Efficient cleavage of the N-O bond of 3,6-dihydro-1,2-oxazines mediated by some alpha-hetero substituted carbonyl compounds in mild conditions

Base-Mediated Fragmentation of Bicyclic Dihydro-3,6-oxazines: Transformation of Nitroso Diels-Alder Cycloadducts

Nitroso Diels-Alder cycloadditions of benzene oxide with various acyl-nitroso derivatives are described. Treatment of these cycloadducts with methyllithium results in a fast fragmentation reaction, leading to highly functionalized cyclic amino alcohols. The mechanism of the reaction and the role of the epoxide in the fragmentation process are investigated. The reaction proceeds via the formation of an unsaturated imine, which tautomerizes to an enamine if no neighboring epoxide is present.

d0a3 Synthon Equivalents for the Stereocontrolled Synthesis of Functionalized 1,4-Amino Alcohol Precursors

This study demonstrates the design, synthesis, and first application of novel aminooxylating reagents that serve as a useful d⁰a³ synthon equivalents for organic synthesis. It has been demonstrated that their reaction with α,β-unsaturated aldehydes performed under aminocatalytic conditions provides a straightforward access to polysubstituted tetrahydro-1,2-oxazine derivatives, direct precursors of functionalized 1,4-amino alcohols. Target tetrahydro-1,2-oxazine derivatives have been obtained with high yields (up to 89%) and with excellent stereocontrol (up to >20:1 dr, > 99.5:0.5 er). Furthermore, the synthetic usefulness of tetrahydro-1,2-oxazines obtained has been demonstrated in various selective transformations.

Intramolecular Inverse Electron-Demand [4 + 2] Cycloadditions of Ynamides with Pyrimidines: Scope and Density Functional Theory Insights

4-Aminopyridines are valuable scaffolds for the chemical industry in general, from life sciences to catalysis. We report herein a collection of structurally diverse polycyclic fused and spiro-4-aminopyridines that are prepared in only three steps from commercially available pyrimidines. The key step of this short sequence is a [4 + 2]/retro-[4 + 2] cycloaddition between a pyrimidine and an ynamide, which constitutes the first examples of ynamides behaving as electron-rich dienophiles in [4 + 2] cycloaddition reactions. In addition, running the ihDA/rDA reaction in continuous mode in superheated toluene, to overcome the limited scalability of MW reactions, results in a notable production increase compared to batch mode. Finally, density functional theory investigations shed light on the energetic and geometric requirements of the different steps of the ihDA/rDA sequence.

Fe-Catalyzed reductive NO-bond cleavage – a route to the diastereoselective 1,4-aminohydroxylation of 1,3-dienes

We describe the iron-catalyzed reductive cleavage of N–O-bonds in oxazines using malononitrile as reductant, which – in sequence with a [4 + 2]-cycloaddition – can be considered as an efficient way for the diastereoselective 1,4-aminohydroxylation of 1,3-dienes.

Ruthenium-catalyzed [2+2] cycloaddition reactions of a 2-oxa-3-azabicyclo[2.2.1]hept-5-ene with unsymmetrical alkynes

Ruthenium-catalyzed [2+2] cycloaddition reactions of a 2-oxa-3-azabicyclo[2.2.1]hept-5-ene with unsymmetrical alkynes were investigated. Yields of up to 90% were obtained though regioselectivity was modest. Select cycloadducts could be separated and used to access a highly functionalized [3.2.0] bicyclic structure through reductive cleavage of the N–O bond. These ring-opened products displayed a chemical exchange phenomenon in 1D carbon NMR and required characterization by 2D NMR techniques.

The nitrosocarbonyl hetero-Diels-Alder reaction as a useful tool for organic syntheses

Organic transformations that result in the formation of multiple covalent bonds within the same reaction are some of the most powerful tools in synthetic organic chemistry. Nitrosocarbonyl hetero-Diels-Alder (HDA) reactions allow for the simultaneous stereospecific introduction of carbon-nitrogen and carbon-oxygen bonds in one synthetic step, and provide direct access to 3,6-dihydro-1,2-oxazines. This Review describes the development of the nitrosocarbonyl HDA reaction and the utility of the resulting oxazine ring in the synthesis of a variety of important, biologically active molecules.

Ring-rearrangement metathesis of nitroso Diels-Alder cycloadducts

Strained nitroso Diels-Alder bicyclo[2.2.1] or [2.2.2] adducts functionalized with alkene side chains of diverse length undergo a ring-rearrangement metathesis process with external alkenes and Grubbs II or Hoveyda-Grubbs II ruthenium catalysts, under microwave irradiation or classical heating, to deliver cis-fused bicycles of various ring sizes, which contain a N-O bond. These scaffolds are of synthetic relevance for the generation of molecular diversity and to the total synthesis of alkaloids. The observation of unexpected reactions, such as epimerization or one-carbon homologation of the alkene side chain, is also reported.

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids were investigated. The use of [RhCl(COD)](2), (+/-)-BINAP, and NaHCO(3) in MeOH at 60 degrees C was found to be the optimized conditions for the ring-opening reactions to give the 1,2-ring-opened products. Various arylboronic acids were examined, and low to moderate yields were obtained and stereoselectivities (trans/cis) from 50:50 to 100:0 were observed.