Enone Olefin [2 + 2] Photochemical Cycloadditions

Abstract

The photochemical [2 + 2] cycloaddition reaction of alkenes to enones, the light‐induced cycloaddition of an excited state enone to a ground state alkene to produce a cyclobutane, is a highly useful reaction in organic synthesis since two new carbon–carbon bonds are formed and a maximum of four new stereogenic centers are introduced into the molecule in the process. The first [2 + 2] photocycloaddition reaction was reported by Ciamician in 1908 when he observed the formation of carvone camphor on exposure of carvone to sunlight for one year. After confirmation of this finding in the late 1950s numerous investigators immediately recognized the value of the reaction for the rapid construction of complex molecular frameworks. The photocycloaddition of a series of Diels–Alder adducts of various dienes and benzoquinones to efficiently produce complex caged structures was subsequently carried out. This included the cycloaddition of a diketone to form a diketone. Meanwhile, a similar photochemical cycloaddition of an enone was utilized in a synthesis of the platonic solid cubane. Shortly thereafter investigations of the intermolecular reaction were initiated through a study of the dimerization of cyclopentenone and the photocycloaddition of cyclopentenone to cyclopentene. Further investigation of the various synthetic and mechanistic aspects of the photocycloaddition resulted in several total syntheses, including caryophyllene and bourbonene, as well as a proposed mechanistic rationale for the reaction. As a result of this pioneering work, largely by Eaton, Corey, and De Mayo, numerous natural and nonnatural products have been prepared by synthetic routes which have a [2 + 2] photocycloaddition as a crucial step in their synthesis. Through further refining over the last 15–20 years, the photocycloaddition reaction has become recognized as an important transformation in the repertoire of the synthetic organic chemist. Both the intermolecular and intramolecular versions of the reaction have been extensively studied. The intramolecular photocycloaddition, like the intramolecular variation of many reactions, has been the subject of much recent work. It is generally thought to be more predictable with regard to regiochemical and stereochemical control, but as will be seen from many of the examples in this chapter, the intermolecular reaction can be highly selective and predictable if the proper reaction participants and conditions are chosen. This chapter reviews the literature on the enone olefin [2 + 2] photocycloaddition since its discovery. Because of the volume of work in the field, there will undoubtedly be some omissions. Examples of all the major types of enone alkene photocycloadditions have been included so that the reader can get a sense of the historical evolution and the current state of the art. Both the inter molecular and the intra molecular variants of this reaction are treated in this chapter. The reader is also referred to a number of excellent reviews on the inter molecular reaction, the intra molecular reaction, and complete treatments which have appeared.

Photochemical [2+2] cycloaddition reaction of enone derivatives with 2-silyloxy-1H-pyrrole derivatives

“…The strained cyclobutane [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and oxetane [21][22][23] structures are important in organic synthesis. After Ciamician discovered the first example of photochemical [2+2] cycloaddition reaction, 24 Paternò, following Ciamician, reported the first photochemical [2+2] oxetane formation reaction using light irradiation.…”

“…25 Since then, many chemists have contributed their efforts to the synthetic and mechanistic studies of photochemical [2+2] cycloaddtion reactions. [1][2][3][4][5][6][7][8][9] Generally, the photochemical [2+2] oxetane formation reactions are called Paternò-Büchi (PB) reactions, in which furan derivatives have played their significant roles as alkene substrates. [21][22][23] To our best knowledge, even though a series of work concerning intramolecular photochemical [2+2] cycloaddition reaction of pyrrole derivatives have been achieved, [26][27][28]34 very few reports on the intermolecular case have been found till now.…”

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

“…[1] Among the established methods available to the synthetic organic chemist, the intramolecular enone-olefin [2+2] photocycloaddition has proven to be a powerful methodology for the construction of cyclobutane-containing ring systems in a single operation. [2] In connection with our study towards the total synthesis of solanoeclepin A, the use of allenes as olefinic partners in the intramolecular enone olefin [2+2] photocycloaddition is of special interest to us. [3] For the construction of the tricyclic core of the natural product we designed a strategy relying on an intramolecular butenolide allene [2+2] photocycloaddition.…”

“…Scheme 1. In contrast to enone allenes like 1 simple enone olefins with a two-atom tether have been the subject of numerous photochemical studies. [2] Among these, substrates such like 3 (X = NR or O) were shown to be easily accessible from the condensation of a cyclic 1,3-diketone with allylamine or allyl alcohol. [5] This ease of preparation attracted our attention.…”

Photocycloisomerization of Boc‐Protected 5‐Alkenyl‐2,5‐dihydro‐1H‐ pyrrol‐2‐ones

“…± Intramolecular enone alkene [2 2] photocycloadditions, i.e., light-induced cycloisomerizations of a,b-unsaturated carbonyl compounds bearing an additional CC bond [1] [2], have attracted the attention of organic chemists as a powerful method for assembling polycyclic molecules. Well-known examples are the carvone 3 carvonecamphor conversion [3] and a key step in the synthesis of cubane [4].…”