Epoxide ring openings stereochemistry

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... 

A ring-expansion reaction based on epoxide ring opening and control of stereochemistry by er-participation of a silyl group has been devised.37 An example... 

Figure 10. Stereochemistry of hydroxylation reactions (1) with dilute alkaline permanganate and (2) through epoxide ring opening, (i) KMn04, NaOH (ii) m-chloroperbenzoic acid, NaHCOs, CH2Ci2 (Hi) CH3COOH (iv) base catalyzed hydrolysis.

Conofoline has also been isolated from another Malaysian Tabemaemontana species (Ervatamia peduncularis) under the name pedunculine [185]. The same plant also furnished another dimeric alkaloid, peduncularidine (307), which shares the common oxygenated tabersonine-P-epoxide moiety of conofoline but differs in the second unit which is now an opened form of (-)-mehranine, with a trans diol functionality at position 14 and 15 instead of a P-epoxide function. The epoxide ring opening by a water molecule is anticipated to occur preferentially at the less hindered carbon (14 ) resulting in a H(14 P), H(15 a) configuration. This proposal was supported by the ROESY spectrum which showed correlations between the axial H(2 ) and H( 17 ) with H(14 ), indicating that the latter has p stereochemistry. 

D-Ribonolactone has also been used as starting material for a synthesis of 6-epi-trehazolin, 7, in which the key cyclopentanoid functionality and stereochemistry are introduced by diastereoselective epoxidation and regioselective epoxide ring opening by azide to convert 5 to 6 (Scheme... 

The stereochemistry of epoxide ring-opening by benzoic acid has been studied using /m j-4-methoxystilbene oxide (297). The monobenzoate formed was converted into the keto-benzoate (298), whose structure had previously... 

Epoxi ester 1 effeetively rearranges to a 1 1 mixture of acetates 2 and 3 under acidic conditions (0.5 M H2SO4 in THF/H2O 9/1). It is believed that the reaction occurs through the formation of an orthoester intermediate 4 tiiat is subsequently hydrolyzed in the acidic medium (Scheme 25.1). This route provides a convenient method for the synthesis of polyhydroxylated compounds with control of the stereochemistry during the epoxide ring-opening step. 

The cleavage reaction occurs in three steps O protonation of the epoxide, Sn2 nucleophilic attack on the protonated epoxide, and deprotonation of the ring-opened product. Draw the complete mechanism. How many intermediates are there Which step determines diol stereochemistry ... 

When epoxides such as tra s-3-hexene-epoxide 1885 are heated to 65 °C with hexamethyidisiiane 857 and potassium methoxide in anhydrous HMPA, trimethylsilyl potassium 1882 is generated in situ to open the epoxide rings and give 1886, which subsequently looses potassium trimethylsilanolate 97 to afford olefins with inverted stereochemistry, for example as cis-3-hexene 1887, in high yield [103]. The reaction also proceeds at 65 °C in THF, rather than HMPA, if 18-crown-6 is added [103a] (Scheme 12.29). 

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

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