Epoxide regioselective

Florio et al. have employed heteroaromatic rings as organyl-stabilizing groups for metalated aziridines as well as for metalated epoxides. Regioselective deprotonation of aziridine 246 with n-BuLi, followed by addition of Mel, gave aziridine 247 (Scheme 5.62) [88]. 

This process has, however, not yet found industrial application. Similarly, poly-isoprenoids can be epoxidized regioselectively in co-position to functional groups (Table 4, No. 16-19) Polymer-bound mediators can also be used in this... 

The reagent cleaves epoxides regioselectively to give 1 -(/1-hydroxy)cyclopropyl phenyl sulfides, which are cleaved by HgCl2 to a,/ -enones (equation II).1... 

The same reducing agents also react with epoxides in SN2-type reactions converting them into alcohols (Figure 17.41). The sterically less hindered C—O bond reacts in unsymmetrical epoxides regioselectively. 

This example makes the matter look deceptively clear-cut. But with epoxides, regioselectivity is not as simple as this because, even with acid catalysts, Sjyj2 substitution at a primary centre is very fast. For example, Br in acid attacks this epoxide mainly at the less substituted end, and only 24% of the product is produced by the cation-stabilized pathway. It is very difficult to override the preference of epoxides unsubstituted at one end to react at that end. 

Bromohydrins. The reagent reacts with epoxides regioselectively to yield a trans-bromohydrin with bromine predominantly at the less hindered position. Acid- and base-sensitive functional groups are stable to the reagent. 

Starting with picrotoxinin (1) Yoshikoshi developed an improved synthesis of ( )-picrotin (2). Epoxidation of picrotoxinin (1) with peracid at room temperature led to a 5 2 mixture of the epimeric epoxides. Regioselective cleavage of the epoxide was achieved with sodium phenylselenyl triethoxy boronate. Radical reduction of the phenyl selenides 414 with stannane completed this three-step sequence to picrotin (2) in 87% overall yield. 

Dienones with extended conjugation undergo peroxy acid epoxidation regioselectively at the yiS-double bond, even if it is less substituted than the a,p-double bond. The epoxidation of (112) is regio-and stereo-selective (equation 39). Attempted epoxidation of (113a) using nucleophilic reagents furnishes polymeric materials. The epoxidation has been carried out with MCPBA (equation 40). 2a... 

The reaction of Grignard reagents with epoxides, at first glance, appears to be an effective method for a two-carbon homoligation. However, there are several competing reactions that limit the use of this method. These deleterious reactions are formation of halohydrins, rearrangements, polymerization of the epoxide, and in asymmetric epoxides, regioselectivity issues. 

S. Y. Tosaki, R. Tsuji, T. Ohshima, M. Shibasaki, Dynamic ligand exchange of the lanthanide complex leading to structural and functional transformation One-pot sequential catalytic asymmetric epoxidation-regioselective epoxide-opening process, J. Am. Chem. Soc. 127 (2005) 2147. 

HydroaUsylation. 2-Alkynoic esters form ketene Al-enolates on treatment with Dibal-H and NMO. The enolate species react with epoxides regioselectively. Further processing leads a-alkyhdene-y-hutyrolactones in either the ( )-form or (Z)-form. 

Non-crosslinked polystyrene with salen ligand Non-crosslinked polystyrene with onium salt residues Non-crosslinked polystyrene copolymerised with a chiral phosphine ligand 14,765 0.5-3.8" 0.7-2.3 CH2Cl2,THF, EtOAc, DMF DMF,DMSO,DMA, toluene, anisole, MeOH,MeCN, diglyme Asymmetric epoxidation Regioselective addition reaction of phenyl glycidyl ether with S-phenyl thioacetate Pt-catalysed asymmetric hydroformylation of olefins Precipitation (methanol) (r) Precipitation (diethyl ether) (r) [68] [142] [143]... 

In order to characterize the nature of the oxygenating species formed in the reactions of H2O2, PhIO, MCPBA, and dioxygen plus aldehyde, we studied stereoselectivity in c/5-stilbene epoxidation, regioselectivity in (+)-limonene and norbornene epoxidations, and intermolecular competitive reactions between cyclohexene... 

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