Hemiacetal Research Articles

Flow Chemistry for Synthesis of 2-(C-Glycosyl)acetates from Pyranoses via Tandem Wittig and Michael Reactions.

C-Glycosyl compounds (C-glycosides) are a class of saccharide derivatives with improved stability over their O-linked counterparts. This paper reports the synthesis of several trans-2-(C-glycosyl)acetates via a tandem Wittig-Michael reaction from pyranoses (cyclic hemiacetals) using continuous flow processing, which gave improvements compared to reactions conducted in round-bottom flasks. Products were isolated in yields of >60% from reactions of benzyl-protected xylopyranoses, glucopyranoses, and galactopyranoses at higher temperatures and pressures, which were superior to yields from batch procedures. A two-step procedure involving the Wittig reaction followed by Michael reaction (intramolecular oxa-Michael) of the unsaturated ester obtained in the presence of DBU was developed. Reactions of protected mannopyranose gave low yields in corresponding reactions in flow due to competing C-2 epimerization.

Reaction engineering blocks ether cleavage for synthesizing chiral cyclic hemiacetals catalyzed by unspecific peroxygenase

Hemiacetal compounds are valuable building blocks in synthetic chemistry, but their enzymatic synthesis is limited and often hindered by the instability of hemiacetals in aqueous environments. Here, we show that this challenge can be addressed through reaction engineering by using immobilized peroxygenase from Agrocybe aegerita (AaeUPO) under neat reaction conditions, which allows for the selective C-H bond oxyfunctionalization of environmentally significant cyclic ethers to cyclic hemiacetals. A wide range of chiral cyclic hemiacetal products are prepared in >99% enantiomeric excess and 95170 turnover numbers of AaeUPO. Furthermore, by changing the reaction medium from pure organic solvent to alkaline aqueous conditions, cyclic hemiacetals are in situ transformed into lactones. Lactams are obtained under the applied conditions, albeit with low enzyme activity. These findings showcase the synthetic potential of AaeUPO and offer a practical enzymatic approach to produce chiral cyclic hemiacetals through C-H oxyfunctionalization under mild conditions.

In vitro screening of natural product-based compounds for leishmanicidal activity.

Leishmaniasis is one of the major parasitic diseases, caused by obligate intracellular protozoa Leishmania, having high mortality as well as morbidity rate. As there is no human licensed vaccine available against leishmaniasis, chemotherapy remains the major way of combating this disease. Many disadvantages are known to be associated with the current drug regime including severe side effects and toxicity, long duration and expensive treatment, and the emergence of resistance. An alternative approach is being utilized to search for active molecules using natural sources, rather than relying on synthetic drugs. Many plant-derived secondary metabolites like phenolic compounds, steroids, quinones, etc. are being extensively investigated for their anti-leishmanial potential. One such group of complex phenolic compounds are diarylheptanoids. These compounds have been shown to exhibit anti-inflammatory, anti-parasitic, anti-fungal, and other pharmacological activities. In the present study, a set of sixteen tetrahydropyran derivatives including three natural products were obtained in lyophilized form. These compounds with trans-2,6-disubstituted tetrahydropyrans, Diospongin A, Diospongin B (isolated from Dioscorea spongiosa) and Centrolobine (Centrolobium sclerophyllum) as parent compounds were synthesized by the reaction of 1-phenyl-1-triemthylsiloxyethylene with six-membered cyclic hemiacetals in the presence of iodine as a catalyst. All the sixteen synthesized tetrahydropyran derivatives were used for toxicity analysis against L. donovani promastigotes, amastigotes and THP-1-derived human macrophages. IC50 values and selectivity index were calculated for all the compounds. Out of these sixteen, five compounds showed the best effect in vitro in terms of both leishmanicidal activity and non-toxicity to human macrophages.

Triethylamine-Promoted Henry Reaction/Elimination of HNO2/Cyclization Sequence of Functionalized Nitroalkanes and 2-Oxoaldehydes: Diversity-Oriented Synthesis of Oxacycles.

The triethylamine-promoted cascade Henry reaction/elimination of HNO2/cyclization reaction of 2-oxoaldehydes with nitroalkanes bearing various remote functionalities is described. Both chiral and achiral nitroalkanes were applicable to this protocol, leading to a variety of oxacycles, such as chromenes, chromanes, cyclic hemiacetals, and polycyclic acetals. An unexpected regioselective photooxygenation occurred without sensitizer during derivatization to convert a derived diene product into a dioxetane by reaction with singlet oxygen, which provided chromen-2-one and benzaldehyde after fragmentation.

A Redox-Relay Heck Approach to Substituted Tetrahydrofurans.

An operationally simple and efficient strategy for the synthesis of substituted tetrahydrofurans from readily available cis-butene-1,4-diol is described. A redox-relay Heck reaction is used to rapidly access cyclic hemiacetals that can be directly reduced to afford the corresponding 3-aryl tetrahydrofuran. Furthermore, the hemiacetals can also serve as precursors to a range of disubstituted tetrahydrofurans, including the calyxolane natural products.

Inside Cover Picture

The alkoxy radicals derived from cyclic hemiacetals are generated through visible‐light‐induced electron transfer with NaI and Ph3P as the catalyst. Subsequent β‐scission generates carbon‐centered radicals, which are trapped by cinnamic acids, aryl alkenes, vinylboronic acid and silyl enol ether to deliver the corresponding C—C bond forming products. More details are discussed in the article by Wang et al. on page 259–264.image

Visible Light‐Promoted Ring‐Opening Alkenylation of Cyclic Hemiacetals through β‐Scission of Alkoxy Radicals

Comprehensive SummaryThe chemistry of alkoxy radicals was extensively explored during the period of 1960s to 1990s, but it has remained dormant for the past few decades. Recently, alkoxy radicals attract the attentions again, because new methods for generating alkoxy radical species have emerged. These newly developed methods are mainly based on the photolysis by visible light under mild conditions, thus allowing for new transformations of the carbon‐centered radical species that are generated from the β‐scission or hydrogen abstraction of the alkoxy radicals. Herein, we demonstrate that the alkoxy radicals derived from cyclic hemiacetals can be generated through visible‐light‐induced electron transfer with sodium iodide and triphenylphosphine as the catalyst. The alkoxy radicals subsequently undergo β‐scission to generate carbon‐centered radicals, which are trapped by cinnamic acids, aryl alkenes, vinylboronic acid and silyl enol ether to deliver the corresponding C—C bond forming products. This catalytic method for ring‐opening alkenylation reaction of cyclic hemiacetal derivatives under visible‐light irradiation conditions demonstrates the compatibility of the visible light‐promoted alkoxy radical generation method with various carbon radical trapping processes. This work opens up new possibilities for the application of alkoxy radicals in organic synthesis.

Enantioselective Michael Addition/Cyclization/Desymmetrization Sequence of Prochiral Cyclic Hemiacetals and Nitroolefins: Synthesis of Chiral Oxygen-Bridged Bicyclic Compounds.

The organocatalytic enantioselective Michael addition of functionalized prochiral cyclic hemiacetals and nitroolefins has been developed under cooperative enamine and hydrogen bond catalysis. The obtained chiral hemiacetal intermediates could be used in the subsequent diastereocontrolled cyclization/desymmetrization divergent process to access (1) 9-oxabicyclo[3.3.1]nonane or 8-oxabicyclo[3.2.1]octane frameworks via oxocarbenium ion-mediated Friedel-Crafts cyclization, and (2) 2,9-dioxabicyclo[3.3.1]nonane frameworks via intramolecular nucleophilic cyclization. Experimental results suggest that there is neighboring group participation controlling the diastereoselectivities of the desymmetrization process.

Synthesis and Ring-Chain Tautomerism of 1-(4-Ethoxyphenyl)-5-formyl-1H-1,2,3-triazole-4-carboxylic Acid: The First Representative of a 5-Formyl-1H-1,2,3-triazole-4-carboxylic Acids Series

Synthesis of the first representative of a 5-formyl-1H-1,2,3-triazole-4-carboxylic acids series – 1-(4-ethoxyphenyl)-5-formyl-1H-1,2,3-triazole-4-carboxylic acid was performed. The 1-azido-4-ethoxybenzene was chosen as a starting reagent in a two-step synthesis, which reacted with the ethyl 4,4-diethoxy-3-oxobutanoate under base catalysis to form ethyl 5-(diethoxymethyl)-1-(4-ethoxyphenyl)-1H-1,2,3-triazole-4-carboxylate with protected formyl and acid groups. By the subsequent saponification of the ester group and removing of acetal protection, the target 1-(4-ethoxyphenyl)-5-formyl-1H-1,2,3-triazole-4-carboxylic acid was obtained. It has been found that the free acid form predominated in the solution under its cyclic 6-hydroxy-1,6-dihydro-4H-furo[3,4-d][1,2,3]triazol-4-one tautomer. According to 1H NMR, cyclic hemiacetal is about 20%.

A 4-hydroxyproline/trimethyl borate system for asymmetric synthesis of triple aldols from double aldol cyclic hemiacetals

We developed a 4-hydroxyproline-catalyzed asymmetric cross-aldol reaction between aldehydes and cyclic hemiacetals derived from copper-catalyzed asymmetric double aldol reaction reported previously by our group. The addition of trimethyl borate, which acted as a ring-opening reagent for hemiacetals, was critical for producing triple aldol compounds with excellent diastereoselectivity.

Catalytic Enantioselective Synthesis of Morpholinones Enabled by Aza-Benzilic Ester Rearrangement.

Chiral morpholinone is an important building block in organic synthesis and a pharmacophore in medicinal chemistry. However, catalytic enantioselective methods for the construction of this N,O-heterocycle remain scarce. We report herein a chiral phosphoric acid-catalyzed enantioselective synthesis of C3-substituted morpholinones from aryl/alkylglyoxals and 2-(arylamino)ethan-1-ols. The reaction proceeds through a domino [4 + 2] heteroannulation followed by a 1,2-aryl/alkyl shift of the resulting cyclic α-iminium hemiacetals. It represents formally an unprecedented asymmetric aza-benzilic ester rearrangement reaction. A concise synthesis of L-742,694, a neurokinin-1 receptor antagonist, featuring this reaction is documented.

Photo-induced radical borylation of hemiacetals via C–C bond cleavage

In this study, we reported a photo-induced radical borylation of hemiacetal derivatives via C–C bond cleavage. This transformation can be realized under mild conditions with simple reaction settings and irradiation of visible light. A series of substrates, including both cyclic and linear hemiacetal derivatives, were effectively transformed to the borylation product in moderate to good yields. Finally, the mechanism was studied in detail by DFT calculations, suggesting insight of the radical borylation process.

Highly Diastereoselective Reactions of Isoxazolidine-4,5-diols with Grignard Reagents: A New Approach to anti,syn-γ-Amino-α,β-diols

AbstractAn investigation of the reaction of 3,4-trans-isoxazolidine-4,5-diols with Grignard reagents is described for the first time. Their resemblance to five-membered cyclic hemiacetals allows them to react as α-hydroxy-β-(hydroxyamino)aldehydes in a highly stereoselective manner, providing anti,syn-γ-(hydroxyamino)-α,β-diols in moderate yields and with good to excellent syn-diastereoselectivities, which can be improved by the addition of anhydrous cerium chloride. The obtained (hydroxyamino­)diols serve as suitable precursors of anti,syn-γ-amino-α,β-diols that represent valuable scaffolds for the synthesis of various biologically active compounds.

Relative contributions of selected multigeneration products to chamber SOA formed from photooxidation of a range (C10–C17) of n-alkanes under high NOx conditions

A series of chamber experiments was conducted to investigate the composition of secondary organic aerosol (SOA) following oxidation of a range of parent n-alkanes (C10–C17) in the presence of NOx. The relative contribution of selected species representing first, second, and higher generation products to SOA mass was measured using a high-resolution aerosol mass spectrometer. Gas chromatography was also used for a limited set of amenable species. Relative contributions varied substantially across the range of investigated alkanes reflecting slight changes in SOA composition. The contribution of first-generation cyclic hemiacetal is minimal toward the small end of the investigated range and gradually increases with n-alkane size. The relative contributions of second generation and higher nitrate-containing species, in contrast, decrease with an increased alkane size. A similar trend is observed for relative contribution of organonitrates to SOA. Finally, SOA yield and composition are sensitive to water vapor concentrations. This sensitivity is limited to a narrow range (dry to ~15% RH) with little, if any, impact above 15% suggesting that this impact may be negligible under ambient conditions. The impact of water vapor also appears to decrease with increasing alkane carbon number.

Insight into the effects of acid characteristics on the catalytic performance of Sn-MFI zeolites in the transformation of dihydroxyacetone to methyl lactate

To investigate the effects of the acid characteristic of Sn-MFI zeolite on catalytic performance in the transformation of dihydroxyacetone to methyl lactate, multiple characterization and experimental methods were employed. Firstly, the transformation between Lewis and Brönsted acid sites was initially confirmed by 31P-TMP NMR method, owing to the introduction of CH3OH by framework Sn species to donate protons. It is found that Lewis acid sites are cooperative with Brönsted acid sites to boost the primary reactions of dihydroxyacetone (DHA) to methyl lactate (ML). Yet, too high ratio of Brönsted acid sites to Lewis acid sites favors the generation of side product pyruvic aldehyde dimethyl acetal (PADA). That is because Brönsted acid sites majorly catalyze the dehydration of DHA, the addition of pyruvaldehyde (PA) with methanol and the addition of hemiacetal (HA) with methanol to PADA, whereas Lewis acidic framework Sn sites are responsible for the isomerization of HA to ML, which can be further verified by the isotopic labeling experiments. Therefore, this study provides a definite and systematical insight on the impacts of acid property on product selectivity.

Photoinduced Fragmentation Borylation of Cyclic Alcohols and Hemiacetals.

A visible-light photoinduced fragmentation borylation of O-phthalimido cycloalkanols with bis(catecholato)diboron is described. Structurally diverse keto and formyloxy alkyl boronic esters are shown to be conveniently prepared by radical-mediated ring opening of cyclic alcohols and hemiacetals, respectively. The reactions proceed under mild conditions in the absence of additives or photocatalysts, display excellent functional group tolerance, and are shown to allow cleavage of 4-, 5-, 6-, and 7-membered ring substrates. The mechanism proceeds via sequential homolytic N-O and C-C bond cleavages, the latter of which involves β-scission of an alkoxy radical, generating a carbonyl and an alkyl radical that is trapped by the diboron reagent. Spectroscopic studies suggest direct photoexcitation of either the phthalimide or diboron substrates with blue light can initiate a radical chain mechanism.

Organocatalytic Cascade Reaction of Aliphatic Enals and Benzoylnitromethane: Synthesis of Enantioenriched Tetrasubstituted Cyclohexene Carbaldehyde

A new example of the reactivity of enals with benzoylnitromethane was studied in a Michael-Michael-Aldol-Dehydration quadruple organocascade reaction. The reaction unexpectedly yielded a tetrasubstituted cyclohexene carbaldehyde with excellent enantiomeric excess when crotonaldehyde was used as the Michael-acceptor, whereas using (E)-Hex-2-enal as the Michael-acceptor formed a cyclic hemiacetal by steering the reaction into the intramolecular formation of the same intermediate via a Michael-Heterocyclization domino reaction.

Visible-Light-Promoted Ring-Opening Alkynylation, Alkenylation, and Allylation of Cyclic Hemiacetals through β-Scission of Alkoxy Radicals.

The alkoxy radicals that are derived from cyclic hemiacetals have been generated through the visible-light-promoted reaction of the corresponding N-alkoxyphthalimides with Hantzsch ester as the reductant. The alkoxy radicals subsequently undergo β-scission of the C-C bond to generate carbon-centered radicals, which are trapped by alkynyl-, alkenyl-, or allylsulfones.

Amine-tethered phenylboronic acid-enabling ring-opening strategy for carbon chain elongation from double aldol cyclic hemiacetals.

The addition of carbon nucleophiles to cyclic hemiacetal forms of double aldols is a promising approach toward the synthesis of structurally attractive 1,3-polyol derivatives. Cyclic hemiacetals are generally unreactive to carbon nucleophiles under neutral conditions, however, because the electrophilic aldehyde function is masked. Here we developed an amine-tethered phenylboronic acid 7g, which transforms double aldol cyclic hemiacetals to ring-opened linear aldehydes. Combined with the previously-developed copper-catalysed asymmetric double aldol reaction (L. Lin, K. Yamamoto, H. Mitsunuma, Y. Kanzaki, S. Matsunaga and M. Kanai, J. Am. Chem. Soc., 2015, 137, 15418), this method produced synthetically useful chiral building blocks containing a 1,3-di- or tri-ol moiety.

Asymmetric Organocatalytic Sequential Reaction of Structurally Complex Cyclic Hemiacetals and Functionalized Nitro-olefins To Synthesize Diverse Heterocycles

Structurally complex cyclic hemiacetals bearing a racemic tetrasubstituted stereocenter have been prepared in a concise manner and were successfully used in an organocatalytic enantioselective sequence to react with functionalized nitro-olefins, providing bicyclic acetal-containing compounds as two separable epimers with excellent stereoselectivity. The reaction showed broad substrate scope, with respect to the starting hemiacetals. Moreover, this protocol allows the synthetic transformation of the products to various interesting heterocyclic compounds with substantial structural diversity and broad functionality.