Nazarov cycloaddition

West and coworkers developed two new domino processes in which a [4+3] cycloaddition (Nazarov electrocyclization) of l,4-dien-3-ones is succeeded by either an... 

The cycloaddition reaction is one important application of Rh catalysts. Recently, Rh-catalyzed intermolecular [5 + 2] cycloaddition/Nazarov cyclization cascade was reported by Wender et al. (Scheme 5.49) [47]. The reaction could be conducted with a single catalyst, such as [(Cj(,Hg)Rh(cod) "SbFg ] (2mol%), and provide an efficient strategy for the facile synthesis of bicyclo[5.3.0]decanes 72. 

SCHEME 549 Rh-catalyzed cascade intermolecular [5 + 2] cycloaddition/Nazarov cyclization reaction. 

An unusual [4+1] cycloaddition gold-catalysed reaction between propargyl tosylates 102 and imines 103 led to the formation of eyclopent-2-enimines 104 (Scheme 5.27) [27], A possible mechanism for this reaction involves a 1,2-migration of the tosylate that generates the 1,3-diene 105 followed by a Nazarov-hke cyclisation. 

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. 

An attractive strategy to functionalized cyclohep-tenes was developed by West al1181 As illustrated in scheme 7, the substrate 30 is exposed to FeCl3 as Lewis acid at -30 °C to induce a Nazarov electrocyd-ization to give the carbocation 31. Under the reaction conditions 31 undergoes a [4+3]-cycloaddition... 

The Nazarov Cyclization Katl L. Habermas, Scott E. Den-mark, Todd K. Jonas Ketene Cycloadditions John Hyatt, Peter W. Reynolds 0-47103101-0 390.00... 

This cycloaddition works best with substituted oxyallyl cations, like the one from the dibromo-ketone 93 which reacts with the morphol ine enamine of cyclohexanone 95 to give the cyclopentenone 96 in excellent yield.24 You should mark the difference in structure between the Nazarov products, e.g. 70, formed in an electrocyclic reaction, and these cycloadducts, e.g. 96. Each strategy has its part to play and must be chosen according to the structure of the target molecule. 

In the elaboration of cyclopentenones the Nazarov cyclization (not a cycloaddition reaction) also offers some advantages. Metal complexes including a V(IV) chelate of salen 60 and the Cu complex of ent-96B have been employed as catalysts. By structural demand of the substrate to induce rearrangement following the cyclization a synthesis of spirocycles is realized. 

Cyclization. The oxyallyl intermediate derived from treatment of a cross-conjugated dienone with FeClj is trapped by an intramolecular [4 + 3]cycloaddition if one of the a-positions of the dienone is connected to a diene. Interesting tricyclic systems can be produced in this Nazarov cyclization. ... 

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