Cycloadditions Diels-Alder-type, acetylene

Cycloadditions. Although 6 exists in the solid state in the s-gauche form, which should favor [4+2] cycloadditions of the Diels-Alder type, to date all attempted reactions with alkenes, acetylenes, and the C=0 bonds of ketones have remained unsuccessful. The reasons for this are probably the steric overcrowding in 6 as well as the large 1,4-separation of 540 pm between the terminal silicon atoms. Thus, it was at first surprising to find that compound 6 reacted with maleic anhydride through a formal [4+4] cycloaddition to furnish the tetracyclic compound 7. ... 

In view of the preference of 24 for the s-gauche form, it seemed worthwhile to examine its behaviour in [4-1-2] cycloadditions of the Diels-Alder type. Howev, until very recently all reactions with olefins, acetylenes, and the C=0 bond of ketones have remained unsuccessful. The effective shielding of the two double bonds of 24 by the bulky aryl groups and, above all, the large 1,4-separation between the terminal silicon atoms appear to be responsible for the failures. 

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

These Michael-type products are similar to structures obtained by Kotsuki [36] by cycloaddition reactions of N-sub-stituted pyrroles 94 with dimethyl acetylene dicarboxylate. If the substituent on the nitrogen is electron donating (R=Me), a primary 1 1 Diels-Alder cycloadduct 95 reacts further to give Michael-type reaction product 96 (Scheme 23). Described substituent influence on the susceptibility of 7-azabicyclo[2.2.1]hept-2-enes to Michael reaction is in good accord with results obtained by Scheeren [37] who has shown that the hi -pressure Diels-Alder reaction of 3-thiosubstituted N-carbomethoxypyrrole derivatives with electron-poor alkenes (12kbar, MeCN, 50°C, 16h) affords 2-thiosubstituted 7-azabicyclo[2.2.1]hept-2-enes in high yields. 

We also consider new achievements in some classical cycloaddition reactions such as the Diels-Alder condensation with acetylenic dienophiles, [2-1-2] cycloadditions with acetylene component leading to creation of cyclobutene ring, and new results in cyclobutene syntheses by [2-1-2] acyclization of phosphorus containing 1,3-butadiene derivatives synthesized starting with propargyl-type alcohols. 

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