Trans selectivity

In the excellent monograph by Desimoni et al., four rules are listed for Diels-Alder reactions, taken from Sauer s work and based on molecular orbital arguments. 

Electron-withdrawing substituents (Z) on dienophiles and electron releasing substituents (X) on dienes 

The diene and dienophile configurations are retained in the adduct (the cis principle). 

The endo transition state is favored over the exo transition state (the endo rule). 

The Z-substituted dienophiles react with 1-substituted butadienes (in normal electron demand Diels-Alder 

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In a separate report, preparation of the lithium enolate of 31 in the presence of indium trichloride and benzaldehyde provided a 77% yield of 32 with complete trans selectivity however, sequential addition of indium trichloride and benzaldehyde provided Barbier-type products. Organotin enolates have also been used in a Darzens-type... 

The above described approach was extended to include the 1,3-dipolar cycloaddition reaction of nitrones with allyl alcohol (Scheme 6.35) [78]. The zinc catalyst which is used in a stoichiometric amount is generated from allyl alcohol 45, Et2Zn, (R,J )-diisopropyltartrate (DIPT) and EtZnCl. Addition of the nitrone 52a leads to primarily tmns-53a which is obtained in a moderate yield, however, with high ee of up to 95%. Application of 52b as the nitrone in the reaction leads to higher yields of 53b (47-68%), high trans selectivities and up to 93% ee. Compared to other metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions of... 

Alternatively, epoxides can be formed with concomitant formation of a C-C bond. Reactions between aldehydes and various carbon nucleophiles are an efficient route to epoxides, although the cis. trans selectivity can be problematic (see Section 9.1.4). Kinetic resolution (see Section 9.1.5.2) or dihydroxylation with sequential ring-closure to epoxides (see Section 9.1.1.3) can be employed when asymmetric epoxidation methods are unsatisfactory. 

Allenols can be converted into terminal vinylepoxides by a Pd(0)-catalyzed insertion of aryl or vinyl halides [81]. The reactions take place with high trans-selectivity and in good yields (Scheme 9.17a). Chiral 2,3-allenols, which can be easily... 

Optically active five- or six-membered cyclic A -acyliminium ions of this type are generated from the a-inethoxy derivatives, easily obtainable through anodic methoxylation of intermediates that are prepared via ex-chiral-pool syntheses from certain natural amino acids. Reaction of 5-substituted five-membered cyclic A -acyliminium ions with various nucleophiles leads to the predominant formation of cw-products with moderate selectivity. The trans-selective reaction with alkyl copper reagents appears to be an exception. 

In a modified procedure, these additions lead to / ,y- and ) <5-unsaturated amino acid derivatives with cis/trans selectivity >99 1 and syn/anti diastereoselectivity >99 1 (see Section I.5.2.4.2.2.4.)108. 

The diastereoselective intramolecular Michael addition of /(-substituted cyclohexcnoncs results in an attractive route to ra-octahydro-6//-indcn-6-ones. The stereogenic center in the -/-position of the enone dictates the face selectivity, whereas the trans selectivity at Cl, C7a is the result of an 6-exo-trig cyclization. c7.v-Octahydro-5//-inden-5-ones are formed as the sole product regardless of which base is used, e.g., potassium carbonate in ethanol or sodium hydride in THF, under thermodynamically controlled conditions139 14°. An application is found in the synthesis of gibberellic acid141. 

A more practical catalytic system is based on the use of air-stable [Fe(TTP)Cl] as catalyst, which can efficiently catalyze the olefination reactions at 80°C in excellent yields and high trans-selectivity [71] (Scheme 18). 

The hetero-Diels-Alder reaction has also utilized dienophiles in which both reactive centers are heteroatoms. Kibayashi reported that the intramolecular hetero-Diels-Alder cycloaddition of chiral acylnitroso compounds, generated in situ from periodate oxidation of the precursor hydroxamic acid, showed a marked enhancement of the trans-selectivity in an aqueous medium compared with the selectivity in nonaqueous conditions (Eq. 12.55).125 The reaction was readily applied to the total synthesis of (—)-pumiliotoxin C (Figure 12.5).126... 

The concept for the synthesis of4-hydroxy-4,5-dihydroisoxazoles by Righi and coworkers was discussed earlier, in Chapter 7. Here, an extension of this methodology by utilizing polymer-bound nitroacetate (hydroxylated Merrifield resin) is described [10], Thus, the one-pot domino oxidation/nitroaldol cyclization of aziridine 10-28 with immobilized nitroacetate 10-29 furnished 10-30 which, after detachment from the resin, led to the desired product 10-31 in good yield and excellent trans-selectivity (Scheme 10.7). 

Nishiyama et al. introduced a new catalyst, the chiral tr<2 i -RuCl2(Pybox-i-Pr)(ethylene) complex (91), which showed for the first time both enantio- and diastereoselectivity (trans-selectivity) at excellent levels in the reactions of terminal olefins (Scheme 66).251-253 With 4-substituted Ru(Pybox-i-Pr) complexes (92), they studied the substituent effect on enantioselectivity... 

Che et al. and Berkessel et al. independently reported that the cyclopropanation of mono- and disubstituted termini olefins with the Ru-porphyrin complex (8) bearing Halterman s ligand proceeded with high enantio- and trans-selectivity (Scheme 67).256-258... 

The methanochromanone 71 is converted with high trans-selectivity into the furo[2,3-fc]chromanone 72 by reaction with symmetric ketones (Scheme 50) <00H(53)657>. 

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