Eight thermal cycloadditions

No concerted thermal 4 + 4 cycloadditions are known photochemical 4 + 4 additions are observed, but in most cases probably occur through biradicals.88 It should be noted that in the thermal butadiene dimerization (Equation 12.34), the eight-membered ring arises through the allowed [3,3]-sigmatropic isomerization of m-divinylcyclobutane.69... 

This reaction is an [8 + 2] cycloaddition and is thermally allowed. In terms of the pi systems, a 10-membered ring is formed, an entropically unfavorable process. However, the nitrogen holds the ends of the eight-electron pi system close together, so the two-electron component can easily reach them. Counting via the nitrogen, a 5-membered ring is formed, a process that is much more favorable. 

As alludet to in Section 5.2.3.1, the intermolecular photoinduced [lit + 2tt] cycloaddition of dienes is an efficient reaction in only a relatively few instances. However, many of the liabilities normally associated with the intermolecular cycload tion process can, in principle, be circumvented by employing the intramolecular version of the coupling. The requisite eight-membered ring could then be accessed from thermal rearrangement of the resultant divinylcyclobutane photoproduct. 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

This cycloaddition is thermally forbidden under the Woodward-Hoffmann rules, but can be achieved by the transition metal catalysed coupling of two dienes (Scheme 11.55). The transformation of butadiene into a variety of products, including cyclooctadiene 11.163, has been known for many years. The [4 + 4] cycloaddition reaction has been applied in an intramolecular fashion to form eight-membered rings (Scheme 11.56). In the presence of a source of nickel(O) and phosphine ligands, tetraene 11.164 cyclizes to the bicyclic product 11.165, mainly as the cu-isomer. The ratio of isomers is dependent on the phosphine. A stereogenic centre present in the tether between the two dienes can result in useful diastereoseleetivity, once again dependent on the phosphines. This chemistry has been used to synthesize asteriscanolide 11.166, which contains a substituted cyclooctane, by intramolecular cycloaddition of tetraene 11.168 to... 

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