Organic Reactions Volume 4 - Sciencemadness Dot Org

22 ORGANIC REACTIONS
9 9
CH OC-CH CH
Il CHCO 0< I I
N + 2|| >0 -* OC-CH N
/ CHCO \ / \
N N CH-CO
Il I I > 0
CH CH CH-CO
6 6
XXXI XXXII
Although the dimerizations of acetylketene, 97 acrolein, 98 methyl vinyl
ketone," and possibly other, similar compounds 184 may be considered
Diels-Alder reactions, these substances do not undergo normal condensations
with other dienophiles such as maleic anhydride.
Reaction of Maleic Anhydride with Alicyclic Compounds. Cyclopentadiene
reacts vigorously with maleic anhydride to give cis-3>6~endomethylene-l,2,3,6~tetrahydrophthalic
anhydride (XXXIII). 5 The adduct
possesses the endo configuration (XI) and is stereochemically
homogeneous. 129 Citraconic anhydride, 5 pyrocinchonic anhydride/ 21
itaconic anhydride, 5 dibromomaleic anhydride, 7 and acetoxymaleic
anhydride 185 react similarly with cyclopentadiene to give the expected
adducts.
XXTLII
Cyclic dienes carrying alkyl, aryl, methoxyl, ethoxyl, and carbomethoxyl
groups as substituents have been employed successfully in the Diels-
Alder reaction. Although these dienes appear to differ somewhat in
degree of reactivity, sufficient investigation has not been carried out
to indicate the relative hindrance afforded by the individual groups.
Ease of condensation also depends to some extent upon the size of the
ring containing the conjugated system. Five- and six-membered carbo-
184 Alder, Offermanns, and Riiden, Ber., 74, 926 (1941).
185 Nylen and Olsen, KgI. Norske Videnskab. Selskabs, Forh., 11, 229 (1938) [C. A., 33,
5361 (1939)].