2, 3-Epoxy alcohols, ring openings, with

Epoxy alcohols, ring openings, with NH4X and NaSPH, 38 259... 

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... 

When dienones such as 55 are subjected to the epoxidation conditions the electron-poorer C=C double bond is selectively epoxidized. The other C=C bond can be functionalized further, for example, it can be dihydroxylated, as shown in the synthesis of the lactone 56 (Scheme 10.11) [82]. Stannyl epoxides such as 57 (Scheme 10.11, see also Table 10.8, R1 = n-Bu3Sn) can be coupled with several electrophiles [72], reduction of chalcone epoxide 58 and ring opening with alkyl aluminum compounds provides access to, e.g., the diol 59 and to phenylpropionic acids (for example 60). Tertiary epoxy alcohols such as 61 can be obtained with excellent diastereoselectivity by addition of Grignard reagents to epoxy ketones [88, 89]. 

Chiral amino alcohols can be prepared by reaction of chiral epoxides with amines. Enantiopure (25, 3.R)-2,3-epoxy-3-phenylpropanol anchored to Merrifield resin has been used for ring-opening with secondary amines in the presence of lithium perchlorate to afford polymer-supported chiral amino alcohols 47 (Eq. 18) [56], By analogy, (2i ,35)-3-(cis-2,6-dimethylpiperidino)-3-phenyl-l,2-propanediol has been anchored to a 2-chlorotrityl chloride resin (48). Although this polymer had high catalytic activity in the enantioselective addition of diethylzinc to aldehydes, the selectivity of the corresponding monomeric catalyst was higher (97 % ee) in the same reaction. 

Because the two regioisomeric products 8a and 8b have almost the same molecular dimensions, it is difficult to discriminate between the two isomers with the geometric constraints imposed by the zeolite pores. Considering that calcium ions are apt to form mainly five-membered chelate complexes with polyhydroxy compounds (Fig. 4b) 32,33) and that calcium zeolites have also been employed as sorbents in carbohydrate separations (ii), it is possible to speculate that in the CaY-supported NaN3 system the epoxy alcohol first forms a coordinated structure around a calcium ion, as shown in Fig. 4a, followed by ring opening with an azide anion at the C-3 position of the epoxy alcohol, giving a stable, five-membered chelate complex with the calcium ion. 

Tanaka et al. 231) prepared (518a) from allylic alcohol (514) via the epoxy silyl ether (515), which was ring opened with diethylaluminum 2,2,6,6-tetramethylpiperidide to afford the 3-ene-l,2-diol (516) Scheme 90). 

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]. 

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. 

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. 

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. 

Ring-Opening Reactions of Epoxy Alcohols with X2-Ti(OPr )n. 

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... 

TABLE 10.1. Ring-opening reaction of 2,3-epoxy alcohols with NH4N3... 

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