Epoxides, anionic opening

Scheme 8.40 BF3 Et20-promoted epoxide ring-opening by ketone enolate anion.

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... 

Epoxide ring opening with fluoride anion... 

Condensation of the carbonyl group with the yUde from trimethylsulfonium iodide leads initially to an addition product. The anion formed on the carbonyl oxygen then internally displaces dimethyl sulfide to give an oxirane yielding epoxide (114-3). Reaction of that intermediate with a 1,2,4-triazine leads to an epoxide ring opening with the consequent incorporation of the second heterocyclic moiety. There is thus obtained fluconazole (114-4) [125]. 

Stereopure epoxide 1 was prepared and treated with 3.6 equivalents of t-butyllithium in THF/HMPA at -78°C. The intention was that formation of the anion at the benzylic carbon would lead to a 4-exo-epoxide ring opening reaction a subsequent [l,2]-silyl shift (Brook rearrangement) would generate the oxetane 2 with stereocontrol at all three stereocentres. Anion formation proceeded smoothly at -78°C, then 1 ml of 1 M hydrochloric acid was added and the product isolated. Obtained pure in 40% yield, this was shown to be the aldehyde 3. No oxetane 2 was obtained. 

Regiocontrolled Corey-Seebach s epoxide ring-opening with 1,3-dithianyl anion... 

The mechanism of the Wharton transposition is very simiiar to that of the Woiff-Kishner reaction. The epoxyhydrazone is first deprotonated, which triggers the faciie and irreversible epoxide ring-opening. The C-N bond of the resuiting vinyi diazene is broken upon another deprotonation, releasing N2 and a vinyl anion, which in turn affords the desired aiiyiic aicohoi. Aiternativeiy, the formation of a vinyi radical has been proposed. ... 

A convenient synthesis of cyclic P-ketophosphonates has been reported from sodium dialkyl phosphites and oc-nitroepoxides. The phosphite anions attack regiospecifically the carbon P to nitro group with epoxide ring opening and elimination of sodium nitrite. All the cyclic dialkyl P-ketophosphonates are isolated in good yields (56-96%, Scheme 7.50). 

The epoxide is opened regioselectively with the anion of the nitrobenzene thiol 66 and the rest of the synthesis is essentially as described earlier (reduction, cyclisation, addition of substituents to O and N). 

A sequence of several steps, first discussed in connection with Scheme 6.13, installs chlorine at Cg. Reaction of 23-8 with basic hydrogen peroxide starts by selective conjugate addition of peroxide anion to the distal end of the diene. That transient intermediate goes on to form the a-epoxide. Diaxial opening of the oxirane with chlorine gives chlorohydrin... 

Compound (E) was prepared from levoglucosan and, after transformation into its tosylate and the corresponding epoxide, was opened with DMSO anion to give a dimethylated alcohol after desulfurization. The alcohol was then transformed into the corresponding lactol. Reduction of the lactol to the expected triol followed by a tandem protection-oxidation process afforded an aldehyde, which was then coupled with the vinyllithium reagent generated from the corresponding vinyl iodide [190]. The major diastereoisomer was transformed, after a... 

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