Cycloisomerization review

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

For reviews encompassing the Pd-cata-lyzed cycloisomerization and reductive cydization of 1,6-enynes, see (a) B.M. Trost, Acc. Chem. Res. 1990, 23, 34 ... 

For an excellent review covering enantio-selective metal-catalyzed cycloisomerization of 1,6- and 1,7-enynes, see I.J.S. Fairlamb, Angew. Chem. Int. Ed. 2004,... 

Enamines (cf. 63JCE194, 82T1975,88MI1,08H(75)1849) play an important role in the syntheses under review, both as target substances (see Schemes 16 and 17) and as precursors (see Scheme 9 and following Schemes 19-21). Thus, noble-metal-catalyzed enyne and diene cyclizations have been described (Scheme 19) palladium-catalyzed cycloisomerization of... 

Compared with cycloisomerization, enyne metathesis as a bond reorganization of an alkene and an alkyne to produce a 1,3-diene is less studied. A recent review by Diver and Giessert highlights some recent advances in synthetic applications, and mechanistic features [60]. 

A selection of the literature is also necessary in order to give the reader an overview of silver chemistry in the field of cycloisomerization reactions. Therefore, this chapter is not intended to be an exhaustive review of the literature, since more recent specialized reviews can be accessed for that purpose.3-10,26... 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

Cycloisomerizations of enynes catalyzed by Pd complexes address fundamental issues of atom-economy and have become valuable tools in synthetic organic chemistry (for leading reviews on Pd-catalyzed cycloisomerizations see [142-146]). 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

Enyne metathesis reactions in the context of natural product synthesis have been reviewed recently by Mori <2007ASC121>. Using the same ruthenium catalyst, a novel tandem diyne cycloisomerization-CM process has been devised to furnish 3,4-divinyl-2,5-dihydrofurans (Equation 57) <1999CC237>. 

Review articles have featured advances in the cycloisomerization of l,n-allenynes and Rn-allenenes, " reactions for synthesis of quaternary centres bearing a nitrogen substituent, " the Ramberg-Backlund reaction transforming a-halosulfones into alkenes, " and ring expansion of l,2-benzisoxazol-3-ones with a Vilsmeier reagent system, POCI3 and dimethylformamide (Scheme 75). " ... 

Recent advances in the cyclizations catalyzed by transition metals and their complexes are reviewed. The catalytic cyclizations discussed here include various carbocyclizations, for example, cycloisomerization, cycloaddition, reductive cyclization, and so on cascade carbocyclizations, for example, cyclotrimerization, silylcarbocyclization, and Heck reaction carbonylative carbocyclizations cyclohy-drocarbonylations intramolecular hydrosilylations intramolecular silylformyla-tions and aldol cyclizations. These reactions serve as efficient and useful methods for the syntheses of a variety of heterocycles and carbocycles that are important... 

It is only in recent years, following what is commonly referred to as the Gold Rush in the scientific literature, that NHC-Au complexes have gained popularity and have seen their potential unfold. The first application in catalysis of a NHC-gold species appeared in 2003, but the real surge in publication only started in 2006. Since then, there is hardly a week without a report on the activity of NHC-Au catalysts in organic synthesis is published. The field is dominated by two main topics of crucial importance in modern catalysis, namely cycloisomerization of polyunsaturated substrates and hydrofunctionalization of 7i-bonds. NHC-Au species have been the subject of several reviews... 

The close relationship of cyclization modes existed in sesquiterpene biosynthetic machinery, with those presented by the metal-catalyzed cycloisomerizations, justify the vast amount of reports on total synthesis of sesquiterpenes, in which the utilization of enyne or diene cycloisomerizations are the key components of their synthetic strategy. This chapter is intended to cover only selected examples on the topic. Special concern is given on covering catalyzed reactions, which are triggering different cyclization modes, only for the construction of sesqniterpene core strnctnres. Assuredly, cycloisomerization reactions are powerfnl tools in providing also other classes of secondary metabolites, as complex terpenoids and alkaloids, in which the readers are referred to more general reviews on the topic [26,30]. 

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