Aldol cyclization reaction

Furthermore, as described by Mori and coworkers, the domino aldol/cyclization reaction of the 3-keto sulfoxide 2-422 with succindialdehyde (2-423) in the presence of piperidine at r.t. afforded the chromone 2-424 which, on heating to 140 °C, underwent a thermal syn-elimination of methanesulfenic acid to provide 2-426 in 22 % overall yield (Scheme 2.100) [227]. This approach was then used for the synthesis of the natural products coniochaetones A (2-425) and B (2-427) [228]. 

By tethering a,p-enones with another carbonyl-containing moiety, Rrische developed rhodium-catalyzed asymmetric 1,4-addition/aldol cyclization reactions." These reactions proceed with high diastereo- and enantioselectivity, furnishing structurally complex cyclic compounds in a single step [Eqs. (3.11-3.13)]. 

An aldol-cyclization reaction leading to the formation of a five-membered ring. 

Zinc(O) is capable of reducing alkyl halides. The interplay of the reductive action of zinc and the ability of aluminum Lewis acids to activate the carbonyl group enabled effective generation of aluminum enolates from a-bromo carbonyl compounds (Scheme 6.25) [45]. This method is convenient for aldol cyclization reaction, producing macrolactones in moderate to high yields. Note that the possibility of a zinc enolate, rather than the aluminum enolate, promoting the actual reactions could not be excluded. 

Macrocyclic diketones can undergo transannular aldol cyclization reactions, giving bicyclic aldols. A representative example is seen in the cyclization of 1,6-cyclodecanedione to the corresponding hy-droazulenone (equation 142). Aldolization of the related cyclodecadienedione (120) has also been examined under mildly basic conditions aldols (121) and (122) are the main products, being formed in a ratio of 1 4 (equation 143). Control experiments with the pure aldols showed that this is the thermodynamic ratio of isomers. 

Why was benzylammonium trifluoroacetate chosen to induce the Aldol cyclization reaction This appears to be an unusual reagent and deserves comment. The bromoacetate moiety remains a problems since it might react with the enolate anion generated during the Aldol condensation. 

While targeting isosteric analogues of natural spirobrassinin and phytoalexin, Wang and co-workers [54] synthesized spirooxindoles in almost optically pure form through an intramolecular aldol/cyclization reaction of a-isothiocyanato imides with isatins catalyzed by a rosin-derived organocatalyst (Scheme 16.27). 

SCHEME 12 Mechanism of tandem aldol cyclization reactions. 

Chen et al. reported that oxindole-type a-isothiocyanato imides 271 had high activities in the aldol-cyclization reaction with inactive simple ketones. With the promotion of thiourea catalyst 1401, the cascade process of either aromatic or aliphatic ketones 206 with 3-isothiocyanato oxindoles 271 proceeded smoothly to afford spirooxindoles 272 bearing a quaternary stereogenic center with perfect stereoselectivity (Scheme 2.72) [102],... 

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