Diels-Alder reaction in the synthesis

Dipolar cycloaddUions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [7, 2] The versatility of this class of pericychc reactions in the synthesis of five-membered-ring heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-ring carbocyclic systems (equation 1)... 

SCHEME 8. Competition between pincer and domino Diels-Alder reactions in the synthesis of pagodane precursurs92... 

Scheme 66 Iminium ion catalysed Diels-Alder reaction in the synthesis of tamiflu...

The ionic liquid [bmim][BF ] is known to catalyze the aza-Diels-Alder reaction in the synthesis of pyrano- and furanoquinolines [190]. This reaction was also catalyzed by the enantiopure bis-imidazolinium salt 67 in 67% yield with an endo. exo ratio of 60 40 (Scheme 69) [191]. The product was obtained as a race-mate. In addition the aza-Diels-Alder reaction with imines and Danishefsky s diene was catalyzed by the salt 67 giving racemic product. The salt and its analogues could be easily prepared via the oxidation of the corresponding aminals [192]. Investigation of the influence of the counter anion in achiral C2-substituted imidazolinium salts, which can be also described as 4,5-dihydroimidazolium or saturated imidazolium salts, in the aza-Diels-Alder reaction showed, that the catalytic activity increased, the more lipophilic the counter anion and therefore the more hydrophobic the salt was [193]. 

Attachment of a crotonate chain to the alcohol (78), through a mixed carbonate ester, sets up the intramolecular Diels-Alder reaction in the synthesis of podophylotoxin.111 The starting benzocyclobutenol (78) was procured by the sequence of steps shown in Scheme 16. A similar route using a carboxylic ester, in place of the nitrile (76), for the benzyne cyclization gave a mixture of cis- and fra/u-benzocyclo-butenes but ester hydrolysis led to the pure trans acid (77).112... 

Evans, D. A. Barnes, D. M. Cationic bis(oxazo-line)Cu(Il) Lewis add catalysts. Enantioselec-tive furan Diels-Alder reaction in the synthesis of ent-shikimic acid. Tetrahedron Lett. 1997, 38, 57-58. 

C Application Diels-Alder Reaction in the Synthesis of Steroids... 

Inverse Electron Demand Diels-Alder Reactions in the Synthesis... 

Several heterodienes and heterodienophiles are used in hetero-Diels-Alder reactions in the synthesis of heterocyclic compounds. For example, aza-butadiene 81 and 81a and oxazoles 82 and 83 are used as heterodienes for synthesis of heterocycles. 

In 2010, Dirk Trauner s group took aldol reaction as the key reaction to construct the core structure of Maoecrystal V [38]. This is the only group who did not use Diels-Alder reaction in the synthesis of [2.2.2] core structure of Maoecrystal V. They took compound 1.7.92 as the raw material, and after several condensation reactions they constmcted compound 1.7.97. In this way, after ozone cleaved the unsaturated double bond of intermediate 1.7.97, it formed aldehyde which reacted with hydroxyl of the side chain to form acetal strucmre and yielded compound 1.7.98. 

Prof. Fengpeng Wang from Sichuan University utilized oxidative dearomatization reaction to construct the precursor of intramolecular Diels-Alder reaction in the synthesis of atisine [12]. As shown in Fig. 2.11, after Diels-Alder reaction, the core structure of atisine could be obtained. After several steps of functional group conversion, they obtained the common precursor of the synthesis used by Prof. Pelletier and completed the formal total synthesis. 

Several papers have concerned syntheses of terpenoids. The skeleton of sesquiterpenes such as eremophilane has been constructed by Naf etal. from the dienone (45), and Taber and Gunn have described a synthesis of ( )-torreyol (47) as outlined from the enone (46). Wilson and Mao have provided a further paper on their use of the intramolecular Diels-Alder reaction in the synthesis of diterpenes. 

Use of the retro-Diels—Alder reaction in the synthesis of chiral pyrrolidine and pyrrolinone products has been described by several groups. The... 

In summary, the efforts of Martin and his group illustrate well the utility of the intramolecular Diels-Alder reaction in the synthesis of yohimbine alkaloids. Through the use of this reaction, partially reduced isoquinoline... 

Countless elegant applications of intramolecular hetero-Diels-Alder reactions in the synthesis of complex natural products have appeared in literature. In particular, the incorporation of a hetero-Diels-Alder cycloaddition into a cascade sequence can lead to extraordinary brevity in the assembly of molecular complexity. An impressive implementation of this strategy was reported by Heathcock in the total syntheses of daphniphyllum alkaloids (Scheme 17.32) [114, 115]. In the course of these studies, it was found that protonation of unsaturated imine 225 led to a smooth intramolecular hetero-... 

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