Photochemical Reactions. 128th Communication. Isolation of a Thermally Labile 2,8-Dioxabicyclo[3.2.1]oct-3-ene Intermediate on Photolysis of 3,4: 5,6-Diepoxy-5,6-dihydro-β-ionone

Photolysis (λ = 254 mm, THF) of the diepoxyenone (E)- 1 at ?78° leads to the 2,8-dioxabicyclo[3.2.1]oct-3-ene intermediate 3 (51%). At ambient temperature 3 undergoes an unexpectedly rapid electrocyclic opening to the triketone 2 in quantitative yield. Compound 3 seems likely to be the intermediate in the acid-catalyzed rearrangement of (E)- 1 → 18 also.     

Photochemische Reaktionen 94. Mitteilung. Vinyloge β-Spaltung bei Epoxy-enonen der Jonon-Reihe

Photoinduced Vinylogous β-Cleavage of Epoxy-enones of the Ionone Series The photochemistry of the α,β-unsaturated γ,δ-epoxy-enones 1–3 is determined by: (i) C(γ)-O-scission of the epoxide (vinylogous β-cleavage of Type A); (ii) C(γ)-C(δ)-cleavage of the oxirane (vinylogous β-cleavage of Type B); (iii) (E/Z)-isomerization of the enone chromophore. In contrast, 4 with tertiary C(β) shows no Type B cleavage. Type A cleavage is induced both by n,π*- and π,π*-excitation and arises probably from the T₁-state, but Type B cleavage is observed only on π,π*-excitation and represents presumably a S₂-reaction. On Type A cleavage 1–4 undergo 1,2-alkyl-shifts to 1,5-dicarbonyl compounds ( 15–18, 25–28, 34 and 35 ) or rearrange to dihydrofuranes ( 7 and 30 ). The isomerization 1→7 proceeds by a stereoselective [1,3]-sigmatropic shift. On Type B cleavage 1–3 isomerize to a bicyclic enol-ether ( 8, 29 ) or to a monocyclic enol-ether ( 9 ; product of a homosigmatropic [1,5]-shift) or undergo fragmentation to isomers such as allenes 10, 22 and 31 or cyclopropenes 11 and 21 . The non-isolated, unstable (Z)-epoxy-enones 14, 19, 24 and 38 isomerize by fragmentation to the furanes 12, 23, 33 and 39 respectively, on contact with traces of acid or by heating. However, for 19 and 4 , Type B cleavage may lead to the furanes 23 and 39 . On UV. irradiation of the epoxy-enone 4 the initially formed (E/Z)-isomers 34 and 35 yield on π,π*-excitation the enones 37 and 40 by a vinylogous β-fragmentation. In addition, on n,π*-excitation 34 isomerizes to 35 , which decarbonylates exclusively to the enone 37 . The reactions of 1–4 with BF₃ · O(C₂H₅)₂ were also studied (see appendix). The epoxy-enones 1 and 2 isomerize by an 1,2-alkyl shift in good yield to the 1,4-dicarbonyl compounds 79 and 81 , whereas 3 gives the 1,4-diketone 83 , and in small amounts the 1,5-diketone 84 . On the other hand, 4 is converted to the fluorohydroxy-enone 85 and to the 1,5-dicarbonyl product 34 , the only isomer in this series which is identical with one of the photoproducts.     

Photochemical Reactions 151st Communication. Photo-oxygenation of (E)- and (Z)-7-Methyl-β-ionone

Photo-oxygenation of (E)-7-methyl-β-ionone ((E)? 1 ) and (E)-8-methyl-β-ionone ((E)? 2 ) gave rise to the formation of the hydroperoxy-enones (E)? 10 and (E)? 15 , respectively, which, in part, underwent intramolecular epoxidation to the hydroxy-epoxy-ketones 11 and 16 , respectively, The product distribution of the photo-oxidation of (Z)? 1 shows a marked influence of the skewed ground-state conformation of the dienone chromophore. Thus, singlet oxygen (¹O₂) was added to C(γ) of the dienone chromophore leading to the spirocyclic peroxy-hemiacetal 12 and to the endoperoxide 13 . In addition, the tricyclic peroxide 14 was formed as a new type of product via primary addition of ¹O₂ to C(γ) of the dienone chromophore. The structure of 14 was established by X-ray crystal-structure analysis of the hemiacetal 22 .     

Photochemische Reaktionen. 97 Mitteilung [1]. Zur Photochemie konjugierter Epoxy-diene II. Versuche mit (E)-β-Jonyliden-epoxiden

Photolysis of Conjugated Epoxy-dienes Direct and sensitized excitation of the (E)-β-ionylidene-epoxides 1 and 4 leads to different types of isomerizations. Thus photocycloelimination to the cyclopropene-ketones 2 and 6 is only achieved by ¹(π, π*)-excitation (λ=254 nm), whereas ³(π, π*)-excitation (λ > 280 nm, acetone) gives selective C(1′), O-cleavage of the oxirane ( 1 → 7 – 10 and 4 → 11 – 13 ). In contrast to 1 the twofold methylsubstituted epoxy-diene 4 shows mainly (E/Z)-isomerization ( 4 → 5 ) on both ¹(π, π*)- and ³(π, π*)-excitation while the isomerizations 4 → 6 and 4 → 11 – 13 are minor processes, only.     

Model simulation of cationic polymerization by an iterative technique

Using a simple, instructive model of living polymerizations, we develop an economic technique for solving a complete set of kinetic equations for active polymers P_n^* with chain length n. The method consists of two steps: (i) solution of the global kinetic equations for the global species P^* which comprises all individual polymers P_n^* together with monomer M and other non-polymeric species, followed by (ii) iterative solution of the original kinetic equations, successively for the individual polymers P₁^*, P₂^*, P₃^*, …, P_n^*. The method is applied successfully to a rather complex, realistic model of cationic polymerizations. In general, our iterative scheme serves to reduce numerical storage requirements, in comparison with traditional direct integrations for all individual kinetic equations. Therefore it may allow simulations of very demanding polymerizations, including very high degrees of polymerization, which cannot be evaluated by traditional techniques.