On '71, n*-excitation the 6-keto-enones 5-8 are isomerized to compounds B (18, 22, 26, 28) via 1,3-acyl shift and to compounds C (19, 23, 27, 29) via 1,2-acyl shift, whereas the p, y , 6, E-unsaturated ketone 9 gives the isomers 32 and 33 by 1,2-and 1,5-acyl shift, respectively. Furthermore, isomerization of 6 to 24, dimerization of 8 to 30 and addition of methanol to 8 (8 -+ 31) is observed. Unlike 7 and 8 the acyclic ketones 5, 6 and 9 undergo photodecarbonylation on '71, 71"-excitation Evidence is given, that the conversion to B as well as the photodecarbonylation of 5, 6 and 9 arise from an excited singulet state, but the conversion to C as well as the dimerization of 8 from the T,-state.
On '71, n*-excitation the 6-keto-enones 5-8 are isomerized to compounds B (18, 22, 26, 28) via 1,3-acyl shift and to compounds C (19, 23, 27, 29) via 1,2-acyl shift, whereas the p, y , 6, E-unsaturated ketone 9 gives the isomers 32 and 33 by 1,2-and 1,5-acyl shift, respectively. Furthermore, isomerization of 6 to 24, dimerization of 8 to 30 and addition of methanol to 8 (8 -+ 31) is observed. Unlike 7 and 8 the acyclic ketones 5, 6 and 9 undergo photodecarbonylation on '71, 71"-excitation Evidence is given, that the conversion to B as well as the photodecarbonylation of 5, 6 and 9 arise from an excited singulet state, but the conversion to C as well as the dimerization of 8 from the T,-state.
“…Although these two kinds of photoreactions are characteristic of their excited states, i.e., [1,3]-acyl shift from singlet and oxadi-π-methane rearrangement from triplet states, respectively, a mixture of products may be obtained due to indiscriminate population of either of the excited states during irradiation. The reaction mechanism of oxadi-π-methane rearrangement and [1,3]-acyl shift have been studied in great detail, and a correlation between excited state spin multiplicity and electronic configuration with reaction type has been established. 30b, 6 …”
A novel, general, and stereoselective route for rapid creation of
functionalized, linearly fused
cis:anti:cis triquinanes from aromatic
precursors has been presented. Cycloaddition of
2-methoxyspiro[cyclohexa-2,4-diene-6-2‘-oxacyclopropan]one with various
dienophiles and the photochemical
reaction of appropriately designed tricyclic systems, having an
α-methoxy β,γ-enone chromophore
upon triplet (3T) sensitization, are the key elements of
our approach. A new and efficient
methodology for the synthesis of endo-annulated
bicyclo[2.2.2]octenones having an α-methoxy β,γ-enone chromophore (23−36) from
o-vanillyl alcohol has been reported. The
2-methoxyspiro[cyclohexa-2,4-diene-6-2‘-oxacyclopropan]one was generated by oxidation of
o-vanillyl alcohol and trapped
with cyclopentadiene (17), spiroheptadiene (18),
and dimethylfulvene (19) to give the adducts
20−22, which were elaborated to a variety of chromophoric
systems 23−36. The structure
and
stereochemistry of the tricyclic chromophoric systems
(20−36) has been established through
their
high-field (300 MHz) 1H NMR, 13C NMR, and COSY
spectra. Triplet-sensitized photochemical
reaction of the chromophoric systems (23,
28−35) has been investigated. The triplet
excitation of
28−34 led directly to the formation of
functionalized triquinanes (43-48) in a single,
stereoselective
step. Interestingly, the irradiation of 23 furnished a
novel tetracyclic product 40, whereas the
irradiation of the substrate 35, having an additional
α,β-enone chromophore, gave a highly unusual
pentacyclic product 55 as a result of participation of the
OCH3 group in the photoreaction.
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