich reaction of pyrrole-based silyl dienolates is introduced that is particularly apt for alkyl- and α-alkoxyalkyl-substituted aldehydes. The reaction course is effectively orchestrated by the Hoveyda-Snapper amino acid-based chiral ligand/silver(I) catalyst combination to produce valuable vicinal diamino carbonyl compounds in high yields, with virtually complete γ-site- and anti-selectivity and significant catalyst-to-product chirality transfer. The utility of the Mannich products can be seen in the synthesis of an unprecedented perhydrofuro[3,2-b]pyrrolone product, an aza-analogue of naturally occurring (+)-goniofufurone.     

Diastereo- and enantioselective palladium-catalyzed dearomative [4+2] cycloaddition of 3-nitroindoles

A Pd-catalyzed asymmetric decarboxylative [4+2] cycloaddition of 3-nitroindoles and vinyl benzoxazinanones is developed through a dearomatization approach. The reaction provides an efficient protocol for constructing a series of chiral tetrahydro-5H-indolo[2,3-b]quinolines in high yields and with excellent diastereo- and enantioselectivities.     

Diastereo- and enantioselective ruthenium-catalyzed hydrohydroxyalkylation of 2-silyl-butadienes: carbonyl syn-crotylation from the alcohol oxidation level

Exposure of alcohols 2a-2j to 2-silyl-butadienes in the presence of ruthenium complexes modified by (R)-SEGPHOS or (R)-DM-SEGPHOS results in redox-triggered generation of allylruthenium-aldehyde pairs, which combine to form products of carbonyl crotylation 4a-4j in the absence of stoichiometric byproducts and with high levels of syn-diastereo- and enantioselectivity. In the presence of isopropanol under otherwise identical conditions, aldehydes 3a-3j are converted to an equivalent set of adducts 4a-4j. Whereas reactions conducted using conventional heating require 48 h, microwave irradiation enables full conversion in only 4 h. Finally, as illustrated in the conversion of adduct 4a to compounds 6a and 6b, diastereoselective hydroboration-Suzuki cross-coupling with aryl and vinyl halides followed by Fleming-Tamao oxidation enables generation of anti,syn-stereotriads found in numerous polyketide natural products.     

Diastereo- and enantioselective synthesis of densely functionalized cyclohexanones via double Michael addition of curcumins with nitroalkenes

The asymmetric double Michael additions of curcumins to nitroalkenes to afford highly functionalized cyclohexanones have been carried out for the first time. A combination of a dihydrocinchonine-thiourea organocatalyst and K₂CO₃ was found to be the most effective in obtaining the desired cyclohexanones in good yield, diastereoselectivity and enantioselectivity.     

Diastereo- and enantioselective syntheses of ansa-metallocenes from metal halide complexes with tropos biphenol and atropos binaphthol ethers

A diastereo- and enantioselective route to synthesize ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)-titanium and -zirconium dichlorides is described using titanium trichloride or zirconium tetrachloride complexes with tropos (chirally flexible) biphenol and atropos (chirally rigid) binaphthol ethers.     

Diastereo- and enantioselective reduction of α,β-diketodithiane with the baker's yeast

The Baker's yeast reduction of 1-(1,3-dithian-2-yl)-1,2-propanedione gave highly enantio-and diastereoselectively (S)-(+)-(1-1,3-dithian-2-yl)-2-hydroxy-1-propanone or (1S,23-(+)-(1)-(1,3-dithian-2-yl-1,2-propanediol, depending on the reaction time. The hydroxy ketone was reduced with diisobutylaluminum hydride to give (1R,2S)-1-(1,3-dithian-2-yl)-1,2-propanediol with high diastereoselectivity. The former (1S,2S)-diol was converted into L-digitoxose.     

Diastereo- and enantioselective hydrogenative aldol coupling of vinyl ketones: design of effective monodentate TADDOL-like phosphonite ligands

We report the first enantioselective reductive aldol couplings of vinyl ketones, which were achieved through the design of a novel monodentate TADDOL-like phosphonite ligand. Specifically, hydrogenation of commercially available methyl vinyl ketone (MVK) or ethyl vinyl ketone (EVK) in the presence of aldehydes 1a-7a using cationic rhodium catalysts modified by chiral TADDOL-like phosphonite ligands AP-I and AP-IV produces aldol adducts 1b-7b and 1c-7c with excellent control of relative and absolute stereochemistry. The absolute stereochemical assignments of the aldol adducts are made in analogy to that determined for the 5-bromophthalimido derivative of aldol adduct 1b and the 2-bromo-5-nitrobenzoate of 3b, which were established by single-crystal X-ray diffraction analysis using the anomalous dispersion method.     

Diastereo- and enantioselective intramolecular C(sp3)-H arylation for the synthesis of fused cyclopentanes

All C-H bonds are not equal: The intramolecular arylation of unactivated C(sp(3))-H bonds in the presence of a chiral Pd/binepine catalyst allows the synthesis of fused cyclopentanes efficiently and in an diastereo- and enantioselective manner (see scheme).