Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides

3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-alpha-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in the facial selectivity of the epoxidation reaction. Thus, the free HO-2 in 3 (the 6-O-acetyl derivative of 5) directs the attack of m-chloroperbenzoic acid from the more hindered alpha face of the molecule to give 2-propyl 6-O-acetyl-3,4-anhydro-alpha-D-allopyranoside (7) accompanied by the beta epoxide 6 as a very minor product. Reverse diastereoselectivity has been obtained when the HO-2 in 3 was substituted by a bulky tert-butyldimethylsilyl (TBS) group. In this case, the major isomer was the 2-O-TBS derivative of 6 (alpha-D-galacto configuration). The ring-opening of sugar epoxides by nucleophilic per-O-acetyl-1-thio-beta-D-glucopyranose (11) was employed as a convenient approach to the synthesis of (1-->3)- and (1-->4)-thiodisaccharides. For example, ring-opening of the oxirane 7 by 11 led to the expected regioisomeric per-O-acetyl thiodisaccharides beta-D-Glc-S-(1-->3)-4-thio-alpha-D-Glc-O-iPr (12) and beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Gul-O-iPr (13). Regioselectivity in the construction of the (1-->4)-thioglycosidic linkage could be achieved by hindering C-3 of the 3,4-anhydro sugar with a bulky silyloxy group at the vicinal C-2. For instance, coupling of the 2-O-TBS derivative of 7 with 11 led regioselectively to the protected thiodisaccharide beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Glc-O-iPr (27). The utility of the approach was demonstrated through the synthesis of sulfur-linked analogues of naturally occurring (laminarabiose and cellobiose) and non-natural disaccharides (i.e., beta-D-Glc-(1-->4)-alpha-D-Gul).     

Ultrasound mediated efficient ring opening of epoxides by in situ generated dithiocarbamates in green reaction media

Ultrasound accelerated a safe, clean and environmentally benign one-pot, three-component synthesis of 2-hydroxy dithiocarbamate, from amine, carbon disulfide and epoxide in good to excellent yields and short reaction time with excellent chemo-, regio-, and stereoselectivities in green reaction media (water, deep eutectic solvent [DES] and polyethylene glycol [PEG]). The one-pot, three-component condensation of primary and secondary amine with carbon disulfide and epoxides under ultrasonic irradiation was developed as a green and fast protocol for the rapid high-yielding preparation of 2-hydroxy dithiocarbamate in green reaction media without organic solvent and tedious workup.     

Tributylphosphine-catalyzed ring-opening reaction of epoxides and aziridines with acetic anhydride

Reactions of aziridines and epoxides with acetic anhydride catalyzed by an organophosphine provided the corresponding esters of β-amino alcohols and 1,2-diols in high yields, respectively.     

Total synthesis of actinomycin D(C1) via a ring-opening reaction of aziridine

Actinomycin D(C₁) has been synthesized by a route involving the ester formation between two peptide fragments, (2S,3S)-1-(2-nitro-3-benzyloxy-4-methylbenzoyl)-3-methyl-2-aziridine-carbonyl-D-valylproline t-butyl ester and N-benzyloxycarbonylsarcosyl-N-methylvaline, via a ring-opening reaction of aziridine. Cyclization, followed by reduction and oxidation, gave actinomycin D(C₁). The synthetic actinomycin D(C₁) was indistinguishable from natural substance with respect to physical properties and biological activity.     

Efficient ring-opening reaction of epoxides and aziridines promoted by tributylphosphine in water

Tributylphosphine was found to be an effective promoting reagent for the ring-opening reaction of various epoxides and aziridines with nucleophile to produce corresponding anti-bifunctional products in moderate to excellent yields in water.     

Efficient synthesis of chlorohydrins: ionic liquid promoted ring-opening reaction of epoxides and TMSCl

The environmentally benign, highly efficient synthesis of chlorohydrins by cleavage of epoxides using TMSCl in ionic liquid, bmimPF₆, has been studied.     

Regioselective hydrostannation of activated alkynes catalyzed by in situ generated copper hydride

The hydrostannation of activated alkynes proceed in good yields and high regioselectivities catalyzed by CuCl, in the presence of potassium tert-butoxide, triphenylphosphine, and tributylstannane, by generating copper hydride in situ.     

Gas-phase Meerwein reaction of epoxides with protonated acetonitrile generated by atmospheric pressure ionizations

Ethylnitrilium ion can be generated by protonation of acetonitrile (when used as the LC-MS mobile phase) under the conditions of atmospheric pressure ionizations, including electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) as well as atmospheric pressure photoionization (APPI). Ethylnitrilium ion (CH₃ - C o mathop N⁺ HCH_3 - C equiv mathop Nlimits^ + H and its canonical form CH₃ - mathop C⁺ = NHCH_3 - mathop Climits^ + = NH) is shown to efficiently undergo the gas-phase Meerwein reaction with epoxides. This reaction proceeds by the initial formation of an oxonium ion followed by three-to-five-membered ring expansion via an intramolecular nucleophilic attack to yield the Meerwein reaction products. The density functional theory (DFT) calculations at the B3LYP/6-311 + G(d,p) level show that the gas-phase Meerwein reaction is thermodynamically favorable. Collision-induced dissociation (CID) of the Meerwein reaction products yields the net oxygen-by-nitrogen replacement of epoxides with a characteristic mass shift of 1 Da, providing evidence for the cyclic nature of the gas-phase Meerwein reaction products. The gas-phase Meerwein reaction offers a novel and fast LC-MS approach for the direct analysis of epoxides that might be of genotoxic concern during drug development. Understanding and utilizing this unique gas-phase ion/molecule reaction, the sensitivity and selectivity for quantitation of epoxides can be enhanced.     

Regioselective intramolecular oxidation of phenols and anisoles by dioxiranes generated in situ

A novel method for regioselective oxidation of phenols and anisoles has been developed in which dioxiranes, generated in situ from ketones and Oxone, oxidize phenol derivatives in an intramolecular fashion. A series of ketones with electron-withdrawing groups, such as CF(3), COOMe, and CH(2)Cl, were attached to phenols, anisoles, or aryl rings via a C(2) or C(3) methylene linker. In a homogeneous solvent system of CH(3)CN and H(2)O, oxidation of phenol derivatives 1-10 afforded spiro 2-hydroxydienones in 24-55% yields regardless of the presence of other substituents (ortho Me, meta Me or Br) on the aryl ring and the length of the linker. Experimental evidences were provided to support the mechanism that involves a regioselective pi bond epoxidation of aryl rings followed by epoxide rearrangement and hemiketal formation.     

Ruthenium-catalyzed regioselective synthesis of 2-substituted indoles via ring-opening of epoxides by anilines

Anilines react with epoxides in dioxane at 180°C in the presence of a catalytic amount of a ruthenium catalyst along with tin(II) chloride to afford 2-substituted indoles in moderate to good yields.     

N-acylaminophenylcyclopropanes in reaction with nitrous acid generated in situ

The reaction of N-acylaminophenylcyclopropanes with HNO₂ proceeds regioselectively with introduction of an N = O fragment into the three-membered ring and formation of the corresponding Δ²-isoxazolines. For ortho-substituted N-acylaminophenylcyclopropanes side processes were observed, caused by the intramolecular participation of the N-acyl group in conversions of the carbenium ions formed on opening the cyclopropane ring under the action of the nitrosating reagent, and by direct insertion of the modified ortho substituent into the three-membered ring.     

Ultrafast organocatalytic ring-opening polymerization of N-sulfonyl aziridine in the melt

An ultrafast approach for controlled synthesis of well-defined polysulfonamides is established through organocatalytic anionic ring-opening polymerization (ROP) of N-sulfonyl aziridine in the melt. Several different organobases are investigated, and it is found that N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) catalyzed ROP of 2-methyl-N-tosylaziridine (TsMAz) gives the desired polymer, while 1,4-diazabicyclo[2.2.2]octane (DABCO) and 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) initiate the polymerization along with initiator to produce uncontrolled polymers. Using PMDETA as the catalyst, poly(2-methyl-N-tosylaziridine) with molecular weight over 100 kg/mol can be synthesized in less than 90 s. Various initiators, including carboxylic acid, N-sulfonyl amide, unactivated amine, phenol, and thiol, are applicable for this protocol to give the molecular weight and end-group controlled polymers under the open-flask condition. Combining this ultrafast ROP with ring-opening metathesis polymerization (ROMP), a brush copolymer is facile synthesized. This approach allows the ultrafast metal-free synthesis of polysulfonamide and expands the scope of initiators for the ROP of N-sulfonyl aziridines.     

Copper(II) tetrafluoroborate catalyzed ring-opening reaction of epoxides with alcohols at room temperature

[reaction: see text] Efficient ring opening of different epoxides by reaction with representative alcohols is presented. These processes were carried out at room temperature and rely on the usefulness of commercial copper tetrafluoroborate as catalyst.     

Chemo- and stereoselectivity in titanium-mediated regioselective ring-opening reaction of epoxides at the more substituted carbon

Chemo- and stereoselectivity in the ring-opening reaction of epoxides with a reagent prepared from allylmagnesium halide and chlorotitanium triphenoxide is described. It has been proven that the allylating reagent can also be used for the reaction of epoxides bearing a tert-butyl ester, amide, or acetal moiety, and that the epoxide cleavage regioselectively takes place at the more substituted carbon in all cases. Interestingly, while the reaction of acyclic 2,2,3-trialkyl epoxides or 3,3-disubstituted 2,3-epoxy alcohol derivatives with the allyltitanium reagent yielded the allylated products as an almost 1:1 diastereomixture, the ring-opening reaction of 2-substituted 2,3-epoxy alcohol derivatives stereospecifically proceeded through the anti pathway. The latter reaction is extremely useful for asymmetric construction of quaternary carbon centers.     

A convergent total synthesis of ustiloxin D via an unprecedented copper-catalyzed ethynyl aziridine ring-opening by phenol derivatives

[reaction: see text] The ustiloxins are a family of heterodetic cyclopeptides that have been isolated from the water extracts of false smut balls on the panicles of rice plants caused by the fungus Ustilaginoidea virens. A concise total synthesis of ustiloxin D has been achieved via an unprecedented ethynyl aziridine ring-opening of phenol derivatives. The longest linear sequence of the synthesis is 15 steps from commercially available compounds.     

The role of early transition metal azides as catalysts of the ring-opening reaction of epoxides with trimethylsilylazide

Complexes of early transition metals (titanium, vanadium, …) catalyse the reaction of epoxides with Me₃SiN₃. When an azido complex is used as precursor, the reaction follows an overall second-order law and the rate constant is proportional to the complex concentration. The regioselectivity (α- or β-cleavage of the ring) is compared to that of the stoichiometric reaction of the transition metal azide with epoxides. It is concluded that the catalytic mechanism involves the intermediary formation of a complex resulting from the nucleophilic attack of the epoxide ring by the N₃ group bonded to the transition metal atom.     

Microwave assisted ring-opening of epoxides with N-biaryl sulfonamides in the synthesis of matrix metalloproteinase-9 inhibitors

A microwave accelerated epoxide ring-opening process with N-biaryl sulfonamides is described. Under this mild, highly efficient condition, an α-hydroxy-β-N-biaryl sulfonamide skeleton is rapidly assembled leading ultimately to a novel series of matrix metalloproteinase-9 inhibitors with single digit nanomolar activities.     

Synthesis of δ-thiolactams by the aza-Diels-Alder reaction of in situ generated allenyltrimethylsilylthioketenes with imines

Allenyltrimethylsilylthioketenes, generated in situ through [3,3] sigmatropic rearrangement of trimethylsilylethynyl propargyl sulfides, underwent facile [4+2] cycloaddition with imines to afford the corresponding δ-thiolactams. The resulting 2-trimethylsilyl-4-methylenetetrahydroquinolidine-2-thione, obtained by the [4+2] cycloaddition using piperideine as a dienophile, was transformed into (±)-lupinine in six steps.     

Efficient synthesis of fluorophosphonylated alkyles by ring-opening reaction of cyclic sulfates

Ring-opening reactions of functionalized 1,2-cyclic sulfates and oxetanes with the phosphonodifluoromethyl carbanion are reported. This approach allows an easy access to fluorinated beta-hydroxyphosphonates that are building blocks in the synthesis of acyclic nucleosides. Synthesis of precursors of nucleoside phosphorylase inhibitors from these alcohols is described.