Stereoselective synthesis directed epoxidation

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

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... 

In this chapter, key studies which have led to the current understanding of the mechanism of this transformation will be discussed, as a basis for dealing with issues of selectivity and choice of experimental variables in subsequent sections. Since the initial development of this reaction, it has evolved in scope to become a primary method for the stereoselective synthesis of epoxides to which an electronegative substituent is directly attached. Several recent variations on the initial protocol which afford functionally diverse molecules are the subject of Section 1.13.3. Section 1.13.5 examines related reactions that afford substituted aziridines instead of epoxides, and Section 1.13.6 examines some recent variants of the re-... 

In conclusion, chiral heterobimetallic lanthanoid compexes LnMB, which were recently developed by Shibasaki et al., are highly efficient catalysts in stereoselective synthesis. This new and innovative type of chiral catalyst contains a Lewis acid as well as a Bronsted base moiety and shows a similar mechanistic effect as observed in enzyme chemistry. A broad variety of asymmetric transformations were carried out using this catalysts, including asymmetric C-C bond formations like the nitroaldol reaction, direct aldol reaction, Michael addition and Diels-Alder reaction, as well as C-0 bond formations (epoxidation of enones). Thereupon, asymmetric C-P bond formation can also be realized as has been successfully shown in case of the asymmetric hydrophosphonylation of aldehydes and imines. It is noteworthy that all above-mentioned reactions proceed with high stereoselectivity, resulting in the formation of the desired optically active products in high to excellent optical purity. 

McIntosh et al. have applied the Ireland-Claisen rearrangement of bis-allyl silyl ketene acetals in studies directed toward the synthesis of the eupomatilones (Scheme 4.135) [128]. The 1,2-transposition of the alkene, which occurred in the rearrangement afforded a reactive vinyl epoxide (cf Scheme 4.83). Stereoselective cyclization of the carboxylic acid onto the vinyl epoxide generated the 5-aryl lactone, which was further manipulated to the putative structure of 5-epi-eupoma-tilone-6. 

Keywords Configurational stability Deep eutectic solvents Direct metalation group Epoxides Natural products Organolithiums Oxetanes Oxygen heterocycles Stereoselective synthesis Tetrahydrofurans... 

Indolines and indoles were prepared by a direct electrochemical reduction of arenediazonium salts. As a result, radical intermediates were generated from which 3,4-disubstituted tetrahydrofuran skeleta were constructed <02OL2735>. A short and stereoselective total synthesis of furano lignans was realized by radical cyclization of epoxides using a transition-metal radical source <02JOC3242>. Other preparations of tetrahydrofurans using radical cyclization include the synthesis of novel amino acids L-bis-... 

Synthetic application includes Paquette s recent s lication in work directed toward the total synthesis of sterpuric acid. Exposure of enol ether (55) to peracid provided a single diastereomer of the silyloxy compound (56) in good yield. It was from this substrate (55) that the first stable trimethylsilyloxy epoxide was obtained (57) and examined by X-ray crystallography. Similarly stereoselective oxygenation of p-keto ester (49) via the corresponding silyl enol ether provided (50), also in 76% yield. Lastly efficient and highly stereoselective a-hydroxylation by this method was employed during studies towards the synthesis of helenanolides (58 to 59). ... 

The effect of a free OH group is usually much more pronounced than that of the protected one. For example, oxiranes 58 and 59 were obtained with high stereoselectivity (directed by the free OH), although both oxy-substituents act in the opposite directions [ 1,44] (O Fig. 12). When the epoxidation is performed not on allylic alcohols but allylic esters the selectivity of this process is low both possible oxiranes are obtained in comparable amounts. The stereoselective formation of the 2-3-epoxides can be achieved also by the Sn2 process. Recently, Lowary proposed an efficient synthesis of 2,3-epoxy-arafci o-furanoside 61 from the parent glycoside 60 in a sequence of reactions presented in Scheme 20 [45]. Such anhydrosugars are convenient precursors for further functionalization at either the C-2 or C-3 position. 

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