Epoxidation 2.3.3- trisubstituted allyl alcohols

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... 

Substrate structure has a dramatic influence on the rate of the Sharpless asymmetric epoxidation. -Disubstituted and trisubstituted allylic alcohols react the most rapidly whereas Z-disubstituted and unsyininetrical disubstituted analogs react much more slowly32. Chemical yields for these substitution patterns are all in the range of 77-87% and enantioselection is in the range of 95% ee except for the slow-reacting Z-disubstituted allylic alcohols that exhibit enantioselectivities in the range of 90 % ee. 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

Oxiranes are transformed by tris(ethylthio)borane to sulfur-containing derivatives.With selenoboranes, terminal or a,)3-disubstituted epoxides yield 3-hydroxyse enides trisubstituted epoxides give allyl alcohols. 

The isomerization shows high regiospecificity. Thus the Z-epoxide (5) is converted in high yield into the disubstituted allylic alcohol (6), whereas the E-epoxide (7) is converted mainly into the trisubstituted allylic alcohol (8). The bulk of the base may contribute to the selectivity of isomerization. 

The methodology described above allows the asymmetric epoxidation of allylic alcohols or cis-substituted conjugated alkenes and the resolution of terminal epoxides. The asymmetric synthesis of trans-di- and trisubstituted epoxides can be achieved with the dioxirane formed from the fructose-derived ketone 64, developed by Shi and co-workers. The oxidizing agent potassium peroxomonosulfate... 

We will describe representative procedures for the epoxidation of a disub-stituted aromatic allylic alcohol (A), a trisubstituted aromatic allylic alcohol (B) and a disubstituted aliphatic allylic alcohol (C). 

Epoxides can also be converted to allylic alcohols using electrophilic reagents. The treatment of epoxides with trisubstituted silyl iodides and an organic base such as DBN gives the silyl ether of the corresponding allylic alcohols.119... 

Trisubstituted epoxides react to give allylic alcohols in which the hydroxyl group is linked to the less substituted carbon. 

High stereoselectivity has been observed during the epoxidation of the allylic alcohol (84) having a trisubstituted double bond and the allylic alcohol (85) having a cir-disubstituted double bond (equations... 

The unusual reactivity of selenoboranes towards epoxides gives new selective routes to /3-hydroxy-selenides and allyl alcohols.Thus, 1,2-epoxyoctane with B(SeMe)3 at 0°C for 0.7 h, followed by aqueous NaHCOa, gave (164) (81%) whereas the same reaction with styrene oxide gave (165) (63%) as the major product. For trisubstituted oxirans, however, the products were allylic alcohols, e.g. (167) (76%) from (166) on treatment with B(SePh)3 at 20 °C for 1.5 h. 

Unsaturated alcohols generally undergo selective oxidation of the alcohol moiety (Reactions 26 and 27) but when an allylic alcohol contained a reactive trisubstituted double bond selective epoxidation of the double bond was observed (Reaction 28). 

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