Palladium‐catalyzed Oxa‐[4+2] Annulation of para‐Quinone Methides

Abstract

A palladium‐catalyzed oxa‐[4+2] annulation of para‐quinone methides with allyl carbonates bearing a nucleophilic alcohol side chain has been developed. This method provided an efficient strategy to the construction of 2‐oxaspiro‐cyclohexadienones via 1,6‐conjugated addition‐mediated allylation in moderate to good yields. Preliminary results on asymmetric derivatives promised potential in the synthesis of enantioenriched frameworks.magnified image

Palladium‐Catalyzed [4+2] Cycloaddition of Hydroxy‐Tethered Allylic Carbonates with Alkenes: Synthesis of Functionalized Tetrahydropyrans

“…Through sequential decarboxylation of allyl carbonates/deprotonation of acidic protons, Trost, [4] Deng, [5] Shao [6] and Chen [7] synthesized new types of Pd‐TMM 1,3‐carbodipoles for [3+n] cycloaddition reactions (Scheme 1b), and Trost [8a,b] and Chen [8c] prepared π‐allyl‐Pd 1,4‐carbodipoles for [4+n] cycloaddition reactions (Scheme 1c). Yao and Lin, [9a,11a] Ni and Song, [9b] Garrec and Archambeau, [11b] and Liu [10] developed oxygen‐containing π‐allylpalladium zwitterionic intermediates, which were generated from hydroxy‐tethered allyl carbonates and worked as 1,4‐C,O‐dipoles for [4+2], [9] [4+3] [10] and [4+4] [11] cycloaddition reactions (Scheme 1d). This type of oxygen‐containing intermediates had good reactivity and provided a useful method for synthesis of biologically important oxygen‐containing heterocyclic compounds.…”

“…[12] Recently, this type of πallyl-Pd intermediates from decarboxylation of 2alkylidenetrimethylene carbonates were exploited to achieve [4 + 1], [13] [4 + 2], [14] and [5 + 4] [15] cycloaddition reactions. Although 1,4-C,O-dipoles could be formed through both deprotonation [9][10][11] and decarboxylation [12][13][14][15] and significant progress has been made in this field, mainly unsubstituted oxygen-containing π-allylpalladium 1,4-C,O-dipoles were used in most cases. [9][10][11][12][13][14][15] Expanding the substitution patterns of oxygen-containing π-allylpalladium zwitterionic intermediates is a very challenging task.…”

“…Although 1,4-C,O-dipoles could be formed through both deprotonation [9][10][11] and decarboxylation [12][13][14][15] and significant progress has been made in this field, mainly unsubstituted oxygen-containing π-allylpalladium 1,4-C,O-dipoles were used in most cases. [9][10][11][12][13][14][15] Expanding the substitution patterns of oxygen-containing π-allylpalladium zwitterionic intermediates is a very challenging task. Most recently, Lu presented a beautiful solution towards this important issue, accomplishing a Pd-catalyzed and visible light-induced asymmetric [4 + 2] cycloaddition of arylsubstituted 2-methylidenetrimethylene carbonates with adiazoketone.…”

Construction of Oxygen‐ and Nitrogen‐based Heterocycles from p‐Quinone Methides

“…A palladium-catalyzed oxa-[4 + 2]-annulation of p-QMs (63) with allyl-carbonates (142), has been developed by Lin, Yao and co-workers, to access 2-oxaspiro-[5.5]-undeca-7,10dien-9-ones (143) in moderate to good yields (Scheme 36). [62] The reaction initially proceeds through an oxidative addition of allylic-carbonate (142) to Pd(0) species, and the resulting Pd-complex adds to 63 in an 1,6-fasion to generate another intermediate, which finally undergoes an intramolecular cyclization, to generate the spirocyclic product 143, as shown in Scheme 36.…”

Enantioselective Domino Reaction of 3-Vinylindole and p-Quinone Methides Enabled by Chiral Imidodiphosphoric Acids: Asymmetric Synthesis of Multisubstituted 3-Indolyl Cyclopenta[b]indoles

“…In recent years, p -quinone methides have emerged as an efficient chemical synthon in many asymmetric transformations due to its vigorous reactivity. − We envisioned that 1,6-addition could yield compounds between 3-vinylindoles and p -quinone methides, and 3-vinylindoles could be transformed into vinyl-iminium species simultaneously. Subsequently, another molecule of 3-vinylindoles would take part in self [3+2] cycloaddition of 3-vinylindole under acid catalysis conditions (Scheme ).…”