Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

Abstract

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

Figure 1

Structures of a few selected historically relevant [2 + 2] photocycloaddition products.

Scheme 1. [2 + 2] Photocycloaddition of an Olefin I via its First Excited Singlet State II (S₁)

Scheme 2. [2 + 2] Photodimerization of an Olefin I via its Excited Cu(I) Complex IV

Scheme 3. [2 + 2] Photocycloaddition of an Olefin I via its First Excited Triplet State VI (T₁)

Scheme 4. Sensitization via Triplet Energy Transfer from a Sensitizer (left) to an Olefin I (right)

Scheme 5. [2 + 2] Photocycloaddition of an Olefin I Mediated by Single Electron Transfer (SET) from a Catalyst (Top) to the Olefin I (Bottom)

Figure 2

Nomenclature to describe regio- and diastereoisomers in [2 + 2] photocycloaddition reactions.

Figure 3

Products of CuOTf-catalyzed [2 + 2] photodimerization reactions performed at λ = 254 nm in the ionic liquid [tmba][NTf₂].

Scheme 6. [2 + 2] Photodimerization Products from Norbornene rac-11 and Influence of a Tether on the Relative Configuration

Scheme 7. Intramolecular Cu(I)-Catalyzed [2 + 2] Photocycloaddition of Diallylsilane 16

Scheme 8. Intramolecular Cu(I)-Catalyzed [2 + 2] Photocycloaddition of Chiral 1,6-Heptadienes rac-19 and rac-20 via the Putative Complexes rac-21 and rac-22

Figure 4

Intramolecular [2 + 2] photocycloaddition of substrate rac-25 to product rac-26 in the synthesis of the nucleoside analogue cyclobut-A (rac-27).

Scheme 9. Conformational Control in the Intramolecular Cu(I)-Catalyzed [2 + 2] Photocycloaddition of Substrate rac-28

Scheme 10. Diastereoselective Cu(I)-Catalyzed [2 + 2] Photocycloaddition with Heteroatoms in the 1,6-Heptadiene Structure

Scheme 11. Cu(I)-Catalyzed [2 + 2] Photocycloaddition of Glucose-Derived 1,6-Heptadienes

Scheme 12. Intramolecular Cu(I)-Catalyzed [2 + 2] Photocycloaddition as an Approach to the Bicyclo[3.2.0]heptane Core of Bielschowskysin (40)

Scheme 13. Baeyer-Villiger Approach to Enantiomerically Pure Bicyclo[4.2.0]octanes from Bridged [2 + 2] Photocycloaddition Products

Scheme 14. Enantiomerically Pure Bicyclo[4.2.0]octanes by [2 + 2] Photocycloaddition and Enantioselective Reduction of an Imide

Scheme 15. Synthesis of Octacyclopropylcubane (51) by Direct Excitation of Diene 50

Scheme 16. Synthesis of Cage Acetal 53 by Xanthone-Sensitized Intramolecular [2 + 2] Photocycloaddition of Substrate 52

Scheme 17. Synthesis of Diazacyclooctanes 55 and rac-56 by Sensitized [2 + 2] Photocyclodimerization of N-acetyl Azetine (54)

Scheme 18. Synthesis of Tetrathiatetraasterane (58) by Intramolecular [2 + 2] Photocycloaddition of Dithiin 57

Scheme 19. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of Axially Chiral Substrate 59

Scheme 20. Schematic Description of Photochemical cis/trans Isomerization via the S₁ or T₁ State

Scheme 21. Synthesis of Cyclophane 64 by a Sequence of two Intramolecular Styrene [2 + 2] Photocycloaddition Reactions

Scheme 22. Synthesis of Pyridinocrownophane 66 by Intramolecular Vinylpyridine [2 + 2] Photocycloaddition

Scheme 23. Synthesis of Cyclobutene rac-68 by Intramolecular [2 + 2] Photocycloaddition of Silyl-Tethered Enyne 67 and Subsequent Removal of the Tether

Scheme 24. Synthesis of Cyclobutylene-Bridged Zirconocene 70 by Intramolecular [2 + 2] Photocycloaddition

Scheme 25. Synthesis of the Methyl Esters rac-72 of (±)-Rhododaurichromanic Acids A and B by Intramolecular [2 + 2] Photocycloaddition Reaction

The diastereomeric ratio refers to the stereogenic center at carbon atom C12. The methyl group can be either up (β) or down (α) relative to the cyclobutane. The former diastereoisomer is the methyl ester of (±)-rhododaurichromanic acid A, the latter of acid B.

Scheme 26. Synthesis of (±)-Melicodenine C (rac-75) by an Intermolecular [2 + 2] Photocycloaddition

Scheme 27. Conversion of (±)-Mahanimbine (rac-76) into (±)-Bicyclomahanimbine (rac-77) by Sunlight Irradiation

Scheme 28. Visible Light Photocatalysis in the Intramolecular [2 + 2] Photocycloaddition of 1-Arylalkenes

Figure 5

Tetrasubstituted cyclobutanes 84, katsumadain C (85), 86, and 87 obtained by [2 + 2] photodimerization.

Figure 6

Association of two molecules of stilbene 89 to 5,5-dihexylbarbituric acid (88) leading to the preferred formation of photodimer 90d.

Scheme 29. Intramolecular [2 + 2] Photocycloaddition of Tethered Stilbenes 91 upon Direct Excitation

Scheme 30. Intermolecular [2 + 2] Photocycloaddition of 9,9′,10,10′-Tetradehydrodianthracene (93) and Acetylene

Scheme 31. Intramolecular [2 + 2] Photocycloaddition of Tetrabenzoheptafulvalene 95

Scheme 32. [2 + 2] Photodimerization of N-Propionyl Dibenz[b,f]azepine 97 upon Direct Excitation

Scheme 33. Intermolecular [2 + 2] Photocycloaddition of Isocoumarin 99 and [2 + 2] Photodimerization of 4H,7H-Benzo[1,2-c:4,3-c′]dipyran-4,7-dione (101)

Scheme 34. De Mayo Reaction of Isoquinolone 103 to Diketone rac-105 and its Transformation to Tetracyclic Product rac-106 with a Galanthan Skeleton (XI)

Figure 7

Structures of chiral template 107 and of its enantiomer ent-107.

Scheme 35. Enantioselective Intramolecular [2 + 2] Photocycloaddition of Isoquinolone 108

Scheme 36. Enantioselective Intermolecular [2 + 2] Photocycloaddition of Isoquinolone (110) with Electron-Deficient Olefins

Scheme 37. Synthesis of Ladderane 113 by Intramolecular [2 + 2] Photocycloaddition of Cyclophane 112

Figure 8

Structures of β-aryl acrylates 114 and 115 and of β-truxinic acid (116) and ε-truxillic acid (117).

Scheme 38. Diastereoselective Synthesis of δ-Truxinate 119 by Intramolecular [2 + 2] Photocycloaddition of Chiral Dicinnamate 118

Figure 9

Structures of diamides 120 and 121, in which a covalent linker induces stereoselective [2 + 2] photodimerization of cinnamic to β-truxinic acid derivatives.

Scheme 39. Intramolecular [2 + 2] Photocycloaddition of Imide 122 to Products 123 and rac-124

Scheme 40. Bis(thiourea)-Mediated Diastereoselective [2 + 2] Photodimerization of Cinnamate 125

Scheme 41. Synthesis of (±)-Dictazole B (rac-130) by Intermolecular [2 + 2] Photocycloaddition

Scheme 42. Synthesis of (−)-Littoralisone (132) by Intramolecular [2 + 2] Photocycloaddition

Scheme 43. Synthesis of the Lignane Natural Product (±)-Tanegool (rac-136)

Scheme 44. Regioselectivity Preferences in the Intermolecular [2 + 2] Photocycloaddition of Enones

Scheme 45. Regioselectivity Preferences in the Intramolecular [2 + 2] Photocycloaddition of Enones

Scheme 46. Stereospecificity of [2 + 2] Photocycloaddition Reactions

Scheme 47. Product Diastereoisomers Formed from [2 + 2] Photocycloaddition Reactions of Cyclic Enones and Their Epimerization

Scheme 48. Intermolecular [2 + 2] Photocycloaddition of β-Functionalized 2-Cyclopentenone 137 and Consecutive Transformations

Scheme 49. Facial Diastereoselectivity in the Intermolecular [2 + 2] Photocycloaddition of Chiral 4-Substituted 2-Cyclopentenones

Scheme 50. Intermolecular [2 + 2] Photocycloaddition of Bicyclic Enone 151 and Structure of (+)-Kelsoene

Scheme 51. Intermolecular [2 + 2] Photocycloaddition of Tricyclic Enone rac-154 En Route to the Synthesis of (±)-Merrilactone A (rac-157)

Scheme 52. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of 2-Cyclopentenone rac-158

Scheme 53. Access to [4.5.5.5]Fenestranes by Intramolecular [2 + 2] Photocycloaddition

Scheme 54. Intramolecular [2 + 2] Photocycloaddition of Enone 162 En Route to (+)-Guanacastepenes A and E

Scheme 55. Diastereoselective Intramolecular [2 + 2] Photocycloaddition Due to Stereogenic Centers in the Tether

Scheme 56. Solvent Influence on the Intramolecular [2 + 2] Photocycloaddition of 2-Cyclopentenone rac-166

Scheme 57. Intramolecular [2 + 2] Photocycloaddition of Substrate rac-169 to the Formal Crossed Product rac-170

Scheme 58. Synthesis of Cage Ketone rac-173 by Intramolecular [2 + 2] Photocycloaddition

Scheme 59. Facial Diastereoselectivity in the Intermolecular [2 + 2] Photocycloaddition of Chiral 2-Cyclohexenones

Scheme 60. Intermolecular Allene [2 + 2] Photocycloaddition to Tricyclic Enones 178 and 180

Scheme 61. Enantioselective Approaches to [2 + 2] Photocycloaddition Products of 2-Cyclohexenone Employing Chiral Auxiliaries or Templates

Figure 10

Structures of selected enones, which were studied by Margaretha et al. in [2 + 2] photocycloaddition reactions.

Figure 11

Photodimerization products of enone 190.

Scheme 62. High Diastereoselectivity in an Intramolecular [2 + 2] Photocycloaddition Reaction Mediated by a Chiral Tether

Scheme 63. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of 2-Cyclohexenones

Scheme 64. Unusual Regioselectivity in the Intramolecular [2 + 2] Photocycloaddition of (−)-α-Pinene-Derived 2-Cyclohexenone 208

Scheme 65. Synthesis of Cage Diketone rac-211 by Intramolecular [2 + 2] Photocycloaddition

Scheme 66. Synthesis of Cage Diketones by Intramolecular [2 + 2] Photocycloaddition of 1,4-Cyclohex-2-enediones

Scheme 67. Intermolecular [2 + 2] Photocycloaddition of Diels–Alder Product 216 with Allene

Scheme 68. Intermolecular [2 + 2] Photocycloaddition of 3-Hexyne to Homobenzoquinone rac-218

Scheme 69. Intermolecular [2 + 2] Photocycloaddition of Chloranil (220) to Cyclooctene

Scheme 70. Undesired Intramolecular [2 + 2] Photocycloaddition of para-Quinone rac-222, an Intermediate in the Synthesis of (±)-Colombiasin A

Scheme 71. Intramolecular [2 + 2] Photocycloaddition of Naphthoquinone 224 to (−)-Elecanacin (225) and its Diastereoisomer 226

Figure 12

Structures of chalcone dimers rac-227, rac-228, and 229.

Figure 13

Structures of multifunctional chalcones 230 and 231 as employed in [2 + 2] photocycloaddition reactions.

Scheme 72. Intermolecular [2 + 2] Photocycloaddition of 232

Scheme 73. [2 + 2] Photodimerization of 3-Arylindenone 235

Scheme 74. Intramolecular [2 + 2] Photocycloaddition of (+)-Sclareolide-Derived Enone 237

Scheme 75. Intramolecular [2 + 2] Photocycloaddition of Acylcyclopentene rac-239

Scheme 76. Intermolecular [2 + 2] Photocycloaddition of Enynone 241 with α-Acrylonitrile

Scheme 77. [2 + 2] Photodimerization of 3-Alkynyl-2-cycloheptenone 243

Scheme 78. Divergent Regioselectivity in the Intramolecular [2 + 2] Photocycloaddition of Furan-3(2H)-ones rac-246

Scheme 79. Synthesis of Biyouyanagins A (250) and B (251) by Intermolecular [2 + 2] Photocycloaddition

Scheme 80. Intermolecular [2 + 2] Photocycloaddition of Enone 253 with Diene 254 En Route to a Formal Total Synthesis of (±)-Erysotrine

Scheme 81. Representative Intermolecular [2 + 2] Photocycloaddition Reactions of 4-Hetero-2-cyclohexenones 256 and 258

Scheme 82. Facial Diastereoselectivity in the Intramolecular [2 + 2] Photocycloaddition of Dihydropyridones rac-261

Scheme 83. Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions of 2,3-Dihydropyridin-4(lH)-ones

Scheme 84. Intramolecular [2 + 2] Photocycloaddition of Pyran-2,4-dione rac-266

Scheme 85. [2 + 2] Photodimerization of Chromone 268

Scheme 86. Photochemical Reaction of Chromone 270 and Ethylene

Scheme 87. Enantioselective Ring Opening/Allylation of [2 + 2] Photocycloaddition Products rac-273

Scheme 88. Intramolecular [2 + 2] Photocycloaddition of 3-Acetoxy-2-cyclopentenone rac-275

Scheme 89. Intramolecular [2 + 2] Photocycloaddition of 3-Alkenyloxy-2-cyclohexenone rac-277

Scheme 90. Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions of 3-Alkenyloxy-Substituted Enones

Scheme 91. Chirality Transfer in the Intramolecular [2 + 2] Photocycloaddition of Axially Chiral Allenes 282

Total yield of (E)- and (Z)-diastereoisomers, the ee refers to the respective (E)-diastereoisomer.

Scheme 92. Synthesis of Pyrroles 287 by a Sequence of Intramolecular [2 + 2] Photocycloaddition and retro-Mannich Fragmentation

Scheme 93. De Mayo Reaction as an Entry to 1,5-Dicarbonyl Compounds such as rac-289

Scheme 94. Intramolecular [2 + 2] Photocycloaddition of Enone 290 as a Pivotal Step in the Synthesis of Manzamine Alkaloids

Scheme 95. Synthesis of 6-Azabicyclo[3.2.1]octan-3-one rac-298 by a Photocycloaddition–retro-Mannich–Mannich Cascade

Scheme 96. Intermolecular [2 + 2] Photocycloaddition Reactions of 5-Substituted 2(5H)-Furanones 299 and 301

Scheme 97. Intermolecular [2 + 2] Photocycloaddition of Ethylene to Bis(butenolide) 303

Scheme 98. Construction of the Bielschowskysin Core by Intramolecular [2 + 2] Photocycloaddition

Scheme 99. Crossed Intramolecular [2 + 2] Photocycloaddition Reaction of rac-307 as an Approach toward the Core Fragment of Solanoeclepin A

Scheme 100. Intramolecular [2 + 2] Photocycloaddition of N-Boc-Protected 4-(Allylaminomethyl)-2(5H)-furanones 309

Scheme 101. Intramolecular [2 + 2] Photocycloaddition of Carbohydrate-Derived 5,6-Dihydro-2(2H)-pyranone 311

Scheme 102. Approach to the Tricyclic Skeleton of (±)-Aquatolide by Intramolecular [2 + 2] Photocycloaddition

Figure 14

Structures of intermolecular [2 + 2] photocycloaddition products of di-α-pyrones.

Figure 15

Structures of coumarin dimer rac-317 and of [2 + 2] photocycloaddition product 318.

Scheme 103. Enantioselective [2 + 2] Photocycloaddition of Coumarins Mediated by a Chiral Lewis Acid

Scheme 104. Enantioselective [2 + 2] Photocycloaddition of Coumarins Mediated by Chiral Thiourea 322

Scheme 105. Enantioselective Intermolecular [2 + 2] Photocycloaddition Reaction of Axially Chiral Amide 323

Scheme 106. Intermolecular [2 + 2] Photocycloaddition of Benzodipyrandione 326

Figure 16

Structures of various maleic anhydride [2 + 2] photocycloaddition products and of (−)-sceptrin (332).

Scheme 107. Formation of Cyclobutene Photocycloaddition Product rac-334 and its Consecutive Reaction to Cyclodecadiene 335

Scheme 108. Pivotal Photochemical Step in the Synthesis of (+)-Byssochlamic Acid

Scheme 109. Access to Cyclobutane 341 via Oxaquadricyclane 339

Scheme 110. Intermolecular [2 + 2] Photocycloaddition of Tetronate 342

Scheme 111. Intramolecular [2 + 2] Photocycloaddition of Various Enones to a Trifluoro-Substituted Olefin

Scheme 112. Diastereotopos-Differentiating [2 + 2] Photocycloaddition of Tetronate rac-346

Scheme 113. Divergent Regioselectivity in the Intramolecular [2 + 2] Photocycloaddition of Tetronates rac-348 and rac-349

Scheme 114. Intermolecular [2 + 2] Photocycloaddition of 1,3-Dioxin-4-one 352 with a Chiral 3-Pyrroline

Scheme 115. Photochemical Key Steps En Route to (±)-Saudin and (±)-Ingenol

Scheme 116. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of 1,3-Dioxin-4-one 360

Scheme 117. Formation of a Strained Bicyclo[2.2.0]hexane Core by an Intramolecular [2 + 2] Photocycloaddition

Scheme 118. Intramolecular [2 + 2] Photocycloaddition of 3-Benzyloxy-2(5H)-furanone 364 En Route to (+)-Lactiflorin

Figure 17

Structures of 3-halo-2(5H)-furanones 366, 367, of ascorbic acid derivative 368, and of (Z)-ligustilide (369).

Scheme 119. Synthesis of (+)-Chloranthalactone F (371) from (+)-Chloranthalactone A (370) by [2 + 2] Photodimerization

Scheme 120. Sequence of [2 + 2] Photodimerization/Intramolecular [2 + 2] Photocycloaddition upon Irradiation of Dihydropyridine 372

Scheme 121. Intramolecular [2 + 2] Photocycloaddition of 2-Azabicyclo[2.2.2]octene rac-374

Scheme 122. Synthesis of the Bielschowskysin Core by Intramolecular [2 + 2] Photocycloaddition

Scheme 123. Synthesis of 4-Fluoro-2,4-methanoproline (rac-380) by Intramolecular [2 + 2] Photocycloaddition

Scheme 124. Intramolecular [2 + 2] Photocycloaddition of β-Aminoalkylidene Malonate 381

Scheme 125. Intermolecular [2 + 2] Photocycloaddition of Fumarate 383 and cis-Stilbene upon Irradiation of the CT Complex

Scheme 126. Facial Diastereoselectivity in the Intermolecular [2 + 2] Photocycloaddition of Pyrrol-2(5H)-one 385

Scheme 127. Intramolecular [2 + 2] Photocycloaddition of Pyrrol-2(5H)-ones rac-388 and rac-390

Scheme 128. Enantioselective Intramolecular [2 + 2] Photocycloaddition of 5,6-Dihydro-1H-pyridin-2-one 392 in the Presence of Chiral Template ent-107

Scheme 129. Intermolecular [2 + 2] Photocycloaddition of Pyridone 394 with Acrylonitrile

Scheme 130. Intramolecular [2 + 2] Photocycloaddition of 2-Pyridones 396

Scheme 131. Enantioselective Intramolecular [2 + 2] Photocycloaddition of 4-Substituted Quinolones

Scheme 132. Enantioselective Intermolecular [2 + 2] Photocycloaddition of Quinolones 402 and 404

Scheme 133. Intramolecular [2 + 2] Photocycloaddition of Quinolone 406 in the Presence of Chiral Sensitizer 407

Figure 18

Structures of chiral sensitizers 409–411 with a 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton.

Scheme 134. Catalytic Enantioselective Intramolecular [2 + 2] Photocycloaddition of Quinolone 412

Scheme 135. Catalytic Enantioselective Intermolecular [2 + 2] Photocycloaddition of Pyridone 414

Scheme 136. Visible-Light-Induced Enantioselective [2 + 2] Photocycloaddition to Products 417 Catalyzed by Chiral Thioxanthone 411

Scheme 137. Frozen Chirality Approach to the Intermolecular [2 + 2] Photocycloaddition of Quinolone Ammonium Salt 418

Scheme 138. Comparison of Batch vs Flow in the Intermolecular [2 + 2] Photocycloaddition of Maleimide (420)

Scheme 139. Sensitized Intramolecular [2 + 2] Photocycloaddition of Dimethylsubstituted Maleimide rac-422

Scheme 140. Intramolecular [2 + 2] Photocycloaddition of Tethered Maleimides 424 to Cage Diimide 425

Scheme 141. Intramolecular [2 + 2] Photocycloaddition of Thiomaleimide 426 upon Direct Excitation

Scheme 142. Chirality Transfer in the Intramolecular [2 + 2] Photocycloaddition of Axially Chiral Maleimide 428

Figure 19

Structures of 2′-deoxyuridine (430a) and thymidine (430b) and of their [2 + 2] photocycloaddition products to 2,3-dimethyl-2-butene.

Scheme 143. Intermolecular [2 + 2] Photocycloaddition of 4-Pyrimidinones 433 and Ethylene

Scheme 144. Intramolecular [2 + 2] Photocycloaddition of 4-Pyrimidinone 435

Scheme 145. Auxiliary-Induced Diastereoselectivity in the [2 + 2] Photocycloaddition to 3-Azabicyclo[3.2.0]heptan-2-ones 438

Scheme 146. Intramolecular [2 + 2] Photocycloaddition of Imide 439 to the Straight Product 440

Scheme 147. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of Axially Chiral Diimide 442

Scheme 148. Intermolecular [2 + 2] Photocycloaddition of 1-Thiocoumarin (444) and Tetrachloroethylene

Figure 20

Structures of thiocoumarin-containing bichromophoric compounds and preferred site of [2 + 2] photocycloaddition reactions.

Scheme 149. Intramolecular [2 + 2] Photocycloaddition of Cyano-Substituted Cyclobutene rac-449

Scheme 150. Diastereoselective Intramolecular [2 + 2] Photocycloaddition of Iminium Salt 451

Scheme 151. Intramolecular [2 + 2] Photocycloaddition of (±)-Ipsdienol (rac-453)

Scheme 152. Intramolecular [2 + 2] Photocycloaddition as an Entry to the Core Fragment of (±)-Artocarpol A

Scheme 153. Ir-Catalyzed Intramolecular [2 + 2] Photocycloaddition Reaction of Dienes 457

Figure 21

Structures of representative electron transfer photocatalysts 459–464.

Scheme 154. Intramolecular [2 + 2] Photocycloaddition Reaction of 2,6-Diphenylhepta-1,6-diene (465)

Scheme 155. Intermolecular [2 + 2] Photocycloaddition Reaction of 2-Vinylbenzofuran (468) and Styrene

Scheme 156. Visible-Light-Induced Intramolecular Oxidative [2 + 2] Photocycloaddition Reactions of Bis(styrenes) 470

5 mol% 462 was used.

Scheme 157. Intermolecular [2 + 2] Photocycloaddition Reaction of Styrenes

Scheme 158. Visible-Light-Induced [2 + 2] Photodimerization of Aromatic Alkenes,,

50 mol% Naphthalene was used as the electron relay compound. 75 mol% Anthracene was used. The reaction was performed in acetone at −45 °C.

Scheme 159. Visible-Light-Induced Intramolecular [2 + 2] Photocycloaddition Reaction of Coumarin 475

Scheme 160. Synthesis of the Core Skeleton of (±)-Nakamuric Acid by Visible-Light-Induced Intramolecular [2 + 2] Photocycloaddition Reaction of Vinylimidazole 477

Scheme 161. Intramolecular [2 + 2] Photocycloaddition Reactions of Bis(enones) 479

Scheme 162. Intermolecular [2 + 2] Photocycloaddition Reactions of Acyclic Enones

Scheme 163. Intramolecular [2 + 2] Photocycloaddition Reaction of Enone 483 Bearing a Cleavable Redox Auxiliary

Scheme 164. Dual Catalysis Strategy in Enantioselective [2 + 2] Photocycloaddtion Reactions of Enones,,,

d.r. = 86/14. d.r. = 88/12. d.r. = 75/25. d.r. = 89/11.

Scheme 165. Intermolecular [2 + 2] Photocycloaddition between para-Benzoquinone 489 and Acenaphthylene (490)

Scheme 166. Intermolecular [2 + 2] Photocycloaddition Reaction between C₆₀ (493) and 4-Vinylanisole

Scheme 167. [2 + 2] Photocycloaddition Reaction of 1,3-Cyclohexadiene and Styrene Catalyzed by the Pyrene-Indole Bichromophoric Complex 496