Looking glass inhibitors: scalable syntheses of DNJ,DMDP, and (3R)-3-hydroxy-l-bulgecinine from d-glucuronolactone and of l-DNJ,l-DMDP,and (3S)-3-hydroxy-d-bulgecinine from l-glucuronolactone. DMDP inhibits β-glucosidases and β-galactosidases whereas l-DMDP is a potent and specific inhibitor of α-glucosidases

A convenient large-scale synthesis of 1-deoxynojirimyin (DNJ) from d-glucuronolactone involves introduction of azide at C-5 with retention of configuration to give 5-azido-5-deoxy-1,2-O-isopropylidene-α-d-glucofuranose as a key intermediate in an overall yield of up to 72%; the same intermediate can be transformed into DMDP [(2R,3R,4R,5R)-2,5-bis(hydroxymethyl)pyrrolidine-3,4-diol] and (3R)-3-hydroxy-l-bulgecinine [(2S,3R,4R,5R)-3,4-dihydroxy-5-hydroxymethyl-l-proline]. l-Glucuronolactone, a readily available l-sugar chiron, may similarly be used to access the enantiomers l-DNJ, l-DMDP, and (3S)-3-hydroxy-d-bulgecinine. A comparison of glycosidase inhibition by DMDP (an inhibitor of β-glucosidases and β-galactosidases) and l-DMDP (a potent and specific α-glucosidase inhibitor) with the corresponding enantiomeric hydroxybulgecinines is reported; DMDP and (3R)-3-hydroxy-l-bulgecinine show weak inhibition of glycogen phosphorylase.     

Sm(OTf)3 as a highly efficient catalyst for the synthesis of 2,3-unsaturated O- and S-pyranosides from glycals and the temperature-dependent formation of 4-O-acetyl-6-deoxy-2,3-unsaturated S-pyranosides and 4-O-acetyl-6-deoxy-3-alkylthio glycals

By using Sm(OTf)₃ as the catalyst, synthesis of 2,3-unsaturated-glycosides has been performed. A series of 2,3-unsaturated glycosides were obtained from 3,4,6-tri-O-acetyl–d-glucal or 3,4-di-O-acetyl-l-rhamnal under mild reaction conditions in good yield and high anomeric selectivity. It was found that under certain conditions, reaction of 3,4-di-O-acetyl-l-rhamnal with thiol leads to temperature-dependent formation of C-1-S and C-3-S product. A temperature-dependent profile of the yield of these two products is given.     

Synthesis of castanospermine

The diastereoselective synthesis of castanospermine is described in 11 synthetic steps from l-xylose. The borono-Mannich reaction between l-xylose, allylamine, and (E)-styrene boronic acid gives a tetrahydroxy amine with the desired configurations for C-6, C-7, C-8, and C-8a in the target molecule. A novel pyrrolo[1,2-c]oxazol-3-one precursor was employed to allow for the control of π-facial diastereoselectivity in an osmium(VIII)-catalyzed syn-dihydroxylation (DH) reaction. A regioselective ring-opening of the cyclic sulfate derivative of the resulting diol then secured the C-1 hydroxyl group of castanospermine with the correct configuration. A Mitsunobu cyclization then provided di-O-benzyl castanospermine and ultimately the final target alkaloid.     

A diastereoselective C–C bond formation at C-5 of d-gulose. A convenient approach to (5S)-5-C-alkyl-β-l-lyxo-hexofuranoses

A flexible approach to the stereoselective synthesis of (5S)-5-C-methyl- and (5S)-5-C-ethyl-β-l-lyxo-hexofuranoses 15a, 22 starting from 1,2:5,6-di-O-isopropylidene-α-d-gulofuranose 3 as the source of chirality is described. The corresponding C-5 alkyl groups were introduced via a Wittig olefination followed by Pd/C-mediated hydrogenation of the conformationally restricted alkenes in a highly diastereoselective manner.     

P2O5-Promoted Efficient and Diastereoselective Synthesis of Substituted 5-Methylene-dihydropyran-2,6-diones and 3-Methylene-3,4-dihydropyran-2-ones

A simple, efficient, and diastereoselective synthesis of 5-methylene-dihydropyran-2,6-diones and 3-methylene-3,4-dihydropyran-2-ones from substrates afforded by the S_N2 reaction between the acetyl derivatives of the Baylis–Hillman adducts and methylacetoacetate or acetylacetone or benzoyl acetone via saponification followed by P₂O₅-mediated cyclization is described.     

A versatile approach to pyrrolidine azasugars and homoazasugars based on a highly diastereoselective reductive benzyloxymethylation of protected tartarimide

A highly diastereoselective synthesis of enantio-enriched all trans-3,4-dibenzyloxyl-5-benzyloxymethyl-2-pyrrolidinone 13a was developed based on SmI₂-mediated benzyloxymethylation of O,O′-dibenzyltartarimide. The versatility of 13a and its antipode as the key building blocks for the asymmetric synthesis of pyrrolidine azasugars and homoazasugars has been demonstrated by elaborating them into naturally occurring DAB 1 (1), LAB 1 (2), N-hydroxyethyl-DAB 1 (4), 6-deoxy-DMDP 7, and 5-epi-radicamine B 36 as well as the reductive ring-opening product 35.     

A convenient synthesis of 1,2-difluoro-1, 2-dideoxyhexoses using xenon fluoride

A method for the addition of fluorine to the double bond in acetylated glycals using XeF₂ in the presence of BF₃ has been shown to provide a convenient route for the synthesis of acetylated 1, 2-dideoxy-1, 2-difluorosugars. The reagent attacks the double bond predominantly from the least hindered side to give a cis adduct, but the reaction also provided other isomers which have been separated by chromatography and their stereo-chemistry determined by ¹⁹F NMR spectroscopy. The proposed mechanism of the reaction involves an initial electrophilic attack by the reagent on C-2 followed by a nucleophilic attack on C-1. Deacetylation of the adducts gave 1, 2-dideoxyl-1, 2-difluorosaccharides, which are of interest in biochemical studies. A crystalline 2-deoxy-2-l-fucose was prepared by selective hydrolysis of the anomeric fluorine atom of 1, 2-dideoxy-1, 2-difluoro-l-fucose.     

A convenient approach toward the synthesis of enantiopure isomers of DMDP and ADMDP

A practical method for the synthesis of five-membered iminocyclitols, pyrrolidine alkaloids bearing multiple hydroxyl substituents, has been developed. All of the eight key intermediates, enantiopure tri-O-benzyl cyclic nitrones, are prepared from four cheap, readily available d-aldopentoses. The nucleophilic addition of cyclic nitrones with vinyl magnesium chloride and TMSCN shows high 2,3-trans stereoselectivity. To construct the 2,3-cis configurations, inversion of the C-2 nitrile group is achieved via an elimination-reduction sequence. Using this approach, five isomers of DMDP and six isomers of ADMDP are prepared efficiently. In the biological evaluation, iminocyclitol 27 is a new and potent inhibitor against β-hexosaminidase with an IC₅₀ value of 0.2 μM.     

An easy and stereoselective synthesis of N-Boc-dolaproine via the Baylis-Hillman reaction

In this communication we report a stereoselective total synthesis of N-Boc-dolaproine (Dap), an amino acid residue of the antineoplastic pentapeptide Dolastatin 10. Our strategy is based on a Baylis-Hillman reaction between N-Boc-prolinal and methyl acrylate, followed by a diastereoselective double bond hydrogenation and hydrolysis of the ester function.     

Studies on the total synthesis of sanglifehrin A: stereoselective synthesis of the C(29)-C(39) fragment

A highly stereoselective synthesis of the C(29)-C(39) fragment of the potent immunosuppressant sanglifehrin A has been accomplished by a sequence involving 16 steps (18% overall yield) from N-propionyloxazolidinone 9. Key steps are a diastereoselective hydroboration, and a diastereoselective epoxidation of an allylic alcohol followed by a 1,5-anti boron-mediated aldol reaction of methyl ketone 4 with chiral aldehyde 5.     

An efficient synthesis of the C1-C14 subunit of (−)-lasonolide A via a target oriented β-C-glycoside formation sequence

An efficient synthesis of the C₁-C₁₄ subunit resident in (−)-lasonolide A is reported herein. The key reaction features that were utilized include a Molander-Reformatsky SmI₂ mediated intramolecular aldol reaction followed by a diastereoselective target oriented β-C-glycoside formation sequence. Lastly, a chemo- and diastereoselective cross-metathesis of a terminal olefin in the presence of a trisubstituted alkene with acrolein and subsequent olefination of the aldehyde moiety allowed for the completion of the (E,E)-diene side chain.     

Zur lokalisierung funktioneller gruppen mit hilfe der massenspektrometrie—VIII : 6-hydroxy-androstan-3,17-dione und 6,17β-dihydroxy-androstan-3-one

5β-androstan-3-ones carrying a 6α-OH group show in their mass spectra a key-ion indicating the loss of water and C-1 to C-4 as C₄H₅O? particle. 6β-OH isomers lose instead C-1 to C-4 in form of C₄H₇O?.In 6α-hydroxy-androstan-3-ones differentiation between the connection of the A/B-ring system is possible, because in 5α-isomers the loss of C-3 to C-7 occurs as a C₅H₆O₂ particle, while the 5β-isomers lose the same C atoms as a C₅H₇O? unit.Compounds with a 6β-OH group in an A/B trans connected ring system show a tendency for thermal water elimination. After rearrangement of the double bond in 4,5 position the typical fragments for 3-keto-Δ⁴-steroids are obtained.Occasionally a strong influence of a 6-OH group on fragmentation reactions in the D-ring system is observed: The presence of a 6α-OH group in an androstan-3,17-dione enhances the loss of C-16 and C-17 in the form of acetaldehydenol. Also the connection of the A/B-ring system may have a considerable influence on this type of reaction: In 6,17β-dihydroxy-androstan-3-ones only by trans connection of the A/B-ring system, C-16 and C-17 are lost with high probability after water elimination.     

Stereoselective synthesis of hexahydroindoles and octahydrocyclohepta[b]pyrroles via gold(I)-catalyzed intramolecular 1,4-hydroamination of cyclic 1,3-dienes

The gold(I)-catalyzed intramolecular hydroamination of cyclohexa-1,3-dienes bearing an arylsulfonamide at the C-5 position proceeds in a 1,4-addition manner to afford hexahydroindole derivatives in a diastereoselective fashion and in good yields, whereas octahydrocyclohepta[b]pyrrole derivatives can be obtained from seven-membered ring substrates under the same reaction conditions. Coordination of the gold(I) species to the 1,3-diene at the double bond adjacent to the arylsulfonamide tether gave an η²-alkene gold complex. The anti-attack of the sulfonamide to the η²-alkene gold complex at the terminal position of the 1,3-diene resulted in the formation of the fused bicyclic ring with a newly formed Au-C bond at the allylic position. Allylic rearrangement of the η¹-allylgold complex followed by protodemetalation provided the fused heterobicyclic skeletons and regenerated the catalyst.     

Influence of an 8-trifluoroacetyl group on flavanol couplings

The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF₃, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl₄ or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4→O→C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4→C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy.     

Synthesis of 4-(thien-2-yl)-substituted coumarins through Lewis acid catalyzed Michael addition of thiophenes to 3-benzoylcoumarins followed by oxidation

3-Benzoyl-4-(thien-2-yl)coumarins have been obtained in good yields according to the S_N^H addition–oxidation protocol, involving the diastereoselective addition of thiophenes at C-4 of 3-benzoylcoumarins under BBr₃ catalysis, followed by oxidation of the intermediate 3,4-trans-3-benzoyl-4-(thien-2-yl)-3,4-dihydrocoumarins with DDQ. This two-step procedure can be regarded as nucleophilic substitution of hydrogen (S_N^H) on the heterocyclic ring of coumarins.     

A stereodivergent approach to 1-deoxynojirimycin, 1-deoxygalactonojirimycin and 1-deoxymannojirimycin derivatives

A stereodivergent synthesis of N-alkylated 1-deoxygalactonojirimycin and 1-deoxymannojirimycin derivatives has been achieved from a protected 1-deoxynojirimycin intermediate having two free OH groups tactically positioned at C-2 and C-4. The inversion of configuration of the secondary alcohols was performed by way of a Swern oxidation followed by a highly diastereoselective reduction using NaBH₄ or L-Selectride.     

Organocatalytic formal [5+1] annulation: diastereoselective cascade synthesis of functionalized six-membered spirocyclic indane-1,3-diones/oxindoles via Michael–aldol reaction

An efficient cascade protocol has been developed for the diastereoselective synthesis of functionalized six-membered spirocyclic compounds. The reaction proceeded smoothly between indane 1,3-diones/oxindoles/coumaranone as the dinucleophilic components and (E)-5-nitro-6-aryl-hex-5-en-2-one as the dielectrophile to give the desired products with reasonable to high chemical yields (30–84%) and high levels of diastereoselectivities (upto >95:5 dr). The reaction proceeded smoothly via cascade Michael–aldol reaction.     

An unexpected high erythro-selection in the Grignard reaction with an N,O-acetal: a concise asymmetric synthesis of indolizidine alkaloid (−)-2-epi-lentiginosine

Starting from commercially available lactone 10, a concise and highly diastereoselective synthesis of (?)-(lS,2R,8aS)-2-epi-lentiginosine (2) is described. The synthesis featured an unexpected highly erythro-selective reaction of Grignard reagent 7 with the protected N,O-acetal 8. The stereochemical outcome of this reaction is contrary to the known results involving the reactions with O-benzyl protected aminofuranosides and aminoglycosides. Thus, this method constitutes an extension of the threo-diastereoselective C–C bond formation methodology.     

Chemoenzymatic synthesis of enantiomerically pure tricyclic benzomorphan analogues

The key step in the synthesis of enantiomerically pure benzomorphan analogous tricyclic amines 2 is the kinetic resolution of secondary alcohol 7 using the lipase from Pseudomonas fluorescence. The (S)-configured alcohol (S)-7 and the (R)-configured ester (R)-8 were obtained in good yield (40% and 46%, respectively) and excellent enantiomeric excess (99% ee and 98.4% ee, respectively). A diastereoselective oxa-Pictet-Spengler reaction of (S)-7 with ethyl glyoxalate (OHC–CO₂Et) followed by a Dieckmann cyclization provided the tricyclic ring system 11, which allowed the diastereoselective introduction of an amino group at the 6-position. The absolute configuration of alcohol (S)-7 was determined with the tricyclic alcohol 13. The quantum mechanically calculated specific optical rotation of (S,S,S)-configured alcohol 13 is in accordance with the measured specific rotation of the synthesized compound. Moreover, X-ray crystal structure analysis of the synthesized compound, determined with the three-beam interference method, proved the (S,S,S)-configuration of 13. The enantiomerically pure dimethylamine 12 showed moderate affinity toward σ₂ receptors.