Oxacyclopropane , from reactions

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) ... 

Oxacyclopropanes also can be prepared from vicinal chloro- or bromo-alcohols and a base. This is an internal SN2 reaction and, if the stereochemistry is correct, proceeds quite rapidly, even if a strained ring is formed ... 

Acidic conditions also can be used for the cleavage of oxacyclopropane rings. An oxonium ion is formed first, which subsequently is attacked by the nucleophile in an SN2 displacement or forms a carbocation in an SN1 reaction. Evidence for the SN2 mechanism, which produces inversion, comes not only from the stereochemistry but also from the fact that the rate is dependent on the concentration of the nucleophile. An example is ring opening with hydrogen... 

Some acid-catalyzed solvolysis reactions of oxacyclopropanes appear to proceed by SN1 mechanisms involving carbocation intermediates. Evidence for the SN1 mechanism is available from the reactions of unsymmetrically substituted oxacyclopropanes. For example, we would expect the conjugate acid of 2,2-dimethyloxacyclopropane to be attacked by methanol at the primary carbon by an SN2 reaction and at the tertiary carbon by an SN1 reaction ... 

What remains is to identify compound B, which gives an oxacyclopropane stereoisomeric to that formed from compound A, but at a slower rate, and D, which gives the same product as C, but also more slowly. Because both types of reactions under consideration require an axial leaving group, it makes sense to flip the chairs of the remaining starting compounds (in which the —Br is currently equatorial) and see what we get. 

The ring-opening process of Equation 8.45 is, of course, simply the reverse of the process by which oxiranes (oxacyclopropanes, epoxides) are formed from halohy-drins (e.g., see item 3,Table 7.6). Further, as written, the processes shown in Schemes 8.90-8.92 are reversible and thus, at least in principle, carbonyl compounds can be converted to enol ethers, acetals (and ketals), and orthoesters. However, while acetals and ketals readily form from alcohols and acids under dehydrating conditions (Chapter 9) and esters undergo exchange reactions with alcohols in the... 

Outline a short synthesis of tra s-2-methylcyclohexanol from cyclohexene. (Hint Review the reactions of oxacyclopropanes in Section 9-9.)... 

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... 

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