Epoxide opening, regioselectivity

Epoxide opening. Regioselective formation of fluorohydrins is observed. 

The reversibility of halohydrin dehalogenase-catalyzed reactions has been used for the regioselective epoxide-opening with nonnatural nucleophiles (an example is given in Scheme 10.34) [133]. The stereoselectivity of the enzyme results in the resolution of the racemic substrate. At the same time, the regioselectivity imposed by the active site geometry yields the anti-Markovnikov product. [128]... 

A limitation of epoxide deoxygenation with functionalized substrates became apparent in the second case where a mixture of olefins was isolated. Thus, the regioselectivity of epoxide opening and elimination of the titanium oxygen species was too low for practical use. 

Porco s synthesis of ( )-kinamycin C (3) constituted the first reported route to any of the diazofluorene antitumor antibiotics. This synthesis invokes several powerful transformations, including a modified Baylis-Hillman reaction, a catalyst-controlled asymmetric nucleophilic epoxidation, and a regioselective epoxide opening to establish the D-ring of the kinamycins. The tetracyclic skeleton was constructed by an... 

When the epoxide is 1,2-disubstituted, steric and electronic effects are responsible for the preferential formation of one product. In this context, benzyl radicals are always produced irrespective of the substitution pattern of the epoxide. For these intermediates, the more reactive tert-butyl thiol is the hydrogen atom donor of choice. Chelation of titanium can be used to good effect for regioselective epoxide opening, as shown in Scheme 12.8 [5d]. 

Epoxide opening proceeded with a regioselectivity of 88 12 to 90 10 for both 33 and 34. These values are in excellent agreement with the calculated... 

Unsymmetrical epoxides (39) can form two isomers, (40) and (41), on reaction with nitrate anion and so raise the issue of regioselectivity. Under acidic conditions terminal epoxides are found to predominantly yield the primary nitrate ester (41) although this is not clear cut and propylene oxide is reported to yield an ill defined mixture of isomers. A comprehensive study on the regioselectivity of epoxide opening with nitrate anion under acidic conditions was conducted on glycidol. ... 

The potential of regioselective epoxide opening is shown in the ex-chiral-pool synthesis of thromboxane B2 from levoglucosan via epoxide opening as the key step. 

Alternatively, epoxide (3R,5S)-14 was opened regioselectively with lithium iodide on silica [29], The crude product was immediately subjected to acetonide protection, which afforded the desired iodide sy -(3R,5S)-13 with a 58% yield (44% from syn-(3R,5S)-5a Scheme 2.2.7.7). Epoxide (3R,5S)-14 was easily obtained from dihydroxy ester syn-(3R,5S)-5a by treatment with DBU (66% yield) [11]. Treatment of sy -(3R,5S)-5a with LiCN in CH2CI2 gave nitrile (3R,5R)-15 in almost quantitative yield [21]. 

The in situ opening is particularly useful for terminal epoxide groups present in compounds such as 2-methylglycidol (6), which is opened regioselectively at the terminal C3-position by a variety of nucleophiles (C6H5SH, sec-amines, and phenols) in the presence of Ti(0-/-Pr)4. 

An epoxide is opened regioselectively to give the most stable carbocation. 

VF epoxide function is not affected by this reaction. Tetrafluorosilane serves as both a Lewis acid and a fluoride source in the subsequent regioselective epoxide opening to compound 36 SiF4 reacts with water to give silicic acid and hydrogen fluoride, and the latter re-... 

Lewis acid-catalyzed (SnCl4) ring opening of the epoxide is regioselective, placing the nucleophile at the more substituted carbon. 

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