Epoxidation base catalyzed

Base-Catalyzed. Ring Opening of Epoxides. Base-catalyzed ring opening of epoxides provides a route to allylic alcohols.150... 

Let us look, for example, at the reaction of ethylene oxide with phenol. Acid catalyzes reaction by converting the epoxide into the highly reactive protonated epoxide. Base catalyzes reaction by converting the phenol into the more strongly nucleophilic phenoxide ion. 

Such copolymers of oxygen have been prepared from styrene, a-methylstyrene, indene, ketenes, butadiene, isoprene, l,l-diphen5iethylene, methyl methacrjiate, methyl acrylate, acrylonitrile, and vinyl chloride (44,66,109). 1,3-Dienes, such as butadiene, yield randomly distributed 1,2- and 1,4-copolymers. Oxygen pressure and olefin stmcture are important factors in these reactions for example, other products, eg, carbonyl compounds, epoxides, etc, can form at low oxygen pressures. Polymers possessing dialkyl peroxide moieties in the polymer backbone have also been prepared by base-catalyzed condensations of di(hydroxy-/ f2 -alkyl) peroxides with dibasic acid chlorides or bis(chloroformates) (110). 

Base-catalyzed epoxide opening is a typical S -2 reaction in which attack of the nucleophile takes place at the less hindered epoxide carbon. For example, 1,2-epoxypropane reacts with ethoxide ion exclusively at the less highly substituted, primary, carbon to give l-ethoxy-2-propanol. 

When the epoxide is to be used, a basic curing agent such as a tertiary amine, R3N, is added to cause the individual prepolymer chains to link together. This "cross-linking" of chains is simply a base-catalyzed epoxide... 

The A-D-ring analog 30a,b (mixture of epimers) has been prepared from the epoxide 3a,b by base catalyzed epoxide cleavage, hydroxymethylenation, and O-alkylation of the butenolide unit using standard conditions. Hydroxymethylenation of keto-ester 7 followed by butenolide addition provided the A-D-ring analog 31. 

Epoxides Can undergo base-catalyzed ring opening ... 

If the epoxide is unsymmetrical, the nucleophile attacks primarily at the less substituted carbon atom in base-catalyzed ring opening. 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

The related syn diol epoxide isomers were synthesized from the 1,2- and 7,8-dihydrodiols of 5-MC by reaction with N-bromoacetamide in moist DMSO followed by base-catalyzed cyclization by the usual Method I procedures (60). 

The simplest model compound is cyclohexene oxide III. Monomers IV, V and VII represent different aspects of the ester portion of I, while monomers VII and VIII reflect aspects of both the monomer I and the polymer which is formed by cationic ring-opening polymerization. Monomers IV-VII were prepared using a phase transfer catalyzed epoxidation based on the method of Venturello and D Aloisio (6) and employed previously in this laboratory (7). This method was not effective for the preparation of monomer VIII. In this specific case (equation 4), epoxidation using Oxone (potassium monoperoxysulfate) was employed. 

Scheme 4-21 shows the preparation of L-threitol and L-erythritol.38 Epoxy alcohols (2J ,3iS)-61 and (2S,3/ )-61. generated by asymmetric epoxidation, are exposed to sodium benzenethiolate and sodium hydroxide in a protonic solvent to undergo base-catalyzed rearrangement, yielding the threo-diol 62 and erythro-diol 63, which can then be converted to the corresponding tetraacetate of l-threitol 67 and L-erythritol 69 through subsequent transformations. 

The base-catalyzed hydration of 2-phenyloxirane involves nucleophilic attack preferentially at C(3) (0-C(3) cleavage), but with only partial regio-selectivity. Acid-catalyzed hydration is mainly by 0-C(2) cleavage. The hydration of 2-phenyloxirane catalyzed by epoxide hydrolase is characterized by its very high regioselectivity for the less-hindered, unsubstituted C(3) [175] [176], involving retention of configuration at C(2). In other words, (R)-and (5)-2-phenyloxirane are metabolized to (/ )- and (S)-l-phenylethane-l,2-diol (10.118), respectively. Substrate enantioselectivity was also character-... 

Fig. 10.27. Mechanistic and stereochemical aspects of the hydration of trans-(R,K)-anethole epoxide in the absence (Reactions a and b) and presence of epoxide hydrolase (Reaction c) [178][179]. The ratio of Reaction a to bis ca. 4 1 in acid- and base-catalyzed hydration to yield the threo- and erythro-diols, respectively. The enzymatic Reaction c also yields preferentially the erythro-diol.

An example of a whole-cell process is the two-step synthesis of an enantiopure epoxide by asymmetric reduction of an a-chloro ketone (Scheme 6.4), catalyzed by recombinant whole cells of an Escherichia coli sp. overexpressing an (R)-KRED from Lactobacillus kefir and GDH from Thermoplasma acidophilum, to the corresponding chlorohydrin, followed by non-enzymatic base-catalyzed ring closure to the epoxide [17]. 

Base-catalyzed reactions, in which the nucleophile provides the driving force for ring opening, usually involve breaking the epoxide bond at the less substituted carbon, since this is the position most accessible to nucleophilic attack.86 87 The situation in acid-catalyzed reactions is more complex. The bonding of a proton to the oxygen weakens the C—O... 

Base-catalyzed ring opening of epoxides constitutes a route to allylic alcohols. 

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