The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

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.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.5b00723

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

Progress in the synthesis and transformations of alkylidenecyclopropanes and alkylidenecyclobutanes.

The first examples of enantioselective intermolecular [2+2] photocycloadditions of isoquinolone with alkenes are reported. Photoreactions were carried out at low temperature in the presence of a chiral hydrogen-bonding template, which effectively shields one face of the substrate through formation of a hydrogen-bonded supramolecular complex. Functionalized cyclobutane products were obtained in excellent yields (86–98%) and with outstanding regio-, diastereo-, and enantioselectivity (88–99% ee).

Enantioselective intermolecular [2+2] photocycloadditions of isoquinolone mediated by a chiral hydrogen-bonding template.

Enantioselective synthesis of pyrrole-annulated medium-sized-ring compounds by an iridium-catalyzed allylic dearomatization/retro-Mannich/hydrolysis sequence is presented. Various substituted pyrrole-annulated seven- and eight-membered-ring products were obtained under mild reaction conditions with moderate to good yields and excellent enantioselectivity. Additionally, these products contain a scaffold widely distributed in natural products and biologically active compounds. The current method provides a convenient way for accessing such pyrrole-anuulated medium-sized-ring compounds.

Iridium-Catalyzed Enantioselective Synthesis of Pyrrole-Annulated Medium-Sized-Ring Compounds.

Formation of bicyclic pyrroles and furans through an enone allene photocycloaddition and fragmentation sequence

Intramolecular Butenolide Allene Photocycloadditions and Ensuing Retro-Ene Reactions of Some Photoadducts

The [2+2] photocycloaddition of allenes 15–18, 23 and 26 was studied. Irradiation of a solution of these substrates in acetonitrile at 300 nm resulted in the clean conversion of the starting materials into a mixture of photoproducts. The major product in all cases was a bicyclic pyrrole or furan fused to an eight membered ring (43–70 % yield). The formation of these products is thought to be a result of a heteroatom-induced fragmentation of the straight adduct (7). This is supported by irradiation of the carbon analogue 32 which al

Pauli J. Wrigstedt

Papers

Formation of bicyclic pyrroles and furans through an enone allene photocycloaddition and fragmentation sequence

Intramolecular Butenolide Allene Photocycloadditions and Ensuing Retro-Ene Reactions of Some Photoadducts

Formation of Bicyclic Pyrroles and Furans Through an Enone Allene Photocycloaddition and Fragmentation Sequence.