Regio- and Diastereoselective [3 + 2]-Spiroannulation of Benzoxazines with Chalcones: A Rh(III)-Catalyzed Redox-Neutral Approach to α-Aroyl Spiro-Indanamines

Abstract

We report an atom-economic Rh­(III)-catalyzed [3 + 2]-spiroannulation reaction between cyclic ketimines and α,β-unsaturated carbonyl compounds, allowing the synthesis of novel spirocycles with concomitant generation of three stereogenic centers in one pot. The reaction does not require any silver additives or external oxidants and is believed to proceed in a redox-neutral manner. A broad substrate scope with good functional group tolerance permitted the synthesis of a vast spectrum of spirocyclic 1,4-benzoxazine derivatives containing polysubstituted α-aroyl-indanamines in good to excellent yields with high diastereoselectivity.

C–H activation-initiated spiroannulation reactions and their applications in the synthesis of spirocyclic compounds

“…Also the ratios of different isomers are provided in brackets in Scheme 44. 39 This cascade reaction begins with C-H bond cleavage to form a rhodacycle intermediate. Subsequently, coordination with chalcone and migratory insertion followed by intramolecular 1,2-addition into the iminyl moiety and final proto-demetalation furnish the spiroannulated product (Scheme 45).…”

Substrate‐Controlled Product Divergence in Iron(III)‐Catalyzed Reactions of Propargylic Alcohols: Easy Access to Spiro‐indenyl 1,4‐Benzoxazines and 2‐(2,2‐Diarylvinyl)quinoxalines

“…Scrutiny of the literature reveals that though a plethora of methods exist for the synthesis of 1,4‐benzoxazines, [5] methods for the synthesis of spiro‐benzoxazines [4e,6] have been limited in number. In particular, reports [4e,6a–d] for the synthesis of spiro‐indenyl 1,4‐benzoxazines are few which rely on the use of costly Rh III ‐catalyzed C−H functionalization/annulation cascade reactions on pre‐functionalized 1,4‐benzoxazines (Scheme 1a). Therefore, the development of alternative cost‐economic methods to synthesize them from simple substrates, preferably based on concurrent formations of 1,4‐benzoxazine and fused indene ring in one pot would be worthwhile.…”

Rhodium(iii)-catalyzed intermolecular [3+3] annulation of benzoxazines with quinone compounds: access to spiro-heterocyclic scaffolds

“…6 In particular, benzoxazines have attracted a great deal of attention in Rh( iii )-catalyzed C–H activation because the nitrogen atom could act as a good directing group (DG) in the process of formation of a key rhodacycle intermediate. 7 Recently, the Rh( iii )-catalyzed cascade C–H activation for spiroannulation and cyclization of benzoxazines with various coupling partners such as activated alkenes, 8 alkynes 9 and diazo ketoesters 10 a have been well established to construct a variety of cyclic five- and six-membered heterocyclic scaffolds (Scheme 1a). To date, many methodologies have been developed to construct spiro[4.5]-benzoxazines via [3+2] annulations.…”