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Alcohols, 2,3-epoxy ring opening

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... [Pg.53]

On one hand, they increase the reaction rate due to an electrophilic assistance for the epoxy ring opening and, on the other, lower the reactivity of the alcoxy anion owing to its solvation and the decrease of its nucleophility. Positive, neutral or even negative effects of the alcohol additives on the reaction rate are governed by the relationship between these two factors. The chain propagation reaction mechanism itself remains trimolecular. [Pg.155]

Zinc-mediated Barbier-t) e addition of 238, followed by Luche s procedure obtained a mixture of the homoallylic alcohol diastereomers 239 and 240. Alcohol 239 was carried through benzoylation, deketalization, silylation, and ozonolysis, to produce C-branched y-lactone 241. Benzoylation of 240 followed by hydroboration, PCC oxidation, debenzoylation, and alkaline-promoted cyclization directly formed C3-branching 2-deoxyfuranose 242 [87] (O Scheme 63). A novel method for stereoselective synthesis of 4 -a-carbon-substituted nucleosides, through epoxidation of 4, 5 -unsaturated nucleosides and SnCU-promoted epoxy ring opening, was... [Pg.338]

Scheme 1.5. Alcohol catalysis of the epoxy-ring-opening reaction by amine groups (the -OH may arise from impurities in the resin, the resin itself, e.g. DGEBA, Scheme 1.4, or following ring-opening of another epoxy group. Scheme 1.3). Reproduced with permission from St John (1993). Scheme 1.5. Alcohol catalysis of the epoxy-ring-opening reaction by amine groups (the -OH may arise from impurities in the resin, the resin itself, e.g. DGEBA, Scheme 1.4, or following ring-opening of another epoxy group. Scheme 1.3). Reproduced with permission from St John (1993).
The hydroboration-oxidation studies of 1,4-epoxy-1,4-dihydronaphthalene (1) with various hydroborating agents such as borane-methyl sulfide (BMS), dicy-clohexylborane, disiamylborane, and 9-BBN have yielded interesting results [1]. The reaction of 1 with dicyclohexylborane or disiamylborane affords the exo alcohol 2 without epoxy ring opening (Eq. 32.1). On the other hand, 9-BBN hy-droboration of 1 (1 1 mole ratio) in THE at 25 °C, followed by oxidation affords a homoallylic alcohol 3 after opening of the epoxy ring (Eq. 32.2). BMS, however, affords a mixture of both the alcohols. [Pg.559]

In general, 2-substituted allylic alcohols are epoxidized in good enantioselectivity. Like glycidol, however, the product epoxides are susceptible to ring opening via nucleophilic attack at the C-3 position. Results of the AE reaction on 2-methyl-2-propene-l-ol followed by derivatization of the resulting epoxy alcohol are shown in Table 1.6.1. Other examples are shown below. [Pg.54]

Sharpless and Masumune have applied the AE reaction on chiral allylic alcohols to prepare all 8 of the L-hexoses. ° AE reaction on allylic alcohol 52 provides the epoxy alcohol 53 in 92% yield and in >95% ee. Base catalyze Payne rearrangement followed by ring opening with phenyl thiolate provides diol 54. Protection of the diol is followed by oxidation of the sulfide to the sulfoxide via m-CPBA, Pummerer rearrangement to give the gm-acetoxy sulfide intermediate and finally reduction using Dibal to yield the desired aldehyde 56. Homer-Emmons olefination followed by reduction sets up the second substrate for the AE reaction. The AE reaction on optically active 57 is reagent... [Pg.59]

The 2,3-epoxy alcohols are often obtained in high optical purity (90% enantiomeric excess or higher), and are useful intermediates for further transformations. For example by nucleophilic ring opening the epoxide unit may be converted into an alcohol, a /3-hydroxy ether or a vicinal diol. [Pg.256]

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. [Pg.239]

A synthetically useful diastereoselectivity (90% dc) was observed with the addition of methyl-magnesium bromide to a-epoxy aldehyde 25 in the presence of titanium(IV) chloride60. After treatment of the crude product with sodium hydride, the yy -epoxy alcohol 26 was obtained in 40% yield. The yyn-product corresponds to a chelation-controlled attack of 25 by the nucleophile. Isolation of compound 28, however, reveals that the addition reaction proceeds via a regioselective ring-opening of the epoxide, which affords the titanium-complexed chloro-hydrin 27. Chelation-controlled attack of 27 by the nucleophile leads to the -syn-diastereomer 28, which is converted to the epoxy alcohol 26 by treatment with sodium hydride. [Pg.54]

The overall transformation of alkenes to alcohols that is accomplished by epoxi-dation and reduction corresponds to alkene hydration. Assuming a nucleophilic ring opening by hydride addition at the less-substituted carbon, the reaction corresponds to the Markovnikov orientation. This reaction sequence is therefore an alternative to the hydration methods discussed in Chapter 4 for converting alkenes to alcohols. [Pg.1110]

The asymmetric oxidation of organic compounds, especially the epoxidation, dihydroxylation, aminohydroxylation, aziridination, and related reactions have been extensively studied and found widespread applications in the asymmetric synthesis of many important compounds. Like many other asymmetric reactions discussed in other chapters of this book, oxidation systems have been developed and extended steadily over the years in order to attain high stereoselectivity. This chapter on oxidation is organized into several key topics. The first section covers the formation of epoxides from allylic alcohols or their derivatives and the corresponding ring-opening reactions of the thus formed 2,3-epoxy alcohols. The second part deals with dihydroxylation reactions, which can provide diols from olefins. The third section delineates the recently discovered aminohydroxylation of olefins. The fourth topic involves the oxidation of unfunc-tionalized olefins. The chapter ends with a discussion of the oxidation of eno-lates and asymmetric aziridination reactions. [Pg.195]

The wide scope application of this transformation arises not only from the utility of epoxide compounds but also from the subsequent regiocontrolled and stereocontrolled nucleophilic substitution (ring-opening) reactions of the derived epoxy alcohol. These, through further functionalization, allow access to an impressive array of target molecules in enantiomerically pure form. [Pg.196]

Ti(OPr1)4-mediated nucleophilic ring opening of 2,3-epoxy-alcohol with primary amine requires more rigorous conditions, and the product is a complex mixture. Lin and Zeng22 found that this problem could be overcome and moderate to good yields could be obtained under weak base conditions by in situ /V-acylation of the aminolysis product with benzoyl chloride. [Pg.205]

Ring-Opening Reactions of Epoxy Alcohols with X2-Ti(OPr )n. [Pg.207]

Using different reagents or under various conditions, 2,3-epoxy alcohols can undergo ring-opening reactions with metallic hydrides, giving 1,3-diols or 1,2-diols. As shown in Scheme 4-16, reduction of 3-substituted 2,3-epoxy alcohols with Red-Al leads to the exclusive formation of 1,3-diols, and this can be applied in the preparation of 1,3-diol compounds.31... [Pg.209]

This approach provides a new method for carbohydrate synthesis. In the synthesis of tetritols, pentitols, and hexitols, for example, titanium-catalyzed asymmetric epoxidation and the subsequent ring opening of the thus formed 2,3-epoxy alcohols can play an essential role. [Pg.212]


See other pages where Alcohols, 2,3-epoxy ring opening is mentioned: [Pg.405]    [Pg.161]    [Pg.397]    [Pg.397]    [Pg.73]    [Pg.397]    [Pg.370]    [Pg.372]    [Pg.376]    [Pg.106]    [Pg.180]    [Pg.55]    [Pg.362]    [Pg.339]    [Pg.301]    [Pg.272]    [Pg.277]    [Pg.290]    [Pg.296]    [Pg.61]    [Pg.462]    [Pg.55]    [Pg.71]    [Pg.60]    [Pg.60]    [Pg.205]    [Pg.206]    [Pg.206]    [Pg.207]    [Pg.73]    [Pg.616]   
See also in sourсe #XX -- [ Pg.462 ]




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2, 3-Epoxy alcohols, ring openings, with

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Epoxy alcohol ring opening intramolecular nucleophile

Epoxy alcohols

Epoxy alcohols, chiral, ring opening

Epoxy ring

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