Photochemistry of Isabelin

The Synthesis of Germacrane Sesquiterpenes and Related Compounds

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 intermolecular and the intramolecular variants of this reaction are treated in this chapter. The reader is also referred to a number of excellent reviews on the intermolecular reaction, the intramolecular reaction, and complete treatments which have appeared.


Keywords:

enone olefin;
photochemical cycloadditions;
intramolecular additions;
intermolecular additions;
regioselectivity;
stereoselectivity;
competing reactions;
scope;
limitation;
experimental procedures;
alkenes;
acetylenes;
cyclohexenones;
benzoquinones;
pyrimidines;
azaenones;
cyclopentanones;
bridged bicyclic dienes;
benzopyranones;
hexadienes;
nitrogen;
oxygen

Enone Olefin [2 + 2] Photochemical Cycloadditions

Aquatolide. A new type of humulane-related sesquiterpene lactone

The elucidation by NMR and chemical methods of the unique structure of betanidin, the aglycon of the red-violet beet pigment betanin, forty years ago at the University of Zurich, Switzerland, was the beginning of my plant chemistry research program. Many of the same chemical and spectral techniques developed in Zurich have been used at The University of Texas at Austin for the structure analysis of members of many other classes of natural products including especially flavonoids, terpenoids, and alkaloids. Investigations at UT-Austin have concerned many topics such as biochemical and molecular systematics, biosynthetic pathways, structure-activity relationships, and the medicinal importance of natural products and included studies of antiviral proteins in the genus Phytolacca and neurotoxic nonprotein amino acids from cycads and other sources. Following the betalain story and an account of the early development of my UT-Austin biochemical systematic program, the Phytolacca and neurotoxin investigations are discussed herein.

Selected Topics from Forty Years of Natural Products Research: Betalains to Flavonoids, Antiviral Proteins, and Neurotoxic Nonprotein Amino Acids†

This chapter is concerned with the light-induced addition of the double bond of α,β-unsaturated ketones to alkenes to give cyclobutanes. The reaction is variously referred to as enone photoannelation, enone (2+2) cycloaddition, the de Mayo-Eaton reaction, or the de Mayo reaction. The last name is, however, usually reserved for the cycloaddition of enolized diketones to alkenes.

Photochemistry of Isabelin

Citations

The Synthesis of Germacrane Sesquiterpenes and Related Compounds

Enone Olefin [2 + 2] Photochemical Cycloadditions

Aquatolide. A new type of humulane-related sesquiterpene lactone

Selected Topics from Forty Years of Natural Products Research: Betalains to Flavonoids, Antiviral Proteins, and Neurotoxic Nonprotein Amino Acids†

Enone Photochemical Cycloaddition in Organic Synthesis

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