Diamines aldehyde donors

Table 2.7 Diamine 8-CF3C02H-catalyzed aldol reactions of a,a-disubstituted aldehyde donors to afford hydroxyaldehydes with a quaternary carbon atom [29].

For the aldol reactions of aldehyde donors using (S)-proline or diamine (S)-8-CF3C02H, the major products and the proposed most suitable transition state that explains the stereochemistries of the products are also shown in Scheme 2.12 [8, 29a]. 

Scheme 2.12 The proposed most suitable transition states of the (S)-proline-catalyzed and diamine 8-CF3C02H-catalyzed aldol reactions of aldehyde donors [29b].

Diamine catalyst 10, which gives excellent results in the reaction of isobutyr-aldehyde with aromatic aldehydes, affords only moderate diastereoselectivity when used with aliphatic aldehyde donors [119]. Even C2-symmetric catalysts fail to give significant improvements [34c, 139]. Wang reported that the use of fluorous (5)-pyrrolidine sulfonamide 97 in such reactions give better diastereoselectivity and can easily be recovered by simple fluorous solid-phase extraction (Scheme 3.22) [122, 123]. 

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. 

Conditions Diamine 8 (0.05 mmol, 0.1 equiv), CF3C02H (0.05 mmol, 0.1 equiv), donor aldehyde (1.0 mmol, 2 equiv), acceptor aldehyde (0.5 mmol, 1 equiv), DMSO (0.5 mL). Entries 1 and 2, 12 h entry 3,... 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Schiff bases commonly employ polyamines as reagents, the donor nature of the ligands depending both on the type of aldehyde or ketone used and the nature of primary amine or diamine. Scheme 1 illustrates some ways of obtaining Schiff bases of different denticities via 1 1 reactions of aldehydes with ketones to yield mixed N, O, S, and Se donor sets. Condensation of aldehydes and ketones with a-amino acids is also known. Where two moles of a carbonyl compounds and one mole of diamine are reacted, more elaborate Schiff bases with higher denticity result, also exemplified in Scheme 1. 

This procedure provides a good method for the construction of l,2-a ft-aldol moieties that are less accessible by the Sharpless asymmetric dihydroxylation (see Scheme 37,0 Scheme 58, O Scheme 92,0 Sect. 11) [138] because the corresponding Z-oleflns are difficult to obtain and show reduced enantioselectivity. The first demonstration of the use of the biologically significant substrate dihydroxyacetone as a donor in organocatalyzed aldol reaction was reported by Barbas 111 and co-workers [139]. The reactions of DHA with protected glyoxal and glycer-aldehydes, in aqueous media and in the presence of enantiomerically pure diamine 24, provide access to pentuloses and hexuloses, respectively (O Scheme 19). 

The Barbas group further identified catalyst Id for asymmetric Michael additions using brine as the reaction medium 10 mol% diamine Id/TFA at ambient temperature afforded q n-Michael adducts in high yields and good to excellent stereoselectivities for cyclic ketones. The stereoselectivities decreased when aliphatic aldehydes were used as donors (Scheme 9.13). Moreover, the same authors demonstrated the first example of sym-metrical/unsymmetrical a,a-diallgrlaldehydes as donors in asymmetric Michael reactions to produce adducts featuring an all-carbon quaternary stereocentre. The use of 30 mol% (S )-l-(2-pyrrolidinylmethyl)pyrrolidine la in combination with an equal amount of TFA yielded products with moderate to high levels of diastereo- and enantioselectivity (Scheme 9.14). ... 

Mimicking the pyruvate aldol reaction has been a long-standing goal for chemists. In 2005, Dondoni and co-workers reported the dimerization of ethyl pyruvate in the presence of a diamine catalyst [53]. This donor also reacts with chloral monohydrate, giving the desired aldol with both moderate enantioselectivity (86% cc) and yield (55%), most likely due to insufficient reactivity [28c]. Pyruvic aldehyde dimethyl acetal is a decent aldol donor for aromatic as well as aliphatic aldehydes (Chart 3.3)... 

Fig. 10 Generation of a set of four multifunctional dynamers PI, P2, P3, and P4 via reversible imine formation between two diamine monomers Ml and M2, and two aldehydes M3 and M4, containing non-covalent supramolecular interaction subunits a donor (Ml), an acceptor (M3), a potential cation binding fragment (Ml and M3) and a neutral linker (the dimethylsiloxane units...

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