Oxirane ring terminal

The opening of the oxirane ring is accompanied by inversion except when the oxirane ring is in the terminal position of an aliphatic chain the ultimate result is equivalent to trans addition to the double bond. Thus cyciohexene yields trans-1 2- f/ohexanediol ... 

The rate of hydrozirconation of a terminal olefin is much faster than the reduction of the oxirane ring. This chemoselectivity was used in the preparation of various cycloalkylmethanols by hydrozirconation of alkenyloxiranes (Scheme 8-28) [217]. 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

A so far still unsolved problem is the direct enantioselective epoxidation of simple terminal olefins. For example the epoxidation of propylene that was achieved with a 41% ee almost twenty years ago by Strukul and his coworkers using Pt/diphosphine complexes is still unsurpassed. Unfortunately such low ee s are of no practical interest. The problem was circumvented by Jacobsen using hydrolytic kinetic resolution of racemic epoxides (Equation 26) and is practised on a multi 100 kg scale at Chirex. The strategy used is to stereose-lectively open the oxirane ring of a racemic chiral epoxide leaving the other enantiomer intact. Reactions are carried out to a 50% maximum conversion. The catalyst belongs to the metal-salen class described above and can be recycled. The products are separated by fractional distillation. 

The high selectivity of catalyst lb toward terminal unfunctionalized alkenes is highlighted by the experiments reported in Fig. 2.2. As a typical example, czs-l,4-hexadiene bears both terminal and cis C=C bonds and in the presence of a stoichiometric amount of m-chloroperbenzoic acid (m-CPBA) leads mainly to czs-4,5-epoxy-l-hexene due to the electrophilic epoxidation of the more electron-rich internal double bond. On the contrary, when the epoxidation is performed with catalyst lb and one equivalent of H2O2, the regioselectivity of the reaction is completely inverted, favoring the product with the terminal oxirane ring. The same applies to frflns-l,4-hexadiene or dienes bearing substituents in the... 

A 5-exo reaction can be observed when ,/Tepoxycyclohexenone is treated with tin(lV) chloride. After a regioselective oxirane ring opening, cyclization follows via axial attack of the alkyne group. The terminal vinyl cation is trapped by chloride and the bicyclo[4 3.0]nonan-2-one system 1 is formed stereoselectively in 94% yield5. 

The SN-2 attack of the alcoholate anion take place preferentially at the a-carbon atom of the oxiranic ring (normal SN-2 attack), which is explained by the low steric hindrance of this atom and by the electron release effect of the methyl group, which increases the electron density at the carbon atom in the (3 position [2, 4, 5, 9-14, 17, 49-54]. An high electron density carbon atom is less susceptible to the attack of the anions. As an immediate consequence, the terminal hydroxyl groups are predominantly secondary, thus proving that this is a predominantly normal SN-2 attack [4, 74] ... 

Therefore, in the present study we have measured CPT, activity in mitochondrial, microsomal, peroxisomal and high-speed supernatant fractions prepared from rat liver on a quantitativle basis. We have also quantified the relative expression of proteins that bind dinitrophenyl (DNP)-etomoxiryl-CoA (an oxirane ring-containing inhibitor of hepatic CPT I") in each of these fractions in the intact liver under conditions in which all three activities were optimally inhibited. Our results indicate that the overt CPT activity of microsomes and peroxisomes is associated with a protein that is very similar, if not identical, to mitochondrial CPT I. In microsomes too, the CPT, activity is associated with a protein of Mr 88,(X)0, but in this membrane fraction, there is evidence that the N-terminal domain has different properties from those of mitochondrial and peroxisomal CPT I. 

With respect to this reaction, weak bonds are formed between the oxygen of the oxirane ring and one hydrogen atom of the amine. This explanation is also supported by Ito and Okahashi [91]. In addition, they have suggested a termination in which boron trifluoride complexes with tertiary amine are produced ... 

Figure 9 Examples of polydimethylsiloxane resins terminated by either oxirane rings 72 or chemical groups containing labile hydrogen atoms, such as carboxylic acid 68 and aliphatic amine 70. These compounds are synthesized by hydrosilylation of carbon-carbon double bonds using hydrogen-terminated polydimethylsiloxanes 66 as starting reactant. This reaction is also employed to produce epoxy-silicone 74 from allyl-...

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