Epoxidation regioselectivity

Regioselective epoxidation of allylic and homo-allylic alcohols... 

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]... 

Displacement of the 3-hydroxyl group of 74 was carried out with Et2NSF3 (DAST) (DAST - diethylaminosulfur trifluoride) in dichloromethane. The expected fluorinated product 75 on treatment with aqueous perchloric acid led to regioselective epoxide ring opening to give 76, which on treatment with hydrazine hydrate at 100 °C for 18 h yielded 3,4-dihydroxy-8-oxo-octahydropyridazino[l,6-r/][l,2,4]triazine-l-carboxylic acid phenylamide 77 (Scheme 3) <1997T9357>. 

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... 

As a further example of a hydroxyl-assisted epoxidation, geraniol and nerol bearing two isolated C=C double bonds were regioselectively epoxidized with TS-1 at the 2-position (near the OH group), as reported by Kumar et al. (795). On the basis of these results, Kumar et al. (195) proposed that the transition state of the epoxidation of allylic alcohols involves coordination of the alcoholic functional group to the Ti active site and that the double bond interacts with one of the peroxidic oxygen atoms, not with the titanium site (Scheme 9). 

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]. 

As described in equation 78, the relatively electron-rich ring double bond of a- or fi-ionone was regioselectively epoxidized, whereas the electron-deficient double bond was not oxidized at all. This result also supports the radical rather than the anionic character of 52. 

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. 

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-... 

Two classes of structures containing azido- and aziridino-hydroxyl-(3-lactam (I and II, respectively, Fig. 21) have been prepared by means of a stereo- and regioselective epoxide ring opening reaction [296],... 

The selective catalytic epoxidation of alkenes has become the most important reaction catalyzed by heme proteins in organic synthesis. As described above, the monoxygenase activity of a heme peroxidase is restricted to CPO due to the open substrate access of the ferryl subunit for this enzyme. HRP catalyzes epoxidations only after mutagenetic variations, as shown for the substrate trans-P-methylstyrene [234]. An exception of this rule is the regioselective epoxidation of (T.TJ-piperylpiperidide, which is successfully catalyzed by native HRP [265]. 

All the polymers of Table III have been applied for the epoxidation of olefins with alkyl hydroperoxides. For example, the polymers with iminodiacetic acid or diethylene triamine groups were used for the regioselective epoxidation of (E)-geraniol with t-BuOOH to the 2,3-epoxide (225), whereas the Mo anchored to the diphenylphosphinopolystyrene catalyst is used in the epoxidation of cyclohexene with t-BuOOH (228). The polymer-supported molybdenyl thioglycolate has also been used for the catalytic oxidation of thiols and phosphines with air or pyridine N-oxide as the oxidant (234). 

Gahagan M, Iraqi A, Cupertino D C et al. (1989) A high activity molybdenum containing epoxidation catalyst and its use in regioselective epoxidation of polybutadiene J Chem Soc Chem Commun issue 21 1688-1690... 

The combination of titanium(IV) fluoride with antimony(lll) fluoride was found to open the epoxide ring regioselectively to yield the more-substituted fluoride, but only one example with a moderate yield was given. In another case, it was reported that regioselective epoxide cleavage could be best achieved with bis(isopropoxy)titanium difluoride. 

Very successful has been the aldolase-catalyzed aldol reaction as exemplified inO Scheme 14 [97]. The required (R)-3-thioglyceraldehyde (20) is obtained from regioselective epoxide ring opening of (5)-glycidaldehyde diethyl acetal with thioacetic acid and its potassium salt. Con-... 

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