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4 Photochemical Electrocyclization Reactions: Synthesis of Fused, Five-Membered Ring Compounds

4 Photochemical Electrocyclization Reactions: Synthesis of Fused, Five-Membered Ring Compounds

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j 8 Formation of a Five Membered Ring



262



X



X







127



X



O2/I2



128



129



X= N.O,S,Se

Scheme 8.37



intermediate 128 (re aromatization being the driving force), in the presence of

oxygen or iodine as oxidants, leads to the formation of cyclized product 129.

The irradiation of diphenyl amine 130 (X ¼ NHPh) [63 65], diphenyl ether 130

(X ¼ O) [66, 67] or diphenyl thioether 130 (X ¼ S) [66] derivatives is reported to cause

efficient cyclization reactions and to produce the corresponding carbazoles, diben

zofurans, and dibenzothiophenes, respectively (Scheme 8.38).

There are also known examples [68] where a corresponding zwitterionic inter

mediate 134 (Scheme 8.39), obtained from the irradiation of aryl amine 133 in

acetonitrile containing aqueous HCl, instead of becoming aromatic through oxida

tion, undergoes protonation to produce dihydrocarbazole intermediate 135 which by

H

X



H







O2/I2



130



X



X



131



132



X= NHPh,O,S

Scheme 8.38



Me

N







MeO



N

Me



OMe

133

MeO



MeO



H



H



MeO



N

Me



134



OMe



H+



OMe



OMe



MeO



OMe

H+



H+



[1,5]-H shift



[1,3]-H shift



N

Me

136



MeO



O



N

Me

Scheme 8.39



138



96%



135



Hydrolysis



N

Me

137



8.4 Photochemical Electrocyclization Reactions





N

Me



Ph



Ph

N

Me



>300 nm



140

73%



139

Scheme 8.40



sequential [1,5] hydrogen and [1,3] hydrogen shift, followed by proton assisted

hydrolysis, gives 1,2,4 trihydro(4aH) carbazole 3 ones (138) in high yield.

Several examples pertaining to the photochemical electrocyclization of com

pounds possessing an aromatic moiety and an olefinic substituent are also known [69,

70]. For example, one of the earliest reports in this series was concerned with the

photocyclization rearrangement of a (N methylanilino) styrene (139) which, on

irradiation in the absence of oxygen, produced 1 methyl 2 phenyl 2,3 dihydroindole

(140) in 73% yield, as depicted in Scheme 8.40.

The stereochemistry of such photocyclizations was also explored [71] by irradiating

enamines of type 141 which gave trans indolines 142 as the major reaction product,

along with minor cis 143 (Scheme 8.41).

Interestingly, the enamine of cyclopentanone 144, upon irradiation [72], gave only

the cis indoline 145 in 52% yield; this indicated the effect of ring strain on the

stereochemistry of the indolines (Scheme 8.42). Mechanistically, several pathways

H



H



N

Me



+



>300 nm



N

H

Me



N

H

Me



141



142



143



71% major



3% minor



Scheme 8.41



H H



N

Me

H



146



144



N

Me



N

H

Me

145

52%



H H



N

Me

Scheme 8.42



147



j263



j 8 Formation of a Five Membered Ring



264



O



O

H H



O





X



[1,4]-H shift



X



N

Et

148



oxidation



N

Et



X



N

Et



50%



150



149



X = -CH-, N

Scheme 8.43



may be considered for such cyclization reactions. For example, an intramolecular

[2 ỵ 2] photocycloaddition of 144 may give cis dihydrocyclobutane 146, which might

change to 145 under photoreaction conditions. Alternatively, this cyclization can also

involve a vibrationally excited ground state via an allowed disrotatory process to give

the cis dihydro intermediate 147 which, on H shift, produces 145. Another mech

anism which should also be considered involves ylide protonation from the photo

reaction medium.

Enaminones of type 148, on irradiation in a mixture of benzene/methanol solvent,

undergo a smooth cyclization (Scheme 8.43) to produce the corresponding indolones

150 in high yield [73 75].

This enaminone cyclization route to indolones has been applied [76 79] for the

synthesis of a variety of aspidospermidine class of alkaloids 153 155 from com

pound 152, which was obtained in very high yield from the photolysis of 151, as

illustrated in Scheme 8.44.

This type of photocyclization strategy has also been utilized [80] to construct

indoline substructures 157 and 159 by irradiating enamine derivatives bearing an

electron withdrawing substituent on the olefinic double bond, such as 156. This

was produced as a mixture of cis and trans diastereomeric indolines, as shown

in Scheme 8.45. The formation of diastereomers (157 and 159) is explained by



O



R2

N

Bn



O



R1



H



R1







N

2



R



N

H

Bn

152

77-95%



151



N

Bn 155

R

N R1



R

N R1



R2



R2

N

Bn

Scheme 8.44



153



N

Bn



154



8.4 Photochemical Electrocyclization Reactions



CN



CN

CO2Et

O



N

Bn



CO2Et







O

N

Bn Bn



Bn



156



157 31%





CN

CN

O Bn



EtO2C



CO2Et







N

Bn



O

N

Bn Bn



158



159 62%



Scheme 8.45



N



NBu

O

N

Bn

160







CH2OBn



N

H

161



Bn

N



O

CH2OBn



N

H



Bn

O

CH2OBn



162

2.4:1 (68%)



Scheme 8.46



considering the photochemical E/Z isomerization of the double bond before the

photocyclization step.

The same group has extended this photocyclization approach to obtain the

corresponding debenzylated spiroindolines 161 and 162 in 68% yield and 2.4 : 1

ratio from the photocyclization of 160 (Scheme 8.46).

The role of the zwitterion intermediate 164 and its rearrangement in the photo

cyclization of aromatic thioethers 163 to arene dihydro thiophene derivatives 165 is

supported [81] by its independent trapping (via 1,3 dipolar cycloaddition) with N

phenyl maleimide to obtain 166 in high yields (Scheme 8.47).

The intramolecular trapping of ylide 164 is also reported [82] in the

photoreaction of 167, which gave unexpected 169 as a major product, as depicted

in Scheme 8.48.

The expected intramolecular 1,3 dipolar cycloaddition product 171 was only a

minor product (3%). The formation of major product 169 was explained through an

intramolecular Michael reaction of the enolate ion.



j265



j 8 Formation of a Five Membered Ring



266



S





H

H

O



S

163



NPh

164

O

O

NH



H



H

S



H

N Ph



S



H



O



O

166



165



Scheme 8.47







S

EtO2C



S



CO2Et



O

O

167



168



S



S



CO2Et

H CO Et

2



S

O



EtO2C



O



O

169

84%



170

5%



171

3%



Scheme 8.48



8.5

Photoinduced Electron Transfer-Mediated Cyclizations:

Synthesis of Five-Membered Carbocyclic and Heterocyclic Ring Systems

8.5.1

Radical Cation-Mediated Carbon Carbon Bond Formation



The reaction of organic radical cations have been the focus of much interest, and their

synthetic reactions including addition to alkenes and nucleophilic capture by

alcohols resulting in carbon carbon and carbon oxygen bond formation, respec



8.5 Photoinduced Electron Transfer Mediated Cyclizations



CH2OH



hν /DCA

DCM



δ+



CH2OH



CH2OH



CH2OH

DCA



DCA

(δ−)



j267



70%



172



173

contact-ion pairs



other isomers



hν/ DCB

CH3CN

Ar

CH2OH



CH2OH



O



O



DCB



Ar = p-C6H4-CN

solvent-separated

ion pairs



3 isomers (26%)



3 isomers (46%)



174



175



Scheme 8.49



tively has been investigated in detail. For example, Roth’s group [83] studied the

electron transfer photochemistry of geraniol and farnesol (Scheme 8.49), where

radical cations were shown to add to olefin and generate five membered ring systems.

The electron transfer photoreaction between DCA and (E) 3,7 dimethylocta 2,6 dien

1 ol (geraniol); (E) 172) in dichloromethane produces mainly cis 2 (2 propenyl)

trans 5 methylcyclopentanemethanol (173, 70%), whereas the photoreaction using

1,4 dicyanobenzene (DCB) in acetonitrile produced all of the possible cyclic ethers

(174, 175).

The difference in product distribution was explained by considering the involve

ment of solvent separated ion pairs of 172 DCB and contact ion pairs 172 DCA.

Similar observations were also made with farnesol.

Demuth et al. [84] have studied, systematically, the PET cyclization of isoprenoid

polyalkenes using sterically hindered electron acceptors such as 1,4 dicyano 2,3,5,6

tetramethyl benzene (TMDCB) in combination with biphenyl (BP) as a cosensitizer

(Scheme 8.50). The cyclization mode (6 endo trig versus 5 exo trig) yielding five

and six membered rings was found to depend on the substitution pattern of the

polyalkenes. For example, the PET reaction of isoprenoid polyalkene 176a in

acetonitrile/water (4 : 1) gave 177 (15 30%), while 176b produced 178 ($50%). The

synthetic scope of such transformations has also been explored with polyalkenes 179

and 181, which produced functionalized bi and tricyclic (all trans fused) com

pounds 180 and 182, respectively (Scheme 8.50). The reaction is initiated by the

regioselective one electron transfer oxidation of the w alkene site of the acyclic

polyalkene, giving rise to a radical cation which is trapped by a nucleophile such

as water or methanol present in the reaction medium. The resulting neutral radical

initiates a cyclization cascade, resulting in a tertiary radical. Termination of the

process is achieved either by hydrogen atom transfer to this radical, or through

reduction of the final radical.



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