Ketalized Diels-Alder type adduct

Natural Diels-Alder-type adducts, including substituted cyclohexene derivatives, prenylbrflavonoids (ketalized Diels-Alder-type adducts), and 2-arylbenzofuran-ketalized Diels-Alder adducts, comprise the fourth main group of structures. Substituted cyclohexene derivatives include (1-a,11-3 ,1-p,11-6 )-dihydrochalcone-dehydrogeranylflavone (393) and (1-a,11-3 ,1-P,11-6 )-dihydrochalcone-dehydrogeranylflavanone (394, 395). 

Ketalized Diels-Alder-type adducts comprise prenyUlavanone-dihydrochalcones (396) prenylflavanone-prenyldihydrochalcones (397) (prenyl)dihydrochalcone-diprenylflavones (398-399) (1-3, 11-3)-... 

Figure 25. KETALIZED DIELS-ALDER-TYPE ADDUCTS Prenylflavanone-Dihydrochalcone, Prenylflavanone-Prenyldihydrochalcone, (Prenyl)Dihydrochalcone-Diprenylflavone, (1-3, 11-3)-Bi-Prenylchalcone, (I-3 ,II-a I-6 ,II-P)-Prenylchalcone-prenyldihydrochalcone, (I-3 ,II-a I-6 ,II-P)-Prenylchalcone-prenyldihydrochalcone, (I-a,II-3 ,I- p,II-6 )-Prenyldihydrochalcone-prenylchalcone, (I-3 ,II-3 )-Dihydrochalcone-prenylchalcone dimer, (I-3, II-5 1-4, 0,11-1 )-Prenyldihydrochalcone-prenylchalcone, Dihydrochalcone-Prenylflavonol Dimer, (I-3",II-8)-Prenyldihydrochalcone-prenylflavanone dimer, (I-a,II-1 ,I-P,II-4 )-Prenyldihydrochalcone-prenylflavanonol dimers, (I-3 ,II-3 )-PrenyIdihydrochalcone-prenylflavanonol dimer, (I-3 ,II-a,I-6 ,II-P)-Prenylflavanonol-prenyldihydrochalcone dimers.

Dorstenia zenkeri Engl, (leaves) (1-3, 11-5 I-4, 0,II-l)-Prenyldihydro-chalcone-prenylehalcone (ketalized Diels-Alder-type adduct) (409). Abegaz et al., 2002[255]. 

Sorocea bonplandii Baillon (roots) Ketalized Diels-Alder type adducts Soroceal (440) sorocein A (437) kuwanol E (435). Brazilian plants. Effects on agonist-induced contractions in the rat uterus and in the guinea pig ileum in vitro. Messana et al., 1991 [269] Calixto etal., 1993[270]. 

Arylbenzofuran-ketalized Diels-Alder adducts include (1-3 ,11-5)-dihydrochalcone-prenyl-2-arylbenzofuran dimers (422-426) a (l-a,ll-3 1-P,Il-6 )-dihydrochalcone-prenyl-2-aiylbenzofuran dimer, a Diels-Alder adduct of a dehydroprenyl-2-aiylbenzofuran with a chalcone (427) a (l-a,ll-3 1-P,ll-6 )-dihydrochalcone-prenyl-2-aiylbenzofuran dimer (428) a 2-arylbenzofuran-prenyldihydrochalcone dimer (429) 2-arylbenzofuran-ketalized Diels-Alder adducts (430-431) Diels-Alder-type adducts of a chalcone and a dehydroprenylstilbene (432-433) and stilbene and diprenyldihydrochalcone dimers (434-438). 

From a biogenetic point of view, mulberrofurans F (131) and G (27) seem to be Diels-Alder type adducts derived from chalcomoracin (103) and mulberrofuran C (130), respectively, by intramolecular ketalization of the carbonyl group with the two adjoining hydroxyl groups as shown in Fig. 30. 

An intriguing dichotomy was revealed in the reactivity of indole (85) with ketals 76a-c (Figure 24). At room temperature, only the Diels-Alder adducts 86a-c are obtained in good yields, but at reflux temperature, only rearomatized Michael-type 1,6-adducts 87a-c are observed [139]. 

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