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Swern elimination

The regiochemistry of this elimination reaction resembles that observed by Davis et al. (see Scheme 9) [23]. The special nature of the bonds in three-mem-bered rings is probably responsible for this exclusive regiochemistry. It is of interest to note that 3,3-dimethylaziridine-2-carboxylic ester indeed leads to the corresponding 3H-azirine ester upon Swern oxidation here there is, of course, no choice. [Pg.102]

A number of modifications were made to meet scale-up requirements. In the preparation of the common intermediate, LiBH4 was used in place of LiAlH4 in Step A-2 and a TEMPO-NaOCl oxidation was used in place of Swern oxidation in Step A-3. Some reactions presented difficulty in the scale-up. For example, the boron enolate aldolization in Step B-l gave about 50% yield on the 20- to 25-kg scale as opposed to greater than 75% on a 50-g scale. The amide formation in Step B-3 was modified to eliminate the use of trimethylaluminum, and the common intermediate 17 could be prepared on a 30-kg scale using this modified sequence. The synthesis of the C(l)-C(6) segment V was done by Steps C-l to C-5 in 66% yield on the scale of several kg. [Pg.1243]

Sometimes, an alkene conjugated with a ketone is introduced during a Swern oxidation.172a 232 This can be explained by an a-chlorination followed by elimination of HC1. [Pg.161]

Sometimes, when a Swern oxidation produces a carbonyl compound possessing a good-leaving group at the 3-position, an in situ elimination occurs, resulting in the generation of a conjugated enone or enal. [Pg.165]

The activation of DMSO by electrophilic reagents such as oxallyl chloride or trifluoroacetic anhydride (TFAA) (among many others) produces an oxidant capable of oxidizing primary alcohols to aldehydes in high yields. This oxidation is called the Swern oxidation and yields the aldehyde (oxidized product) by reductive elimination of dimethylsulfide (reduced product) and proceeds under mild, slightly basic conditions. It is a second widely used and effective oxidative method for the production of aldehydes from primary alcohols. [Pg.193]

Recently, a cyclization-elimination route to carbohydrate-based oxepines is proposed (Scheme 11). After silyl protection and hydroboration/oxidation, starting hept-l-enitols 31 give the protected heptan-l-itols 32. Swern oxidation of the latter followed by sequential acetal formation/cyclization provide methyl 2-deoxyseptanosides 33 that undergo elimination reactions to give the carbohydrate-based oxepines 34 <2005JOC3312>. [Pg.55]

The unsubstituted hydrazones derived from aromatic ketones and aldehydes are converted to the corresponding alkyl chlorides, in high yield, under Swern oxidation conditions. In this unusual oxidation/reduction sequence, the substrate undergoes a net reduction. Unsubstituted hydrazones derived from cyclohexyl ketones yielded elimination products. The mechanism in Scheme 7 has been postulated.111... [Pg.104]

An entirely different approach to the introduction of isolated double bonds is illustrated by the selective silylation of the 3-hydroxy group of 155 and Swern oxidation at C-2 to give 156 following spontaneous elimination. Final conversion of the enone to the free sugar 157 is accomplished by carbonyl reduction that induces an ester migration (Scheme 17).176... [Pg.87]

Fig. 17.13. Mechanism of the Swern oxidation of alcohols. The actual reagent is an "activated DMSO" (compound B or D), which reacts with an alcohol with formation of A or C, respectively. Dissociation leads to the sulfonium salt E, which is then converted into the sulfonium ylide F after NEt3 addition and raising the temperature from -60 to -45 °C. /3-Elimination via a cyclic transition state generates the carbonyl compound and dimethyl sulfide from F. Fig. 17.13. Mechanism of the Swern oxidation of alcohols. The actual reagent is an "activated DMSO" (compound B or D), which reacts with an alcohol with formation of A or C, respectively. Dissociation leads to the sulfonium salt E, which is then converted into the sulfonium ylide F after NEt3 addition and raising the temperature from -60 to -45 °C. /3-Elimination via a cyclic transition state generates the carbonyl compound and dimethyl sulfide from F.
Alkyl-2//-aziridine-2-carboxylates 867 have been oxidized with the Swern reagent to afford 2//-azirine-2-carbox-ylates <1995TL4665>. Oxidation of either the (Z)- or the ( )- isomers of 867 provides the same 2//-azirine-2-carboxylate 868, where the integrity of the stereogenic center at C-2 is retained. This regioselectivity results from the unexpected removal of the apparently less acidic C-3 proton during the base-induced ty -elimination of the A-dimethylsulfonium intermediate (Scheme 217). [Pg.95]

The DIBAL reduction of 11 provides alcohol 28 which is oxidized to the corresponding aldehyde 12 under Swern conditions. Sometimes the intermediate of DIBAL reductions eliminate aluminum alkoxide... [Pg.248]

In the first step of this reaction sequence, the primary alcohol 21 is oxidized to the corresponding aldehyde 38 in a Parikh-Doering oxidation which is related to the Swern oxidation. In general, this type of oxidation is conveniently carried out by addition of a solution of pyridine-SOs complex in DMSO to a mixture of the alcohol, DMSO and NEts. It can be assumed that dimethyl sulfoxide and sulfur trioxide react to form 0-dimethylsulfoxonium sulfate 40, which then further reacts with primary alcohol 39 to give 0-alkyl dimethylsulf-oxonium intermediate 41. Then, sulfonium salt 42 is formed and subsequently deprotonated by NEts to give sulfonium ylide 43. Finally, an intramolecular p-elimination occurs to provide the desired aldehyde 44 and dimethyl sulfide. [Pg.262]

In chemical oxidation or reduction the redox reagent and the substrate often form a covalent or ionic bond, for example, an ester in chromic acid oxidation [8], a sulfonium methylide in the Swern oxidation [9], cyclic esters in the svn dihydroxylation with OSO4 [10], or in the selenium dioxide oxidation of ketones and aldehydes [11]. In electrochemical processes the substrate must diffuse from the bulk of the solution to the electrode and compete there with other components of the electrolyte by competitive adsorption for a position at the electrode surface [12]. The next step is then generation of the reactive intermediate by electron transfer at the electrode that reacts with a low activation energy to the products. In chemical oxidations or reductions one finds a reductive or oxidative elimination of the intermediate with a higher activation energy. [Pg.208]

The next phase of the synthesis involved the transposition of aldol adduct 61 to the protected "aldol" adduct 60. (3-Hydroxyketone 61 was subjected to conditions (NaBH4, AcOH) which effected a direct reduction of the carbonyl moiety of 61 and thereby introduced the axial C(9) hydroxyl functionality of 67 with complete stereocontrol through an intramolecular delivery of hydride within an alkoxide intermediate at C(7). After diprotection of both hydroxyl groups of 67, chemoselective deprotection of hydroxyl at C(7) and Swern oxidation, ketone 60 was isolated. The enolate derivative of 60 could be stereoselectively p-methoxybenzylated, and the resulting ketone was reduced to the wrong equatorial alcohol 68. The C(7) stereogenic center was inverted by treatment of the nosylate derivative of 68 with rubidium acetate to afford the desired acetate 69 accompanied by the syn elimination product (15%). [Pg.26]

Oxidations involving chromium (VI) reagents such as H2Cr04 are simple to carry out and have been widely used. These reactions involve formation of chromate esters, and include an elimination step similar to the general mechanisms shown in Section 12.4A. Chromium (VI) is a carcinogen and an environmental hazard, however. For this reason, methods like the Swern oxidation and others are increasingly important. [Pg.554]

In Zhu s approach, the advanced intermediate 34 was subjected to Swern oxidation and tetra- -butyIammonium fluoride (TBAF)-mediated deprotection of the silyl ether, yielding a mixture of aldehyde 35 and hemiaminal 36. Treatment of this mixture with 0.01% v/v methanesulfonic acid in dichloromethane initiated a process consisting of acyhminium formation, deprotonation, and domino P-elimination/cydization. Thus, dehydration of hemiaminal 36 initially led to iminium ion 37, which formed enamide 38 under loss of a proton. Elimination of the sulfur side chain then resulted in conjugated iminium ion 39, which was set up to undergo a phenolic Mannich cych2ation. [Pg.529]

The enantiomer of 72 (ent-72) was synthesized from the same starting material 106 fScheme 12.37). Benzylation of a hydroxy group in 106 followed by acetal hydrolysis afforded 140, which was converted into 5-enop)Tanoside 141 by the conventional method. The Ferrier carbocyclization of 141 generated 142 as a diastereomeric mixture in 83% yield. Protection of the hydroxy group in 142 as a THP ether and subsequent reduction of the ketone carbonyl gave 143. 0-Mesylation of 143 followed by acidic work-up afforded 144. Swern oxidation of 144 was accompanied by the p-elimination of the OMs group to furnish ent-72 in 93% yield. Cyclohexenone ent-72 could be used for the synthesis of natural enantiomer of actinoboline. [Pg.471]

Ubukata s synthesis [51] started with ammonolysis followed by Swern oxidation of ( )-55 [52] to give nitrile aldehyde 56. Wittig-Horner reaction of 56 with phosphonate 58 derived from glutarimide acetic acid 57 [27] gave 59. The nitrile was converted to amide 60, which was subjected to selenolac-tamization [53] to afford 61. Oxidation-elimination of the selenenide moiety gave 48 a diastereomeric mixture. [Pg.191]


See other pages where Swern elimination is mentioned: [Pg.496]    [Pg.551]    [Pg.101]    [Pg.9]    [Pg.150]    [Pg.271]    [Pg.298]    [Pg.560]    [Pg.88]    [Pg.185]    [Pg.352]    [Pg.1924]    [Pg.1926]    [Pg.368]    [Pg.457]    [Pg.43]    [Pg.213]    [Pg.585]    [Pg.818]    [Pg.351]    [Pg.210]    [Pg.425]    [Pg.129]    [Pg.222]    [Pg.220]    [Pg.1213]    [Pg.228]   
See also in sourсe #XX -- [ Pg.146 , Pg.153 ]




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