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Aldol chemistry

A nice and convergent approach to both compounds makes use of RCM to form the 5-membered building block 71, which mimics the carbohydrate part of the nucleosides. The necessary diene precursor 69 is readily assembled via Evans aldol chemistry. RCM then affords the ring in almost quantitative yield (69->70), leaving the chiral centers and the free hydroxyl group intact. Removal of the chiral auxiliary by reductive cleavage, attachment of the base by means of jt-allylpalladium chemistry, and a final deprotection step complete these highly efficient syntheses [46]. [Pg.72]

The asymmetric center in fragment 3 was efficiently installed using Kiyooka s chiral Lewis acid promoted Mukaiyama aldol chemistry (Scheme 2.86). Commer-... [Pg.129]

The other two unenolizable esters we mentioned on p. 728 undergo cross-condensations with ketones. Unlike formaldehyde, formate esters are well behaved—no special method is necessary to correspond with the Mannich reaction in aldol chemistry, Here is what happens with cyclohexanone. [Pg.731]

The asymmetric synthesis of (—)-denticulatin A (30) shows an interesting application of the boron aldol chemistry (Scheme 6) [23]. In a group-selective aldol reaction between the weso-aldehyde 27 and (5)-28, the hydroxyalde-hyde 29 was formed with > 90 % de, which spontaneously cyclized to the lactol 31. The configuration at the stereocenters of C-2 and C-3 in 29 is in accordance with the induction through the sultam auxiliary as well as with preference of an a-chiral aldehyde to react to the ant/-Felkin diastereomer in an aldol reaction which is controlled by the Zimmermann-Traxler model [24, 25]. [Pg.14]

Reductive aldol chemistry was investigated for the formation of 2,2-disubstituted-3-pyrrolines. Thus for example, Birch-type reduction of pyrrole 56 with lithium di-t-butylbiphenyl (LiDBB) in the presence of bis(methoxyethyl)amine (BMEA), quenching the unreacted LiDBB with 1,2-dibromoethane, treatment with magnesium bromide, and addition of isobutyraldehyde gave a >20 1 mixture of anti-57 and syn-SS <03OBC3749, 03TL1095>. [Pg.135]

An exploration of selectivity in the context of intramolecular aldol chemistry. [Pg.216]

In this Chapter, we provide an overview of the various aldol control elements available for achieving synthetically useful stereoselectivity and then analyze some representative syntheses of polyketide natural products (particularly macro-lide targets) which are based primarily on the strategic use of aldol chemistry. These examples are chosen to illustrate the variety of aldol processes that have been applied to structurally complex targets and are taken largely from the recent literature (1989-1999). This selection covers some of our own research along with important contributions from other groups. [Pg.249]

As shown in Scheme 9-57, the Ci-Cf, ketones 202 and 203 have both been prepared by aldol chemistry. The synthesis of ketone 202 used the Braun auxiliary 204 in a lithium-mediated aldol reaction and afforded adduct 205 in 75% yield and 98% ds [75]. This synthe.sis can be compared with the related Reformatsky reaction of imide 206, again controlled by an auxiliary attached to the enolate... [Pg.282]

These examples illustrate that biocatalytic strategies can initiate innovative solutions for problems like cross aldol chemistry, regio- and stereoselective reduction of... [Pg.112]

We are interested in this instance in the stereochemistry between the y- and 8-positions. To use aldol chemistry here would require a carbonyl at the p-posit ion and it is conspicuous by its absence. We shall have to put it in. Disconnection 72 of serriconin and FGI leads to an acid 74 which is looking a little more like the kind of substrate we need for an aldol reaction. [Pg.409]

The synthesis starts with the thioester 64 to get the cis boron enolate 75 we need for the syn product Once the adduct 76 has been made, the sulfur has done its work and can be removed. Transesterification with MeOH and HgCl2 and straightforward steps lead to iodide 78. Once this iodide has reacted with the enolate of pentanone, there will be no trace of the carbonyl that originally allowed the aldol chemistry to be performed. [Pg.410]

Traditionally, aldol reactions were carried out under protic conditions, such that the enolate was formed reversibly (see Volume 2, Chapter 1.5). An added measure of control is possible if one uses a sufficiently strong base that the enolate may be quantitatively formed prior to addition of the electrophile. The renaissance that has occurred in the aldol reaction in the last two decades has been mainly due to the development of methods for the formation and use of preformed enolates. The simplest enolates to prepare are those associated with lithium and magnesium, and there now exists a considerable literature documenting certain aspects of lithium and magnesium enolate aldol chemistry. This chapter summarizes the aldol chemistry of preformed enolates of these Group I and Group II metals. Other chapters in this volume deal with boron enolates, zinc enolates, transition metal enolates and the related chemistry of silyl and stannyl enol ethers. [Pg.181]

Carreira s recent work is an extension of earlier studies [5] in which a titanium(IV) complex, prepared in situ from tridentate ligand ( )-l and Ti(Ot Pr)4, was found to catalyze Mukaiyama aldol reactions with high enantioselectivities. The chiral ligand used in both the ene and aldol chemistry is prepared from 3-bromo-5-ferf-butylsalicylaldehyde and 2-amino-2 -hydroxy-l,T-binaphthol. This... [Pg.24]

Figure 28 Demonstration of reaction tuning in zeolites acetaldehyde undergoes complex aldol chemistry on activated HZSM-5 but selectively forms crotonaldehyde at 353 K when water is coadsorbed. Figure 28 Demonstration of reaction tuning in zeolites acetaldehyde undergoes complex aldol chemistry on activated HZSM-5 but selectively forms crotonaldehyde at 353 K when water is coadsorbed.
Florhydral has the methyl group p to the aldehyde. One possible approach is an asymmetric conjugate addition, but again asymmetric reduction of the acid (or allyhc alcohol) is preferable, since the required alkene is easy to make by aldol chemistry. Here we show one example with the acid and one with the alcohol, but either are possibihties in both cases. [Pg.502]


See other pages where Aldol chemistry is mentioned: [Pg.15]    [Pg.129]    [Pg.328]    [Pg.253]    [Pg.239]    [Pg.230]    [Pg.226]    [Pg.70]    [Pg.270]    [Pg.261]    [Pg.136]    [Pg.22]    [Pg.215]    [Pg.615]    [Pg.615]    [Pg.7]    [Pg.326]    [Pg.301]    [Pg.315]    [Pg.799]    [Pg.830]    [Pg.301]    [Pg.315]    [Pg.799]    [Pg.830]    [Pg.159]    [Pg.186]    [Pg.157]    [Pg.632]   
See also in sourсe #XX -- [ Pg.157 ]




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