Nazarov rearrangement

The rearrangement of divinyl ketones to cyclopentenones (the Nazarov rearrangement) can be promoted with BF3-Et20. This transformation has recently been applied to good effect in the construction of the hydroazulene core found in guanacastepene. Other variants of the Nazarov reaction have been developed and are usually coupled with a second event, such as the reductive... 

Combining this cycloisomerization reaction with met-aUa-Nazarov rearrangement gave tetracyclic cyclopropanes 334 in good yield. The stereoselectivity was very high, and a single isomer was isolated (Scheme 1.161) [231]. 

Shi, F.-Q., Li, X., Xia, Y, Zhang, L. and Yu, Z.-X. (2007) DFT Study of the Mechanisms of In Water Au(I)-Catalyzed Tandem [3,3]-Rearrangement/Nazarov Reaction/[l,2]-Hydrogen Shift of Enynyl Acetates A Proton-Transport Catalysis Strategy in the Water-Catalyzed [l,2]-Hydrogen Shift. Journal of the American Chemical Society, 129, 15503-15512. 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

A very new example for the combination of an Au -catalyzed [3,3]-rearrangement and a Nazarov reaction has been disclosed by Zhang and coworker. Thus, cyclopen-tenones could be easily achieved by converting en-ynyl acetates in the presence of AuCl(PPh3)/AgSbF6 [320]. 

Cyclopenta-l,4-dioxanes 95 are formed in high yields through the acid-catalysed rearrangement of the dioxolanes 94 in which electrocyclisation of a hydroxypentadienyl carbocation, akin to a Nazarov cyclisation, is involved (Scheme 62) <00CEJ4021>. 

These container molecules exhibit large openings on each triangle face. That is why different exchange processes can potentially take place. In the literature various applications are described such as encapsulation of neutral molecules or cations [44-74]. Reactive or unstable species could also be stabilised [63, 67, 75-78]. The cages can serve as a catalyst, e.g. for the Aza Cope rearrangement [54, 66, 79-81] and for the hydrolysis of several compounds [82-85]. They have also been used for C-H bond activation [54, 57, 61, 66, 69] or Nazarov cyclisation [86],... 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

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