The synthesis of the pyranonaphthoquinones dehydroherbarin and anhydrofusarubin using Wacker oxidation methodology as a key step and other unexpected oxidation reactions with ceric ammonium nitrate and salcomine.
The synthesis of two closely related pyranonaphthoquinones, dehydroherbarin and anhydrofusarubin, is described. The construction of the naphthalene nuclei was achieved using the Stobbe condensation reaction using 2,4-dimethoxybenzaldehyde and 2,4,5-trimethoxybenzaldehyde as their respective starting materials. Two key steps en route include a PIFA-mediated addition of a methoxy substituent onto the naphthalene skeleton and a Wacker oxidation reaction to construct the benzo[g]isochromene nucleus. Two interesting oxidation reactions of the intermediate isochromene enol ether of 7,9-dimethoxy-3-methyl-1H-benzo[g]isochromene-5-ol were observed. Treatment of the substrate with salcomine resulted in the formation of (3-formyl-4-hydroxy-6,8-dimethoxynaphthalene-2-yl)methyl acetate, while treatment of the same substrate with CAN resulted in the formation of racemic (3R,4R)-3-hydroxy-7,9-dimethoxy-3-methyl-5,10-dioxo-3,4,5,10-tetrahydro-1H-benzo[g]isochromen-4-yl nitrate.
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa.
Adushan
A
Pillay
Amanda L
AL
Rousseau
Manuel A
MA
Fernandes
Charles B
CB
de Koning
D002563:Cerium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D009285:Naphthoquinones / Q000138:chemical synthesis* / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction
D009285:Naphthoquinones; D009942:Organometallic Compounds; C580782:anhydrofusarubin; D002563:Cerium; C001334:herbarin; C003955:N,N-ethylenebis(salicylideneiminato)cobalt(II); C004653:ceric ammonium nitrate