Epoxides bonding

Of interest is a recent report of a rapid synthesis of efaroxin (51), a potent, selective O2 adrenoceptor antagonist, using Darzens Reaction. Accordingly, a-bromoester 48 was condensed with aldehyde 47. The glycidic ester (49) was then hydrogenated to reduce the more labile epoxide bond to give alcohol 50. Subsequent standard transformations subsequently lead to a completed 4-step synthesis of efaroxin. o... 

Identify the substitution pattern of the two epoxide carbon atoms—in this case, one carbon is secondary and one is primary. Then recall the guidelines for epoxide cleavages. An epoxide with only primary and secondary carbons usually undergoes cleavage by SN2-like attack of a nucleophile on the less hindered carbon, but an epoxide with a tertiary carbon atom usually undergoes cleavage by backside attack on the more hindered carbon. In this case, an S]sj2 cleavage of the primary C—O epoxide bond will occur. 

In contrast, hydrolyzed silane compounds, presumably adsorbed as oligomeric films, confer corrosion resistance in both hydrating and Cl environments. These inhibitors can also couple with applied epoxy primer or adhesive formulations to further protect the metal against corrosion by strengthening the metal-epoxide bond. The organosilanes do not appear to affect the curing process, e.g., % crosslinking, of the polymeric epoxy systems. 

Base-catalyzed reactions, in which the nucleophile provides the driving force for ring opening, usually involve breaking the epoxide bond at the less substituted carbon, since this is the position most accessible to nucleophilic attack.86 87 The situation in acid-catalyzed reactions is more complex. The bonding of a proton to the oxygen weakens the C—O... 

The diol-epoxide contains a reactive carbon center, namely the C-10 position, which can open to form a carbonium ion that is susceptible to nucleophilic attack (Scheme 3.4). The predominant nucleophile among DNA bases is guanine, which preferentially interacts with the carbonium ion at the N2-amine position of guanine to form the BaP-N 2-guanine adduct. The epoxide bond of the diol-epoxide metabolite is particularly resistant to hydrolysis because it is located in the Bay region of the BaP molecule, where steric hindrance prevents the attack of hydrolytic enzymes, such as epoxide hydrolase. 

In practice the first two classes merge into a single concept of structural rearrangement concerted with epoxide opening, because no attempt seems to have been made to detect an-chimeric assistance by kinetic studies. The reactions which follow a concerted pathway are recognised when product studies show a stereospecific involvement of bonding electrons trans to the breaking epoxide bond. 

Epoxides bonded to a chain of carbon atoms can also be named as derivatives of oxirane, the simplest epoxide having two carbons and one oxygen atom in a ring. The oxirane ring is numbered to put the O atom at position one, and the first substituent at position two. No number is used for a substituent in a monosubstituted oxirane. 

Evolved gas analysis showed that the epoxide-bonded wood had... 

Competition between an alkene and an ether oxygen for participation in epoxide opening is evident in the reactions of trichothecenes (illustrated by the skeletal structure 20 equation 8). In aqueous acid, participation of the ether oxygen is favored and (21) is formed, probably due to preferred cleavage at the tertiary center under the more electrophilic conditions. Under neutral or basic conditions, the diminished role of intermolecular electrophile (water) results in unusual scission of the primary epoxide bond, with formation of (22). ... 

The material is derived by ring-opening of the epoxide bond of Leukotriene A by cysteine. 

The acid-catalyzed ring opening of an epoxide always results in a frans-1,2,-diol, with the two carbons formerly part of the epoxide bonded to each of the two hydroxyl groups. 

Whenever a chelating effect is polarizing one epoxide bond particularly, thus activating one carbon atom of the epoxide and preparing it for nucleophilic attack, there is a good chance quite easily to overcome steric hindrance and to establish quaternary carbon centers. This will be of special value if it can be achieved in an already quite crowded region of a molecule. 

If R in 103 is assumed to be a simple unbranched hydrocarbon chain and one operates with a mild oxophilic Lewis acid, both epoxide bonds will be polarized to a certain extent, but we will be far away from the cation status. 

Polarization of the higher substituted epoxide bond (see 170) is followed by ring contraction placing the positive charge next to the silicon group, which leads to the cyclohexene 171 [49]. 

Base-catalyzed reactions, in which the nucleophile provides the driving force for ring opening, usually involve breaking of the epoxide bond at the less substituted... 

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