Epoxides generation

Table 6.2 Examples of epoxides generated by AE, applied to different primary allylic alcohols showing all eight basic substitution patterns.

Hydrogenation of carbon dioxide in the presence of an epoxide generates a mixture of the diol, its formate esters, and the cyclic carbonate. While the reaction has been shown to operate in high yield (1300 TON for the cyclic carbonate Eq. (10)) [93], the fact that a mixture is generated and that the cyclic carbonate could be made more cleanly in the absence of H2 makes the reaction uninteresting for synthesis. Sasaki s group showed that this reaction in the presence of an amine base gives CO rather than cyclic carbonate (Eq. (11)) [94]. The epoxide then serves as a trap for the water. 

The prototype for this structural class is 2-methyl-2-propen-l-ol (methallyl alcohol), from which asymmetric epoxidation generates optically active 2-methyloxiranemethanol. Like gly-cidol, 2-methyloxiranemethanol has been difficult to obtain by stoichiometric asymmetric epoxidation, but with the use of the catalytic version, reasonable quantities, are now produced [4] and the compound has become commercially available. In situ derivatization also can be used to recover this epoxy alcohol from the epoxidation reaction. Progress in the isolation of 2-methyloxiranemethanol is reflected in entries 1-3 of Table 6A.3, and the results of in situ derivatization are revealed by entries 4-6. The enantiomeric purity of 2-methyloxiranemethanol produced in this way is very good (92-95% ee), and improvement to 98% ee is observed after recrystallization of the 4-nitrobenzoate derivative. 

Recently, silicon tetrafluoride [207] and tetrabutylphosphonium fluoride [208] have been used to prepare fluorohydrins from epoxides. Generation of a superacid is required for the ring opening of a perfluorinated oxirane [209] (Figure 3.42). 

Allylmagnesium halides under the influence of ClTi(OiPr)3 add at the more-substituted position of unsymmetrical epoxides, generating quaternary carbon centers <2005T6726>. 

HMPA at 65 C for 3 hours affords the corresponding trans and cis olefins, respectively. Backside attack of Mc3SiK on the cis (and trans) epoxides generates the threo- (and erythro-) jS-alkoxysilanes, respectively, followed by syn elimination (Peterson elimination) to give the olefin with inverted stereochemistry. 

Sml2X that is produced as a result of this process then serves to open the epoxide, generating the iodohy-drin. Although this appears to be a likely scenario, a more direct route involving a Finkelstein reaction between the brsamarium iodide salts - cannot be ruled out (Scheme 2). 

Jacobsen successfully extended the Cr(salen)-catalyzed ARO of raeso-epoxides with TMSN3 to the reaction of terminal epoxides, generating valuable 1-amino-... 

The strong base catalyzed phenolic cured epoxy reaction proceeds via the phenoxide ion which reacts with epoxide, generating the alkoxide ion. The high basicity of the alkoxide ion reacts with phenolic to regenerate the phenoxide ion and the reaction repeats itself. 

An alternative way to HMHM-structures such as 32 is shown in Scheme 6. The Co-symmetrical ditosylate 24 is converted into the mono-acetonide 29 which forms the epoxide 30 on treatment with base. The cuprate attack on the epoxide generates a second epoxide 31 via tosylate elimination. With excess cuprate 31 is opened to give 32 directly (4). 

A rapid route to N-substituted trihydroxypiperidines (l,5-dideoxy-l,5-imino-pentitols) by nucleophilic opening of bis-epoxides derived from d- and L-ara-binitol, D-xylitol and related compounds has been reported. The 3-0-benzyl-Zn s-epoxides, generated by tosylation of the primary hydroxyl groups followed by treatment with sodium hydride and benzyl bromide, were treated with primary amines to yield piperidine/pyrrolidine mixtures, whose composition was found to be dependent on the nature of the alditol precursor, the amine and the reaction... 

The expulsion of a nitrogen atom leads to an amide that reacts intramolecularly with the epoxide, generating two five-membered fused cycles and a monoalkyl carbonate that looses CO2, yielding the first product. 

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