Glycols formation from epoxides

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

In many cases almost quantitative yields are reported for the formation of v/c-dinitrate esters from the reaction of simple alkyl and dialkyl epoxides with dinitrogen pentoxide (Table 3.2). Some of the products formed include ethylene glycol dinitrate (2) (96%), 1,2-propanediol dinitrate (8) (96 %), 2,3-butanediol dinitrate (94 %) and 1,2-butanediol dinitrate (96 %). Reaction times are of the order of 5-15 minutes. 

Subsequent work by Price and Berti,1407 ou the other hand, Jailed to produce epoxides from the cyclic sulfites of either dr- or front-1,2-diphenylothvlene glycol. A similar Jack of epoxide formation... 

Complex (23) also catalyzes reaction (147). In addition to the products (120)-(122), the diformate ester of 1,2-propylene glycol was also formed.582 Other epoxides were studied and the complex [CoCl(PPh3)3] catalyzed the formation of the lactone (122) only. Formate complexes were believed to be present, resulting from the insertion of C02 into a rhodium-hydride bond (equation 149). The formate complex then initiates the catalytic cycle. The general mechanism given in Scheme 54 was proposed for the reaction. 

Catalytic systems containing Te02, HBr and AcOH have been used industrially by Oxirane to convert ethylene to ethylene glycol via the formation of mono- and di-acetate (equations 131 and 132).359-361 The overall yield from ethylene to ethylene glycol is more than 90%, making this reaction competitive with respect to the older silver-catalyzed ethylene epoxidation process. 

An interesting synthesis of JH1 170 starting from furan was published by Cavill et al.145). The use of a glycol precursor (193) prior to the oxirane formation guarantees the generation of a uniform 10,11-epoxide, in contrast to the usual m-chloroper-benzoic acid oxidation145) (Scheme 34). 

The cyclization in Step B is an improvement of Butler s procedure for the synthesis of which employs less convenient reagents, KNH and l-bromo-3-chloroacetone acetal. Beside the acetals derived from neopentyl glycol, those derived from ethanol, 1,3-propanediol and 2,4-pentanediol have been synthesized by the present method. The second part of Step B involves the formation and the electrophilic trapping of cyclopropenyl anion 2, which is the key element of the present preparations. Step B provides a simple route to substituted cyclopropenones, but the reaction is limited to alkylation with alkyl halides. The use of lithiated and zincated cyclopropenone acetal, on the other hand, is more general and permits the reaction with a variety of electrophiles alkyl, aryl and vinyl halides, Me3SiCl, Bu3SnCl, aldehydes, ketones, and epoxides. Repetition of the lithiation/alkylation sequence provides disubstituted cyclopropenone acetals. 

The formation of 5a-hydroxy-6 , 7 -epoxides can be explained, from 56,6B-epoxides as given in the scheme. This was explained from the following reason. 4-Deoxy-5B,6B-epoxides are less in number. Transdiaxial opening of epoxide forms glycol at C-5 and C-6 position. Dehydration at C-6 followed by epoxidation from the rear side produces 5o-hydroxy-6a,7a-epoxides. 

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