K2S2O8-promoted rearrangement of nitrones for the synthesis of benzo[d]oxazoles

An efficient route for the synthesis of valuable benzoxazoles has been developed through self-oxidative cyclization with N–O bond cleavage.

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.

The LANCA three-component reaction to highly substituted β-ketoenamides - versatile intermediates for the synthesis of functionalized pyridine, pyrimidine, oxazole and quinoxaline derivatives

The LANCA three-component reaction of lithiated alkoxyallenes LA, nitriles N and carboxylic acids CA leads to β-ketoenamides KE in good to excellent yields. The scope of this reaction is very broad and almost all types of nitriles and carboxylic acids have successfully been used. The alkoxy group introduced via the allene component is also variable and hence the subsequent transformation of this substituent into a hydroxy group can be performed under different conditions. Enantiopure nitriles or carboxylic acids can also be employed leading to chiral KE with high enantiopurity and dinitriles or dicarboxylic acids also lead to the expected bis-β-ketoenamides. β-Ketoenamides incorporate a unique combination of functional groups and hence a manifold of subsequent reactions to highly substituted heterocyclic compounds is possible. An intramolecular aldol-type condensation reaction efficiently furnishes pyridin-4-ols PY that can be further modified by palladium-catalyzed reactions, e.g., to specifically substituted furopyridine derivatives. Condensations of β-ketoenamides with ammonium salts or with hydroxylamine hydrochloride afford pyrimidines PM or pyrimidine N-oxides PO with a highly flexible substitution pattern in good yields. The functional groups of these heterocycles also allow a variety of subsequent reactions to various pyrimidine derivatives. On the other hand, acid-labile alkoxy substituents such as a 2-(trimethylsilyl)ethoxy group are required for the conversion of β-ketoenamides into 5-acetyl-substituted oxazoles OX, again compounds with high potential for subsequent functional group transformations. For acid labile β-ketoenamides bearing bulky substituents the acid treatment leads to acylamido-substituted 1,2-diketones DK that could be converted into quinoxalines QU. All classes of heterocycles accessed through the key β-ketoenamides show a unique substitution pattern - not easily accomplishable by alternative methods - and therefore many subsequent reactions are possible.

Design of a Mesoscale Continuous-Flow Route toward Lithiated Methoxyallene

The unique nucleophilic properties of lithiated methoxyallene allow for C-C bond formation with a wide variety of electrophiles, thus introducing an allenic group for further functionalization. This approach has yielded a tremendously broad range of (hetero)cyclic scaffolds, including precursors to active pharmaceutical ingredients. To date, however, its valorization at scale is hampered by the batch synthesis procedure, which suffers from serious safety issues. Hence, the attractive heat- and mass-transfer properties of flow technology were exploited to establish a mesoscale continuous-flow route toward lithiated methoxyallene. An excellent conversion of 94 % was obtained, corresponding to a methoxyallene throughput of 8.2 g h^-1 . The process is characterized by short reaction times, mild reaction conditions and a stoichiometric use of reagents.

Synthesis of substituted oxazoles via Pd-catalyzed tandem oxidative cyclization

An easy approach to access multisubstituted oxazoles via tandem oxidative cyclization is developed.

Synthesis of highly functionalized 2,2'-bipyridines by cyclocondensation of β-ketoenamides - scope and limitations

The scope of a flexible route to unsymmetrically functionalized bipyridines is described. Starting from 1,3-diketones 1a–e, the corresponding β-ketoenamines 2a–e were converted into different β-ketoenamides 3a–g by N-acylation with 2-pyridinecarboxylic acid derivatives. These β-ketoenamides were treated with a mixture of TMSOTf and Hünig’s base to promote the cyclocondensation to 4-hydroxypyridine derivatives. Their immediate O-nonaflation employing nonafluorobutanesulfonyl fluoride provided the expected 4-nonafloxy-substituted bipyridine derivatives 5a–g in moderate to good overall yields. The bipyridyl nonaflates are excellent precursors for palladium-catalyzed reactions as demonstrated by representative Suzuki and Sonogashira couplings. Thus, a library of specifically substituted bipyridine derivatives was generated, showing the versatility of the simple 1,3-diketone-based approach to this important class of ligands.

Gold-catalyzed tandem reactions of amide-aldehyde-alkyne coupling and cyclization-synthesis of 2,4,5-trisubstituted oxazoles

We report the first cationic gold(i)-catalyzed one-pot reaction of amide, alkyne and aldehyde followed by cyclization, to successfully access highly substituted oxazoles derivatives in good yields. A single catalyst allows the occurring of this multi-step reaction atom- and step-economically, with water as the only theoretical side product.

Dense or porous packing? Two-dimensional self-assembly of star-shaped mono-, bi-, and terpyridine derivatives

The self-assembly behavior of five star-shaped pyridyl-functionalized 1,3,5-triethynylbenzenes was studied at the interface between an organic solvent and the basal plane of graphite by scanning tunneling microscopy. The mono- and bipyridine derivatives self-assemble in closely packed 2D crystals, whereas the derivative with the more bulky terpyridines crystallizes with porous packing. DFT calculations of a monopyridine derivative on graphene, support the proposed molecular model. The calculations also reveal the formation of hydrogen bonds between the nitrogen atoms and a hydrogen atom of the neighboring central unit, as a small nonzero tunneling current was calculated within this region. The title compounds provide a versatile model system to investigate the role of multivalent steric interactions and hydrogen bonding in molecular monolayers.

Palladium-catalyzed imidoylative cyclization of α-isocyanoacetamides: efficient access to C2-diversified oxazoles

A novel procedure for the synthesis of C2-diversified oxazoles, through palladium-catalyzed imidoylative cyclization of α-isocyanoacetamides with aryl, vinyl, alkynyl halides, or triflates, was developed. Migratory insertion of isocyanide into a Csp3-palladium(II) intermediate in a cascade process was also realized, generating alkyl-substituted oxazoles. Therefore, oxazoles functionalized at the C2 position with sp, sp(2), and sp(3) hybridized carbon atoms are accessible by applying this method.

Alkoxyallenes as building blocks for organic synthesis

Alkoxyallenes are unusually versatile C3 building blocks in organic synthesis. Hence this tutorial review summarizes the most important transformations, including subsequent reactions and their applications in the synthesis of relevant compounds, e.g. natural products. The reactivity patterns involved and the synthons derived from alkoxyallenes are presented. Often alkoxyallenes can serve as substitutes of acrolein or acrolein acetals, utilisation of which has already led to interesting products. Most important is the use of lithiated alkoxyallenes which smoothly react with a variety of electrophiles and lead to products with unique substitution patterns. The heterocycles or carbocycles formed are intermediates for the stereoselective synthesis of natural products or for the preparation of other structurally relevant compounds. The different synthons being put into practice by the use of lithiated alkoxyallenes in these variations will be discussed.

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

An extension of the substrate scope of the Flögel-three-component reaction of lithiated alkoxyallenes, nitriles and carboxylic acids is presented. The use of dicarboxylic acids allowed the preparation of symmetrical bis(β-ketoenamides) from simple starting materials in moderate yields. Cyclocondensations of these enamides to 4-hydroxypyridine derivatives or to functionalized pyrimidines efficiently provided symmetrically and unsymmetrically substituted fairly complex (hetero)aromatic compounds containing up to six conjugated aryl and hetaryl groups. In addition, subsequent functionalizations of the obtained heterocycles by palladium-catalyzed couplings or by oxidations are reported. We also describe the simple synthesis of a structurally interesting macrocyclic bispyrimidine derivative incorporating a 17-membered ring, whose configuration was elucidated by DFT calculations and by subsequent reactions.

Synthesis of substituted oxazoles from enamides

2,5- and 2,4,5-substituted oxazoles were prepared in one-pot by the NBS/Me₂S-mediated cyclization of enamides in the presence of base.