ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Irradiation in the n→π* absorption band of the α,β-unsaturated γ,δ-epoxyketone 5 in ethanol at -65° exclusively afforded the rearranged ene-dione 13, whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14, and 15 were obtained. Selective π→π* excitation of 5 at -78° and + 24° led to similar product patterns. The 9β,10β-epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation.Neither the epoxyketones 5 and 7 nor the photoproducts 13-15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16, 18, and 19, respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift (→ 16) when photolyzed in alcoholic solvents instead of benzene.The selective transformations 5 → 13 (at -65° and n → π* excitation) and 7 → 14+15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55, the likely primary photoproducts resulting from epoxide cleavage in the triplet-excited compounds 5 and 7, via the transition states 54, 56, and 57. The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53. The similar effect of π → π* excitation even at -78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15, as compared with 13, is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation.
Additional Material:
2 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19720550312
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