Potassium epoxide opening with

The first example of an intramolecular aldol cyclopenta-annulation on a carbohydrate system has been described by Jenkins and co-worirers/ Epoxide 26 was converted to 27 ia axial epoxide opening with allyl magnesium chloride and subsequent diastereoselective enolate methylation, Wacker oxidation of 27 then providing diketone 28. Treatment with potassium r-butoxide led to cyclopenta-annulation and dehydration of the intermediate aldol product gave 29, confirmed by X-ray structure analysis. 

The direction of ring-opening of 5,10-epoxy-9(ll)-enes by bases depends upon the reaction conditions.235 Thus the a-epoxide (288) loses a proton from C-12 with potassium t-butoxide to give the 5a-hydroxy-9,11-diene (289), but lithium dial-kylamides favour formation of the 10a-hydroxy-4,9(1 l)-diene (290). These and related results suggest that the lithium dialkylamides are poorly dissociated, and favour a 1,2- rather than a 1,4-epoxide-opening reaction.235... 

The [Mn(salen)]-catalyzed epoxidation of chromene derivatives was discovered to occur with exceptional enantioselectivity [74]. Chromene derivatives bearing 2,2-disubstitution appear to combine all the important substrate characteristics required for a highly enantioselective epoxidation. The synthetic utility of the enantioenriched epoxychroman products is increased by the predictable regio- and stereochemical outcome of epoxide ring opening with a variety of nucleophiles. These two features were highlighted in the synthesis of the selective potassium channel activator BRL 55834 [78]. Catalyst loadings as low as... 

The next key series of reactions results in the transformation of 625 to 626. The first step is a Sharpless epoxidation of the allylic alcohol. This is followed by a regiospecific oxirane ring opening with potassium benzyloxide to introduce the required oxygen that will eventually... 

Regioselective opening of epoxide 228 with potassium cyanide in ethanol—buffer furnishes the jS-hydroxy nitrile 235. This undergoes a Reformatsky reaction [83] with tert- mXy bro-moacetate/zinc followed by a highly stereoselective ketone reduction with sodium borohy-dride/diethylmethoxyborane to provide the chiral ester 236. Subsequently, 236 is converted in four steps to optically active NK-104 (237), a highly potent HMG-CoA reductase inhibitor [84] (Scheme 55). 

The free hydroxy group in 880 is first protected as a benzoate, ring opened with the lithium anion of 1-heptyne, and finally tosylated to afford 881, which is deprotected and cyclized to the epoxide with potassium carbonate in methanol. A Lindlar reduction of the triple bond provides the cw-olefin present in ( —)-(6Z,97 ,10iS)-9,10-epoxyheneicosadec-6-ene (882). In order to prepare the enantiomer 884, 880 is directly ring opened with the lithium anion of 1-heptyne, and then tosylated to afford 883, which is cyclized and selectively reduced to (+ )-(6Z,95, 10/ )-9,10-epoxyheneicosadec-6-ene (884) [250] (Scheme 192). 

The synthesis of triticonazole is only described in patents as a one-pot sequence [57] Knoevenagel condensation of 4-chlorobenzaldehyde on 2,2-dimethylcyclopentanone gives the a,) -unsaturated ketone, which enters a Corey-Chaykovsky epoxidation reaction to afford the epoxide which in turn is opened with the potassium salt of 1,2,4-triazole (Scheme 17.10). 

A decisive solvent effect is also observed with other a,/ -epoxy ketones. Specifically, 3jS-hydroxy-16a,17a-epoxypregn-5-en-20-one and its acetate do not react with thiocyanic acid in ether or chloroform. However, the corresponding thiocyanatohydrins are formed by heating an acetic acid solution of the epoxide and potassium thiocyanate. As expected, the ring opening reaction is subject to steric hindrance. For example, 3j6-acetoxy-14f ,15f5-epoxy-5) -card-20(22)-enoIide is inert to thiocyanic acid in chloroform, whereas the 14a,15a-epoxide reacts readily under these conditions.Reactions of 14a,15a-epoxides in the cardenolide series yields isothiocyanatohydrins, e.g., (135), in addition to the normal thiocyanatohydrin, e.g., (134). 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

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). 

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