Epoxides monosubstituted

This reaction works well with monosubstituted epoxides ... 

Jacobsen also showed that 2,2-disubstituted epoxides underwent kinetic resolution catalyzed by (salen)Cr-N3 complex 3 under conditions virtually identical to those employed with monosubstituted epoxides (Scheme 7.34) [64]. Several epoxides in this difficult substrate class were obtained with high ees and in good yields, as were the associated ring-opened products. The kinetic resolution of TBS-... 

At first, the reaction was characterized as most effective in the epoxidation of cw-disubstituted olefins.86 Later, the scope of this reaction was expanded to include the highly enantioselective synthesis of hYwiv-dis ubstitutcd8 7 and tri-substituted epoxides,88 as well as certain monosubstituted epoxides.89 The first example of nondirect asymmetric epoxidation of tetrasubstituted olefins has also appeared.90... 

The table, which collects representative examples, shows that monosubstituted epoxides afford homoaUyhc alcohols resulting from the attack to the less substituted carbon atom (runs 1, 5 and 7). HomoaUyhc alcohols are useful intermediates in several important total synthesis." Disubstituted epoxides fail to react (run 4). Styrene oxide leads to a mixture of homoaUyhc alcohols (run 2) and ally lie epoxides give mixture of 1,2- and 1,4-opening product, with predominance of the 1,4 product (run 3, 6 and 8). 

Epoxides also undergo the Ritter reaction in good yields with retention of configuration via a episulfonium intermediate 190a (double-inversion process). For monosubstituted epoxides, the yields of oxazolines are lower due to nondis-criminatory attack of the nitrile on both the primary and the secondary carbon atom of the episulfonium intermediate. Complete retention of configuration is still observed despite the lower yield (Scheme 8.54). 

Other reagents have also been used to effect carbon-carbon bond formation. For example, chiral monosubstituted epoxides (93) can be regioselectively carbomethoxylated under relatively mild conditions with CO/H, in the presence of the salen complex 69. The reaction proceeds with retention of chirality about the secondary epoxide carbon and represents a new route to chiral hydroxy esters 94 <99JOC2164>. 

Deprotonation of complex 1 with butyllithium at — 78 °C generates the enolate species 2 (described in Section 1.1.1.3.4.1.1.), which reacts with electrophiles while in the anti conformation (acyl oxygen anti to carbon monoxide oxygen). Enolate 2 is inert to 1,2-epoxypropane (3a) at — 78 °C, but in the presence of a Lewis acid, rapid reaction ensues leading to preferred alkylation of the least hindered site of the epoxide13. Reaction of the enolate 2, derived from the racemic complex 1, with racemic monosubstituted epoxides results in preferential formation of one of two possible diastereomers this can be termed a double enantiomer-differentiating reaction. 

Enolate species 6, derived from 1-oxopropyl complex 5, reacts similarly with monosubstituted epoxides. Under the influence of diethylaluminum chloride, only the diastereomers 7 and 8 were observed in the reaction mixture 7 was the major product. The use of boron trifluoride - diethyl ether complex instead of diethylaluminum chloride caused a complete loss of stereocontrol at C , producing a 50 50 mixture of diastereomers 7 and 8, but stereocontrol at C was retained as no other diastereomers were produced. The major diastereomer produced is consistent with the intermediacy of a transition state like that represented in Newman projection C which has the usual anti-E-snolate geometry and lacks the R methyl gauche interaction of structure D. 

Reactions of the enolate derived from enantiomerically pure 5 with enantiomerically pure monosubstituted epoxides 3 are also shown in the table on p 952 it is noteworthy that very high selectivities are obtained regardless of the relative configurations of the two chiral reactants. 

NOES (1999) National Occupational Exposure Survey 1981-83. Unpublished data as of July 1999. Cincinnati, OH, Department of Health and Human Services, Public Health Service, National Inshtute for Occupational Safety and Health Nomeir, A.A., Silveira, D.M., Ferrala, N.F., Markham, P.M., McComish, M.F., Ghanayem, B.l. Chadwick, M. (1995) Comparahve disposition of 2,3-epoxy-l-propanol (glycidol) in rats following oral and intravenous administration. J. Toxicol, environ. Health, 44, 203-217 Norppa, H., Hemminki, K., Sorsa, M. Vainio, H. (1981) Effect of monosubstituted epoxides on chromosome aberrations and SCE in cultured human lymphocytes. Mutat Res., 91, 243-250... 

Epoxides such as 10 can be prepared in high enantiomeric purity, by, inter alia, kinetic resolution. David Hodgson of Oxford University has demonstrated (J. Am. Chem. Soc. 2004, /26,8664) that on exposure to LTMP, monosubstituted epoxides are smoothly converted into the corresponding alkoxy carbenc or alkoxy carbenoid. When an alkene is available for insertion, the cyclopropane, in this case 11, is formed with high diastereocontrol. This represents a powerful new approach to enantioselective ring construction. It is possible that in the absence of a target alkene, the intermediate alkoxy carbene could divert to intramolecular C-H insertion, which might also proceed with substantial diastereocontrol. 

Norppa, H., Hemminki, K., Sorsa, M. Vainio, H. (1981) Effect of monosubstituted epoxides on chromosome aberrations and SCE in cultured human lymphocytes. Mutat. Res., 91, 243-250... 

The discovery that certain catalysts are capable of producing crystalline polymer from optically inactive (racemic) monosubstituted epoxides has caused a good deal of excitement in recent years because it seems virtually certain that such crystallinity must originate from an asymmetric synthesis of the polymer chain (24). 

Epoxides can react smoothly with allylsilane reagents. For example, 1,3-bis-trimethylsilyl-Tpropene can react with one of a variety of epoxides under Lewis acid catalysis to generate a highly functionalized alkenol (Equation 35). Yields are modest, with best results being obtained using monosubstituted epoxides <1998TL529>. Trimethyl silyl... 

A final group of fp -carbanions which have been found to react well with at least monosubstituted epoxides are the vinylalanes formed either by direct hydroalumination or by zirconium-catalyzed car-boalumination of a terminal alkyne. Both species are insufficiently reactive to couple with epoxides and must first be converted into their corresponding ate complexes, usually by treatment with n-butylli-thium (Scheme 30). 

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