Hetero-Diels-Alder -catalyzed

Metal-catalyzed asymmetric hetero Diels-Alder reactions of unactivated dienes with glyoxylates 98PAC1117. 

Catalytic enantioselective hetero-Diels-Alder reactions are covered by the editors of the book. Chapter 4 is devoted to the development of hetero-Diels-Alder reactions of carbonyl compounds and activated carbonyl compounds catalyzed by many different chiral Lewis acids and Chapter 5 deals with the corresponding development of catalytic enantioselective aza-Diels-Alder reactions. Compared with carbo-Diels-Alder reactions, which have been known for more than a decade, the field of catalytic enantioselective hetero-Diels-Alder reactions of carbonyl compounds and imines (aza-Diels-Alder reactions) are very recent. 

This chapter will try to cover some developments in the theoretical understanding of metal-catalyzed cycloaddition reactions. The reactions to be discussed below are related to the other chapters in this book in an attempt to obtain a coherent picture of the metal-catalyzed reactions discussed. The intention with this chapter is not to go into details of the theoretical methods used for the calculations - the reader must go to the original literature to obtain this information. The examples chosen are related to the different chapters, i.e. this chapter will cover carbo-Diels-Alder, hetero-Diels-Alder and 1,3-dipolar cycloaddition reactions. Each section will start with a description of the reactions considered, based on the frontier molecular orbital approach, in an attempt for the reader to understand the basis molecular orbital concepts for the reaction. 

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

The two transition states in Figs 8.5 and 8.6 correspond in principle to a metal-catalyzed carho-Diels-Alder reaction under normal electron-demand reaction conditions and a hetero-Diels-Alder reaction with inverse electron-demand of an en-one with an alkene. The calculations by Houk et al. [6] indicated that with the basis set used there were no significant difference in the reaction course. 

Fig. 8.6 The calculated transition-state structure for the reaction of acrolein with butadiene leading to formation of vinyldihydropyran by a hetero-Diels-Alder adduct catalyzed by BH3 using a RHF/3-21G basis set [6]...

In a combined experimental and theoretical investigation it was found that the / -alkyl group in the dienophile gave a steric interaction in the transition-state structure which supported the asynchronous transition-state structure for the Lewis acid-catalyzed carbo- and hetero-Diels-Alder reactions. The calculated transition-state energies were of similar magnitude as obtained in other studies of these BF3-catalyzed carbo-Diels-Alder reactions. 

Very few theoretical studies of hetero-Diels-Alder reactions have been performed [25] and, furthermore, theoretical studies of the Lewis acid-catalyzed hetero-Diels-Alder reactions are even more limited. 

The mechanism for the hetero-Diels-Alder reaction of benzaldehyde 9 with the very reactive diene, Danishefsky s diene 10, catalyzed by aluminum complexes has been investigated from a theoretical point of view using semi-empirical calculations [27]. The focus in this investigation was to address the question if the reaction proceeds directly to the hetero-Diels-Alder adduct 11, or if 11 is formed via a Mukaiyama aldol intermediate (Scheme 8.4) (see the chapter dealing with hetero-Diels-Alder reactions of carbonyl compounds). 

The number of theoretical investigations of hetero-Diels-Alder reaction is very limited. The few papers dealing with this class of reactions have shown that the influence of the Lewis acid on the reaction course can to a high extent be compared to those found the carbo-Diels-Alder reactions. At the present stage of investigations, however, more work is needed if we are to understand the influence and control of selectivity in Lewis acid-catalyzed hetero-Diels-Alder reaction - we are probably at the beginning of a new era in this field. 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

Lewis-acid-catalyzed asymmetric hetero-Diels-Alder cycloaddition of a 1-thiabuta-1,3-diene with chiral A/-acryloyl and A/-crotonyl oxazolidinone dienophile [105]... 

Lithium catalyzed hetero-Diels-Alder reactions. Cyclocondensation of N-protected ot-amino aldehydes with 1-methoxy-3-fert-butyldimethylsilyloxybutadiene in the presence of lithium perchlorate [104]... 

Johannsen M., Yao S., Graven A., Jorgensen K. A. Metal-Catalyzed Asymmetric Hetero-Diels-Alder Reactions of Unactivated Dienes With Glyoxylates Pure Appl. Chem. 1998 70 1117-1122... 

Wada E., Yasuoka H., Pei W., Chin U., Kanemasa S. Lewis Add-Catalyzed Stereoselective Hetero Diels-Alder Reactions of (E)-l-Phenylsulfonyl-3-Alken-2-Ones With Vinyl Ethers. Synthetically Equivalent to Stereoselective Michael Type... 

Togni A., Pastor S. D. Cooperativity of Chirality in Homogeneous Catalysis The Gold(I)-Catalyzed Aldol Reaction and the Vanadium(IV)-Catalyzed Hetero-Diels-Alder Cycloaddition Chirality 1991 3 331-340... 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Jorgensen et al. [84] studied how solvent effects could influence the course of Diels-Alder reactions catalyzed by copper(II)-bisoxazoline. They assumed that the use of polar solvents (generally nitroalkanes) improved the activity and selectivity of the cationic copper-Lewis acid used in the hetero Diels-Alder reaction of alkylglyoxylates with dienes (Scheme 31, reaction 1). The explanation, close to that given by Evans regarding the crucial role of the counterion, is a stabilization of the dissociated ion, leading to a more defined complex conformation. They also used this reaction for the synthesis of a precursor for highly valuable sesquiterpene lactones with an enantiomeric excess superior to 99%. 

Similar transformations have been performed with Danishefsky s diene and glyoxylate esters [85] catalyzed by bis (oxazoHne)-metal complexes to afford the hetero Diels-Alder product in 70% isolated yield and up to 72% ee. Jorgensen [86,87] reported a highly enantioselective, catalytic hetero Diels-Alder reaction of ketones and similar chiral copper(II) complexes leading to enantiomeric excesses up to 99% (Scheme 31, reaction 2). They also described [88] a highly diastereo- and enantioselective catalytic hetero Diels-Alder reaction of /I, y-imsaturated a-ketoesters with electron-rich alkenes... 

Bolm et al. [108] prepared a C2-symmetric bis (sulfoximine) as ligand for the copper-catalyzed hetero-Diels-Alder reaction. The stereogenic sulfur atom being located near the AT-coordinating atom, these structures were assumed to be promising for asymmetric catalysis. Their Hgand (79 in Scheme 43) was synthesized by palladium-catalyzed N-aryl imination from 1,2-dibromobenzene and (S)-S-methyl-S-phenylsulfoximine with Pd2dba3 in 70% yield. 

The spiro dimer of a-tocopherol (9, see also Fig. 6.4) is formed as mixture of two diastereomers by dimerization of the o-QM 3 in a hetero-Diels-Alder reaction with inverse electron demand. Both isomers are linked by a fluxion process (Fig. 6.22), which was proven by NMR spectroscopy.53 The detailed mechanism of the interconversion, which is catalyzed by acids, was proposed to be either stepwise or concerted.53-55... 

Tocopheryl)propionic acid (50) is one of the rare examples that the o-QM 3 is involved in a direct synthesis rather than as a nonintentionally used intermediate or byproduct. ZnCl2-catalyzed, inverse hetero-Diels-Alder reaction between ortho-qui-none methide 3 and an excess of <2-methyl-C,<9-bis-(trimethylsilyl)ketene acetal provided the acid in fair yields (Fig. 6.37).67 The o-QM 3 was prepared in situ by thermal degradation of 5a-bromo-a-tocopherol (46). The primary cyclization product, an ortho-ester derivative, was not isolated, but immediately hydrolyzed to methyl 3-(5-tocopheryl)-2-trimethylsilyl-propionate, subsequently desilylated, and finally hydrolyzed into 50. 

The montmorillonite KlO-catalyzed aza-Diels-Alder reaction of Danishefsky s diene with aldimines, generated in situ from aliphatic aldehydes and p-anisidine, proceeded smoothly in H20 or in aqueous CH3CN to afford 2-substituted 2,3-dihydro-4-pyridones in excellent yields (Eq. 12.47).115 Also, complex [(PPh3)Ag(CBiiH6Br6)] was shown to be an effective and selective catalyst (0.1 mol% loading) for a hetero-Diels-Alder reaction with Danishefsky s diene and the reaction showed a striking dependence on the presence of trace amounts of... 

Similar aza-Diels-Alder reactions of Danishefsky s diene with imines or aldehydes and amines in water took place smoothly under neutral conditions in the presence of a catalytic amount of an alkaline salt such as sodium triflate or sodium tetraphenylborate to afford dihydro-4-pyridones in high yields (Eq. 12.49).117 Antibodies have also been found to catalyze hetero-Diels-Alder reactions.118... 

Using water as the solvent enhanced the rate of the hetero-Diels-Alder reaction relative to the dimerization of cyclopentadiene. In addition, the reaction is much faster at a low pH, which implies that the reaction is acid catalyzed. The 5,5-fused system generated has been used in the total synthesis of several bioactive compounds, including the anti-HIV agent (—)-carbovir (Eq. 12.51)121 and the hydroxylactone moiety of mevinic acids (Eq. 12.52).122... 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

In recent years, many novel MCRs - including Michael addition-initiated three-component domino sequences [10], Knoevenagel/hetero-Diels-Alder-based MCRs [11], radical chain MCRs [12], transition metal-catalyzed Pauson-Khand MCRs [13], as well as Petasis MCRs [14], have been added to the chemisf s armamentarium and successfully applied to all fields of organic synthesis. 

Many examples of asymmetric reactions catalyzed by copper complexes with chiral ligand systems have been reported. In particular, various copper-bis(oxazoline) catalysts (e.g., complexes (H) to (L), Scheme 48) are effective for carbon-carbon bond-forming reactions such as aldol,204 Mukaiyama-Michael, Diels-Alder,206 hetero Diels-Alder,207,208 dipolar cycloaddition,209,210... 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

Schaus et al.41 have also reported an asymmetric hetero Diels-Alder reaction of Danishefsky s diene 10042 with aldehyde 101 catalyzed by chromium(III) complex 99 bearing a similar chiral salen ligand. Product 102 is obtained in moderate to good yield and stereoselectivity (Scheme 5-31 and Table 5-5). 

TABLE 5-5. Hetero Diels-Alder Reaction Catalyzed by 99... 

Figure 26. Proposed stereochemical model for the hetero-Diels-Alder reaction of ethyl vinyl ether and acylphosphonate catalyzed by 55c-Cu(II) complex.

Scheme 22. Effect of the enol ether alkyl group in the catalyzed hetero-Diels-Alder reaction. [Adapted from (221).]...

Scheme 23. Copper-catalyzed hetero Diels-Alder reactions of glyoxylates and 1,3-dienes. [Adapted from (226, 227).]...

Cycloaddition. Recently )V-vinyloxazol id i nones were presented as new potential chiral dienophiles in inverse hetero-Diels-Alder reactions to activated 1-oxabutadienes, leading with high diastereocontrol to heteroadducts under Eu(fod)3-catalyzed conditions (Scheme 49).17... 

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