Photocyclization, stilbenes

Stilbenes, photocyclization of, 30, 1 StiUe reaction, 50, 1 Stobbe condensation, 6, 1 Substitution reactions using organocopper reagents, 22, 2 41, 2 Sugars, synthesis by glycosylation with sulfoxides and sulfinates, 64, 2 Sulfide reduction of nitroarenes, 20, 4 Sulfonation of aromatic hydrocarbons and aryl halides, 3, 4 Swem oxidation, 39, 3 53, 1... 

The key step in the synthesis of the azaphenanthrene alkaloid eupoulaur-amine 45, is an intramolecular SRN1 reaction of 3-bromo-Af-methyl-A-(2-oxo-2-phenyl-ethyl)-isonicotinamide 44, followed by in situ stilbene photocyclization, and further methylation (Sch. 46). 

Further extensions of the stilbene photocyclization are seen in analogous reactions of compounds containing the imine chro-mophore (e.g. 3,71) or an amide group (3.72). The amide reaction can be considered formally as giving a zwitterion intermediate, which undergoes proton shifts and oxidation to form the observed product. Non-oxidative cyclizations that start with either N-vinyl aromatic carboxamides (C=C— N— CO—Ar) or N-aryl a,p-unsaturated carboxamides (Ar— N—CO—C=C) have been extensively used to make quinoline or isoquinoline alkaloids and their derivatives a fairly simple example is given in (3.73). 

The photochemical reactions of some a-arylvinyl bromides (31), in acetic acid in the presence of sodium acetate and tetraethylammonium bromide (labeled with 8zBr) give nucleophilic substitution, reductive debromination, cis-stilbene photocyclization, and oxidation as the primary pathways to product... 

Compound 37b was synthesized using a stilbene photocyclization approach key steps in this route are the preparation of the aldehyde 40 and the final stilbene photocyclization (Scheme 7). The methyl group is incorporated to favor formation of the desired photocyclized product. The route is convergent and affords 1-triptycyl[4]helicene 37b in 20% overall yield (4 steps). 

Based on the experience obtained from the synthesis of the [4]helicene 37b, we initially sought to prepare 4-(9-triptycyl)phenanthrene (36) also using a stilbene photocyclization strategy. Unfortunately, various photochemical approaches failed in the final photocyclization step. Given these results, we... 

Carruthers and Stewart were the first to extend the process of stilbene photocyclization to the syntheses of condensed thiophenes. As noted previously, the four monomethyl derivatives (1- to 4-) of 16, where the methyl group is located in the D ring, were prepared by Tilak and Pschorr thiannula-tions (vide supra). Carruthers and Stewart101 used photocyclization to prepare samples of five other monomethyl derivatives (5- to 9-) (substituents... 

Discotic phenanthrene derivatives have been prepared by Scherowsky and Chen [39]. Their synthesis is illustrated in Scheme 14. The phosphonium salt prepared by bromination of trimethoxybenzyl alcohol and reaction of the benzyl bromide with triphenyl phosphine was deprotonated and a Wittig reaction with 2,4-dimethoxy-or 2,3,4-trimethoxybenzaldehyde produced the stilbene. Photocyclization gives the phenanthrene, which can be demethylated with boron tribromide and acylated using the Steglich method. 

Z-Stilbene also undergoes photocyclization to 4a,4b-dihydrophenanthrene via an electrocyclization. ... 

V-Phenylbenzylimines undergo stilbene-type photocyclizations (equation 83) to yield heterocyclic compounds143. The reactions usually take place via the iminium salt and need oxidants like oxygen or iodine. Six-electron electrocyclic reactions have been observed for 1-aza-l,3-dienes144 (equation 84). 

The photocyclization has been found not to occur with stilbenes substituted with acetyl, dimethylamino, or nitro groups, lodo substituents are replaced by hydrogen by photolysis in cyclohexane solution. w-Substituted stilbenes give mixtures of 2- and 4-substituted phenanthrenes which generally are difficult to separate. 

Phosphoryl-stabilized anions, 25, 2 Photochemical cycloadditions, 44, 2 Photocyclization of stilbenes, 30, 1 Photooxygenation of olefins, 20, 2 Photosensitizers, 20, 2 Pictet-Spengler reaction, 6, 3 Pig liver esterase, 37, 1 Polonovski reaction, 39, 2... 

Polycyclic aromatic compounds, synthesis by photocyclization of stilbenes, 30, 1... 

The influence of steric effects on the photocyclization process is clearly discernible for the ortho and meta substituted stilbenes. In these cases steric repulsion intervenes as the two C atoms forming the new bond approach. This steric repulsion opposes the stabilizing interactions which promote the cyclization process. As a result the reactivity is decreased. Other consequences of such superimposed steric... 

The l,3,5-triene-l,3-cyclohexadiene interconversion is a six-electron electrocycli-zation that requires a cis central double bond to occur.245 An important application of this rearrangement is the photocyclization of cis-stilbene to dihydrophenanthrene [Eq. (4.45)], which is usually further oxidized to phenanthrene 249... 

F. B. Mallory and C. W. Mallory, Organic Reactions, vol. 30. Wiley (1984). This is an extensive compilation and discussion of the photocyclization reactions of stilbene and related systems. 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

The photocyclization of stilbene and its simple derivatives to phenanthrene and substituted phenanthrenes is now a well-documented and useful synthetic reaction the scope and mechanism of this reaction have been thoroughly reviewed by Stermitz.344 There... 

The photocyclization of stilbenes to phenanthrenes has been used in the synthesis of the aporphine ring system347,348 conversion of the... 

Numerous examples of stilbene-to-dihydrophenanthrene photocyclization incorporating oxygen and sulfur heterocycles have been reported. Oxidation to the phenanthrene is usually effected by added iodine or by oxygen. Thus, irradiation of 2,3-diphenylchromone (37) results in the formation of phenanthro[9,10 -2,3]chromone (38)29 analogous photocyclizations have... 

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