Study of metal and acid catalysed deprotection of propargyl ethers of alcohols via their allenyl ethers

Glyco-DNA: Enzymatic Synthesis of Base-Modified and Hypermodified DNA Displaying up to Four Different Monosaccharide Units in the Major Groove

A portfolio of six modified 2'-deoxyribonucleoside triphosphate (dNTP) derivatives derived from 5-substituted pyrimidine or 7-substituted 7-deazapurine bearing different carbohydrate units (d-glucose, d-galactose, d-mannose, l-fucose, sialic acid and N-Ac-d-galactosamine) tethered through propargyl-glycoside linker was designed and synthesized via the Sonogashira reactions of halogenated dNTPs with the corresponding propargyl-glycosides. The nucleotides were found to be good substrates for DNA polymerases in enzymatic primer extension and PCR synthesis of modified and hypermodified DNA displaying up to four different sugars. Proof of concept binding study of sugar-modified oligonucleotides with concanavalin A showed positive effect of avidity and sugar units count.

Red Algal Molecules - Synthesis of Methyl Neo-β-carrabioside and Its S-Linked Variant via Two Synthetic Routes: A Late Stage Ring Closure and Using a 3,6-Anhydro-d-galactosyl Donor

Methyl neo-β-carrabioside has been synthesized for the first time, employing either a late stage ring closure to install the required 3,6-anhydro-bridge or a suitable 3,6-anhydro-galactosyl donor to form the unfavored 1,2-cis-equatorial α-linkage. Using the late stage ring closure approach, an S-linked analogue of methyl neo-β-carrabioside was also realized. These compounds have applications in the identification and characterization of marine bacterial exo-α-3,6-anhydro-d-galactosidases that have specific activity on red algal neo-carrageenan oligosaccharides, such as those found in both family 127 and 129 of the glycoside hydrolases. In addition a biochemical assay using the synthesized methyl neo-β-carrabioside and the marine bacterial exo-α-3,6-anhydro-d-galactosidase ZgGH129 demonstrates that the minimum substrate unit for the enzyme is neo-β-carrabiose.

Brønsted Acid-Catalyzed Aza-Ferrier Reaction of N,O-Allenyl Acetals: Synthesis of β-Amino-α-methylene Aldehydes

A Tf₂NH-catalyzed aza-Ferrier reaction of N,O-allenyl acetals was reported. This protocol provided various types of β-amino-α-methylene aldehydes as the products. The N,O-allenyl acetal substrates were easily prepared by base-induced isomerization of N,O-propargyl acetals with Triton B. The N,O-propargyl acetals were prepared from the corresponding aldehydes or lactams. Further synthetic applications of the products were also described.

A new method testing the orthogonality of different protecting groups

A new test was elaborated to identify a new set of orthogonal protecting groups. With the developed method eight different protecting groups were tested under various deprotection conditions and the complex reaction mixtures were analysed by HPLC. The developed method allows for quick identification of orthogonality using simple model structures.

Base-mediated cascade rearrangements of aryl-substituted diallyl ethers

Two base-mediated cascade rearrangement reactions of diallyl ethers were developed leading to selective [2,3]-Wittig-oxy-Cope and isomerization-Claisen rearrangements. Both diaryl and arylsilyl-substituted 1,3-substituted propenyl substrates were examined, and each exhibits unique reactivity and different reaction pathways. Detailed mechanistic and computational analysis was conducted, which demonstrated that the role of the base and solvent was key to the reactivity and selectivity observed. Crossover experiments also suggest that these reactions proceed with a certain degree of dissociation, and the mechanistic pathway is highly complex with multiple competing routes.

Direct entry to erythronolides via a cyclic bis[allene]

The complexity and low tractability of antibiotic macrolides pose serious challenges to addressing the problem of resistance through semi- or total synthesis. Here we describe a new strategy involving the preparation of a complex yet tractable macrocycle and the transformation of this macrocycle into a range of erythronolide congeners. These compounds represent valuable sectors of erythromycinoid structure space and constitute intermediates with the potential to provide further purchase in this space. The routes are short. The erythronolides were prepared in three or fewer steps from the macrocycle, which was prepared in a longest linear sequence of 11 steps.

Synthesis of glucoconjugates of oleanolic acid as inhibitors of glycogen phosphorylase

Synthesis and biological evaluation of glucoconjugates of oleanolic acid, linked by either a triazole moiety or an ester function, as novel inhibitors of glycogen phosphorylase have been described. Several triterpene-glycoside conjugates exhibited moderate-to-good inhibitory activity against rabbit muscle GPa. Compound 12 showed the best inhibition with an IC(50) value of 1.14 microM. Structure-activity relationship (SAR) analysis of these inhibitors is also discussed. Possible binding modes of compound 12 were explored by molecular docking simulations.

Traceless chiral auxiliaries for the allene ether Nazarov cyclization

The key stereochemical factors that determine transfer of asymmetry from the chiral auxiliary to the cyclopentenone in the allene ether version of the Nazarov reaction have been elucidated. On the basis of the new insights into the mechanism, two highly effective chiral auxiliaries were designed and prepared.

Alkynyl polysaccharides: synthesis of propargyl potato starch followed by subsequent derivatizations

Potato starch propargyl ethers (PgS) with degrees of substitution (DS) from 0.1 to 2.2 have been prepared by etherification of starch with sodium hydroxide or Li dimsyl in Me(2)SO and propargyl bromide. DS values and substituent distribution were determined after hydrolysis and acetylation by GC-MS. The order of reactivity was 2>6>>3, with O-3 substitution being preferably observed in the trisubstituted units. Repeated analysis of the starch derivatives revealed that propargyl residues were lost during storage, a phenomenon that was not fully understood until now. Selected PgS were further functionalized: (a) O- and C-methylated to O-(2-butynyl)-O-methyl starch (BMS), (b) in a Mannich type reaction with diethylamine and formaldehyde to yield O-(4-diethylamino)-2-butinyl starch (DEABiS), (c) in a 1,3-dipolar cycloaddition with benzyl azide ('click-chemistry') to a N-benzyltriazole derivatized starch (BTrS), and (d) with carbon dioxide to O-(3-carboxy)-2-butinyl starch (CBiS). While the yield of carboxylation was only poor, conversion was high or nearly quantitative for reactions a-c. Thus, it is demonstrated that starch propargyl ethers are valuable intermediates for the preparation of functional polysaccharides.

Bistrimethylsilylpropargylic Ether: A Versatile Ambident Synthon to Access Substituted Allenyne Ethers and α-Substituted Bispropargylic Alcohols

The reactivity of 1,5-bis(trimethylsilyl)propargylic ethers 3 toward bases and electrophiles was investigated. Bispropargylic ethers 4, substituted allenyne ethers 6-8, and alpha-substituted bispropargylic ethers 9 were prepared in good yields, respectively, by protodesilylation, isomerization, or metalation/alkylation of bispropargylic protected alcohol 3. The ambident behavior of metalated synthon 3 was discussed and rationalized. Removal of the protecting groups of 9 easily afforded useful alpha-substituted bispropargylic alcohol.

Enhanced diastereoselectivity in beta-mannopyranosylation through the use of sterically minimal propargyl ether protecting groups

[reaction: see text] 2-O-Propargyl ethers are shown to be advantageous in the 4,6-O-benzylidene acetal directed beta-mannosylation reaction. The effect is most pronounced when the O3 protecting group is a bulky silyl ether or a glycosidic bond; however, even with a 3-O-benzyl ether, the use of a 2-O-propargyl ether results in a significant increase in diastereoselectivity. The beneficial effect of the propargyl ether is thought to be a combination of its minimal steric bulk, as determined by a measurement of the steric A-value and of its moderately disarming nature, as reflected in the pKa of propargyl alcohol. Conversely, the application of a 3-O-propargyl ether in the benzylidene acetal directed mannosylation has a detrimental effect on stereoselectivity, for which no explanation is at present available. Deprotection is achieved by base-catalyzed isomerization of the propargyl ether group to the corresponding allenyl ether, followed by oxidative cleavage with N-methylmorpholine N-oxide and catalytic osmium tetroxide.

A Hierarchy of Aryloxide Deprotection by Boron Tribromide

Aryl propargyl ethers and esters are cleaved selectively in the presence of aryl methyl ethers and esters by boron tribromide in dichloromethane. Under the same conditions, allyl ethers undergo very rapid Claisen rearrangement, and benzyl ethers are also cleaved more rapidly than propargyl. A mechanism involving intramolecular delivery of bromide to the propargyl terminus is proposed. [reaction: see text]

Mild oxidative one-pot allyl group cleavage

[reaction: see text]. A new one-pot method is described for the removal of O- and N-allyl protecting groups under oxidative conditions at near neutral pH. The allyl group undergoes hydroxylation and subsequent periodate scission of the vicinal diol, followed by repetition of this reaction sequence on the enolic form of the aldehyde intermediate.