Alcohols epoxide opening

The enantioseiective hydrogenation of a-amino ketones has been applied extensively to the synthesis of chiral drugs such as the / -agonist SR 58611 (Sanofi Cie). m-Chlorstyreneoxide was obtained via carbene-induced ring closure of the amino alcohol. Epoxide-opening by a chiral amine obtained via a ruthenium-catalyzed hydrogenation of an enamide has led to the desired compound where... 

Amino alcohols. Epoxide opening by amines catalyzed by SmClj is regioselective and stereoselective. The nucleophiles attack from the less substituted carbon atoms. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

The configurations assigned to (8) and (9) were established by comparison with the products resulting from epoxidation of 3-methyl-5a-cholest-2-ene followed by reduction with lithium aluminum hydride to the alcohol (9). The usual /ra 5-diaxial epoxide opening requires that the hydroxyl group, formed by reduction, is axial as shown in (9). 

The AE reactions on 2,5,5-trisubstituted allyl alcohols have received little attention, due in part the limited utility of the product epoxides. Selective ring opening of tetrasubstituted epoxides are difficult to achieve. Epoxide 39 was prepared using stoichiometric AE conditions and were subsequently elaborated to Darvon alcohol. Epoxides 40 and 41 were both prepared in good selectivity and subsequently utilized in the preparation of (-)-cuparene and the polyfunctoinal carotenoid peridinin, respectively. Scheme 1.6.12... 

A careful analysis of this problem led to the identification of an exceedingly simple solution (see Scheme 10). The Masamune-Sharpless solution to the threo 2,3-diol problem actually takes advantage of the ready availability of the erythro 2,3-diol diastereoisomer. As we have seen in Scheme 9, erythro 2,3-diols such as 20 can be conveniently assembled from trans allylic alcohols via sequential SAE and Payne rearrangement/epoxide opening reac-... 

These epoxide-opening conditions were originally developed by Sharpless and coworkers for the regiocontrolled opening of 2,3-epoxy alcohols [30]. It has been proposed that ligand exchange of the substrate with isopropoxide forms a covalently bound substrate-titanium complex (Chart 3.3). Nucleophilic attack on this complex at the 3-position is favored over attack at the 2-position. In the case of 49,... 

Corey s sulfonium ylide methodology32 was then adopted. As shown in Scheme 7-51, conversion of ketone 161 to spiroepoxide 165 proceeded with high yield, and a Lewis acid-induced epoxide opening gave the desired allylic alcohol 164 with perfect yield. 

Titanocene-Catalyzed Reductive Epoxide Opening to Alcohols... 

Based on the labeling experiments, a plausible mechanism involving mthenium vinylidene intermediates SS is proposed in Scheme 6.21. Cydization of this vinylidene intermediate leads to the formation of the epoxy carbenium 56, which then undergoes an epoxide opening to form l,4-dien-3-ol 57. A subsequent pinacol rearrangement of this alcohol furnishes ketone 58, providing the required skeleton for the observed phenol product 54. 

---