Aldol reaction stereocontrol

Still s synthesis of monensin (1) is based on the assembly and union of three advanced, optically active intermediates 2, 7, and 8. It was anticipated that substrate-stereocontrolled processes could secure vicinal stereochemical relationships and that the coupling of the above intermediates would establish remote stereorelationships. Scheme 3 describes Still s synthesis of the left wing of monensin, intermediate 2. This construction commences with an aldol reaction between the (Z) magnesium bromide enolate derived from 2-methyl-2-trimethylsilyloxy-3-pentanone (21) and benzyloxymethyl-protected (/ )-/ -hydroxyisobutyraldehyde (10).2° The use of intermediate 21 in aldol reactions was first reported by Heathcock21 and, in this particular application, a 5 1 mixture of syn aldol diastereoisomers is formed in favor of the desired aldol adduct 22 (85% yield). The action of lithium diisopropylamide (LDA) and magnesium(n) bromide on 21 affords a (Z) magnesium enolate that... 

The value of 2-acyl-1,3-dithiane 1-oxides in stereocontrolled syntheses has been extended to the enantioselective formation of (3-hydroxy-y-ketoesters through ester enolate aldol reactions <00JOC6027>. 

Acyclic stereocontrol has been a striking concern in modern organic chemistry, and a number of useful methods have been developed for stereoregulated synthesis of conformationally nonrigid complex molecules such as macrolide and polyether antibiotics. Special attention has therefore been paid to the aldol reaction because it constitutes one of the fundamental bond constructions in biosynthesis. 

The frequent occurrence of /Thydroxy carbonyl moiety in a variety of natural products (such as macrolide or ionophore antibiotics or other acetogenics) has stimulated the development of stereocontrolled synthetic methods for these compounds. Indeed, the most successful methods have involved aldol reactions.13... 

One of the key features of such stereocontrolled aldol reactions is the predictability of the absolute stereochemistry of the enantiomers (or diastereo-mers) that will be formed as the major products. The preferred intermediate for an archetypal aldol reaction, proceeding by way of a metal enolate, can be tracked using the Zimmerman Traxler transition state and the results from the different variations of the aldol reaction can be interpreted from similar reasoning, and hence predictions made for analogous reactions1129]. 

Aldol reactions have also been used as a means of macrocychzation in total synthesis and were quite successful in some cases. However, over a broader spectrum of substrates, the results are unpredictable at best and yields and stereochemical outcome vary greatly. The predominant reasons are difficulties in selective enolate formation in multi-carbonyl compounds, competing and equilibrating retro-aldolizations—especially with polyketides, which often possess several aldol moieties—and intermolecular instead of intramolecular reaction preference. Whereas most of these drawbacks may be overcome, substrate-independent stereocontrol plays a crucial role. At least one new stereocenter is formed during a macroaldolization, and because of the folding constraints involved, its configuration cannot be adequately predicted. Therefore, this can be useful in special cases but with the current possibilities is not the method of choice for a general diversity-oriented synthesis. 

Cross aldol reactions of silyl enol ethers with acetals (25 - 26, and 27 - 28) are also mediated by EGA. The reaction runs smoothly at —78 °C in a CH2CI2— —LiClO —Et NClO —(Pt) system. At an elevated temperature protonation of both enol ether and acetal occurs competitively to give 28 in a poor yield. Table 5 summarizes yields and diastereoselectivities of 28 obtained by EGA, TiCl TMSOTf and TrtClO 5 . The EGA method is superior to TiCl with regard to the stereocontrol, and comparable with TMSOTf and TrtClO in both stereocontrol and yield. 

Evans [369] and Masamune [370,371] have pioneered the use of thioesters for stereocontrol of the aldol reaction. The accompanying scheme summarizes the reactions of boron and silicon enolates of t-butyl thiopropanoate with aldehydes [372]. Both reactions are stereoconvergent. (Z) and (E)-enolates afford the same diastereoisomer syn with the boron... 

Whereas the thermodynamic route described above relied on reagent control to establish the spongistatin C19 and C21 stereocentres, the discovery of highly stereoselective 1,5-anti aldol reactions of methyl ketones enabled us to examine an alternative,16 substrate-based stereocontrol route to 5. Regioselective enolisation of enantiomerically pure ketone 37, derived from a readily available biopolymer, gave end... 

A new tandem Michael-aldol reaction of a,ft-unsaturated compounds bearing a chalcogenide or thioamide group with electrophiles has been reviewed.163 The product o -(o -hydroxyalkyl)enones - Morita-Baylis-Hillman (MBH) adducts - can be formed with significant stereocontrol when an optically active thione is used. 

High anti-diastereoselectivity is observed for several aromatic imines for ortho-substituted aromatic imines the two newly formed stereocenters are created with almost absolute stereocontrol. Aliphatic imines can also be used as substrates and the reaction is readily performed on the gram scale with as little as 0.25 mol% catalyst loading. Furthermore, the Mannich adducts are readily transformed to protected a-hydroxy-/8-amino acids in high yield. The absolute stereochemistry of the Mannich adducts revealed that Et2Zn-linked complex 3 affords Mannich and aldol adducts with the same absolute configuration (2 R). However, the diastereoselectiv-ity of the amino alcohol derivatives is anti, which is opposite to the syn-l,2-diol aldol products. Hence, the electrophiles approach the re face of the zinc enolate in the Mannich reactions and the si face in the aldol reactions. The anti selectivity is... 

Stereocontrol of aldol reactions of 4-phenylsulfanyltetrahydrothiopyran-4-carbaldehyde 283 forms an essential feature of syntheses of l-oxa-8-thiaspiro[4.5]decanes (Scheme 38) <1996TL4581>. 

The use of different acid functionalities on pyrrolidine-derived catalysts has improved the reaction rate of some aldol reactions. For example, pyrrolidine-based tetrazole derivative 9 (Fig. 2.2) catalyzed many aldol reactions with rates faster than proline, with similar stereocontrol [16, 18b, 24, 55]. The faster reaction rates with tetrazole derivative 9 in DM SO as compared with proline were attributed to the lower pKa of the tetrazole moiety as compared to the carboxylic acid group in DMSO (tetrazole pKa(DMSO) 8.2 acetic acid pKa(DMSO) 12.3) [55, 56]. In addition, tetrazole derivative 9 is more soluble than proline in many organic solvents. A higher actual concentration of the catalyst in the solution phase of a reaction mix-... 

In the (S)-proline-catalyzed aldol reactions, the addition of a small amount of water did not affect the stereoselectivities [6]. However, a large amount of water often resulted in products with low enantiomeric excess water molecules interrupt the hydrogen bonds and ionic interactions critical for the transition states that lead to the high stereocontrol. For example, in the (S)-proline-catalyzed aldol reaction of acetone and 4-nitrobenzaldehyde in DMSO, the addition of 10% (v/v) water to the reaction mixture reduced the ee-value from 76% (no water) to 30% [6]. Note that the addition of a small amount of water into (S)-proline-catalyzed reactions often accelerates the reaction rate, and the addition of water should be investigated when optimizing these reactions [61]. 

Although methods for stereocontrol of the aldol reaction are well documented, including diastereofacial selectivity in reactions of chiral enolates,25 stereocontrol in Mannich reactions appears to have received relatively little attention.26-30... 

SCHEME 125. Stereocontrolled aldol reaction and lactone formation594... 

The first step of the total synthesis of 31 is the (7v )-proli nc-calal yzcd aldol reaction between 4 and 32, which gave the aldol adduct 33 with a good yield (69%) and nearly perfect stereocontrol (>96% de, >99% ee, Scheme 10). The same results were observed when the reaction was carried out on a 40-mmol scale yielding 5.22 g of 33 without a decrease of selectivity. The free hydroxyl group of 33 was quantitatively protected as MOM-ether. After hydrogenolytic debenzylation the aldehyde-ketone was obtained after Dess-Martin oxidation followed by a double Wittig reaction to provide the bisolefine 34 in 41% yield over 4 steps (Scheme 10). 

The most direct method for the preparation of polyol frameworks is without doubt the aldol reaction. The diastereofacial selectivity of the reaction can be controlled by /J-alkoxy groups in both the methylketone enolate and the aldehyde. As investigations by Evans [6] and Paterson [7] and their groups have demonstrated, the correct selection of enolization conditions and the protective group for the )8-hydroxy group are important for the stereocontrol of the reaction. 

Scheme 4. Stereocontrolled aldol reaction according CH2CI2. -78 C. TES = triethylsilyl, TMS = tri-...

A synthetic route to Q, /i-disubstituted cycloalkenones via a four step one-pot synthesis employed (4a) for RCM and then oxidative rearrangement giving products in low to moderate overall yields as a way to access estrogen receptor ligand tetrahydrofluorenones (equation 29)3 Crimmins reported an asymmetric aldol-oleftn metathesis approach to the synthesis of functionalized cyclopentenes exploiting the acyclic stereocontrol of the aldol reaction with efficient (2a) catalyzed RCM (equation 30)3 ... 

Aldol Reactions. The dibutyl boryl enolates of chiral acylox-azolidinones react to afford the syn-aldol adducts with virtually complete stereocontrol (eq 32). 14,43.61-64 Notably, the sense of induction in these reactions is opposite to that predicted from the analogous alkylation reactions. This reaction is general for a wide range of aldehydes and imide enolates. - Enolate control overrides induction inherent to the aldehyde reaction partner. 

Mukaiyama Aldol Condensation. As expected, the chiral titanium complex is also effective for a variety of carbon-carbon bond forming processes such as the aldol and the Diels-Alder reactions. The aldol process constitutes one of the most fundamental bond constructions in organic synthesis. Therefore the development of chiral catalysts that promote asymmetic aldol reactions in a highly stereocontrolled and truly catalytic fashion has attracted much attention, for which the silyl enol ethers of ketones or esters have been used as a storable enolate component (Mukaiyama aldol condensation). The BINOL-derived titanium complex BINOL-TiCl2 can be used as an efficient catalyst for the Mukaiyama-ty pe aldol reaction of not only ketone si ly 1 enol ethers but also ester silyl enol ethers with control of absolute and relative stereochemistry (eq 11). ... 

The reagent (2) is probably the most versatile chirally modified acetate enolate. Good results have also been obtained with the Mg enolate of 2-acetoxy-l,l,2-triphenylethanol and with boron enolates derived from 2,4-dialkylborolanes Chiral Fe-acetyl complexes, which can be considered as acetate equivalents, give impressive stereocontrol upon enolization and aldol reaction. ... 

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