Photochemical reactions. 143rd communication. Photochemistry of 5,6-epoxy-1,3-dienes in the ionone series. Influence of a hydroxy group in the 7-position

The synthesis and photolyses of the epoxydiene (E)- 5 are described. On triplet excitation (λ > 280 nm, acetone), (E)- 5 undergoes initial cleavage of the C(5) O bond leading to the intermediate c. Presumably an H-shift ( c → e ) followed by the fragmentation of the 1,4-diradical e leads (via the enol 37 ) to the diketones (E)- 6 and (Z)- 12 . Alternatively cleavage of the C(6) C(7) bond of c furnishes the diradical intermediate d which reacts by recombination leading to (E)- 13A + B, 16 , and 17A + B , or by an H-shift to the enol intermediate 38 . The latter undergoes an aldol-type reaction to (E/Z)- 14A + B and (E/Z)- 15A + B , as well as a photochemical [2 + 2]-cycloaddition to 18 . On singlet excitation (λ = 254 nm, MeCN), (E)- 5 undergoes photocleavage to the carbene intermediates f and g . The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 22 as the main product. From the carbene intermediate g , compounds 23, 24 , and 25 arise by carbene insertion into the neighboring C C or C H bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)- 20 , is formed via the ylide intermediate h .     

Photochemical reactions. 144th Communication. Photochemistry of 5,6-epoxy-1,3-dienes in the ionone series: Influence of a methoxy group in position 7

On triplet excitation (λ > 280 nm, acetone), the epoxydiene (E)- 5 undergoes initial cleavage of the C(5)? O bond of the oxirane and subsequent cleavage of the C(6)? C(7) bond leading to the diradical intermediate e which reacts by recombination furnishing the cyclic compounds (E/Z)- 6 , (E/Z)- 7,8 , and 9 . Alternatively, a H -shift leads to the aliphatic methyl-enol ether 10 which undergoes a photochemical [2+2]-cycloaddition to compounds 12 and 13 , the main products on triplet excitation of (E)- 5 . On singlet excitation (λ = 254 nm, MeCN), (E)- 5 undergoes cleavage to the carbene intermediates f and g . The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 14 as the main product. From the carbene intermediate g , the methyl-enol ether 15 arises by carbene insertion into the neighboring C? H bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)- 16 , and compounds 17A+B are formed via the ylide intermediate h . Finally, the cyclobutene 18 is the product of an electrocyclic reaction of the diene side chain.     

Photochemical reactions. 148th communication. Photochemistry of acylsilanes: Preparation,photolyses, and thermolyses of α,β-unsaturated silyl ketones

The syntheses, photolyses, and thermolyses of the α,β-unsaturated silyl ketones (E/Z)-7, (E)- 8 , and (E)- 9 are described. On n,π*-excitation (λ > 347 mm), the aforementioned compounds undergo (E/Z)-isomerization followed by γ-H abstraction. The intermediate enols are trapped intermolecularly by siloxycarbenes leading to the dimeric acetals 27A + B, 30A + B , and 31A + B . In addition, the acylsilanes (E/Z)- 7 undergo photoisomerization by δ-H abstraction furnishing the acylsilanes 29A + B . Flash vacuum thermolyses (FVT) of (E/Z)- 7 , (E/Z)- 8 , and (E)- 9 give rise to intramolecular reactions of the siloxycarbene intermediates. Thus, FVT (520°) of (E)- and (Z)- 7 selectively leads to the enol silyl ethers 32 and (E)- 33 , respectively, arising from carbene insertion into an allylic C–-H bond. FVT of (E/Z)- 8 (560°) and (E)- 9 (600°) affords the trienol silyl ethers 34A + B and the cyclic silyl ethers 37A + B , respectively, which are formed by CH insertion of the siloxycarbenes. As further products of (E)- 8 and (E)- 9 , the bicyclic enol ethers 35 and 36 are formed, presumably via siloxycarbene addition to the cyclohexene C?C bond.     

Photochemische Reaktionen. 117. Mitteilung [1] Zur Photochemie von 5,6-Epoxydienen und konjugierten 5,6-Epoxytrienen

Photochemistry of 5,6-Epoxydienes and of Conjugated 5,6-Epoxytrienes On singulet excitation (δ = 254 nm) the 5,6-epoxydiene 6 and the conjugated 5,6-epoxytrienes 7 and 8 exclusively give products arising from cleavage of the C, C-bond of the oxirane (cf. 6 → 9 , 10 , 11 ; 7 → (E)- 15 , 16 , 17 ; 8 → 18 (A+B) , 19 (A+B) , 20 , 21 ). The dihydrofuran compounds 11 and (E/Z)- 15 are formed by cyclization of a ketonium-ylide a and d , respectively. Photolysis of a gives the carbene b which yields the cyclopropene 9 , whereas d forms photochemically the carbenes f and g which yield the methano compounds 16 and 17 . The isomeric cyclopropene derivatives 20 and 21 are products of the intermediates h and i , respectively, which are formed by photolysis of the ylide e . The cyclopropene 21 isomerizes by intramolecular cycloadditions to 18 (A+B) and 19 (A+B) . - On triplet excitation (λ?LD nm; 280 nm; acetone) 6 undergoes cleavage of the C(5), O-bond and isomerizes to 12 and 14 . However, 7 is converted by cleavage of the C, C-bond of the oxirane to yield 15 . On treatment with BF₃O(C₂H₅)₂ 6 gives 14 , whereas 7 yields 22 , and 8 forms 23 and 24 .     

Photochemical reactions. 146th communication. Photochemistry of conjugated methano-epoxydienes: Participation of the neighboring cyclopropane ring in product formation via carbonyl ylide and carbene intermediates

On singlet excitation (λ = 254 nm, THF, pentane or hexane), the diastereoisomeric methano-epoxydienes (E)- 6 and (E)- 7 undergo interconversion and yield the products 8 – 11 . The main process is the cleavage of the oxirane ring to the vinyl carbene intermediate e which undergoes addition to the adjacent double bond furnishing the cyclopropene 8 . The alternative carbene intermediate f is evidenced by the formation of the cyclobutene 10 . For the fragmentation leading to 11 , the carbene f as well as the dipolar species h are considered as possible intermediates. On triplet sensitization (acetone, λ > 280 nm), (E)- 7 shows behavior typical of epoxydienes, undergoing fission of the C? O bond of the oxirane ring and isomerization to the compounds 13 , 14 and (E/Z)- 15 .     

Photochemical reactions. 133rd Communication. Photochemistry of epoxydienes of the ionone series: Influence of a methyl group at the diene side chain on the oxirane cleavage

On triplet excitation (λ > 280 nm, acetone), the epoxydiene (E)- 2 undergoes (E)/(Z)-isomerization exclusively, leading to the conformers (Z)- 2A and (Z)- 2B . On singlet excitation (λ = 254 nm), apart from (Z)- 2A + B , the cyclobutenes 3A + B are formed. However, the epoxydiene (E)- 2 does not undergo reactions leading to carbene and C,O-bond cleavage products, which are normally observed on singlet and triplet excitation, respectively, of the epoxydienes of the ionone series.     

Photochemical reactions, 136th communication. Photochemistry of conjugated epoxyenones and dienes of the ionone series: Influence of a neighboring spiro-oxirane function

The preparation and photolyses of the diepoxyenones (E)- 8 and (E)- 9 as well as the diepoxydiene (E)- 10 are described. On ¹π,π^*-excitation (λ = 254 nm), the diastereoisomeric diepoxyenones (E)- 8 and (E)- 9 undergo isomerization via the ylide intermediate f and the carbene intermediate g leading to the primary photoproducts 17A and 18–21 (Scheme 8). On ¹n, π^*-excitation (λ > 347 nm), (E)- 8 shows behaviour typical of epoxyenones undergoing C(γ), O-bond cleavage of the oxirane and isomerization to compounds 22 , (E/Z)- 23 and (E)- 24 (Scheme 10). On singlet excitation, the diepoxydiene (E)- 10 , is cleaved to the carbonyl ylide j and the carbenes 1 and m (Scheme 11). The carbonyl ylide j fragments via the dipolar intermediate k to the acetylenic dienone (E)- 31 . The carbene 1 , showing behaviour typical of vinyl carbenes, furnishes the cyclopropene 30 . The alternative carbene m , however, undergoes an insertion reaction into the neighboring oxirane C,C-bond leading to the proposed but not isolated oxetene 43 , which is further transformed to the products 33A _ B by an intramolecular cycloaddition.     

Photochemical reactions. 132nd communication. Photochemistry of 5,6-epoxy-5,6-dihydro-7-methyl-β-ionine: Influence of a methyl group at the enone side chain on the oxirane cleavage

¹n, π*-Excitation of the γ,δ-epoxy-enone (E)- 3 leads exclusively to the conformers (Z)- 3A + B . On ¹π, π*-excitation of (E)- 3 , in addition to (Z)- 3A + B , products 6–9 arising from a carbene intermediate e are formed. However, products of an isomerization via C(γ), O-bond cleavage of the oxirane were not formed on either mode of excitation. On thermolysis, at 80° the conformer (Z)- 3A is transformed into (Z)- 3B , which on photolysis returns to (Z)- 3A and (E) -3 . At 160°, however, (Z) -3B rearranges to the isomers 6, 10 and 11 .     

Photochemical reactions. 152nd Communication. Photochemistry of acylsilanes; photolysis and thermolysis of cyclopropyl silyl ketones

The photolysis and thermolysis of the Cyclopropyl silyl ketones 3, 4 , and 5 are described. On n,π* excitation, the silyl ketones 3 and 4 undergo a Norrish-type-II reaction involving γ-H abstraction, cyclopropyl ring cleavage followed by retro-enolization to the acylsilanes 6 and (E/Z)- 12 , respectively. As a common product of 3 and 4 , the dihydrofuran 7 is formed via the alternative C(α)-C(β) cleavage of the cyclopropyl moiety. Compounds 6 , 7 , and (E/Z)- 12 are new types of acylsilane photoproducts. The irradiation of acylsilane 5 gave the analogous dihydrofuran 15 as the only product. On photolysis of 3 and 4 , products 8A + B and 13A + B , derived from a siloxy carbene intermediate, were found as well. On thermolysis of 3 and 4 , the acylsilanes 6 (80%), and (E)- 12 (33%) and (Z)- 12 (34%), respectively, are formed as the only products. Their formation may occur via a [1, 5] sigmatropic H-shift. The thermolysis of 5 gave the diene 16 whose formation can be explained by insertion of a siloxycarbene into the neighboring cyclopropane leading to the cyclobutene 28 as thermally unstable intermediate.     

Photochemical Reactions. 129th Communication. Photochemistry of a conjugated diepoxydiene: Product formation via carbonyl ylide and carbene intermediates with participation of neighbouring epoxy functions

On singlet excitation (λ=254 nm, acetonitrile) the diepoxydiene (E)- 7 undergoes photocleavage to the carbonyl ylide VII and the carbenes X and XI . The carbonyl ylide VII rearranges to the thermally labile dioxabicyclo [3.2.1]octene 20 or fragments via VIII to the aldehyde 9 and propyne. The carbene X , showing behaviour typical of vinyl carbenes, undergoes addition to the adjacent double bond furnishing the cyclopropene 11 . The carbene XI , however, undergoes an insertion reaction into the neighbouring oxirane C,C-bond leading to the oxetene (E)- 21 which can be isolated at ?78°, but at room temperature is rapidly transformed to the aldehyde 10 . On triplet excitation (acetone, λ>280 nm), however, (E)- 7 shows the typical behaviour of epoxydienes, undergoing C, O-cleavage of the oxirane and isomerization to 22, 23 and (E/Z)- 24 .     

Photochemical reactions. 147th Communication. Further investigation of the photochemistry of 5,6-epoxy-5,6-dihydro-β-ionone: Product formation via a carbonyl-ylide intermediate

On π,π*-excitation of the epoxyenone (E)- 1 (λ = 254 nm, MeCN), in addition to the previously isolated compounds 2 – 9 , the new products 10 – 12 , derived from the ylide intermediate c were isolated. Further evidence for the ylide c was obtained by the rapid racemization of the optically active epoxyenone (?)-(E)- 1 .     

Photochemical reactions. 139th Communication. Photochemistry of acylsilanes: 1. Siloxcarbene formation versus γ-H-abstraction

The Syntheses and the photolyses of the acylsilane 1 and the corresponding methyl ketone 2 are described. On n,π*-excitation, the silyl ketone 1 as well as the methyl ketone 2 undergo a Norrish type II reaction involving γ-H-abstraction and fragmentation to the diene 12 , and acetone ( 20 ) or the acylsilane 26 , respectively. The methyl ketone 2 , but not the acylsilane 1 , isomerizes to cyclobutanols ( 21A - D ). Additionally, compound 1 shows photochemical behavior typical of acylsilanes undergoing rearrangement to the siloxycarbene intermediate c . Insertion of c into the O? H-ond of the enol 28 leads to compound 13 . Initial trapping of the siloxycarbene c by H₂O, however, gives rise to the formation of compounds 16 – 18 . As minor photolysis products of 1 , compounds 14 and (Z)- 15 were formed; however, on vapor phase thermolysis (520°) of 1 , compounds 14 and (E/Z)- 15 were obtained in 92% combined yield. To a small extent the acylsilane 1 also undergoes Norrish type I cleavage leading to the acid 19 .     

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

The metalloradical activation of ortho‐benzallylaryl N‐tosyl hydrazones with [Co(TPP)] (TPP=tetraphenylporphyrin) as the catalyst enabled the controlled exploitation of the single‐electron reactivity of the redox non‐innocent carbene intermediate. This method offers a novel route to prepare eight‐membered rings, using base metal catalysis to construct a series of unique dibenzocyclooctenes through selective C_carbene?C_aryl cyclization. The desired eight‐membered‐ring products were obtained in good to excellent yields. A large variety of aromatic substituents are tolerated. The proposed reaction mechanism involves intramolecular hydrogen atom transfer (HAT) to Co^III–carbene radical intermediates followed by dissociation of an ortho‐quinodimethane that undergoes 8π cyclization. The mechanism is supported by DFT calculations, and the presence of radical‐type intermediates was confirmed by trapping experiments.     

Density function studies on the PtCl2‐catalyzed asymmetric cycloisomerization reaction of hydroxylated enynes

The PtCl₂‐catalyzed asymmetric cycloisomerization reaction of hydroxylated enynes was studied using density functional theory (DFT). All structures have been optimized completely at the B3LYP/6‐311G(d,p) level. As shown, the cycloisomerization reaction is exothermic. The cycloisomerization reaction mainly undergoes the formation of catalyst‐hydroxylated enzyme coordination, the asymmetric cyclopropyl platinum carbene, catalyst–cyclopropyl enol coordination, and catalyst–cyclopropyl ketone coordination. The chirality‐limiting step for the asymmetric cycloisomerization reaction is the formation of the asymmetric cyclopropyl platinum carbene, and the rate‐determining step for this reaction is the formation of the catalyst–cyclopropyl ketone coordination. The dominant products predicted theoretically are (R,S) ‐syn_5a, in agreement with the experiment. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Photochemische Reaktionen. 71. Mitteilung. Die Photoisomerisierung eines spirocyclischen α, β-Epoxyketons

On direct UV. irradiation and on triplet sensitization with acetophenone the spirocyclic epoxyketone (R)-(?)- 9 undergoes racemization (Φ^313/334 0.014, Φ^Sens 0.0060) and rearrangement to the enantiomeric spiro-β-diketones (R)-(+)- 14 (Φ^313/334 0.068, Φ^Sens 0.0037) and (S)-(?)- 14 (Φ^313/334 0.024, Φ^Sens 0.0023). The quantum yield data show that triplet reaction due to intersystem crossing is unimportant on direct irradiation, and they exclude that one common diradical intermediate of type d (Scheme 8) for the three reaction paths is involved in both the singlet and the triplet reaction. The postulate of photolytic Cα? O epoxide cleavage to intermediates of type d for the rearrangement requires that the rate of rearrangement is greater than the rate of rotation around the Cα? Cβ; bond in a given d , and that the rate difference is greater in singlet-generated d than in the triplet analogue. Reclosure of diradicals d and/or photolytic Cα? Cβ cleavage to diradical e and reclosure can account for the racemization of 9 . The optically active spiro-β-diketone 14 was found to racemize also on direct irradiation and on triplet sensitization. Furthermore, both 14 and the isomeric β-diketone 20 , which was obtained by UV. irradiation of the homocyclic epoxyketone 19 , photochemically isomerize to the enol lactones 23 and 21 , respectively.     

Glycosylidene Carbenes. Part 24 Reactivity modulation by protecting groups of the addition of glycosylidene carbenes to electron-rich alkenes

The reactivity of glycosylidene carbenes derived from pivaloylated vs. benzylated diazirines 1 and 2 towards enol ethers have been examined. The pivaloylated 1 led to higher yields of spirocyclopropanes than the benzylated 2. Among the enol ethers tested, dihydrofuran 6 proved most reactive, yielding 71–72% of the spiro-linked tetrahydrofuran 7 , while the benzylated diazirine 2 afforded only 33% of the analogue 8 (Scheme 1 ). Other enol ethers proved much less reactive. The addition of 1 and 2 to the dihydropyran 10 and the 2, 3-dihydro-5-methyl-furan 15 gave low yields of single cyclopropanes (→ 12 , 14 , and 16 ), and the glycals 17 and 18 , and (E)-1-methoxy-oct-1-ene ( 23 ) did not react. The main products of these reactions were the azines (Z, Z)- 11 and (Z, Z)/( E, E)- 13. Similarly, 1 and 2 reacted poorly with (Z)-1-methoxyoct-1-ene ( 24 ), leading to cyclopropanes 25 / 26 / 27 and 28 / 29 / 30 / 31 (Scheme 2). Main products were again the azines (Z, Z)- 11 and (Z, Z)/(E, E)- 13 . The structure of 70 and 25 was established by X-ray analysis (Figs. 1 and 2). The mechanism of addition of glycosylidene carbenes to enol ethers is discussed, AMI Calculations indicate that the LUMO_carbene/HOMO_alkoxyalkene interaction is dominant at the beginning of the reaction, while the transition states are characterized by a dominant interaction of the doubly occupied, sp²-hybridized orbital of the carbene with the LUMO of the enol ether. The relative reactivity of the carbenes towards either the enol ethers or the diazirines determine type and yields of the products.     

Photochemical reactions. 141st communication. Photochemistry of α,β-unsaturated δ,ϵ-epoxyketones of the ionone series

On n,π*- as well as on π,π*-excitation, the 4,5-epoxy-α-ionones (E)- 1 , (E)- 2 , and (E)- 3 undergo (E)/(Z)-isomerization and subsequent γ-H-abstraction leading to the corresponding 4-hydroxy-β-ionones (E/Z)- 9 , (E/Z)- 13 , and (E/Z)- 17 as primary photoproducts. On photolysis of (E)- 3 , as an additional primary photoproduct, the β,γ-unsaturated σ,?-epoxy ketone 18 was obtained. The other isolated compounds, namely the 2H-pyrans 10A + B and 14A + B as well as the retro γ-ionones 11 and 15A + B , represent known types of products, which are derived from the 4-hydroxy-β-ionones (E/Z)- 9 and (E/Z)- 13 , respectively.     

Photochemical reactions. 134th Communication. Photochemistry of 5,6-epoxy-5,6-dihydro-3,4-methano-β-ionone: Influence of a cyclopropane ring on the reactivity of an ylide intermediate

On ¹n,π*-excitation(λ > 347 nm), the diastereomeric methanoepoxyenones (E)- 6 undergo isomerization via C,O-cleavage of the oxirane leading to diastereomeric photoproducts ((E)- 5 →(E/Z)- 13 and 14 ; (E)- 6 →(E/Z)- 16 and 17 ). On ¹π,π*-excitation (λ = 254 nm) of either (E)- 5 ) or (E- 6 the photoproducts 9, 10 and 11 are formed. By laser flash photolysis (λ = 265 nm) the ylide intermediate 3 was detected, with a lifetime of 10 μs in MeCN at ambient temperature. Stern-Volmer analysis of the ylide quenching by MeOH disclosed that compounds 9 and 10 , but not 11 , arise from the ylide intermediate e .     

Theoretical Analysis of the Heterocyclic [4+2] Cycloaddition Between Pyridinium Ion and Enol Ether

Dearomative heterocyclic [4+2] cycloaddition between the N-(2,4-dinitrophenyl)pyridinium ion of nicotinamide and an enol ether was analyzed by Density Functional Theory (DFT) calculations. The calculation revealed that the reaction undergoes stepwise bond formation rather than occurring in a concerted manner. The experimental products were found to be both kinetically and thermodynamically favored. The calculated transition states and intermediate suggested that the high diastereoselectivity is derived from the electrostatic interaction between the 2-nitro group of the pyridinium ion and the hydrogen of the enol ether.     

A new approach to c-glycoside congeners: Metal carbene mediated methylenation of aldonolactones.

Versatile intermediates for the synthesis of C-glycosides and oxygen heterocycles are readily available through titanium mediated methylenation of aldonolactones.Aldonolactones may be alkylated using the titanium carbene complex 2. The resulting enol ethers are of value as potential substrates for glycosidase enzymes. A high yield two step process for alkylation of these enol ethers was described. The products and procedures are relevant to the synthesis of mycotoxins, including citreoviridin and aurovertin B.