Epoxides stereospecific deoxygenation

Heterolytic cleavage of oxiranes with Grignard reagents 89UK401. Stereospecific deoxygenation of epoxides to olefins 87H(26)1345. 

It has recently been reported that selenoamide is a useful reagent for the stereospecific deoxygenation of epoxides under mild conditions.This deoxygenation can be applied to various epoxides, i.e. mono-, di- and tri-substituted epoxides and cyclic epoxides. A suggested reaction path is illustrated in Scheme 34. 

Other reductions using selenium compounds involve the stereospecific deoxygenation of epoxides by use of selenocyanates or selenoamides [178]. 

The stereospecific desulfurization of thiiranes may be effected by carbenes (70). Epoxides are deoxygenated with much lower efficiency (71). 

Olefinic compounds can be regenerated from epoxides by a number of stereospecific deoxygenation reactions and these are illustrated for two sets of... 

In the presence of more conventional bases carbene production is supressed. The decomposition of the chlorodifluoromethyl anion has been shown to be reversible and the yield of difluorocyclopropane is increased by increasing the alkene concentration. Epoxides upon treatment with dichlorocarbene afford cyclopropanes stereospecific-ally. The reaction proceeds by stereospecific deoxygenation of the epoxide to give olefin (41) which is subsequently trapped by the carbene. 

Epoxides are stereospecifically deoxygenated to alkenes with inversion of stereochemistry upon treatment with PhMe2SiLi in THF.4 trimy-Stilbene is obtained in 83% yield and >99% stereoselectivity from the reaction of oxide of ciy-stilbene with PhMc2 SiLi. Similarly, irons-stilbene oxide is converted to c/s-stilbene. 

More frequent than the deoxygenation of epoxides to alkenes is reduction of epoxides to alcohols. Its regiospecificity and stereospecificity depend on the reducing agents. 

These reactions can be carried out catalytically because the tellurium precipitated during the deoxygenation of the oxiranes will react rapidly with added sodium diethyl phosphite. Lithium 0,0-diethyl tellurolophosphate is a more powerful deoxygenation reagent and is able to convert certain internal epoxides to olefins in a stereospecific manner3. 

The P(m) reagents have an even greater affinity for sulfur than they do for oxygen and there are a number of useful synthetic procedures based on desulfurization.9 In contrast to the deoxygenation of epoxides, the desulfurization of thiiranes is stereospecific and represents a potentially useful synthetic route to alkenes. Unfortunately, however, thiiranes were not easily obtained until the... 

TiCU plays a dual role in effecting epoxide opening and subsequent deoxygenation with allylsilane as depicted in Eq. (277) [641]. The reaction is highly stereospecific. 

By use of Me3SiK, some mono-, di-, and trisubstituted epoxides can be deoxygenated stereospecifically, with inversion of stereochemistry [Eq. (49)1 (96, 97). Thus, reaction of cis and trans epoxides with Me3SiK in... 

Deoxygenation of epoxides.2 The reagent, in combination with trifluoroacetic acid, converts epoxides into olefins at room temperature rapidly and stereospecifically with preservation of the stereochemistry about the C-C bond of the epoxide. 

Deoxygenation of epoxides. n-ButyUithium (2-3 eq.) reacts with ferric chloride in THF at -78° to form a black, soluble iron species that converts epoxides into olefins in about 60-90% yield. The reaction is not stereospecific for example, the oxide of c -stilbene is converted into cu-stilbene and trans-stilbene in the ratio 89 11. ... 

Deoxygenation of epoxidesf Iron carbonyl (1 equiv.) in N,N-dimethylacet-amide or tetramethylurea deoxygenates epoxides (2 hours, 145°). No reaction occurs in refluxing THF. The reaction is not stereospecific irany-stilhene oxide is converted into both tra/w-stilbene (56%) and c/s-stilbene (22%). Epoxides of... 

Deoxygenation of epoxides. Epoxides are converted into olefins by reaction with this reagent in THF at 20. The reaction is stereospecific. Thus the oxide of trans-stilbene is converted into c/j-stilbene (75% yield, >97% stereospecific), and the oxide of c/s-stilbene is converted into truns-stilbene (83%, yield, > 99%, stereospecific). 

A stereospecific method for deoxygenating epoxides to alkenes involves reaction of the epoxide with the diphenylphosphide ion, followed by methyl iodide. The method results in overall inversion of the alkene stereochemistry. Thus,... 

Deoxygenation of Epoxides. The reagent deoxygenates epoxides in a stereospecific manner, although the reaction is limited to aryl-substituted epoxides (eq 6). A related reagent, [ -Bu3SnAlMe3] Li+, has broader applicability. ... 

On treatment with PhaPL, u/c-iodohydrins give high yields of olefins via stereospecific trani-elimination from the phosphorylated iodohydrin. A more useful extension of this process is the one-flask deoxygenation of epoxides to olefins via uic-halohydrins using 1.1 molar equivalents each of Ph3P.HI and PhaPL. The reduction takes place at or below room temperature, in high yield (91—98%), and with better than 98% retention of configuration. 

Further approaches to AZT have been reported. The anhydronucleoside (74) was obtained directly from thymidine by two sequential Mitsunobu reactions, and, after treatment with LlNb and deacylation, gave AZT in 73% overall yield.86 The same group has also used D-xylose to prepare AZT, by condensation of l,2-di-0-acetyl-3,5-di-(7-benzyl-D-xylofuranose with silylated thymine, and deoxygenation at C-2 to give a known intermediate for AZT. 87 In an approach from acyclic precursors, the Sharpless epoxidation product (75) was manipulated as indicated in Scheme 11 the stereospecificity in the cyclization, which gave only the p-anomer, was rationalized... 

Epoxides and oxetans are deoxygenated to give olefins in a non-stereospecific manner triplet intermediates are probably involved, though not necessarily exclusively. The desulphurization of sulphides is described, the radiceils formed undergoing subsequent reaction. 

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