Oxacyclopropanes formation

B Oxacyclopropane formation from a C=C double bond MCPBA or any other peroxycarboxylic acid may be used. 

Figure 9-6 Only the anti conformation of a 2-bromoalkoxide allows for oxacyclopropane formation. The two gauche conformers cannot undergo intramolecular backside attack at the bromine-bearing carbon.

Oxacyclopropane Formation from an Alkene Through the Haloalcohol... 

D Oxidation with Peroxidic Compounds. Oxacyclopropane (Oxirane) Formation... 

The grouping C—O—C—O—C is characteristic of an acetal or a ketal (see Section 15-4E), but it also can be regarded as an ether with two ether links to one carbon. Compared to other ethers (except for the oxacyclopropanes), substances with the C—O—C—O—C group are very active toward acidic reagents, as pointed out in connection with their formation from alcohols (Section 15-4E) and their use as protecting groups for the OH function (Section 15-9C). 

Reactions with sulfur ylides proceed differently. The products are oxacyclo-propanes (oxiranes) —not alkenes. The addition step proceeds as with the phosphorus ylides, but the negatively charged oxygen of the dipolar adduct then displaces the sulfonium group as a neutral sulfide. This is an intramolecular Sn2 reaction similar to the formation of oxacyclopropanes from vicinal chloroalcohols (Section 15-11C) ... 

CH2CH2OH (An example of 2-carbon homologation formation of an alcohol two carbons longer than a starting organometallic reagent, by addition to oxacyclopropane)... 

As anticipated by the formation of oxiranes (oxacyclopropanes, epoxides) from halohydrins, the four-membered oxygen-containing ring, oxetane (oxacyclobutane), can be prepared by heating 3-chloropropanol (HOCH2CH2CH2CI) with potassium hydroxide (KOH) (Equation 8.51). 

Ring opening of oxacyclopropanes is also catalyzed by acids. The reaction in this case proceeds through initial cyclic alkyloxonium ion formation followed by ring opening as a result of nucleophilic attack. 

Alkynes can also be prepared from other alkynes. The reaction of terminal alkynyl anions with alkylating agents, snch as primary haloalkanes, oxacyclopropanes, aldehydes, or ketones, results in carbon-carbon bond formation. As we know (Section 13-2), such anions are readily prepared from terminal alkynes by deprotonation with strong bases (mostly alkyllithium reagents, sodinm amide in liqnid ammonia, or (jrignard reagents). Alkylation... 

Epoxides are reactive entities due to the strain of the oxacyclopropane moiety. They can be formed from a variety of precursors but this chapter will be limited to the formation of epoxides by the oxidation of double bonds. If the double bond bears three or four different substitutes, epoxidation creates one or two chiral centers. In almost all epoxide-containing natural products these chiral centers were not created by chance and the products are nonracemic but contain an excess of one enantiomer. The chirality of the epoxides is caused by the chirality and the regioselectivity of the forming enzyme. This is a general attribute of enzymes making them ideal tools for enantiopure syntheses. 

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