Epoxide groups, ring-opening

One of the first indications that the antiinflammatory potency of the corticoids could be increased was the observation that incorporation of a 9a-fluoro group in hydrocortisone resulted in a tenfold increase in activity. Treatment of hydrocortisone acetate (170a) with phosphorus oxychloride in pyridine yields the corresponding olefin, 172. This, on being subjected to the reaction sequence depicted in the transformation of 104 to 108 (addition of HOBr, closure to the epoxide and ring opening with HF),... 

The suggested reaction mechanism involves a nucleophilic attack of the imine nitrogen at the activated triple bond, followed by a proton exchange, to give a benzimidazolinium system which, by intramolecular attack at the carbonyl group, leads to an epoxide that ring opens to the observed product. For the ethyl derivative (R = Et) a tub conformation could be established by X-ray crystallographic analysis.33... 

These results can now be used to consider what happens when a diol epoxide attacks DNA. The epoxide group will open and trans addition will occur. The product (XXVI) will have the DNA substituted adjacent to the bay region (particularly if it is hindered so that the epoxide group is made more reactive) and will lie axial to the PAH ring system. This means that the plane of the PAH and the alkylated base of DNA must have a perpendicular relationship to each other as indicated in Figure 19. In this Figure those sites in... 

The vinyl silane 247 was unmasked to the ketone 250 by epoxidation, subsequent ring opening of the epoxide with HF in pyridine and concurrent cleavage of the THP and TBS protecting group (Scheme 39). The Stork-Jung annulation was completed through the treatment of the diketone 250 with sodium methoxide to mediate the intramolecular aldol con-... 

Allylic alcohols can also be obtained from epoxides by ring opening with a selenide anion followed by elimination via the selenoxide (see Section 6.8.3 for discussion of selenoxide elimination). The elimination occurs regiospecifically away from the hydroxy group.116 117 118... 

When the protonated epoxide undergoes ring-opening, the CH,OH molecule attacks from the backside of the C. The nearby, newly formed OH group hasn t moved out of the way and blocks approach from the frontside. This leads to inversion at the chiral carbon. Since there was no change in group priorities, the configuration in the product is (S). 

The pathway by which the reactions are considered to occur involves attack of the enolate anion at the 1 0 bond of o-iodosyl benzoic acid followed by reductive elimination of o-iodobenzoic acid upon addition of methoxide to the carbonyl group. Ring opening of the epoxide thus formed yields the hydroxy dimethyl acetal ... 

D. Epoxide (Oxirane) Ring Openings III Electrophilic Main Group Species... 

The characteristic thing about macromolecules, we have said, is their great size. This size has little effect on chemical properties. A functional group reacts much as we would expect, whether it is in a big or little molecule an ester is hydrolyzed, an epoxide undergoes ring-opening, an allylic hydrogen is susceptible to abstraction by free radicals. 

Henkel Kgaa, Fatty Alcohol Derivatives with Vicinal Hydroxy andAUcoxy Groups. Production by High Pressure Hydrolysis of Ester Prepared hy Acylation, Epoxidation and Ring Opening, DE Patent 3,829,735 (1990). 

Similarly, the other enantiomer of the epoxide was ring-opened with the Grignard derivative of 5b to yield alcohol 20 in 76 % yield (Scheme 3.8). Subsequent ZrCLj-catalysed deprotection and cyclization generated cyclic acetal 21 in 96 % yield. Finally, as before, this acetal was oxidized to lactone 22 in an 82 % yield followed by hydrogenolysis of the benzyl group to form (H-)-5-muricatacin lb in a 75 % yield. 

Several bicyclic-, aliphatic-, and aryl-meso-epoxides are ring-opened by 5-methoxymercaptobenzothiazole in the presence of a chiral phosphoric acid catalyst (TRIP), giving fra 5 -l,2-hydroxysulfides in yields between 52 and 98% with 46 to 85% ee Several different groups are tolerated in the reactions. Two possible transition states are proposed. 

In the case of epoxy acetate 1, a neighboring group participation mechanism should proceed by initial protonation of the epoxide, followed by backside attack of the acetoxy group, ring opening and subsequent formation of acetoxonium ion 5 (Seheme 25.3). 

The same group also realized the tandem epoxidation and ring-opening reaction on Mn(Salen)-based MOF prepared using Lj2 as the organic linker and Zn as the inorganic node [118]. 

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