Photochemical rearrangements 1.5- diene

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

Dienes carrying alkyl or aryl substituents on C-3 can be photochemically rearranged to vinylcyclopropanes in a reaction called the di-n-methane rearrange-... 

Pirkle, W. H. Smith, S. G. Koser, G. F. Stereospecificity and wavelength dependence in the photochemical rearrangement of spiro[2.5]octa-4,7-dien-6-ones to quinone methides. j. Am. Chem. Soc. 1969, 91, 1580-1582. 

It is apparent from the quantity of material included in this chapter that there is an extensive body of work concerning the utilization of diene and polyene photochemistry in a synthetic setting. The unique behavior of the excited chromophores permits the application of powerful new methods for the construction of complex molecules. Unusual photochemical rearrangements and photocycloaddition pathways often lead to substantial increases in molecular complexity, allowing such processes to serve as key strategic steps in target oriented syntheses. 

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

Studies of di-ir-methane photochemical rearrangements have been one of the main areas of research in organic photochemistry for many years (for reviews, see Refs. 1-4). The first example of a reaction of this type was reported by Zimmerman in 1967 in the sensitized irradiation of barrelene 1 that yields semibullvalene 2 [5] (Scheme 1). The reaction has been extended to a large number of acyclic and cyclic 1,4-dienes that yield the corresponding vinylcyclo-propanes on irradiation, in the di-ir-methane (DPM) version of the rearrangement. This reaction also takes place when a vinyl unit is replaced by an aryl group. A few representative examples of DPM rearrangements are shown in Scheme 1 [6-9]. 

The photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

A Transoid 1,3-Diene. In contrast to the considerable number of photochemical rearrangements of homoannular dienes only one photochemical rearrangement of a transoid heteroannular diene has been reported. Irradiation of 3,5-cholestadiene (Formula 400) in ethanol gives the pentacyclic ether (Formula 401) (176). 

Scheme 5 illustrates the photochemical rearrangement of 4//-1,2-diazepine 23 to 6//-1,4-diazepine 25, which occurs via an aza-di-7i-methane reaction with l,2-diazabicyclo[3.2.0]hepta-2,6-diene 24, as the intermediate <1996CRV3065>. 

An electrocyclic photochemical rearrangement of oxepins to cyclobutene ring-containing valence isomers has also been reported. Thus 2-oxabicyclo-[3.2.0]hepta-3,6-diene has been isolated upon photoisomerization of 1. Similarly, when 1-benzoxepin 100 was irradiated, the valence tautomer3,4-benz-2-oxabicyclo[3.2.0]-hepta-3,6-diene was formed. ... 

The photochemical rearrangement of a cyclohexa-1,3-diene with a s-cis fixed diene unit to give the corresponding bicyclo[3.1.0]hex-2-ene has been reported. Thus the photolysis of dehyd-roergosterol acetate in benzene solution led to photodehydroergosterol 5. ... 

Alkyl-allyl complexes of isomeric systems can be interconverted and thus be used in isomerization of vinylcyclopropanes. Ethyl 4-azabicyclo[5.1.0]octa-2,5-diene-4-carboxylate (20) reacts with pentacarbonyliron to give complex 21, which photochemically rearranges to complex 23. Carbonylation of both products 21 and 23 leads to ethyl 9-oxo-2-aza-bicyclo[3.3.1]nona-3,7-diene-2-carboxylate (22). While complex 21 upon heating regenerates the starting material, complex 23 gives the isomeric product 24. In contrast to iron, with rhodium only the endo-complex 25 is formed. ... 

The primary process is the formation of (5), which can be isolated but very easily decomposes to (6) and 1.5-cyclooctadiene. Absorption of another light quantum converts (5) - -(7), in a third photochemical step (7) can be isomerized to 1.3-cyclooctadiene iron tricarbonyl (8), which can be prepared directly from 1.3-cyclooctadiene, too. Reaction (7)- - (8) comprises an example of the photochemical rearrangement of a photochemically formed metal carbonyl complex. Such reactions occur quite frequently and can be of preparative value (see section F. 2,3). In (6) the diene is acting as a bridging ligand, other cases of this type of coordination are found in 164,228). 

The photochemical rearrangement of cyclo-octa-1,5-diene in the presence of [Rh2(Cl)2( -l,5-cod)2] gives the 1,4-diene as the exclusive primary photoproduct but this undergoes subsequent isomerization to other compounds as the reaction... 

Photochemical rearrangement reactions of 1-aza-1,4-dienes and 2-aza-1,4-dienes from their triplet states to form corresponding cyclopropylimines are known as the aza-di-n-methane rearrangements [62]. 

The different Antarafacial and suprafacial sigmatropic shifts of H-atom or other groups which are photochemically allowed have been studied under photochemical rearrangements of 1, 5-diene. 

In the above example, substituted 1, 5-diene gives (A) as major product on photochemical rearrangement. Why ... 

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