Diols epoxide ring-opening

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

Figure 8. Epoxide ring opening reactions of bay-region diol epoxides and epoxides from dibenzo[a,h] acridine, and epoxides from benzo[a]acridine and benzo[c]acridine. Figure adaptedfrom reference 27. 

Figure 15. Epoxide ring opening reactions of epoxides and diol epoxides from benzo[a]pyrene, 4-azabenzo[a]pyrene and 10-azabenzo [a]pyrene.

Treatment of the /3-keto ester 220 with sodium ethoxide at elevated temperature triggered off an epoxide ring opening by / -elimination that was followed by the desired Knoevenagel condensation to afford the tricyclic product 206 (Scheme 34). The enone moiety in the intermediate 221 did not show a propensity for deprotonation and, therefore, the ketone carbonyl function of the enone moiety was available for a Knoevenagel condensation. The reduction of the p-keto ester (206) to the corresponding diol was the next objective. Treatment of the TES-protected -keto ester (TES-206) with DIBAH afforded the diastereomeric diols 222 and 223 in a moderate diastereoselec-tivity in favour of the undesired diastereomer 222. The diastereomers were separated and the undesired diastereomer 222 was epimerized to 223 by a sequence that consists of Mitsunobu inversion and benzoate ester reduction [98, 99]. 

From the NMR spectrum of copolymers produced from cyclohexene oxide and carbon dioxide it is difficult to assess low levels of asymmetric induction, i.e., low degrees of desymmetrization in the epoxide ring-opening step. In order to determine the extent of asymmetric induction it is necessary to hydrolyze the copolymer leading to the tra s-cyclohexane-l,2,-diol and examine the enantiomeric excess (4) [22]. Figure 4 shows the NMR spectrum in the carbonate region of atactic copolymer produced from cyclohexene oxide and CO2 using an achiral (salen)CrX catalyst. 

The compound (70) by epoxide ring-opening with hydrogen fluoride, followed by photocatalysed addition of acetylene to the A16-bond and lithium aluminium hydride reduction, gave (71). Protection of the 1,2-diol system in (71) as the acetonide followed by oxidation, dehydration, and regeneration of the diol grouping produced... 

The primary type of epoxide reaction remains the nucleophilic ring-opening reaction. The ring opening reactions of epoxides with oxygen nucleophiles represents an important method for the synthesis of differentially O-substituted 1,2-diols. The ring opening of epoxides with... 

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