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Photochemische Reaktionen. 104. Mitteilung [1]. Zur Photospaltung der C,C‐Oxiranbindung konjugierter γ, δ‐Epoxy‐enone: Photolyse von 4‐Methyliden‐5,6‐epoxy‐5,6‐dihydro‐β‐jonon
Author(s) -
Frei Bruno,
Wolf Hans Richard,
Jeger Oskar
Publication year - 1979
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19790620532
Subject(s) - chemistry , bicyclic molecule , cyclopropene , allene , ether , enone , medicinal chemistry , adduct , methanol , pentane , photochemistry , photodissociation , stereochemistry , organic chemistry , catalysis
The photoinduced cleavage of the C,C‐oxirane bond of γ, δ‐epoxy‐enones: UV.‐irradiation of 4‐methylidene‐5,6‐epoxy‐5,6‐dihydro‐β‐ionone On 1 n , π*‐excitation (λ ≥ 347 nm, pentane) 5 gives the isomeric bicyclic ether 10 in 75% yield (s. Scheme 2 ). In methanol the photoconversion of 5 to 10 is strongly reduced (12%) in favour of the formation of the methanol adduct 11 (43%). On photolysis in aqueous acetonitrile 5 is converted to the bicyclic ether 10 (9%), the dihydrofurane 12 (18%) as well as to the triketones 13A and 13B (7%), and 14 (23%). On 1 π, π*‐excitation (λ = 254 nm) in pentane no 10 is formed, but 5 isomerizes to the tricyclic cyclopropyl compound 16 (59%), the allenic product 17 (10%), and the cyclopropene compound 18 (12%; s. Scheme 3 ). Photolysis in methanol furnishes 11 (63%), and 18 (4%), but no tricyclic cyclopropyl compound 16 . In a secondary photoreaction (λ = 254 nm) the dihydrofurane 12 is isomerized to the bicyclic cyclopropyl compound 20 . Evidence is given that the products 11 and 13 are formed by solvent addition to an intermediate ketonium ylide b (s. Scheme 12 ). The presence of b is further proven by the formation of 12 , a product of an electrocyclization of b . On photofragmentation of b carbenoids d and e are presumably formed (s. Scheme 14 ). 1,2‐Hydrogen shift in d yields the allene derivative 17 , and cyclization of d gives the cyclopropene compound 18 . On the other hand, e cyclizes to the non isolated cyclopropene compound 69 which is transformed to 16 by an intramolecular [4 + 2]‐cycloaddition. The present investigation shows that the photochemistry of 5 is determined by photoinduced C,C‐bond cleavage of the oxirane ring. This is in sharp contrast to the photochemistry of conjugated γ, δ‐epoxy‐enones without the additional double bond in ε, ζ‐position, where selective photocleavage of the C(λ), O‐bond is observed.