Alkylidenecyclopropanes as Dienes

Although l,l-disubstituted-l,3-dienes are quite unreactive in the Diels-Alder reaction, allylidenecyclopropanes exhibit a good reactivity especially toward activated dienophiles. The strain present in the alkylidenecyclopropane moiety is responsible for the reactivity enhancement observed in these compounds. The literature concerning the parent diene and few analogs until 1984 has been thoroughly reviewed by Krief [31]. Since then, some other examples of variously-substituted allylidenecyclopropane reacting as dienes in [4 -f 2] cycloadditions were published. 

Diels-Alder reactions of 2-(trimethylsilyloxy)allylidenecyclopropane (112) with electrophilic dienophiles gives 3-silyloxy spiro[2.5]oct-2-enes or the corresponding hydrolysis compounds spiro[2.5]octan-3-ones (Tables 11 and 12) [32]. 

While doubly activated dienophiles gave moderate to good yields under thermal conditions (Table 11), mono activated alkenes such as cyclohexenone (123) failed to react at temperatures up to 150 °C but underwent cycloaddition under Lewis-acid catalysis even if with poor yields (Table 12, entries 1-3) [32]. 

More substituted and unactivated dienophiles such as 3-acetoxy-2-methyl-cyclopentenone, did not react with 112 even in the presence of BF3 etherate. 4-Acetylspiro[2.1.2.3]decen-9-one (128) could also be isolated under these con- 

The related l-methyl-2-t-butyldimethylsilyloxy allylidenecyclopropane (129) reacted with activated dienophiles at ambient temperature or even below (Table 13, entries 2-4) to give moderate to good yields of the corresponding [4 -i- 2] cycloadducts 133-135. Only dimethyl acetylenedicarboxylate (113) required heating to 110 °C to give good yields of the adduct 132 (Table 13, entry 1) [32]. Adduct 135 from p-quinone (entry 4) tends to oxidize by purification on silica gel in the air. 

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For the more reactive allylic halides 4 a and 4 b the salt of diethyl hydrazinodicarboxylate was used as the nucleophile in preference to hydrazine.Deazetization of the resultant 4,5-dihydro-37/-pyrazoles 5 gave a mixture of alkylidenecyclopropanes 6 and 7. It should be noted that attempts to make 4-isopropylidene-4,5-dihydro-37/-pyrazole (5 b) by the more obvious route (by the reaction of diazomethane with 3-methylbuta-l,2-diene) failed. This route has been modified for the synthesis of various deuterated derivatives of 5 and used to make optically active (3/ ,57 )-3,5-dimethyl-4-methylene-4,5-dihydro-3/f-pyrazole, but all of the cyclopropanes produced from thermolysis of this compound were racemic. A similar route has also been used in the synthesis of 7,7-dimethyl-6b,7a-dihydro-7//-cycIopropa[fl]acenaph-thylene. ... 

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