Electrocyclization cascade

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

POLVQUINANES PRODUCED FROM DIISOPROPYL SQUARATE VIA AN ELECTROCYCLIC CASCADE... 

Scheme 3-22 Intra-intraraolecular Heck-bTr-electrocyclization cascades [172],...

Application of carbon-carbon cross-coupling/67C-electrocyclization cascade reactions in pyridine synthesis is well known. Larock and coworkers developed a palladium-catalyzed coupling of vinylic imines 279 with terminal alkynes 280 followed by subsequent copper-catalyzed cyclization of 281 to give aryl, vinyl, and alkyl-substituted pyridines 282 in moderate yields. ... 

Salem, B. and Suffert, J. (2004) A 4-exo-dig cyclocarbopaUadation/87r electrocyclization cascade Expeditions access to the Tricyclic core structures of the ophiobohns and aleurodiscal. Angew. Chem. Int. Ed., 43, 2826-30. 

Scheme 8.23 Intra-intramolecular Heck-6jt-electrocyclization cascades [318].

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

N. Majumdar, K. A. Korthals, W. D. Wulff, J. Am. Chem. Soc. 2012, 134, 1357-1362. Simultaneous synthesis of both rings of chromenes via a benzannulation/O-quinone methide formation/electrocyclization cascade. 

Other examples of sequences where electrocyclization is the final step include the atom-economical syntheses of [4.6.4.6.] fenestradienes. A tandem hydrogenation/8 t-> 6 t electrocyclization cascade from en-yn-enes 110 linked to a preexistent cyclobutane-containing tricycle was developed (Scheme 5.40) [118, 119]. Overall yields for the sequence range from 63-93%, leading to these fenestradienes 112 with full diastereoselectivities. These results may be explained by the high torquoselectivities observed during the 8jt-conrotatory process that forms the cyclooctatrienering intermediate 111 [120]. 

Scheme 542 Tandem aza-Cope/enolate formation/electrocyclization cascade.

Hulot, C., Amiri, S., Blond, G. L, Schreiner, P. R., Suffer , J. (2009). Understanding the torquoselectivity in 8x-electrocyclic cascade reactions synthesis of fenestradienes versus cyclooctatrienes. Journal of the American Chemical Society, 131, 13387-13398. 

In 2004, the group of Sorensen reported an efficient enantioselective total synthesis of (-l-)-harziphilone (6) using an MBFT strategy (Scheme 13.1), which allowed the direct conversion of an acyclic polyunsaturated diketone 1 to the required bicyclic ring system of the target molecule in one step through a DABCO-catalyzed double conjugate addition reaction followed by a 6x-electrocyclization cascade reaction [2]. 

Scheme 1.10 Stepwise or double [2-H2]cycloaddition/retro-4jt-electrocyclization cascades to synthesize [4]dendralenes [46, 47].

An Electrocyclization Cascade in Nature Immunosuppressants from Streptomyces Cultures... 

SCHEME 6.31 Palladium-catalyzed Heck/StiUe coupling/6jt-electrocyclization cascade. 

The electrocyclization cascade that assembles the endiandric acids, sequential thermal 871-comotation followed by 671-disrotation, is a recurring theme in natural product... 

Cueto M, D Croz L, Mate JL, San-Martin A, Darias J. Ely-siapyrones from Elysia diomedea. Do such metabolites evidence an enzymatically assisted electrocyclization cascade for the biosynthesis of their bicyclo[4. 2. 0]octane core Org. Lett. 2005 7 415 18. 

The very rmusual bicydo[4.2.0]octane core of the polypropionates isolated from Placobranchus ocellatus and By-sia diomedea (Figure 23.29) is surmised to originate from an 8jt-6jt electrocyclic cascade (Cueto et al., 2005 Manzo et al, 2005a). The often simultaneous presence of 1,3-cyclohexadienic and bicyclohexenic derivatives results in a photorearrangement of the former into the latter, as has been demonstrated in vivo with Placobranchus ocellatus (Ireland and Scheuer, 1979) (Figure 23.30). 

A facile electrocyclization cascade has been reported to occur when diynes (46) are halogenated with NBS or NIS in the presence of a catalytic amount of AgBp4 (Scheme 3). The initial attack by X+ apparently generates the vinylic cation (47) that undergoes cyclization via (48) to produce the benzylic alcohol (49). The silver(I) catalyst then promotes aromatization with a concomitant hydration, giving rise to the halo-substituted benzo[a]fluorenols (50). ... 

Some reactions of particular interest are some examples of tandem reactions such as (i) the Stille coupling/elimination sequence with a 1,1-dibromo-l-alkene in the preparation of Callipeltoside [89] or (ii) the Stille coupling/electrocyclization cascade in the synthesis of immunosuppressants (Scheme 6.12) [90] or (iii) distan-nylated reagents such as 1,2-vinylditin and 1,4-dienyltin used in double Stille reaction sequences [91] and peculiar cascade reactions [92]. 

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