6n-Electrocyclizations

A 6n electrocyclic closure could be implicated in the photocyclisation reaction reported for the N-amino pyridinium ion (257) and some of its ring substituted derivatives to give (258). This type of reactivity was also seen for compound (233). A summary of the previously published work and of new results for the 67t photocyclisation of photochromic ethylenes substituted by derivatives of cyclopentadiene anion and pyridinium cation has appeared.The basic skeleton involved in the rearrangement is shown in structure (259) which is in photochemical equilibrium with (260). 

M, Es-Samti H. A thermal 6n electrocyclization strategy towards taiwaniaquinoids. First enantiospeciUc synthesis of (—)-taiwaniaquinone G. J. Chem. Soc., Chem. Commun. 2009 592-594. 

Scheme 8.63 The Heck reaction/6n-electrocyclization cascades and its application in the synthesis of modified steroids [240, 393].

Smith, et al. developed an organocatalyzed 6n electrocyclization of benzaldi-mines, as precursors to 2-aza-pentadienyl anions, prepared in situ from anilines 248 and aldehydes 249, to give functionalized indolines in high yields and enantioselectivities, Scheme 3.80 [101], 

Scheme 5 Synthesis of echinochrome A a typical example for An-6n electrocyclic ring opening and ring closure [67]

The first example of a chiral Bronsted acid, (154), that efficiently catalyzed asymmetric 6n electrocyclization of a,j8-unsaturated hydrazones (194) to pyrazolines (195) in high yields and enantioselectivities has been presented by List and co-workers (Scheme 54)7  

Relatively few examples of photochemically induced 6n -electrocyclizations of azahexa-l,3,5-trienes to azacyclohexa-1,3-dienes have been documented the reverse process involving ring cleavage, however, is more common. The formation of azalumazin-7-ones 19 from triazahexatrienes 20 under oxidative conditions can be regarded as an example of such a process,18 but 

Scheme 14.40 Total synthesis of shimalactones A and B (260 and 261) by Trauner and coworkers applying a domino Stille/8n/6n-electrocyclization reaction.

The 877-electrocyclization of carbonyl ylides of type B, e.g., the ylide derived from oxirane 145, was less favored than the 6n electrocyclization to yield a mixture of three compounds 146 and isomers 147 (in a ratio of 3 7). The annulated vinylfuran derivative 147 formed as mixture of isomers (cis-trans 4 3) while the stereochemistry of the single isomer of 146 has been established by X-ray analysis of the Diels-Alder adduct (Scheme 44) [81TLC194953 85CB4035]. 

Conformationally constrained bispropargyl sulfones with an ort/io-alkenyl moiety under basic condition undergo isomerization to monoallene followed by a 6n-electrocyclization. Further isomerization and 6a -electrocyclization has been reported to result in bis-naphthyl sulfones (Scheme 40).  

A synthesis of bicyclo[5.3.0]decatrienes through a Rh(I)-catalysed cycloisomerization of 3-acyloxy-4-ene-l,9-diynes has been reported " to proceed by [l,2]-acyloxy migration, 6n electrocyclization, migratory insertion, and reductive elimination. The overall process viewed as a intramolecular 5 -I- 2-cycloaddition with concomitant [l,2]-acyloxy migration (Scheme 146). 

The mechanism of the dearomatization-cyclization of aromatic amides takes place by initial lithiation at the ortho position of the aromatic ring. This species is in equilibrium with the more stable a-amide lithiated compound, which forms the final product by means of a 6n electrocyclic ring closure. 

More recently, alkaloid derivatives have also found application in other transformations such as electrocyclization reactions. For instance. Smith and co-workers have shown that treatment of benzaldimine 101 with 10 mol% of cin-chonidinium chloride 102 in a biphasic system of aqueous potassium carbonate and toluene gives cyclized product 103 in 99% yield and 97% ee. The sense of stereoinduction within this 6n electrocyclization (as represented by 104) was tentatively rationalized using a modification of the 

Early in 1992, Negishi et al. reported a palladium-catalyzed cascade involving alkynes toward benzene derivatives [38], Recently, Blond et al. reported a palladium-catalyzed cascade reaction toward strained aromatic polycycles 100 [39] (Scheme 6.22). Bromoenediynes 98 undergo A-exo-dig cyclocarbopalladation and subsequent 5-exo-dig cyclization to afford the vinylpalladium intermediate 99, which possibly undergoes a 6n-electrocyclization and a syn 3-H elimination to furnish the products observed. 

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