Oxiranes, nucleophilic substitution with

In stereoselective antitheses of chiral open-chain molecules transformations into cyclic precursors should be tried. The erythro-configurated acetylenic alcohol given below, for example, is disconnected into an acetylene monoanion and a symmetrical oxirane (M. A. Adams, 1979). Since nucleophilic substitution occurs with inversion of configuration this oxirane must be trens-conilgurated its precursor is commercially available trans-2-butene. 

Attempts at nucleophilic substitution of 20-halogeno-substituents in the halogenohydrins (28) were uniformly unsuccessful, even with ammonia or phthalimide anion. Oxiran ring-closure occurred in all cases. 

With silyl-substituted oxiranes, dibal-H favors the primary alcohol and Bu 3A1H favors the secondary alcohol. These observations have been interpreted in terms of the timing of the hydride transfer to one of the oxirane carbons. dibal-H, which exists as a Lewis complex in donor media (R3N-A1H(Bu )2, or R20-A1H(Bu )2) acts as a nucleophilic hydride source, which preferentially attacks the least-hindered carbon. With Bu 3A1, complexation with the oxirane oxygen precedes isobutene elimination and the generation of the Al—H bond. A considerable carbocation character is acquired in the transition state, hence formation of the primary alcohol is favored. It is worthy of note that trialkylstannyl-substituted oxiranes are reduced with Red-Al invariably at the oxirane... 

In one case a spirocyclobutyl-substituted vinyl oxirane has been converted to an a-ethylidenecyclo-pentanone (91TL3395). Vinyl oxiranes are coupled with organostannanes in the presence of (CH3CN)2PdCl2 to furnish allylic alcohols in good yields (88JA4039,89T979). Heteroatom nucleophile... 

Perfluorooxiranes in which a different perfluoroalkyl group substitutes for the CF3 of HFPO can also function as sources of difluorocarbene. In the presence of nickel powder, for example, oxirane 32 reacts with iodine to give difluorodiiodomethane (33) in high yield, accompanied by small amounts of oligomeric diiodides." Yields are very low in the absence of nickel, and it is suggested that the reaction occurs on the surface of the metal with a nucleophilic nickel-carbene complex. 

This process occurs as a concerted reaction, is stereospecific, and thus corresponds to the Sn2 mechanism of nucleophilic substitution at an sp -C-atom. For example, from ds-2,3-dimethy-l-oxirane, ( )-threo3-aminobutan-2-ol is obtained by reaction with NH3 ... 

Through two successive nucleophilic substitutions an oxirane (epoxide) function can be introduced at the expense of the initial diol. When the reaction is carried out with a diol as precursor, a diepoxide is formed ... 

Hydroxyalkyl ether cellulose [hydroxyethyl- (HEC) and hydroxypropyl cellulose (HPC)] are prepared in a different way. They use the capability of the alkoxide groups of alkali-cellulose to undergo a nucleophilic substitution when reacted with oxiranes ... 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

Polyfluoroalkyl- andperfluoroalkyl-substituted CO and CN multiple bonds as dipolarophiles. Dmzo alkanes are well known to react with carbonyl compounds, usually under very mild conditions, to give oxiranes and ketones The reaction has been interpreted as a nucleophilic attack of the diazo alkane on the carbonyl group to yield diazonium betaines or 1,2,3 oxadiazol 2 ines as reaction intermediates, which generally are too unstable to be isolated Aromatic diazo compounds react readily with partially fluorinated and perfluorinated ketones to give l,3,4-oxadiazol-3-ines m high yield At 25 °C and above, the aryloxa-diazolines lose nitrogen to give epoxides [111]... 

Substitution of an additional nitrogen atom onto the three-carbon side chain also serves to suppress tranquilizing activity at the expense of antispasmodic activity. Reaction of phenothia zine with epichlorohydrin by means of sodium hydride gives the epoxide 121. It should be noted that, even if initial attack in this reaction is on the epoxide, the alkoxide ion that would result from this nucleophilic addition can readily displace the adjacent chlorine to give the observed product. Opening of the oxirane with dimethylamine proceeds at the terminal position to afford the amino alcohol, 122. The amino alcohol is then converted to the halide (123). A displacement reaction with dimethylamine gives aminopromazine (124). ... 

A postulated mechanism for nucleophilic attack in enzymatic hydrolysis suggests that electron-withdrawing substituents on the phenyl ring must accelerate the reaction. This is seen in Table 10.1 when comparing 2-phenyl-oxirane with 2-(4-nitrophenyl)oxirane. A systematic study of a few aryl-substituted phenyloxiranes confirmed this inference [175],... 

Thus, 2-furfuryl vinyl ether 6a is extremely sensitive to cationic activation (16) because of its very pronounced nucleophilic character, but the polymerization is accompanied by some gel formation due to abundant alkylation of the furan rings pendant to the macromolecules. This structural anomaly is not encountered with the 5-methylated monomer 6b (16) precisely because electrophilic substitutions take place predominantly at C5 and are therefore impossible with this monomer. A similar difference of phenomenolo was observed with the 2-fiiryl oxiranes 4a and 4b (17). 

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