Epoxides, ether formation from

The reductive ether formation from keto epoxides is an acid-catalyzed process (Eqs. 234407 and 235408). 

For synthetic applications, it is important to realize that ether formation from epoxides can be run as a catalyst-controUed or a configuration-controlled reaction and that in certain cases one can also benefit from kinetic versus thermodynamic control. 

For 1,2-disubstituted epoxides, the regiochemical outcome of nucleophilic attack becomes less predictable. However, in the case of epoxy ethers chelation control can be used to deliver the nucleophile preferentially to the epoxide carbon away from the ether moiety. Thus, treatment of epoxy ether 61 with an imido(halo)metal complex, such as [Cr(N-t-Bu)Cl3(dme)], leads to the clean and high-yielding production of the chlorohydrin 64. The regioselectivity is rationalized in terms of initial formation of a chelated species (62), followed by attack at C-3 to form the more stable 5-membered metallacyclic alkoxide 63 <00SL677>. 

Since this reaction proceeds according to the SN2 mechanism, inversion takes place at the carbon atom involved (Fig. 2-19). The ring size of epoxides can vary from three- to six-membered rings. The three-membered derivatives belong to the most important subclass of internal ethers and are termed oxirans. A prerequisite for oxiran formation is obviously coplanarity and trans position of the reacting groups (Fig. 2-20). 

Epoxidations. Formation of chiral epoxides from enol ethers derived from protected glucopyranosyl derivatives has been reported. 1,2-Epoxyalkylphosphonates are obtained from epoxidation of vinylphosphonates. Generation of dimethyldioxirane at high pH (10.5-11.5) is advantageous. ... 

The site-selectivity of oxidations by mCPBA is demonstrated in the conversion of (4 R = Me or Ph) into the corresponding ene epoxide (5). The product is sensitive to acid, so that the conversion is accomplished in a basic two-phase medium. Normal epoxidation of (6) with mCPBA leads to (7), The stereochemistries for such reactions are shown in the predominant formation of the 3-epoxide (8) (81%) from the parent alkene, with 12% of the a-product. Similar epoxidation of the cannabinol (9) leads to a less stereo-specific isomer distribution of 27.3% and 18.2%. Remarkable stereoselectivity has been shown in the epoxidation of the 14,15-unsaturated oestratrienes (10). Whereas oxidation of 17j3-esters and 17/3-ethers gave 14a,15a-epoxides (< 59%), the 17j3-urethane derivatives displayed a s /w-directive effect to yield 14/3,15j3-epoxides (< 87%). 

Related Sc(OTf)3-catalyzed reactions, i.e. alkylation of silyl enol ethers with sulfur dioxide adduct of 1-methoxybutadiene, that of lithium enolates with epoxides, and benzopyran formation from o-hydroxybenzaldehydes and dimethox) ropane have also been reported. 

Crystallization of cis—1,4-polyisoprene from solution at -65 C has been carried out it is therefore possible that block copolymer preparation by epoxidation, bromination or some other reaction could be accomplished with lamellas of this polymer. Lamellar crystallization of cellulose, of amylose and of polyacrylic acid have been reported substitution reactions such as acetylation or ether formation with the hydroxyl groups and esterfication of the acid groups are possible reactions to carry out with lamellas of those polymers. The use of nonaqueous systems may be better suited to prevent swelling, and therefore, attack of the crystalline regions. It should also be possible to react poly(vinylalcohol) lamellas in suspension with acids or anhydrides to form vinyl-alcohol-vinyl ester block copolymers or with phosgene to obtain chloroformate groups which can undergo further reactions. 

Easily prepared from the appropriate monosaccharide, a glycal is an unsatu-rated sugar with a C1-C2 double bond. To ready it for use in potysaccharide synthesis, the primary -OH group of the glycal is first protected at its primary -OH group by formation of a silvl ether (Section 17.8) and at its two adjacent secondary - OH groups by formation of a cyclic carbonate ester. Then, the protected glycal is epoxidized. 

The C2-symmetric epoxide 23 (Scheme 7) reacts smoothly with carbon nucleophiles. For example, treatment of 23 with lithium dimethylcuprate proceeds with inversion of configuration, resulting in the formation of alcohol 28. An important consequence of the C2 symmetry of 23 is that the attack of the organometallic reagent upon either one of the two epoxide carbons produces the same product. After simultaneous hydrogenolysis of the two benzyl ethers in 28, protection of the 1,2-diol as an acetonide ring can be easily achieved by the use of 2,2-dimethoxypropane and camphor-sulfonic acid (CSA). It is necessary to briefly expose the crude product from the latter reaction to methanol and CSA so that the mixed acyclic ketal can be cleaved (see 29—>30). Oxidation of alcohol 30 with pyridinium chlorochromate (PCC) provides alde-... 

---