Regioselective Diels-Alder reaction

Sequential radical cyclizations are also featured in an efficient and clever synthesis of the cedrane framework 83 (see Scheme 15).30 Compound 81, the product of a regioselective Diels-Alder reaction between isoprene (79) and nitroethylene (80), participates in a nitroaldol reaction (Henry reaction) with 5-methyl-4-hexenal in the presence of a basic resin to give 82. Because the nitro group in... 

Fig. 10. Enzymatic regioselective Diels-Alder reaction of 22 and 23. The catalyst with a small nano-sized space selectively produces e/ido-adduct, 24, and the other catalyst with a large space produces exo-adduct 25...

Tamariz et al. reported the synthesis of mukonine (11) based on a regioselective Diels-Alder reaction of N-phenyl-4,5-dimethylidene-2-oxazolidinone (634) with methyl propiolate (635). The diene 634 was prepared in moderate yield from the condensation reaction of 2,3-butanedione (632) with phenyl isocyanate (633). In an optimized reaction procedure using drastic basic hydrolytic conditions (KOH/ MeOH), followed by methylation with dimethyl sulfate, the adduct 636, was... 

Rishirihde B (57) (Figure 7.10) was isolated by Iwaki and co-workers from Strep-tomyces rishiriensis OFR-1056 [24], and this compound exhibits antithrombotic activity through selective a2-macroglobuhn inhibition. Allen and Danishefsky synthesized 57 utiHzing regioselective Diels-Alder reaction of the quinodimethide... 

The most efficient and regioselective Diels-Alder reactions are those involving electronically compatible dienes and dienophiles. It might be instructive to mention the rapid syntheses of olivacine [179] and catharanthine [180] benefit from the well-... 

Diels-Alder reactions.1 A key step in a synthesis of 4-demethoxydaunomy-cinone (5) involves a regioselective Diels-Alder reaction of anthracene-1,4,9,10-tetrone (2) with (1) to give the desired adduct (3) in 79% yield. A similar reaction with 2-acetoxy-1,3-butadiene affords only a 58% yield of the corresponding adduct. The product (rather unstable) is aromatized and hydrolyzed to give 4 in high yield. 

Regioselective Diels-Alder reactions.1 The phenylthio group controls the re-gioselectivity of the reaction of 1 with monosubstituted dienophiles. 

Fig. 15.25. Regioselective Diels-Alder reactions with a 2-substituted 1,3-diene I comparison of the effects exerted by one or two dienophile substituents.

Fig. 15.28. RegioseLective Diels—Alder reactions with a 2-substituted diene II selectivity increase by way of addition of a Lewis acid.

These same features can ensure regioselective Diels-Alder reactions. The same orbital of the dienophile is used and, if the HOMO of the diene is also unsymmetrical, the regioselectivity of the reaction will be controlled by the two largest coefficients bonding together. 

Scheme 10 illustrates an analogous regioselective Diels-Alder reaction with 3-nitrocycIohexenone (17) serving as a cycloalkynone dienophile equivalent. ... 

Although mixtures are often obtained, usually one predominates, the one indicated above, but selectivity depends on the nature of the substituents on both diene and alkene. This regioselectivity, in which the ortho or para product is favored over the meta, has been explained by molecular-orbital considerations. When X = NO2, regioselectivity to give the ortho or para product was very high at room temperature, and this method, combined with subsequent removal of the NO2 (see 19-67) has been used to perform regioselective Diels-Alder reactions. 

SCHEME 25.5(F) Regioselective Diels-Alder reactions under varying reaction conditions (Reference 59). 

In the few cases studied,103 unsymmetrical and electron-deficient 1,2,4,5-tetrazines participate in predictably regioselective Diels-Alder reactions with electron-rich dienophiles [Eq. (34)]. 

A regioselective Diels-Alder reaction between 1,2,4-alkaltrienes and conjugated dienes occurs in the presence of (dba)3Pd2-CHCl3. ... 

Moreover, 2//-pyrans can undergo [4+2] cycloadditions with activated multiple bonds. For instance, compound 3 undergoes a regioselective Diels-Alder reaction with propiolic ester to adduct 6 which yields methyl 2,4-dimethylbenzoate by cycloreversion and acetone elimination ... 

Weeratunga G, Prasad GKB et al (1990) Regioselective Diels-Alder reactions of N-cyanoindoIe-4, 7-diones elaboration of the A-ring of the kinamycins on a BC ring template. Tetrahedron Lett 31 5713-5716... 

Regioselective Diels-Alder reactions. The intramolecular [4+2]cycloaddition of... 

Diels-Alder cycloadditions have greater synthetic flexibility when suitable substituents on the diene are available for elaboration to functionality which is otherwise difficult to build into the structure. To this end, several schemes to novel 1,3-dienes have been described this year. Hagemann s ester (79) is a useful intermediate in synthesis of terpenoids. The ester is now conveniently available by regioselective Diels-Alder reaction of 1-methyl-l,3-bis(trimethylsiloxy)buta-1,3-diene (80) with ethyl acrylate. The yield is 61% overall after 5 days heating in xylene at 170—180 C, followed by hydrolysis of the moisture-sensitive adduct (81). The diene (80) is readily prepared from acetylacetone and trimethylsilyl chloride in the presence of triethylamine-zinc chloride in an ether-benzene mixture. The related diene (82), similarly prepared from 2-formylbutan-3-one, is used to give the useful cyclic dienophile (83). These sequences are outlined in Scheme 18. ... 

Diels-Alder Reactions.- A regioselective Diels-Alder reaction between the diene (77) and the quinone (78) gives an adduct which is a key intermediate in a synthesis of the left-hand portion of the... 

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