Revisiting the armed-disarmed concept: the importance of anomeric configuration in the activation of S-benzoxazolyl glycosides

The unexpectedly high reactivity of a (2-benzoxazolyl) per-O-benzoyl-beta-D-thioglucoside and related donors in reactions promoted by copper(II) trifluoromethanesulfonate is revealed, by comparison with the unreactive alpha-anomer, to be the result of neighboring group participation. Revision of the armed-disarmed concept for glycosyl donors is not required.     

Versatile solid-phase synthesis of s-arylisothioureas

Resin-bound amines 4a-m condense with di(benzotriazol-1-yl)methanimine 6 to give 1H-benzotriazole-1-carboximidamide resins 7a-m, which subsequently react with thiols 9a'-g' followed by cleavage affording nonprotected isothioureas 1aa'-mg' in high yields and with good purities. Analogous reactions with secondary amines activated with EtMgBr lead to guanidines 2a,b,e-g in moderate yields. Resin-bound isothioureas 1 are converted by acyl chlorides or carboxylic acids into acyl derivatives 12a-n in high yields and with good purities.     

S-benzoxazolyl as a stable protecting moiety and a potent anomeric leaving group in oligosaccharide synthesis

As a part of a program for developing new versatile building blocks for stereoselective glycosylation and convergent oligosaccharide synthesis, we demonstrated that S-benzoxazolyl (SBox) glycosides are stable toward major protecting group manipulations employed in carbohydrate chemistry. On the other hand, they can be glycosidated under relatively mild reaction conditions to afford either 1,2-trans or 1,2-cis-linked disaccharides. Selective and chemoselective activations of the SBox moiety were also proved to be feasible, which was demonstrated by synthesizing a number of oligosaccharide sequences.     

Versatile enantiocontrolled synthesis of (+)-fostriecin

Fostriecin, a potent protein phosphatase inhibitor and antitumor agent, has been enantioselectively synthesized in naturally occurring form via a versatile route, which also allows one to secure all possible stereoisomeres of the C1-C13 fragment including the C11 stereocenter and the geometry of the delta 12-double bond.     

Versatile synthesis of oxindole-1,3-dicarboxamides

A new, high-yielding synthesis of oxindole-1,3-dicarboxamides was elaborated starting from 1-phenoxycarbonyl-3-ethoxycarbonyl-2-oxindole and 1,3-diphenoxycarbonyl-2-oxindole. This method permits also the preparation of N,N,N′-tri- and N,N,N′,N′-tetrasubstituted oxidole-1,3-dicarboxamides, families of compounds that are unknown in the literature. The scope and limitations of the methodology have also been investigated, and a remarkable selectivity has been observed among the amines used in the amidation steps.     

Koenigs-Knorr synthesis of cycloalkyl glycosides

Cadmium carbonate was found to be a useful promoter in the Koenigs-Knorr synthesis of 2-(4-methoxybenzyl)cyclohexyl-beta-D-glycopyranosides. Using this promoter model glucoside and galactoside derivatives of cyclic (i.e., secondary) alcohols were synthesized in 50-60 % overall yields. Diastereoisomeric mixtures of products were obtained in these syntheses, which started from racemic isomers of 2-(4-methoxy-benzyl)cyclohexanol. The prepared compounds have been purified and characterized by their (1)H- and (13)C-NMR spectra, as well as by their IR and MS spectra, in order to use them as reference compounds in planned subsequent research.     

Versatile synthesis of rare nucleotide furanoses

Direct activation of unprotected thioimidoyl furanosides yielded in only one step and few minutes a panel of rare uridine 5'-diphospho-furanoses. Diastereoselectivity of the reaction was tightly connected with reaction time, temperature, and nature of the furanosyl donor. This approach was totally selective since no ring expansion from the initial five-membered ring to the more stable pyranose form was observed.     

Stereocontrolled synthesis of 2,3-anhydro-beta-d-lyxofuranosyl glycosides

[reaction: see text] The stereocontrolled synthesis of 2,3-anhydro-beta-D-lyxofuranosyl glycosides from thioglycoside 2 and glycosyl sulfoxide 3 is reported.     

Short synthesis of phenylpropanoid glycosides calceolarioside-B and eutigoside-A

A convenient 4-step synthesis of calceolarioside-B 1 and eutigoside-A 2 in high overall yield is described. The key step involved the regioselective, Me₂SnCl₂-catalyzed O-6 acylation of unprotected 2-phenylethyl-β-d-glucosides 5a–b with cinnamoyl chlorides 6a–b in excellent yields. Acylation at O-6 is selective with the acid chlorides used. This work serves as a model for the convenient synthesis of phenylpropanoid glycosides acylated at O-6.     

Versatile synthesis of 3,4-b diheteropentalenes

We describe a new route for the synthesis of thieno[3,4-b]thiophene, alkyl derivatives thereof, seleno[3,4-b]thiophene, and thieno[3,4-b]furan made from inexpensive starting materials, such as thiophene-2-carboxylic acid and furan-2-carboxylic acid. Such fused heterocycles are of great interest for low band gap organic semiconductors and applications including OLEDs, organic photovoltaic cells, and electrochromics.     

Versatile synthesis of nanometer sized hollow silica spherest

Using the controlled precipitation of silicic acid on functionalized polystyrene latexes, nanometer sized silica-coated spheres could be prepared and subsequently modified to allow dispersion in non-aqueous solvents; removal of the interior polymer by calcination resulted in the formation of hollow silica spheres.     

Versatile asymmetric synthesis of the kavalactones: first synthesis of (+)-kavain

[reaction: see text] Three asymmetric pathways to the kavalactones have been developed. The first method is chiral auxiliary-based and utilizes aldol reactions of N-acetyl thiazolidinethiones followed by a malonate displacement/decarboxylation reaction. The second approach uses the asymmetric catalytic Mukaiyama additions of dienolate nucleophile equivalents developed by Carreira and Sato. Finally, tin-substituted intermediates, prepared by either of these routes, can serve as advanced general precursors of kavalactone derivatives via Pd(0)-catalyzed Stille couplings with aryl halides.     

Versatile and expeditious synthesis of aurones via Au I-catalyzed cyclization

Aurones are conveniently formed in a three-step procedure including a goldI-catalyzed cyclization of 2-(1-hydroxyprop-2-ynyl)phenols as a highly regio- and stereoselective key step. A wide diversity of derivatives can be obtained starting from substituted salicylaldehydes. Synthesis of natural 4,6,3',4'-tetramethoxyaurone and structure revision of two natural products (dalmaisione D and 4'-chloroaurone) were achieved.     

Versatile synthesis of 3,5-disubstituted 2-fluoropyridines and 2-pyridones

5-bromo-2-fluoro-3-pyridylboronic acid (3) was prepared in high yield by ortho-lithiation of 5-bromo-2-fluoropyridine (1), followed by reaction with trimethylborate. Suzuki reaction of 3 with a range of aryl iodides gave 3-monosubstituted 5-bromo-2-fluoropyridines 4 in excellent yields. A second Suzuki reaction utilizing the bromo constituent of 4 with aryl and heteroaryl boronic acids provided 3,5-disubstituted 2-fluoropyridines 5, which in turn could be converted to the corresponding 2-pyridones 6.     

Experiments on the synthesis of carotenoid glycosides

Isozeaxanthin was treated with trifluoroacetic acid and tetra-O-benzoyl-1-thio-β-d-glucose to afford β,β-carotene-4,4′-bisthioglucoside isomers in good yields. The deprotected compounds are mimetics of naturally occurring thermoxanthins and can show favourable effects against oxidation stress.     

Chemoenzymatic synthesis of enantiomerically enriched aminoalkenols and glycosides thereof

1-t-Butoxycarbonylamido-3-pentene-1-ol 3 and 2-azido-4-phenyl-3-butene-1-ol 4 were enantiomerically enriched by enzymatic acetylation using various lipases and esterases (CHIRAZYM) to give acetylated compounds 5 and 7, respectively. Compound 3 gave the best results (E=94) with Candida antarctica A lipase (CHIRAZYM L-5), whereas 4 could not be separated into the enantiomers with satisfactory E values. The absolute configurations were proven for both compounds via independently prepared derivatives. Both enantiomers of 5, as well as racemic 7, were N-deblocked and condensed with octonic acid derivatives 14 to give the corresponding C-glycosides 17 and 22 after deprotection of the intermediates in good overall yield. Compound 4 was similarly condensed with glucose imidate 11 to give the diastereomeric O-glycosides 13 after deprotection. The latter glycosides were prepared as precursors for the generation of the corresponding aldehydes as substrates for aldolase catalyzed reactions.     

Facile and efficient synthesis of chiral sulfoxide esters: Versatile tool in asymmetric synthesis

Facile and efficient synthesis of sulfoxide esters using menthols as chiral auxiliary is described. Phenylthio/benzylthio/naphthylthioacetic esters act as an efficient substrate for chiral sulfoxides via oxidation in one step. The structural and stereochemical aspects of target product were established on the basis of various spectroscopic studies, namely FT-IR, NMR (¹H NMR and ¹³C NMR) and elemental analysis. This method is simple, fast, convenient and very efficient.     

Versatile soluble oligomeric styrene supports for peptide synthesis

Soluble oligomeric styrene supports are reported here with high loading capacities of 1.5–1.6 mmol/g similar to resins used in solid phase peptide synthesis. Oligoether and alkyl chains are incorporated into the scaffold to improve the support solubility and act as spacers between the attachment sites. Amino acids have been attached to the support in 59–85% yields and 0.87–1.3 mmol/g loading. The supports have been used to synthesize tri‐ to hexapeptides in 38–64% yields using only 2 equivalents of coupling reagents, which is much lower than the amount of reagents typically used in solid phase synthesis. A modular synthetic approach is used to obtain the supports so that any efficient styrene‐based attachment site can be readily incorporated into our soluble support scaffold. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2501–2509     

Total synthesis of cucurbitoside-like phenolic glycosides by double fluorous and acyl mixture synthesis

The first and simultaneous total syntheses of cucurbitosides A, B, G, and I, seguinosides C and D, and two unnatural analogs were achieved using the technique of fluorous mixture synthesis. The eight precursors of cucurbitoside-like phenolic glycosides were prepared by glycosylation of a mixture of two glucopyranosyl acceptors bearing different fluorous benzyl groups with a mixture of four apiofuranosyl donors bearing benzoyl, 3-methylbutyryl, 4-benzyloxybenzoyl, and 4-nitrobenzoyl groups, followed by a single run of HPLC with serially connected Fluophase^® RP and Inertsil^® ODS-3 columns. Finally, the individual pure disaccharide precursors were detagged to yield the eight cucurbitoside-like phenolic glycosides.