Diastereoselective Synthesis of Benzoxanthenones

Abstract

An oxidative catalytic vanadium(V) system was developed to access the naturally nonabundant diastereomers of carpanone from the corresponding alkenyl phenol monomer in one pot by tandem oxidation, oxidative coupling, and 4+2 cyclization. This system was applied to the synthesis of two other analogues of carpanone. Mild oxidizing silver salts were used as the terminal oxidant to minimize background oxidation which produces the natural diastereomer of carpanone. Further, the first examples of enantioselective oxidative benzoxanthenone formation are reported. Solvent polarity has a strong effect on enantioselectivity, consistent with a mechanism involving binding of vanadium Schiff base catalysts to the alcoholic moiety of the alkenyl phenols during the cyclization step.

Visible-Light-Induced Oxidative Coupling of Phenols and Alkenylphenols with a Recyclable, Solid Photocatalyst

“…This result compares favorably to a copper-catalyzed reaction using air as the oxidant (60% yield) Scheme b). , …”

“…22 ortho-Alkenylphenol 8 underwent β−β coupling followed by Diels−Alder cyclization to form the natural product carpanone (Scheme 7b). 23,24 In conclusion, we have developed a selective oxidative phenol and alkenylphenol coupling protocol by means of light and a TiO 2 photocatalyst. This photocatalyst is inexpensive and environmentally benign.…”

Catalytic Oxidative Coupling of Phenols and Related Compounds

“…This combination of reagents causes β–β homocoupling of the ortho -alkenyl phenols (Scheme ). However, this adduct is not stable and is poised for an intramolecular Diels–Alder reaction to generate the carpanone scaffold. All prior reports on this oxidative coupling generate the endo product after the Diels–Alder reaction, which corresponds to the carpanone natural product stereochemistry .…”

Vanadium(V) Complex-Catalyzed One-Pot Synthesis of Phenanthridines via a Pictet-Spengler-Dehydrogenative Aromatization Sequence

“…Furthermore, there are a few reports on the cooperative effect of vanadium(V) complexes in synthesis [24][25][26][27][28]. Recently, we developed chiral oxovanadium complexes [29] that act as redox and/or Lewis acid catalysts for the enantioselective oxidative coupling of arenols [30][31][32][33], Friedel-Crafts-type reactions [34], and a sequential cascade reaction that affords oxahelicenes [28]. As part of our ongoing research, we envisioned a one-pot synthesis of phenanthridine derivatives 5 using vanadium(V) complexes 6 as Lewis acid/oxidation catalysts (Scheme 1).…”

“…Thus, Lewis acidic vanadium(V) complex 6 would promote a Pictet-Spengler reaction of imine intermediate 3 formed by the condensation of aniline 1 and benzaldehyde 2 to afford tetrahydroisoquinoline 4. A subsequent dehydrogenative aromatization of 4, assisted by the same vanadium(V) complex, now operating as a one-electron oxidant, would yield the desired phenanthridine product 5 [28,35].…”