Showing papers by "Andrey A. Tabolin published in 2023"

Nitronate-aryne cycloaddition as a concise route to stereochemically complex fused benzisoxazolines and amino alcohols.

Abstract: The reaction of cyclic nitronates (isoxazoline N-oxides and 5,6-dihydro-4H-1,2-oxazine N-oxides) with Kobayashi's aryne precursors affords tricyclic benzene-fused nitroso acetals as a result of [3 + 2]-cycloaddition. The process is regio- and stereoselective in most cases and produces the target cycloadducts possessing up to four contiguous stereogenic centers. These nitroso acetals were shown to be convenient precursors of valuable polysubstituted aminodiols through catalytic hydrogenolysis of the N-O bonds. Also, the action of protic acids resulted in an unusual fragmentation of the cyclic nitroso acetal moiety through heterolytic N-O bond cleavage and Beckmann-type reaction. Using this acid-mediated reaction, the synthesis of a hitherto unknown hexahydrobenzo[4,5]isoxazolo[2,3-a]azepine scaffold was accomplished.

[3+3]-Annulation of Cyclic Nitronates with Vinyl Diazoacetates: Diastereoselective Synthesis of Partially Saturated [1,2]Oxazino[2,3-b][1,2]oxazines and Their Base-Promoted Ring Contraction to Pyrrolo[1,2-b][1,2]oxazine Derivatives

Abstract: A rhodium(II)-catalyzed reaction of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with vinyl diazoacetates proceeds as a [3+3]-annulation producing bicyclic unsaturated nitroso acetals (4a,5,6,7-tetrahydro-2H-[1,2]oxazino[2,3-b][1,2]oxazines). Optimization of reaction conditions revealed the use of Rh(II) octanoate as the preferred catalyst in THF at room temperature, which allows the preparation of target products in good yields and excellent diastereoselectivity. Under basic conditions, namely, the combined action of DBU and alcohol, these nitroso acetals undergo ring contraction of an unsaturated oxazine ring into the corresponding pyrrole. Both transformations can be performed in a one-pot fashion, thus constituting a quick approach to oxazine-annulated pyrroles from available starting materials, such as nitroalkenes, olefins, and diazo compounds.