https://chemistry.mdma.ch/hiveboard/rhodium/pdf/archive/merbst/quinoline1999.pdf

Quinoline, quinazoline and acridone alkaloids

Natural and synthetic chromenes, fused chromenes, and versatility of dihydrobenzo[h]chromenes in organic synthesis.

Cobalt(III)–carbene radicals, generated through metalloradical activation of salicyl N-tosylhydrazones by cobalt(II) complexes of porphyrins, readily undergo radical addition to terminal alkynes to produce salicyl–vinyl radical intermediates. Subsequent hydrogen atom transfer (HAT) from the hydroxy group of the salicyl moiety to the vinyl radical leads to the formation of 2H-chromenes. The Co(II)-catalyzed process can tolerate various substitution patterns and produces the corresponding 2H-chromene products in good isolated yields. EPR spectroscopy and radical-trapping experiments with TEMPO are in agreement with the proposed radical mechanism. DFT calculations reveal the formation of the salicyl–vinyl radical intermediate by a metalloradical-mediated process. Unexpectedly, subsequent HAT from the hydroxy moiety to the vinyl radical leads to formation of an o-quinone methide intermediate, which dissociates spontaneously from the cobalt center and easily undergoes an endocyclic, sigmatropic ring-closing re...

Metalloradical Approach to 2H-Chromenes

Syntheses of chromenes and chromanes via o-quinone methide intermediates

Improved synthesis of aryl 1,1-dimethylpropargyl ethers

α,α-Dimethylpropargyl ethers of simple phenols are readily prepared in high yields. Thermal rearrangement of them affords 2,2- dimethylchromens, also in high yields. These reactions are used to prepare the natural products, ageratochromen, evodionol methyl ether, lapachenole, seselin, luvangetin, and braylin. A base-catalysed Wessely-Moser rearrangement is reported.

Synthesis of 2,2-dimethylchromens

Improved syntheses of certain O-and N-methyl derivatives of 1,3-dihydroxy-acridone are described. Thermal rearrangement of the appropriate 3-α,α-dimethyl-propargyl ethers in the series yields products which may be methylated to acronycine.

A synthesis of acronycine

Synthesis of 2,2-dimethylchromenes

The effect of the solvent and the counter-ion on the ring alkylation of 3- and 4-methoxyphenols by isopentenyl bromide has been examined. Under strictly anhydrous conditions good and consistent yields of the o- isopentenylphenols were obtained with the combination potassium- toluene, but sodium-toluene was more convenient. Boron trifluoride etherate was more satisfactory than stannic chloride, zinc chloride, or phosphoric acid in catalysing the condensation of 3- and 4-methoxy-phenols with 2-methylbut-3-en-2-ol to the corresponding o-isopentenylphenols. Aryl α,α-dimethylpropargyl ethers were partially hydrogenated to the corresponding α,α- dimethylallyl ethers which rearranged in boiling N,N-diethyl-aniline to o-isopentenylphenols in high overall yield.

E. Ritchie

Papers

Synthesis of 2,2-dimethylchromens

A synthesis of acronycine

Synthesis of 2,2-dimethylchromenes

Synthesis of o-isopentenylphenols

Synth. von acronycin