Dienophiles trimethylsilyl groups

Simple dienes react readily with good dienophiles in Diels-Alder reactions. Functionalized dienes are also important in organic synthesis. One example which illustrates the versatility of such reagents is l-methoxy-3-trimethylsilyloxy-1,3-butadiene (.Danishefsky s diene) 1 Its Diels-Alder adducts are trimethylsilyl enol ethers which can be readily hydrolyzed to ketones. The /j-mcthoxy group is often eliminated during hydrolysis. 

There is a paucity of functional group reactions associated with the pyrrolopyrazines. A few examples may be found within the references describing their syntheses. However, a reaction has been reported which describes the [4 + 2] cycloaddition of pyrrolopyrazines. Ring expansion of 2-phenyl-2//-pyrrolo[3,4-/ ]pyrazine (32) occurs through cyclocondensation of the trimethylsilyl ether (33) with, amongst other dienophiles, methyl acrylate, and a mixture of quinoxalines (34) is obtained in 38% yield (Scheme 2) <86JHC1641>. 

A comparative study of the temperature dependence of the Diels-Alder reaction between [(trimethylsilyl)methyl]cyclopentadiene and dienophile groups confined in selfassembled monolayers or in pulsed plasma polymer layers has been done. The reactivity of dienophile groups confined in pulsed plasma polymer thin films is compared with the behavior of dienophile groups in monolayers, because of their well-known arrangement properties. 

Reactive Enophile in [4 + 2] Cycloadditions. Vinylketenes are not effective as dienes in Diels-Alder reactions because they undergo only [2 + 2] cycloaddition with alkenes, as predicted by frontier molecular orbital theory. However, silylketenes exhibit dramatically different properties from those found for most ketenes. (Trimethylsilyl)vinylketene (1) is a relatively stable isolable compound which does not enter into typical [2 + 2] cy do additions with electron-rich alkenes. Instead, (1) participates in Diels-Alder reactions with a variety of alkenic and alkynic dienophiles. The directing effect of the carhonyl group dominates in controlling the regiochemical course of cycloadditions using this diene. For example, reaction of (1) with methyl propiolate produced methyl 3-(trimethylsilyl)sahcylate with the expected regiochemical orientation. ProtodesUylation of this adduct with trifluoroacetic acid in chloroform (25 °C, 24 h) afforded methyl salicylate in 78% yield (eq 2). 

Another example relates to the plasma polymerization of maleic anhydride on silicon substrates [55]. After functionalization of the anhydride groups of the polymer with dienophile groups such as allyl amine groups, bicyclo[2.2.1]hept-2-ene groups are attached to the surface via the Diels-Alder cycloaddition of a diene. The cycloaddition of cyclopentadiene or [(trimethylsilyl)methyl]cydopentadiene to the cyclic imide-functionalized plasma polymer is shown in Scheme 1.17. In this case, the extent to which maleic anhydride groups are converted is controllable, such that the number density of functionalized groups can be tailored to comply with the adhesion between different types of solid surface. 

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