Alder s. Diene syntheses

Alder s. Diene syntheses Aldimines s. Metallo aldimines Aldol condensation s. a. Retroaldol reaction —, intramolecnlar -, diazoketols, cyclic by - 26,938 Aldol-type condensation, intramolecnlar, to heterocycles 26,935 Aldonolactones... 

Alder s. Diene syntheses Aldimines (s. a. Metallo aldimines)... 

Alder s. Diene syntheses Aldol condensation —. isocyclics by — 12, 735... 

Alder s. Diene syntheses Aldofuranoses 19, 50 Aldonic acid lactones... 

Diels-Alder reaction s. Diene syntheses l,l -Dienamines... 

Dieckmaiin cyclization 21, 910 22, 898/0 Diels-Alder reaction s. Diene syntheses Dienamines s. a. Allenylamines... 

Hetero Diels-Alder reactions with imino dienophiles have been employed as key step in several syntheses of naturally occuring alkaloids. With regard to stereoselective transformations, the approach to (S)-anabasin worked out by Kunz et al. impressively illustrates the high utility of natural carbohydrates as source of chirality in asymmetric synthesis [505]. The N-galactosyl imine 7-28 underwent a Lewis acid catalysed aza Diels-Alder reaction with Danishefsky s diene which proceeded with excellent induced diastereoselectivity to yield the adduct 7-29. A short sequence then afforded the desired alkaloid 7-30. This work also deals with the suitability of several other dienes and imino dienophiles for such transformations (Fig. 7-7). 

Of course, this was only a theory—until in 1982 K.C. Nicolaou s group synthesized the proposed endiandric acid precursor polyene—and in one step made both endiandric acids D and E, plus endiandric acid A, which arises from a further pericyclic reaction, an intramolecular Diels-Alder cycloaddition of the acyclic diene on to the cyclohexadiene as dienophile. 

To build the central ABC core present in 1, two main synthetic strategies have been used Diels-Alder reactions and asymmetric synthesis. This tricyclic subunit was chosen as the first synthetic target because it contains all five stereogenic carbons. A central pyrrolo-isoquinoline skeleton (ABC) was initially synthesized by the high-pressure Diels-Alder reaction of 3-alkyl-5,6-dihydro-2-pyridones with Danishefsky s diene, followed by deprotection and spontaneous pyrrolidine ring closure (see Scheme 16) [77]. 

The other recent examples of asymmetric syntheses involving Danishefsky s diene focused on hetero Diels-Alder reactions. Shibasaki and Feng separately reported asymmetric reactions with carbonyl dienophiles. Shibasaki demonstrated successful asymmetric reactions of ketones using a chiral Cu(I)-Walphos catalyst. Feng used a chiral A,iV -dioxide/In(OTf)3 catalyst in asymmetric cycloaddition reactions of aldehydes. Imine dienophiles are also amenable to asymmetric Diels-Alder reactions with Danishefsky s diene. Wulff reported enantioselective reactions using a VAPOL-B(OPh)3 catalyst system, while Snapper and Hoveyda disclosed silver-catalyzed enantioselective aza Diels-Alder reactions. ... 

By using a variant of Rawal s azadiene Diels-Alder cycloaddition chemistry [103,104], it was possible to synthesize several Kornfeld ketone analogues with substitution patterns that are difficult to otherwise obtain. Thus, heating a mixture of furan 280a and Rawal s diene (281) in CH3CN at reflux for 2h resulted in the formation of a 2 1-mixture of diastereomeric amines 282 that was immediately treated with HF at room temperature to unmask the enone 283 (Scheme 13.64) [105,106]. The crude reaction mixture was then heated at reflux in toluene for 30 min to effect... 

Recently, Carreno, Urbano and coworkers were also able to synthesize almost enantiopure [7]helicene bisquinones 4-58 and 4-59 (96% ee) by reaction of the sulfoxide (S,S)-4-55 with the diene 4-56 in dichloromethane at -20 °C (Scheme 4.12) [19]. This six-step domino process includes a double Diels-Alder reaction, sulfoxide elimination, and aromatization of the rings B and F of the intermediate 4-57 to give 4-58, which could be oxidized to the fully aromatized 4-59. 

Beccalli et al. reported a synthesis of carbazomycin B (261) by a Diels-Alder cycloaddition using the 3-vinylindole 831 as diene, analogous to Pindur s synthesis of 4-deoxycarbazomycin B (619). The required 3-vinylindole, (Z)-ethyl 3-[(l-ethoxy-carbonyloxy-2-methoxy)ethenyl]-2-(ethoxy-carbonyloxy)indole-l-carboxylate (831), was synthesized starting from indol-2(3H)one (830) (620). The Diels-Alder reaction of the diene 831 with dimethyl acetylene dicarboxylate (DMAD) (535) gave the tetrasubstituted carbazole 832. Compound 832 was transformed to the acid 833 by alkaline hydrolysis. Finally, reduction of 833 with Red-Al afforded carbazomycin B (261) (621) (Scheme 5.99). 

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