Regioselectivity epoxy alcohol reduction

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

Fig. 17.43. Regioselective reduction of enantiomerically pure epoxy alcohols to enantiomerically pure diols. 1,3-Diols are formed with Red-Al and 1,2-diols are formed with DIBAL...

Regioselective reduction of 2,3-epoxy alcohols.4 These epoxy alcohols can be converted into enantiomcrically pure 2-alkanols by reduction of their tosylatcs with DIBAH (3 equiv.) in CH2CI2 at -15—- 25°. 

The preparation of the C33-C35 segment [137] began with the Horner-Emmons olefination of wo-butyraldehyde, reduction, and asymmetric epoxida-tion [39] leading to epoxy alcohol 234 (Scheme 33). Dithiane epoxide ring opening (regioselectivity 12 1), primary alcohol reduction, and secondary alcohol protection completed the synthesis of this segment ( 235). 

Photooxygenation of vinylsilanes affords, after reduction, /3-silyl allylic alcohols regioselective-ly in good yields i4a c. Moreover, because of the steric demand of the silyl group, the titanium-catalyzed oxyfunctionalization described leads in high yield to (R, S )-epoxy alcohols80. 

For epoxy-protected ketones, highly regioselective ring opening leading to 3-keto alcohols was attained (equation 10). In the reduction of a diepoxide, 1,4-diol derivatives were predominantly obtained (equation 11). ... 

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