PPh₃-Mediated [4 + 2]- and [4 + 1]-Annulations of Maleimides with Azoalkenes: Access to Fused Tetrahydropyridazine/Pyrrolidinedione and Spiro-dihydropyrazole/Pyrrolidinedione Derivatives

Abstract

Unprecedented PPh-mediated [4 + 2]- and [4 + 1]-annulation of maleimides with in situ formed azoalkenes have been successfully developed, affording fused tetrahydropyridazine/pyrrolidinedione and spiro-dihydropyrazole/pyrrolidinedione derivatives in good yields under mild reaction conditions. Maleimides serve as C2 synthons in the [4 + 2]-annulation using 1,2-dichloroethane as the solvent in the presence of 20 mol % of PPh. With a stoichiometric amount of PPh in acetone, maleimides serve as C1 synthons, and the in situ formed phosphorus ylide is the key intermediate to realize this [4 + 1]-annulation.

Visible Light Mediated Photocatalytic N-Radical Cascade Reactivity of γ,δ-Unsaturated N-Arylsulfonylhydrazones: A General Approach to Structurally Diverse Tetrahydropyridazines

“…Reactions were monitored by TLC using silica-gel on TLC-PET foils Sigma Aldrich, 2-25 µm, layer thickness 0.2 mm, medium pore diameter 60 Å. 1 H and 19 F NMR spectra were recorded at NMR Jeol ECZR 600 MHz, 13 C NMR spectra at 150 MHz, in CDCl3 or d 6 -DMSO. 68 Data were reported as follows: chemical shifts in ppm from Me4Si as an internal standard, integration, multiplicity, coupling constants (Hz); and assignment.…”

“…68 Data were reported as follows: chemical shifts in ppm from Me4Si as an internal standard, integration, multiplicity, coupling constants (Hz); and assignment. 13 C NMR spectra were measured with complete proton decoupling. DEPT experiments were carried out with a DEPT 135 sequence.…”

“…10 Moreover, they are versatile intermediates in organic synthesis. 11 As a consequence, many efforts have been dedicated to new strategies for the construction of these nitrogen heterocycles: among those, [4 + 2] cycloaddition of azoalkenes, [12][13][14][15] 1,2-diaza-1,3dienes, 9,16 or diazenes, 17 and [3 + 3] formal cycloadditions 18 exploiting cyclopropyl derivatives and hydrazones. 19,20 Nevertheless, the need for available starting materials and a good compatibility of reaction conditions with several functional groups has been the driving force for the use of other cyclization strategies.…”

Recent Advances in Phosphine‐Promoted (4 + 1) Annulation Reactions

“…In 2017, Wang and co‐workers reported a similar PPh 3 ‐mediated (4 + 1) annulation of maleimides with in situ formed azoalkenes . The α‐halo N ‐acyl hydrazones 85 were used as the precursors of azoalkenes, which allowed the formation of highly functionalized spiro‐dihydropyrazole/pyrrolidinedione derivatives 86 in high yields with exclusive chemoselectivity.…”

Unexpected Synthesis of 2,3,5,6‐Tetrahydro‐1H‐pyrrolo[3,4‐c]pyridine‐1,3,6‐triones by a Double Michael Addition/CS2 Extrusion/Double Cyclization Sequence

“…The utility and versatility of 1,2‐diaza‐1,3‐dienes (DDs) in the construction of a variety of heterocyclic rings are well known and are demonstrated by significant activity in this field over recent years . In particular, they can react as Michael acceptors in conjugated additions, and they can also be employed in cycloaddition reactions with a wide range of partners , …”