Diels -Alder reaction with -crotonic acid

Rao and colleagues [80] reported the first example of baker s yeast-catalyzed Diels-Alder reaction. Reactions of cyclopentadiene (1) with dienophiles 119 and 120 (Scheme 4.21) in the presence of baker s yeast at pH 7.2 afford prevalently the exo adduct with the exception of crotonic acid 120a. 

Simple diastereoselectivity may also occur in Diels-Alder reactions between electron-poor dienophiles and cyclopentadiene (Figure 15.30). Acrylic acid esters or fraus-crotonic acid esters react with cyclopentadiene in the presence or absence of A1C13 with substantial selectivity to afford the so-called emfo-adducts. When the bicyclic skeleton of the main product is viewed as a roof the prefix endo indicates that the ester group is below this roof, rather than outside (exo). However, methacrylic acid esters add to cyclopentadiene without any exo.endo-selectivity regardless whether the reaction is carried out with or without added A1C13 (Figure 15.30, bottom). 

Crotonic acid esters of cellulose undergo addition reaction with cycloaliphatic amines, like morpholine or piperidine and with aliphatic primary amines. Unsaturated polymers can also undergo Diels-Alder reactions. One example is a reaction of hexachlorocyclopentadiene with polycyclopentadiene ... 

In an early application, the aqueous Diels-Alder reaction of diene carboxy-lates was used to effect a very concise synthesis of the vemolepin precursor 5.2 [60]. This compound had previously been prepared by Schlessinger via another route in 11 steps from ethyl crotonate [61]. Reaction of the substituted methacrolein 5.3 with sodium ( )-3,5-hexadienoate (5.4) in water proceeded at 50 C to give the Diels-Alder endo adduct 5.5 along with a minor amount of the exo adduct (not shown) quantitatively in a ratio of 10/1 (Scheme 1.4). Sodium borohydride was added directly to this reaction mixture to produce the 6-hydroxycarboxylate 5.6, which lactonized spontaneously upon acidic workup. This one-pot operation resulted in the isolation of vemolepin AB-ring... 

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