Diastereoselective synthesis of 3-tert-butyl-3,4-dihydropyrazolo[5,1-c][1,2,4]triazine-3,4-diyl dicarboxylates

The reactions of 3-tert-butyl-7-R¹-8-R²-pyrazolo[5,1-c][1,2,4]triazines (R¹ = H, Br; R² = Me, n-Bu) with N-bromosuccinimide in the presence of R³CO₂H (R³ = Me, t-Bu, Ph) afforded novel diastereomerically pure 3-tert-butyl-7-R¹-8-R²-3,4-dihydropyrazolo[5,1-c][1,2,4]triazine-3,4-diyl dicarboxylates. The structures of the isolated products were established on the basis of IR, ¹H, ¹³C, 2D NOESY NMR, high resolution mass spectrometry and X-ray single-crystal analysis. The steric and mechanistic origins of the observed regio- and stereoselectivity were also discussed.     

Preparation of phenyl‐modified natural rubber in latex stage

Phenyl‐modified natural rubber was prepared in latex stage by bromination of deproteinized natural rubber followed by Suzuki‐Miyaura cross‐coupling reaction. First, the bromination of natural rubber was carried out using N‐bromosuccinimide in latex stage. The bromine atom content increased as amount of N‐bromosuccinimide increased. Second, the allylic bromine atom was replaced with a phenyl group using phenyl boronic acid in the presence of a palladium catalyst, according to the Suzuki‐Miyaura cross‐coupling reaction in latex stage. The resulting products were characterized by nuclear magnetic resonance (NMR) spectroscopy. Signal at 7.13 ppm was assigned to the phenyl group of the product, while signals at 3.98, 4.14, and 4.44 ppm were assigned to the remaining allylic brominated cis‐1,4‐isoprene units. The estimated phenyl group content and the conversion of the Suzuki‐Miyaura cross‐coupling reaction were 1.32 and 23.7 mol%, respectively. Glass transition temperature (T_g) of deproteinized natural rubber increased from ?62°C to ?46.7°C, when the phenyl group was introduced into the rubber.     

Atropselective Dibrominations of a 1,1′‐Disubstituted 2,2′‐Biindolyl with Diverging Point‐to‐Axial Asymmetric Inductions. Deriving 2,2′‐Biindolyl‐3,3′‐diphosphane Ligands for Asymmetric Catalysis

On the ¹H NMR timescale, 2,2′‐biindolyls with (R)‐configured (1‐alkoxyprop)‐2‐yl, (1‐hydroxyprop)‐2‐yl, or (1‐siloxyprop)‐2‐yl substituents at C‐1 and C‐1′ are atropisomerically stable at <0 °C and interconvert at >30 °C. A 2,2′‐biindolyl (R,R)‐ 17 a of that kind and achiral (!) brominating reagents gave the atropisomerically stable 3,3′‐dibromobiindolyls (M)‐ and/or (P)‐ 18 a at best atropselectively—because of point‐to‐axial asymmetric inductions—and atropdivergently, exhibiting up to 95 % (M)‐ and as much (P)‐atropselectivity. This route to atropisomerically pure biaryls is novel and should extend to other substrates and/or different functionalizations. The dibromobiindolyls (M)‐ and (P)‐ 18 a furnished the biindolyldiphosphanes (M)‐ and (P)‐ 14 without atropisomerization. These syntheses did not require the resolution of a racemic mixture, which distinguishes them from virtually all biaryldiphosphane syntheses known to date. (M)‐ and (P)‐ 14 acted as ligands in catalytic asymmetric allylations and hydrogenations. Remarkably, the β‐ketoester rac‐ 25 c was hydrogenated trans‐selectively with 98 % ee; this included a dynamic kinetic resolution.     

Benzyl Borane NHC Adducts: Beyond B−C Bond Scission

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B−C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B−C bond are poorly investigated although several other reactive sites in benzyl–boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr−BH₂Bn [IiPr=:C{N(iPr)CH}₂, Bn=CH₂C₆H₅] is demonstrated. Dihalogenation of the BH₂ entity is observed with BCl₃ and BBr₃, whereas BI₃ either affords IiPr−BHI₂ or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH₂ group was demonstrated with elemental bromine Br₂. The brominated product IiPr−BBr₂−CHBr−C₆H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr−BI−CH(BI₃)−C₆H₅.     

Catalytic Asymmetric Bromination of Unfunctionalized Olefins with H2O as a Nucleophile

The dimeric cinchona alkaloid (DHQD)₂PHAL is used to catalyze an effective asymmetric bromohydroxylation of unfunctionalized olefins with H₂O as nucleophile an N‐bromobenzamide as a bromine source. A variety of optically active bromohydrins are formed with up to 88 % ee.     

Synthesis of 3-iodobenzo[b]thiophenes via iodocyclization/etherification reaction sequence

An efficient method was developed for the synthesis of benzo[b]thiophene core structure using a two-step, multicomponent reaction resulting in favorable yields. This convergent reaction requires room temperature thus eliminating the harsh reaction conditions usually associated with the synthesis of benzo[b]thiophene ring structures. Used in many drugs that treat conditions such as osteoporosis, asthma, and fungal infections, the benzo[b]thiophene core structure is of extreme importance to medicinal and pharmaceutical research. The key step in this process involves the use of an iodine as a cyclizing agent that acts simultaneously as a catalyst. Alcohols such as alkyl, allyl, propargyl, and benzylic are employed to prepare a diverse library of 3-iodobenzo[b]thiophenes.     

TBAI-mediated regioselective 5-exo-dig iodinative oxocyclization of 2-alkynylbenzamides for the synthesis of isobenzofuran-1-imines and isobenzofurans

Alkyne-based regioselective functionalization is an important transformation method for the synthesis of a wide variety of organic products. In this work, a regioselective TBAI-mediated oxidative 5-exo-dig iodo-oxycyclization of 2-alkynylbenzamide is used for the synthesis of various isobenzofuran derivatives with excellent functional group tolerance and high reaction efficiency. We hypothesized that using water in a mixed solvent could change the reaction pathway and realize a high reaction regioselectivity. Furthermore, the application of the developed procedure was demonstrated by the synthesis of phthalazin-1(2H)-one and aryl-substituted isobenzofurans.     

A macroporous polymer-supported cyclic anhydride for efficient sequestration of amines

The efficient removal of primary and secondary amines from organic solutions using a macroporous polymer-supported anhydride is described. The sequestering of primary amines by the anhydride via polymer-bound amide formation is completed within 2-4 h at room temperature. Secondary amines require typically 4 h for complete sequestration.     

Quantum-chemical modelling of the reactivity of charcoal surface double bonds

LCAO MO SCF calculations using the 3-21G basis were used to study the electronic and three-dimensional structure of several polycyclic aromatic hydrocarbons as models for the surface of activated charcoal. Localized carbon-carbon double bonds capable of adding electrophilic reagents were found on the periphery of these molecules. The reactivity of the peripheral C=C bonds was evaluated relative to consecutive bromination reactions of the C₅₄H₁₈ molecule. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 1, pp. 30–34, January–February, 2008.     

Iodine-mediated electrophilic cyclization of 2-alkynylbenzaldoximes leading to the formation of iodoisoquinoline N-oxides

Reaction of 2-alkynylbenzaldoximes 1 with 5 equiv of iodine in EtOH at room temperature gives the corresponding iodoisoquinoline N-oxides 2 in good to excellent yields. The cyclization proceeds very smoothly and quickly without any additives such as bases under very mild reaction conditions.