Diterpenes dienophile

Diels-Alder reaction of the furan derivative 148 with homochiral bicyclic enone 149 is the key step [56] in the total synthesis of the diterpenes jatropho-lone A and B, 151 and 152, respectively, isolated from Jatropha gossypiifolia L [57], Initial efforts to carry out the cycloaddition between 148 and 149 under thermal or Lewis-acid conditions failed due to diene instability. Application of 5kbar of pressure to a neat 1 1 mixture of diene and dienophile afforded crystalline 150 with the desired regiochemistry (Scheme 5.23). Subsequent aromatization, introduction of the methylene group, oxidation and methylation afforded (-l-)-jatropholones 151 and 152. 

Diels-Alder Reaction Dienophiles Difluoroallene Dioxoindoles Dioxolanyliums Discovery of Diels-Alder reaction Discovery of orbital symmetry conservation rule Diterpenes Domino Drtmane sesquiterpenes Compilation 0031, 9804, 9742, 9741. 9737, 9705, 9410, 9406, 9306, 9218, 9118 9126 9705 9833, 9604 9222 9222 0003 0033, 0024, 9835, 9014 9116... 

The benzyne route to 1-cyanobenzocyclobutenes has been employed for the synthesis of a myriad of complex natural products. The cyano group in (74) was used to attach the dienophile as well as for building die imino bridge of (75), an important precursor for some diterpene alkaloids (Scheme 15).110... 

The ABCD ring system of the diterpene alkaloid atisine was constructed by T. Kametani et al using an intramolecular Diels-Alder cycloaddition reaction as the key step. The dienophile was obtained by the traditional Hofmann degradation of the corresponding dimethylamino precursor. The diene was prepared by the kinetic enolization of the cyclohexenone system with LDA. 

Compound (51) appeared to come from a Diels-Alder reaction between a diterpene diene and ochraceolide A as a dienophile. The possibility of the presence of enzymatic system with hetero or Diels-Alderase activity in Celastraceae was suggested once again, based on the structure of (51)... 

Other examples of the use of intramolecular Diels-Alder reactions in preparative chemistry have appeared the o-quinodimethane (186), produced as a transient intermediate in the pyrolysis of the analogous benzocyclobutene, cyclizes to give (187 65%), the desulphurization of which (Raney nickel) affords a potential intermediate for the preparation of tetracyclic diterpenes. Elevated temperatures result in the isomerization of (188) into the internal [4 -I- 2] adduct (189) in which it is clear that a cyano-group has acted as a dienophile (189) is the result of a subsequent double-bond shift, and the structure was validated by crystallographic structure analysis. Bridged y-lactams have been successfully prepared by the intramolecular Diels-Alder reactions of NN-diallyl- and iV-(l,l-dimethylprop-2-yn-l-yl)amides of various penta-2,4-diene carboxylic acids. Thus, pyrolysis of (190) gave an 80%... 

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