Cycloadditions with Trimethylenemethane Equivalents

Stereoselective [3-r 2]-cycloadditions of trimethylenemethane equivalents provide an effective route to substituted five-membered carbocycles not otherwise easily prepared [38-41], Trimethylenemethane diradicals have commonly been accessed from bicyclic diazenes such as 127 not otherwise easily prepared [38, 102], Little showcased that with substrate 129, incorporating an embedded stereocenter, tricyclopentanoid 132 was formed with high yield and diastereoselectivity (dr=99 l. Scheme 18.22) [103], The predominant formation of diastereomer 132 was attributed to a kinetically controlled trapping of the energetically favored intermediate radical 130, in which A] 3 interactions involving the silyloxy substituent are minimized. 

The intermediates generated from transition metal induced cleavage of methylenecyclopropanes are, in contrast to other trimethylenemethane equivalents, capable of undergoing [3 -I- 2] cycloaddition not only with activated, i.e. electron-deficient alkenes, but also with unsaturated, even nonstrained, hydrocarbons. Some of the reactions summarized in this section are also briefly discussed in Sections 2.2.2.2. and 2.2.2.3.I. because homodi- and homooligomerization of the methylenecyclopropane, as well as [2-1-2] cycloadditions, may efficiently proceed as side reactions of the [3 -f 2] cycloadditions. 

D.iii. Trimethylenemethane Equivalents. Palladium catalysts can be used to convert trimethylsilyl acetate 390 to a trimethylene methane (TMM, 391) equivalent. Reaction with alkenes via [3+2]-cycloaddition (sec. 11.11) generates cyclopentanes (this process constitutes a quinane annulation reaction).229 in this reaction, the trimethylsilyl unit is a carbanion equivalent and acetate is a carbocation equivalent. In one example, Trost reacted 390 and 392 with palladium acetate and triisopropyl phosphite [P(Oi-Pr)3] to generate 393 in... 

Methylene-6-oxabicyclo[3.2.2Inon-8-en-7-ones from 2-pyrones via 4-i-3]-cycloaddition with a trimethylenemethane equivalent... 

Developments in the field of stereoselective 1,3-dipolar cycloadditions and their synthetic applications are outlined in Sections 18.2 to 18.4 [22, 23, 28-34]. Subsequently, related [2-i-3]-cycloadditions with 1,3-diradicals and trimethylenemethane equivalents that furnish the corresponding five-membered carbocycles are covered (Section 18.5) [38—41]. The chapter closes with a collection of [2-i-2]-cycloadditions involving ketenes (Section 18.6) [19, 35, 36], photochemical olefin cycloadditions, and Paterno-Biichi reactions (Section 18.7) [36, 37],... 

Trimethylenemethane is a special type of alkene that does not exist as the free compound. Various synthetic equivalents to the synthon 43 shown below have been reported. Trost, in particular, has exploited these compounds in 1,3-dipolar cycloaddition reactions.138 139 A metal-bound, isolated trimethylenemethane species was recently reported by Ando (Scheme 6). It resulted from the complexation of an ero-methylenesila-cyclopropene with group 8 carbonyls (Fe, Ru).140,140a The structure was proved by X-ray crystal structure analysis.29Si NMR data were consistent with the -structure shown. 

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