Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp³)–H to construct C–C bonds

• Hui Yu and
• Feng Xu

Beilstein J. Org. Chem. 2023, 19, 1259–1288, doi:10.3762/bjoc.19.94

• ether α-C–H bond. In the presence of Cu(II), the C(sp2)–C(sp3) coupling of pyridine N-oxides and coumarins with cyclic ethers could be achieved under mild conditions (Scheme 13) [63][64]. These reactions do not all follow the reaction mechanism of the oxidative olefination of simple ethers. The role of

Pyridine C(sp²)–H bond functionalization under transition-metal and rare earth metal catalysis

• Haritha Sindhe,
• Malladi Mounika Reddy,
• Karthikeyan Rajkumar,
• Akshay Kamble,
• Amardeep Singh,
• Anand Kumar and
• Satyasheel Sharma

Beilstein J. Org. Chem. 2023, 19, 820–863, doi:10.3762/bjoc.19.62

• cross-coupling of pyridine N-oxides with nonactivated secondary (2°) alkyl bromides [51]. The cross-coupling is difficult to achieve as the Pd-catalyzed SN2 process is sensitive towards the steric bulk of the secondary or tertiary alkyl electrophiles. The optimized conditions for the ortho-alkylation of

• pyridine N-oxides 9 with nonactivated secondary (2°) alkyl bromides 10 required 5 mol % of the Pd(OAc)2dppf catalyst, Cs2CO3 (2.0 equiv) as base in toluene at 100 °C as shown in Scheme 3. Under these conditions, the reaction provided diverse 2-alkylpyridine derivatives 11 in moderate to good yields

• organoboron coupling partners, Wu and co-workers [91] reported a protocol for the Cu-catalyzed C–H arylation of pyridine N-oxides 9 with arylboronic esters 114 and prepared C2-arylated pyridines 115 in moderate to good yields (Scheme 22). By using an inexpensive Cu catalyst, the method allows for the simple

Deoxygenative C2-heteroarylation of quinoline N-oxides: facile access to α-triazolylquinolines

• Geetanjali S. Sontakke,
• Rahul K. Shukla and
• Chandra M. R. Volla

Beilstein J. Org. Chem. 2021, 17, 485–493, doi:10.3762/bjoc.17.42

• to amination and amidation, there are few reports on metal-free C2-heteroarylation of pyridine N-oxides. In 1984, Rogers demonstrated the synthesis of 2-triazolylpyridines from 2-azidopyridines and phenylacetylene [51]. Along the same lines, Keith reported methodologies for the C2-imidazolylation and

• pyridine N-oxides [54]. Despite the versatility of these methods, the above reports involve the use of external additives for activating the N-oxides and suffer from other disadvantages, including prolonged reaction time, high temperature and limited substrate scope. At the same time, with the advent of Cu

Activated carbon as catalyst support: precursors, preparation, modification and characterization

• Melanie Iwanow,
• Tobias Gärtner,
• Volker Sieber and
• Burkhard König

Beilstein J. Org. Chem. 2020, 16, 1188–1202, doi:10.3762/bjoc.16.104

• amines, imines, amides, pyridine nitrogen and pyrrole nitrogen or as oxidized nitrogen species, e.g., pyridine-N-oxides [10]. Díaz-Terán et al. examined the surface of the samples (surface groups, chemical state of the elements, metal content and distribution) during the activation process of

Host–guest complexes of conformationally flexible C-hexyl-2-bromoresorcinarene and aromatic N-oxides: solid-state, solution and computational studies

• Rakesh Puttreddy,
• Ngong Kodiah Beyeh,
• S. Maryamdokht Taimoory,
• Daniel Meister,
• John F. Trant and
• Kari Rissanen

Beilstein J. Org. Chem. 2018, 14, 1723–1733, doi:10.3762/bjoc.14.146

• :1 v/v, no significant chemical shift changes were observed for nine of the twelve pyridine N-oxides. The above results clearly show the strong influence of DMSO in interfering with the host–guest complexation between BrC6 and the aromatic N-oxides. However, with guests such as 5 and 9, endo cavity

Cross-coupling of dissimilar ketone enolates via enolonium species to afford non-symmetrical 1,4-diketones

• Keshaba N. Parida,
• Gulab K. Pathe,
• Shimon Maksymenko and
• Alex M. Szpilman

Beilstein J. Org. Chem. 2018, 14, 992–997, doi:10.3762/bjoc.14.84

• lithium enolate followed by a second SET step to complete the transformation (Scheme 1a) [16][17]. A different approach, developed by Maulide, relies on the highly efficient umpolung of amides into enolonium species using triflic anhydride, a pyridine base and pyridine N-oxides (Scheme 1b). These

The synthesis of functionalized bridged polycycles via C–H bond insertion

• Jiun-Le Shih,
• Po-An Chen and
• Jeremy A. May

Beilstein J. Org. Chem. 2016, 12, 985–999, doi:10.3762/bjoc.12.97

• cyclopropene 97 reacted [93][94] to form the same products as the hydrazone 96 did directly when heated to 140 °C. Gold The work by the May group was soon followed by gold-promoted carbene/alkyne cascades. These cascades rely on Zhang’s discovery that the use of pyridine N-oxides allow for the formation of

A convergent, umpoled synthesis of 2-(1-amidoalkyl)pyridines

• Tarn C. Johnson and
• Stephen P. Marsden

Beilstein J. Org. Chem. 2016, 12, 1–4, doi:10.3762/bjoc.12.1

• substitution of suitably-activated pyridine N-oxides by azlactone nucleophiles, followed by decarboxylative azlactone ring-opening. The synthesis obviates the need for precious metal catalysts to achieve a formal enolate arylation reaction, and constitutes a formally ‘umpoled’ approach to this valuable class

• of bioactive structures. Keywords: azlactones; pyridines; pyridine N-oxides; substitution; Introduction Pyridines constitute the most frequently observed class of heterocycles found in pharmaceutical products [1]. As such, there is significant demand for synthetic methods that enable access both to

• expensive bespoke ligands, based upon the electrophilic activation of pyridine N-oxides and subsequent reaction with acidic carbon nucleophiles [16][17][18][19][20]. Specifically, we have demonstrated that α-pyridyl,α-alkylamino acid derivatives can be prepared in a one-pot three component coupling between

The Flögel-three-component reaction with dicarboxylic acids – an approach to bis(β-alkoxy-β-ketoenamides) for the synthesis of complex pyridine and pyrimidine derivatives

• Mrinal K. Bera,
• Moisés Domínguez,
• Paul Hommes and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2014, 10, 394–404, doi:10.3762/bjoc.10.37

• reactions of β-ketoenamides 14 and 20 with hydroxylamine hydrochloride provided the symmetric bis(pyrimidine-N-oxide) 28 in 39% yield or the mono-pyrimidine-N-oxide 30 in 54% yield (Scheme 6). The acetoxylation of 2- and 4-alkyl substituted pyridine-N-oxides by treatment with acetic anhydride is known as

• β-ketoenamides 14 and 20 with hydroxylamine hydrochloride to pyridine-N-oxides 28 and 30 and their subsequent Boekelheide rearrangements furnishing functionalized bis(pyrimidine) derivative 29 and pyrimidine/pyridine conjugate 31. Riley oxidation of bis(pyrimidine) derivative 23a and conversion of

Gold-catalyzed regioselective oxidation of propargylic carboxylates: a reliable access to α-carboxy-α,β-unsaturated ketones/aldehydes

• Kegong Ji,
• Jonathan Nelson and
• Liming Zhang

Beilstein J. Org. Chem. 2013, 9, 1925–1930, doi:10.3762/bjoc.9.227

• carboxylate; Introduction We reported in 2010 [1] that α-oxo gold carbenes could be conveniently generated as reactive intermediates in gold-catalyzed intermolecular oxidation of alkynes. By using pyridine N-oxides [1] and later 8-substituted quinoline N-oxides [2] as the external oxidants, this approach

Organocatalyzed enantioselective desymmetrization of aziridines and epoxides

• Ping-An Wang

Beilstein J. Org. Chem. 2013, 9, 1677–1695, doi:10.3762/bjoc.9.192

• enantioselectivities. In the presence of some achiral phase-transfer catalysts or phosphines such as TBAB and P(t-Bu)3, the vicinal chlorohydrins were obtained in high yields. Up to now, three classes of organocatalysts including chiral phosphoramides, chiral phosphine oxides, and chiral pyridine N-oxides were used in

• -oxides The breakthrough of the organocatalyzed enantioselective desymmetrization of meso-epoxides was the employment of chiral pyridine N-oxides as catalysts. In 2001, Fu [87] and colleagues demonstrated the utilization of selected chiral pyridine N-oxides in the enantioselective desymmetrization of meso

• -epoxides. These novel chiral pyridine N-oxides possess a plane of chirality in a ferrocenyl backbone. The steric hindrance of the Fe(η5-C5Ar5) group on catalysts (Figure 16, OC-78 to OC-80) is very crucial to the enantioselectivities of desymmetrization of meso-epoxides. The increase of steric hindrance of