Aldehydes chiral deriv

Among chiral auxiliaries, l,3-oxazolidine-2-thiones (OZTs) have attracted much interest for their various applications in different synthetic transformations.2 Such simple structures, directly related to far better known chiral oxazolidinones,11,12,57 have been explored in asymmetric Diels-Alder reactions and asymmetric alkylations, but mainly in condensation of their /V-acyl derivatives with aldehydes. Chiral OZTs have shown interesting characteristics in anti-selective aldol reactions58 or combined asymmetric addition. 

A diastereoselective synthesis of vicinal diamines has been described79. The aldehydes 56 derived from chiral amino acids 55 were converted into the A-benzylimines 57 and the latter were treated with organometallic reagents R2M in the presence of cerium(III)... 

In order to obtain stereoselective formation of the chiral centers C-3, C-7, and C-14, we explored the use of chiral derivatives of our indoloaze-pine 153. Earlier, we had already found that the A(-benzylindoloazepine ester 215 rearranged on heating to an a-methylene lactam 216, indicating the possibility of thermal generation of an intermediate secondary amine indoloacrylate 217. It was also found that this intermediate could be trapped with a variety of aldehydes. Thus, D-seco D-E-trans vincadiffor-mine congeners (218) could be obtained by condensation of the indoloaze-pine 215 with aldehydes at 100°C (Scheme 54) (132). Consequently, introduction of a chiral substituent onto N of the indoloazepine 153 prior to condensation with our 4-ethyl-4,5-dihydroxypentanal acetonides 186 and 187 appeared to be an option for chiral induction in the formation of cen-... 

The reaction of a, -unsaturated ketones 93, obtained from j -ketoesters 36 and chiral aldehydes 94 [derivatives of D-glyceral (92TL3809,93T7133), and D-mannitol (93T7133)], with MN have been used for the first time in the synthesis of optically pure 2-aminopyrans 95. Yields and diastereos-electivity were low however, recrystallization afforded one of the stereoisomers in a nearly pure state (d.e. 80-100%) (Scheme 27). 

The direct homologation technique was then extended to the synthesis of various uncommon carbohydrate structures. Thus, higher sugars of the L-series were obtained Starting from 2,3-0-isopropylidene-4-O-benzyl-L-threose (43) [39c] (Scheme 13), and the amino tetrose 47 and pentose 48 were prepared from the a-amino aldehyde 46 derived from L-serine [46a] (Scheme 14). These amino sugars were used as chiral building blocks for the... 

Asymmetric induction in the cyclopropanations of unsaturated substrates with methylene has been extensively investigated. A propensity of the Simmons-Simth and related reagents to make coordination to basic atoms is most frequently exploited. Treatment of a,/J-unsaturated aldehyde acetals derived from the aldehydes and chiral dialkyl tartrates or 2,4-pentanediol, with diiodomethane/diethylzinc in hexane, produces cyclopro-panecarboxaldehyde acetals with high diastereoselectivity (equation 69)109 110. Uniformly good diastereoselectivity has also been realized in the cyclopropanations of chiral acetals... 

Additions to Chiral Derivatives of a,fMJnsaturated Aldehydes and Ketones... 

In 1997, Kobayashi and colleagues reported the first truly catalytic enantioselective Mannich-type reactions of aldimines 24 with silyl enolates 37 using a novel chiral zirconium catalyst 38 prepared from zirconium (IV) fert-butoxide, 2 equivalents of (R)-6,6 -dibromo-l,l -bi-2-naphthol, and N-methylimidazole (Scheme 13) [27, 28], In addition to imines derived from aromatic aldehydes, those derived from heterocyclic aldehydes also worked well in this reaction, and good to high yields and enantiomeric excess were obtained. The hydroxy group of the 2-hydroxyphenylimine moiety, which coordinates to the zirconium as a bidentate ligand, is essential to obtain high selectivity in this method. 

Aminals are another class of saturated heterocycles that form very readily under thermodynamic control aminals are nitrogen analogues of acetals. They are usually made by refluxing a 1,2-diamine with an aldehyde in toluene (no acid catalyst is needed because the nitrogens are very nucleophilic), and this makes a very useful way of forming a chiral derivative of an achiral aldehyde. Here is an example the diamine is made from the amino acid proline. The product has a new chiral centre, and it forms as a single diastereoisomer because the phenyl ring prefers to be on the exo face of the bicyclic system (see Chapter 33). 

Chiral derivatives or compounds having a prochiral group in the reactive center are frequently present among substrates, amines, and aldehydes used as reagents in Mannich synthc.sis. When at least two out of the three reagents are chiral or prochiral, the resulting Mannich base will be made up of a diastcrcomeric mixture of products. The main possible combinations of chiral or prochiral couples of reactants leading to diastereo-meric derivatives are reported in Fig. 36. 

Keywords Cyclocondensation, Danishefsky s diene. Aldehydes, Pyrone derivatives. Chiral Lewis acids, Enantioselectivity... 

When the boron ligands and the aldehyde are both chiral, the inherent stereoselectivities of each partner may be either matched or mismatched (Chapter 1). In principle, a chiral aldehyde could derive facial selectivity from either the Felkin-Anh-Heathcock model (Figures 4.8 and 4.10) or the Cram-chelate model (Figure 4.11). However, because the boron of these reagents can accept only one additional ligand, chelation is not possible. Therefore only the Felkin-Anh-Heathcock effects... 

Synthesis of the chiral Cj-Cio fragment was started from the aldehyde 40 derived from cis-butendiol (Scheme 9). Brown s crotylboration (30) of 40 using (-)-(E)-crotyldiisopinocampheyl-... 

It is clear that the Meyers aldehyde synthesis provides a facile route not only to a variety of aldehydes but also to esters, acids, and chiral derivatives. The disconnections available by this methodology include... 

Baldwin [78] synthesised the chiral y-lactam 123 starting with D-penicill-amine, and the aldehyde 121 derived from L-aspartic acid (Scheme 42). 

For reactive aldehydes, proline-derived 7V-sulfonylcarboxamides were investigated as catalysts by Ley and co-workers [45], Berkessel et al. [46], and Kokotos and co-workers [47]. Not only did amino acid derivatives promote the aldol reaction, but also chiral diamines in the presence of an acid were also found to be effective [48]. The yield and enantioselectivity were the same as for the proline-catalyzed reactions. In combination with polyoxometalate acid, a diamine could be used in 0.33 mol%, but for less reactive aldehydes the yields are still low [49]. 

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