Dienamine intermediates

Vinylogous forma mides have been obtained by double Vilsmeier reactions of methyl ketones (53,54), 4-methylpyridine (53,54), and olefinic (55) compounds. The dienamine intermediates were demonstrated in the latter cases. 

Similarly, the addition of organometallic reagents to pyridines leads to dienamine intermediates, which can be demonstrated (167-170). 

In the generation of 3,5-dibenzylpyridine from piperidine and benzalde-hyde, the formation of enamine and dienamine intermediates must also be involved (191-193). 

Diarylprolinol silyl ether catalyst was able to promote enantioselective allqrlations of a,p-unsaturated aldehydes with stabilised carbcations as electrophiles. Dienamine intermediates were trapped by the electrophile in two positions. The y-disubstituted aldehydes were not good substrates for y-substitution. However, linear unbranched and p-substituted a,p-unsaturated aldehydes preferred y-substitution. ot-Substituted aldehydes were unreactive under secondary amine catalysis (Scheme 8.45). 

SCHEME 2.16. Generation of dienamine intermediates from unsaturated iminium ions. O... 

In 2009, the Christmann group reported the application of a dienamine intermediate for Rauhut-Currier-type reactions (775). In this case, the a-position of the dienamine acts as the nucleophile in an intramolecular cyclization reaction giving access to functionalized monocyclic compounds. The applicability of this strategy was illustrated in the synthesis of (7 )-rotundial (193), a mosquito repellent from the leaves of Vitex rotundifolia (175). Hence, an organocatalytic Rauhut-Currier-type reaction of dialdehyde 192 catalyzed by 20 mol% (5)-138 gave (7 )-rotundial (193) directly in good enantioselectivity, albeit in only a moderate yield (Scheme 46). 

Asymmetric inverse-electron-demand Diels-Alder reactions controlled by the dienophile HOMO and the diene LUMO have seldom been reported. Through a HOMO-activation strategy by the formation of dienamine intermediate H between crotonaldehyde (2a) and chiral amine 36 to raise the dienophile HOMO energy, Chen and coworkers successfully developed the first organocatalytic aU-carbon-based inverse-electron-demand Diels-Alder reaction to produce multifunctional cycloadducts 35 with up to 99% ee (Scheme 38.11) [19]. 

The enantioselective organocatalytic functionaUzation of carbonyl compounds at the 7 position represents a highly challenging and persistent problem for asymmetric synthesis [33]. Recently, Melchiorre and coworkers [34] reported a chiral Bronsted acid-assisted dienamine catalysis for the direct asymmetric y-alkylation of a-substituted linear a,(l-unsaturated aldehydes (Scheme 43.22). In this reaction, the chiral phosphoric acid 106 can induce the formation of a chiral contact ion-pair from alcohol 103 [35], which may synergistically engage in a matched combination with the chiral covalent dienamine intermediate derived from chiral primary... 

In 2012, Talavera et al. reported a formal [2h-2] reaction combining a secondary amine catalyst and a hydrogen-bonding donor catalyst for the synthesis of functionalized cyclobutane-fused tetrahydropyran derivatives [35]. The reaction proceeded via a nucleophilic dienamine intermediate resulting from the interaction of secondary amine and y-hydrogen-containing a,p-unsaturated aldehydes, which reacted reversibly with nitroalkenes 117 to give rise to cyclobutanes 118 (Scheme 9.39). Further... 

Scheme 4.27 Synthesis of WH-1,2,3-triazoles via push-pull dienamine intermediate.

In 2013, Ramachary and Shashank reported the synthesis oftrisubstituted 1,2,3-triazoles 93 via dienamine intermediate by treating the cycUc enones 78 with aryl azides 2 in the presence of 10 mol% of pyrroUdine 86 as the catalyst (Scheme 4.33) [36]. The reaction was performed in DMSO as solvent at room temperature. Both substituted and simple Hagemann s esters reacted in a similar manner. Various substituents on the aryl ring of the azide component were tolerated. Both electron-deficient (NO2, CFj, C02Et, CN, and Br) and neutral (CHg and H) aryl azides (Ar-Ng) 2 furnished the bicyclic Af-aryl-l,2,3-triazoles 93 in excellent to... 

Scheme 4.33 Synthesis of 1,4,5-trisubstituted 1,2,3-triazoles via dienamine intermediate.

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