2000
DOI: 10.1002/(sici)1099-0690(200005)2000:10<1871::aid-ejoc1871>3.0.co;2-3
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Cyclohexa-1,2,4-triene from 1-Bromocyclohexa-1,4-diene

Abstract: 1‐Bromocyclohexa‐1,4‐diene (3) was prepared from trans‐4,5‐dibromocyclohexene by elimination of hydrogen bromide. Treatment of a solution of 3 in furan or 2,5‐dimethylfuran with KOtBu afforded the tetrahydroepoxynaphthalenes 4 and 5, respectively. The structure of these products is evidence for the title cycloallene (isobenzene 1) being the reactive intermediate. The compounds 4 and 5 were dehydrogenated by DDQ to the known dihydroepoxynaphthalenes 6 and 7, respectively. These conversions unambiguously confirm… Show more

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Cited by 19 publications
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“…The Hopf cyclization stands out as one of the least exploited pericyclic reactions in organic synthesis (Figure a). It is an atom-neutral, thermally allowed 6π-electrocyclization that converts cis -hexa-1,3-diene-5-ynes and their benzannulated analogues into aromatic rings via two consecutive hydrogen shifts involving highly strained cyclohexa-1,2,4-triene (“1,2,4-isobenzene”) intermediates (Figure a). , Its rate-determining step is not the initial 6π electrocyclization (step 1, Figure a), but the ensuing 1,2-H shift (step 2, TS1 ). , While a photochemical variant has been described, its use has been limited to suitable scaffolds. The thermally induced Hopf cyclization has been disadvantaged by its high activation barrier, necessitating harsh reaction temperatures: >200–250 °C for nonbenzannulated dienynes ,, and >300 °C for benzannulated dienynes. ,,, …”
Section: Introductionmentioning
confidence: 99%
“…The Hopf cyclization stands out as one of the least exploited pericyclic reactions in organic synthesis (Figure a). It is an atom-neutral, thermally allowed 6π-electrocyclization that converts cis -hexa-1,3-diene-5-ynes and their benzannulated analogues into aromatic rings via two consecutive hydrogen shifts involving highly strained cyclohexa-1,2,4-triene (“1,2,4-isobenzene”) intermediates (Figure a). , Its rate-determining step is not the initial 6π electrocyclization (step 1, Figure a), but the ensuing 1,2-H shift (step 2, TS1 ). , While a photochemical variant has been described, its use has been limited to suitable scaffolds. The thermally induced Hopf cyclization has been disadvantaged by its high activation barrier, necessitating harsh reaction temperatures: >200–250 °C for nonbenzannulated dienynes ,, and >300 °C for benzannulated dienynes. ,,, …”
Section: Introductionmentioning
confidence: 99%