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Vicinal stereochemistry reactions

Two additional syntheses that did not address the vicinal stereochemistry problem are outlined here. They differ from the Mori synthesis in regard to the chemistry used to introduce the sidechain, but are similar in that they rely on reactions of a 4-substituted cyclohexanone to introduce the vicinal stereochemical relationship. [Pg.168]

The first clear attempt to address the stereochemical problem presented by the juvabiones was reported by Birch. The plan was to use enone 33 to desymmetrize the 4-substituted cyclohexanone intermediates encountered thus far. The vicinal stereocenters in enone 33 were to be derived from 34 via a retro-aldol condensation. As we will see, 34 was to be prepared via a Diels-Alder reaction, a reaction that frequently has been used to establish vicinal stereochemistry with a high degree of selectivity. [Pg.168]

A conceptually different approach to the juvabiones would be to develop reactions of cyclohexenone (127) with nucleophiles of type 128 that afford conjugate adducts of type 129 with good control over relative (vicinal) stereochemistry and absolute stereochemistry. This plan is conceptually related to the Pearson approach (Juvabione-12) but uses the electrophile needed to directly connect with the now familiar 3-substituted cyclohexanone endgame for juvabione synthesis. We will look at three different approaches that adapt this general strategy. [Pg.194]

Disulfonate esters of vicinal diols sometimes undergo reductive elimination on treatment with sodium iodide in acetone at elevated temperature and pressure (usually l(X)-200°). This reaction derived from sugar chemistry has been used occasionally with steroids, principally in the elimination of 2,3-dihy-droxysapogenin mesylates. The stereochemistry of the substituents and ring junction is important, as illustrated in the formation of the A -olefins (133) and (134). [Pg.344]

In a recent total synthesis of the novel neurotrophic agent merrilactone A (22, Scheme 4) by Inoue and Hirama [24], key intermediate 21 with the cis-bicyclo[3.3.0] octane framework embedded within the caged pentacycle 22 was elaborated from cyclobutane 18 by a sequence of RCM and immediate cleavage of the resulting bicyclic vicinal diol 19 to raeso-diketone 20. Cyclooctenedione 20 then underwent regioselective transannular aldol reaction at low temperature (LHMDS, THF, -100 °C) to produce a 3 1 mixture of isomers in 85% combined yield. The major isomer 21 with the required stereochemistry was then converted into the racemic natural compound ( )-22 in 19 steps. [Pg.278]

Corsaro and co-workers studied the reaction of pyridazine, pyrimidine, and pyrazine with benzonitrile oxide and utilized H NMR spectral analysis to determine the exact structure of all the cyclized products obtained from these reactions <1996T6421>, the results of which are outlined in Table 1. The structure of the bis-adduct product 21 of reaction of pyridazine with benzonitrile oxide was determined from the chemical shifts of the 4- and 5-isoxazolinic protons at 3.76 and 4.78 ppm and coupled with the azomethine H at 6.85 ppm and with the 5-oxadiazolinic H at 5.07 ppm, respectively. They determined that the bis-adduct possessed /(-stereochemistry as a result of the large vicinal coupling constant (9.1 Hz). Similarly, the relative stereochemistry of the bis-adducts of the pyrimidine products 22-25 and pyrazine products 26, 27 was determined from the vicinal coupling constants. [Pg.714]

Incorporation of the carboxylic acid group into the substrate also had an effect on the stereochemistry of the Alder-ene products. Trost and Gelling60 observed diastereoselectivity in the palladium-catalyzed cycloisomerization of 1,7-enynes when the reactions were conducted in the presence of A,A-bis(benzylidene)ethylene diamine (BBEDA, Figure 2). They were able to synthesize substituted cyclohexanes possessing vicinal (Equation (53)) and... [Pg.579]

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). [Pg.12]

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. [Pg.150]

The reaction mechanism of a-amylases is referred to as retaining, which means that the stereochemistry at the cleaved bond of the carbohydrate is retained. Hydrolysis of the glycosidic bond is mediated by an acid hydrolysis mechanism, which is in turn mediated by Aspl97 and Glu233 in pig pancreatic amylase. These interactions have been identified from X-ray crystallography. The aspartate residue has been shown to form a covalent bond with the Cl position of the substrate in X-ray structure of a complex formed by a structurally related glucosyltransferase. " The glutamate residue is located in vicinity to the chloride ion and acts as the acidic catalyst in the reaction. The catalytic site of a-amylases is located in a V-shaped depression on the surface of the enzyme. [Pg.277]

Treatment of the alcohol ( ) with trifluoromethylsulfonic anhydride (triflic anhydride) at -78 C afforded the ester (1 ) which could be isolated and characterized. We knew from previous experience (2J that sulfonyl esters vicinal to an isopropylidene acetal are relatively stable. The triflate T,) reacted cleanly with potassium azide and 18-crown-6 in dichloromethane at room temperature. The crystalline product [68% overall from (1 )] was not the azide ( ) but the isomeric A -triazoline ( )- Clearly the initially formed azide (18) had undergone intramolecular 1,3-cyclo-addition to the double bond of the unsaturated ester (21- ). The stereochemistry of the triazoline (1 ), determined by proton nmr spectroscopy, showed that the reaction was stereospecific. There are several known examples of this reaction ( ), including one in the carbohydrate series ( ). When the triazoline was treated with sodium ethoxide ( ) the diazoester ( ) was rapidly formed by ring-opening and was isolated in 85% yield, Hydrogenolysis of the diazo group of (M) gave the required pyrrolidine ester ( ) (90%). [Pg.109]

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. [Pg.63]

The reaction of phenyl-substituted alkenes (2-phenylprop-l-ene, ( )-l-phenylprop-l-ene, 1,1-diphenylethene, 1,1-diphenylprop-l-ene) with F-Teda BF4 (6) in the presence of various alcohols results in the formation of vicinal fluoro alkoxy adducts with Markovnikov-type regioselec-tivity.89,94 The stereochemistry of the fluorination-methoxylation addition reaction is slightly syn predominant in the case of (Z)-stilbene, indene, and dibenzosuberenone, while equal amounts of both diastereoisomers are formed in the case of ( )-l-phenylprop-l-ene and acenaphthylene. [Pg.464]

Oxacyclopropanes also can be prepared from vicinal chloro- or bromo-alcohols and a base. This is an internal SN2 reaction and, if the stereochemistry is correct, proceeds quite rapidly, even if a strained ring is formed ... [Pg.663]


See other pages where Vicinal stereochemistry reactions is mentioned: [Pg.235]    [Pg.125]    [Pg.166]    [Pg.174]    [Pg.200]    [Pg.442]    [Pg.454]    [Pg.451]    [Pg.230]    [Pg.667]    [Pg.168]    [Pg.1036]    [Pg.1036]    [Pg.548]    [Pg.65]    [Pg.209]    [Pg.223]    [Pg.421]    [Pg.236]    [Pg.739]    [Pg.94]    [Pg.228]    [Pg.245]    [Pg.87]    [Pg.495]    [Pg.239]    [Pg.141]    [Pg.262]    [Pg.952]    [Pg.976]    [Pg.85]    [Pg.804]   
See also in sourсe #XX -- [ Pg.119 ]




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