Enynes ketenes

Cyclobutenones and enyne ketenes undergo a similar cyclization through intermediate diradicals as illustrated in Eq. 3 and discussed by the author. The preparation of cyclobutenones is described (2). 

Wang, K. K. Cascade Radical Cyclizations via Biradicals Generated from Enediynes, Enyne-Allenes, and Enyne-Ketenes. Chem. Rev. 

An analogous cyclization of enyne-ketenes to produce phenolic diradicals like 99 was first reported by Moore and coworkers [35] (Fig. 10). 

In another example from Moore, cyclobutenone 148, rearranges to give enyne-ketene 149. The resulting diradical was intercepted by a tethered alkyne, which in turn led to spirocycle 153 (Scheme 27) [49],... 

This reaction was first reported by Moore in 1985. It is a synthesis of phenol (or quinone) derivatives by means of the thermal cyclization of enyne-ketenes between C2 and Cy via an aryl/phenoxy biradical intermediate. Therefore, it is generally known as the Moore cyclization. Occasionally, it is directly referred to as the Moore reaction. 

Thermal generation of a biradical by cyclization of enyne-ketenes (Moore) or of enyene-allenes (Myers) ... 

Moore cyclization is formulated as a cyclization of enyne-ketenes 1 to diradicals 2. This process usually proceeds by thermal induction, leading to the formation of a benzene ring. 

In 1985 Harold W. Moore and co-workers (University of California, Irvine), described the generation and chemistry of 2-alkenylethynyl ketenes, which were accessible from the corresponding alkynylcyclobutenones. For example, 4-alkynyl-4-trimethylsililoxycyclobutenone 3 forms an enyne-ketene 4. The conjugated ketene 4 undergoes ring closure to produce the diradical 5 (or related zwitterion) which, in turn, proceeds to the substituted... 

The computational comparison of various pathways of thermal reactions of an analog, in which a heteroatom replaces the central olefinic bond, with possible transformations of parent enyne-ketene 1, was carried out and showed little relevance. ... 

Enediynyl ethyl ethers like 12 have been applied by Wang s group as precursors of enyne-ketenes 13, which underwent the Moore cyclization reaction to form diradicals. The intermediate 14 and (after 1,5-hydrogen shift) new diradical 15, form, upon the cascade transformations, the final products mainly chromanol 16 and spiro ketone 17. The latter is a result of subsequent intramolecular reaction of intermediates o-quinones methide and spiro ketone (not illustrated). ... 

Moore et al. investigated the thermal rearrangements of differently substituted cyclobutenones. Reactions of 4-alkenyl-4-hydroxycyclobute-nones such as 18, in which the triple bond is replaced by a double bound, are complementary to the ring expansions of 4-alkynyl-4-hydroxycyclobute-nones and provide a route to the differently substituted benzoquinones, such as aurrantiogliocladin 19. The reaction proceeds via enyne-ketene 20. Since cyclization produces a derivative of hydroquinone 21, an additional oxidation step is required that is accomplished with the use of cerium ammonium nitrate on silica. This ring expansion process is independent of the... 

Organometallic reagents were used for the synthesis of bicyclic aromatic compounds via Moore-type cyclization. Rahm and Wulff described the new synthesis of 5-hydroxyindolines with the use of a chromium carbene complex bearing alkynyl substituent 22." The amino-tethered bis-alkynyl carbene complex 22 was transformed into indoline 23 by thermolysis in the presence of a hydrogen source. The low yield of product 23 was improved when the reaction was carried out in the presence of the electrophile, added to protect the phenol function. This process involves the insertion of one carbon monoxide group from the chromium complex into the skeleton of an eneyne compound 24. The resulting enyne-ketene 25 undergoes a cycloaromatization reaction to afford the 1,4-diradical intermediate 26. Subsequent demetalation yields product 23."... 

Another example of the application of the chromium carbene complex for the synthesis of benzannulated compounds was described by Herndon and Wang." The coupling of substituted carbene chromium complex 27 with conjugated enediyne 28 results in the formation of intermediate enyne-ketene 29, which undergoes the Moore cyclization to produce the intermediate chromium-complexed diradical 30. The... 

Since 4-alkynylcyclobutanones, precursors of enyne-ketene, are relatively readily available, subsequent transformation of enyne-ketenes to aromatic diradicals provide interesting synthetic opportunities. 

Conjugated systems, including 1,3-butadienes, diene-allenes, enyne-allenes, enediynes, dienediynes, and enediallenes, were synthesized via organoboranes. A wide array of cascade cyclization reactions involving high energy intermediates, such as o-isotoluenes, biradicals, o-quinodimethanes, benzocyclobutadienes, and enyne-ketenes, were initiated from these unsaturated compounds. 

In the cases where R is an ethoxyl group, thermolysis at 132 C induced a retro-ene reaction with the elimination of an ethylene to generate enyne-ketenes. Specifically, 72a was converted to the enyne-ketene 73, which then underwent a Moore cyclization reaction to form the biradical 74 (Scheme 17) (27). A 1,5-hydrogen shift afforded the biradical 75, which decayed via intramolecular routes to give the chromanol 76, the phenols 77 and 78, and the spiro ketone 79. 

Wang, K.K. (1996) Cascade radical cyclizations via biradicals generated from enediynes, enyne-allenes, and enyne-ketenes. Chemical Reviews, 96(1), 207-222. 

Masch, P.W., Remenyi, C., Helten, H. and Engels, B. (2002) On the regioselectivity of the cyclization of enyne-ketenes a computational investigation and comparison with the Myers-Saito and Schmittel reaction. Journal of the American Chemical Society, 124(8), 1823-1828. 

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