Diastereoselective Synthesis of Tetrabenzohydrofuran Spirooxindoles via Diethyl Phosphite-Mediated Coupling of Isatins with o-Quinone Methides

Metal-Free Regioselective Oxa-Michael Approach to Access Spirooxindole-Fused Tetrahydrofuran/Tetrahydropyran through [3 + 2]/ [4 + 2] Spirocyclization of Methyleneindolinones with Haloalcohols

An efficient one-pot metal-free, base-catalyzed method has been developed for the regioselective [3 + 2]/[4 + 2] annulation reactions of electrophilic methyleneindolinones with haloalcohols to furnish spirooxindole derivatives under mild reaction conditions. This reaction afforded the corresponding products with two contiguous stereocenters including a quaternary center in good to excellent yield (up to 95%) with moderate to good diastereoselectivities (up to 12.5:1 dr) with complete regioselectivity.

Sequential In Situ-Formed Kukhtin-Ramirez Adduct and P(NMe2)3-Catalyzed O-Phosphination of α-Dicarbonyls with P(O)-H

O-Phosphination of α-dicarbonyls via sequential in situ formation of a Kukhtin-Ramirez adduct and a P(NMe2)3-catalyzed process has been exploited for the synthesis of α-phosphoryloxy carbonyls. A range of P(O)-H derivatives, including diarylphosphine oxides, arylphosphinates, and phosphinates, are competent candidates to be introduced into the α-dicarbonyls in this transformation, and various α-phosphoryloxy carbonyls are obtained. This approach possesses advantages of mild conditions, simple operations, atom economy, high efficiency, and gram-scale synthesis, which make it promising in the synthesis toolbox.

Chiral Lewis Acid-Catalyzed Asymmetric Multicomponent Michael Reaction through [1,2]-Phospha-Brook Rearrangement

The multicomponent catalytic asymmetric Pudovik addition/[1,2]-phospha-Brook rearrangement/Michael reaction sequence of isatins, phosphites, and 4-oxobutenoates was realized. A series of oxindole derivatives containing two contiguous stereocenters was obtained in high yields and excellent stereoselectivities (up to >99% yield, >95:5 dr, >99% ee) using a chiral Lewis acid catalyst. A possible catalytic model is presented to illustrate the stereocontrol.

Stereoselective Reductive Coupling Reactions Utilizing [1,2]-Phospha-Brook Rearrangement: A Powerful Umpolung Approach

[1,2]-Phospha-Brook rearrangement entails the generation of α-oxygenated carbanions via the umpolung process. Recently, these anionic species have been widely utilized for several C-C bond forming strategies, providing various useful frameworks that are difficult to access through conventional approaches. However, the application of this powerful methodology in the development of chiral strategies is still at the nascent stage due to challenges involved in controlling chemoselectivity and enantioselectivity. This synopsis provides a detailed summary of diastereo- and/or enantioselective chemical transformations using [1,2]-phospha-Brook rearrangement.

Brønsted acid-mediated tandem cyclization of triarylphosphines and in situ generated ortho-alkynyl quinone methides: access to heterocyclic quaternary phosphonium salts

Heterocyclic Quaternary Phosphonium Salts (HQPS) have emerged as promising chemicals for organic synthesis and medicinal chemistry. However, the present synthetic methodology of this type of compound is still limited. Here, we report a deconstructive reorganization strategy based on Brønsted acid-mediated tandem 1,4 addition/intramolecular cyclization of triphenylphosphine derivatives and in situ generated o-AQMs for the first time. This protocol provides a novel approach to heterocyclic quaternary phosphonium salts. The method also features a non-metal catalyst, mild reaction conditions, high efficiency and wide substrate scope. Moreover, a series of obtained heterocyclic phosphonium salts can be converted to isotopically labelled 2-benzofuran compounds directly by simple deuteration reactions.

PPh3-Triggered Tandem Synthesis of 2,3-Disubstituted Benzofuran Derivatives from o-Quinone Methides with Acyl Chlorides

A PPh₃-triggered tandem strategy for the efficient synthesis of valuable 2,3-disubstituted benzofuran derivatives in generally good to high yields from aryl or alkyl acyl chlorides and o-quinone methides has been developed. This method features mild reaction conditions, simple operation, and a broad substrate scope.

Substrate-Controlled Regioselectivity Switchable [3 + 2] Annulations To Access Spirooxindole Skeletons

The additive-free [3 + 2] annulation from isatins, amino acids with 2-styrylbenzoxazoles, was described, providing a series of functional and structurally complex 3,3'-pyrrolidinyl-spirooxindole derivatives containing four contiguous and two quaternary stereogenic centers in high yields (up to 95%) and excellent diastereoselectivities (up to >25:1 dr). Interestingly, the reaction exhibits switchable regioselectivity depending on the substrate of amino acids. With proline or thioproline as the substrate, the reaction afforded α-regioselective spirooxindole skeletons. In contrast, when piperidine acid is the substrate, the reaction provided γ-regioselective spirooxindole skeletons.

Diastereoselective 1,6-Addition of α-Phosphonyloxy Enolates to para-Quinone Methides

The addition of α-ketoamide to p-quinone methide initiated by dialkylphosphite in the presence of organic base 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) is explored. Coupling of dialkylphosphites to α-ketoamides in the presence of a base follows [1,2]-phospha-Brook rearrangement, generating corresponding α-phosphonyloxy enolates that are subsequently seized by p-quinone methides (p-QMs). The two-step one-pot 1,6-conjugate addition provides effective access to a series of isatin-incorporated phosphate-bearing 1,6-adducts having two vicinal tertiary carbons with up to 90% yield and >20:1 dr.

Multicomponent Reactions (MCRs) with o-Quinone Methides

This article presents a comprehensive overview of multicomponent reactions (MCRs) that proceed via ortho-quinone methide intermediates (o-QM) generated in the reaction medium. Examples of applications involving these highly reactive intermediates in organic synthesis and biological processes (e. g., biosynthetic pathways, prodrug cleavage and electrophilic capture of biological nucleophiles) are also described. QMs are often generated by eliminative processes of phenol derivatives or by photochemical reactions, including reversible generation in photochromic substances. This class of compounds can undergo various reaction types, including nucleophilic attack at the methide carbon, with subsequent rearomatization, and react with electron-rich dienophiles in inverse-electron demand hetero-Diels-Alder reactions. Its versatile reactivity has been explored in the context of cascade reactions for the construction of several classes of substances, including complex natural products.

Acid-promoted formal [3 + 2] cyclization/N,O-ketalization of in situ generated ortho-alkynyl quinone methides: access to bridged 2,3-cyclopentanoindoline skeletons

An acid-catalyzed formal [3 + 2] cyclization/N,O-ketalization of in situ formed ortho-alkynyl quinone methides (o-AQMs) and tryptophol derivatives was developed.

Coupling partner-dependent unsymmetrical C–H functionalization of N-phenoxyacetamides leading to sophisticated spirocyclic scaffolds

In this paper, a coupling partner-dependent unsymmetrical C–H functionalization of N-phenoxyacetamides leading to the formation of sophisticated spirocyclic scaffolds is presented.