Cholesterol, stereoselective epoxidation

Fe(acac)3] has been reported to have catalytic activity for stereoselective / -epoxidation of cholesterol and analogues with H202.153... 

For the synthesis of oxiranes with more complex structures, the peracid method is combined with other epoxidation procedures examples are the syntheses of ( )-crotoxirane, ( )-epicrotoxirane, and ( )-isocrotoxirane ° or the preparation of cis-trioxatris-(a)homotropylidene. o-Sulfoperbenzoic acid has been used for the stereoselective epoxidation of cholesterol. The selective epoxidation of cholest-5-en-3-one too has been examined. In the synthesis of 25-hydroxycholesterol selective epoxidation occurs on and 26 is formed. The epoxidation of olefin propellanes 27 and 28 can be achieved with MCPBA. As a consequence of the secondary orbital interaction, syn-attack is more marked in the case of 28. ° Epoxypropelladiene can be synthesized in accordance with Eq. 12. ... 

The 1,4-addition of alkyl cyanocupratcs to epoxides to form cyclic a,ft-unsalurated ketones provides a stereoselective method for conversion of dehydro-. cpiandrosteronc into cholesterol (4) and other related steroids. Thus the keto epoxide 2 (obtained from dehydroepiandrosterone in 55% overall yield in three steps) reacts with lithium isohcxylcyanocupratc to give the adduct 3, which can be converted to cholesterol (4) in several known steps.27... 

Although Pt-alkene complexes are normally not as reactive as the corresponding Pd complexes toward nucleophilic addition, platinum can serve usefully as a catalyst in these reactions.59 The following cyclization was mediated by a Pt-PPP-pincer complex 22, demonstrating Markovnikov addition at each step of the cyclization (equation 8.42).60 The reaction is stereoselective and mimics the cyclization of squalene epoxide to lanosterol, a key step in the biosynthesis of cholesterol. 

Epoxidation of cholesterol This reaction can be effected with 02 and isobu-tyraldchydc as rcductant in quantitative yield in CH2CI2 at room temperature. This reaction provides the a- and /3-oxidcs as a 1 1 mixture. Addition of a (tctraphenyl-porphrinato)nickel(ll) catalyst docs not affect the rate, but enhances -stereoselectivity (74 26). 

The natural occurrence of 24,25- and 25,26-dihydroxyvitamin D metabolites has led to studies of their synthesis and configurational assignment via the initial preparation of the correspondingly substituted cholesterol derivatives. Initial approaches to these compounds were carried out on oxidized forms of A24- or A25-steroids (98, 142, 154, 155, 169, 170) or by reduction of the ketol (46) (5i). In these cases, almost equal amounts of the diasteroisomeric epoxides or diols were obtained because the newly formed asymmetric position was too distant from a chiral center (i. e., at C-17 or C-20) for stereoselective induction. 

Stereoselective jS-epoxidation of cholesterol and its derivatives can be carried out in acetonitrile with [Fe(acac)3] and H2O2 in good yield under mild conditions [428] Eq. (269). Thus, a variety of interesting reactions between hydroperoxides and olefins of which we have mentioned here only a few, are catalyzed by metal complexes in solution. 

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