Synthetic methods aldol type reactions

The activation of the carbonyl group by Lewis acids was another leap made in the 1960s as typified by Mukaiyama-aldol reaction. In sharp contrast to the conventional carbonyl addition reactions that had been run under basic conditions, this new method allowed the addition of various nucleophiles under acidic conditions with high chemo- and stereocontrol and, consequently, the scope of the carbonyl addition reaction was extensively expanded. The Lewis acid-promoted ally-lation with allylmetals and ene reaction also received as much attention as the aldol-type reaction. It should be further pointed out that the catalytic versions of asymmetric reactions, which represent one of the most exciting topics in recent synthetic chemistry, owe their development strongly to the Lewis acid activation protocol. The design of a variety of chiral ligands for metals has produced luxuriant fruits in this field. 

There is some similarity between the cracking of petroleum and the cracking of biomass. However, biomass is more complex chemically both in terms of structrual types and functional groups. In petrochemistry, hydrocarbons are fractionated and they are then functionalized by oxidation, halogenation, nitration and other chemical processes so as to add value. The commodity chemicals are then built up into more complex molecules using such popular synthetic methods as Friedel Craft reactions, Michael and aldol condensations, and Heck and Suzuki couplings. The speciality products of these reactions are then further elaborated into formulations for use in everyday applications ranging from personal care... 

Aldol reactions provide a valuable synthetic method for forming carbon-carbon bonds. They can be adapted to extend the length of a carbon chain, to form cyclic compounds, and to provide intermediates that can be transformed into more useful materials. An important feature of these intermediates is that functional groups useful for later reactions are located close to or on the carbons of the newly formed C-C bond. There is an almost bewildering number of variations on the aldol reaction and we shall not mention all of them. The main thing to recognize in all of these reactions is that the acceptor molecule always is a carbonyl compound, best an aldehyde, sometimes a ketone, even an ester (see Section 18-8E). The donor molecule is some type of carbanion usually, but not always, an enolate anion. However, any substance that has a... 

Treatment of cycloprop [c]pyrans 520 with acid leads to chromans 521 with a 5,8-substitution pattern, which is difficult to obtain by other synthetic methods (Scheme 114). This novel reaction is believed to proceed via a retro-hDA opening of the dihydropyran ring to afford 522 followed by an acid-catalyzed aldol-type cyclization and dehydration (Scheme 114) <20040L3191>. 

Three-component coupling reaction of a-enones, silyl enolates, and aldehydes by successive Mukaiyama-Michael and aldol reactions is a powerful method for stereoselective construction of highly functionahzed molecules valuable as synthetic intermediates of natural compounds [231c]. Kobayashi et al. recently reported the synthesis of y-acyl-d-lactams from ketene silyl thioacetals, a,/l-urisalu-rated thioesters, and imines via successive SbCl5-Sn(OTf)2-catalyzed Mukaiyama-Michael and Sc(OTf)3-catalyzed Mannich-type reactions (Scheme 10.87) [241]. 

The reactions described in this chapter include some of the most useful and frequently employed synthetic methods for carbon-carbon bond formation aldol and Claisen condensation reactions, the Wittig and related olefination reactions, and the Robinson annulation. All of these processes involve, at some point, the addition of a carbon nucleophile to a carbonyl group. The type of product which is isolated depends on the nature of the substituent (X) on the carbon nucleophile, the substituents (A and B) on the carbonyl group, and the ways in which A, B, and X affect the reaction pathways available to the intermediate formed in the addition step. 

The reaction with a, -epoxy carbonyl compounds 12 leads to the corresponding reductive ring-opened products, -hydroxy carbonyl compounds 13, in good yields (Scheme 22). Electrochemically generated benzeneselenolate [21,22] and sodium phenylseleno(triethoxy)borate (1) [35, 36], have been applied for this type of reaction as a nucleophilic selenolate. In the latter case, the reaction mechanism was suggested as shown in Scheme 23 [36]. Reaction of 1 with 12 first produces the ring-opened adduct 14, which is then reacted with an excess amount of 1 to produce the final product 13. This method is important as a simple synthetic procedure to aldols and -hydroxy esters that are rather difficult to obtain by other methods. The reactions have been extended to the reduction of more functionalized a, -epoxy carbonyl compounds [37] and have been successfully applied for the synthesis of several natural products [38]. 

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