Epoxide scission, synthesis

The first synthesis of a thio sugar by epoxide scission to be reported was the preparation of 1,2-0-isopropylidene-6-thio-D-glucose (67, R = H) from 5,6-anhydro-l,2-0-isopropylidene-a-D-glucofuranose (66) by the action of hydrogen sulfide. The product was isolated as the crystalUne... 

Mode of synthesis A. cyanohydrin, by way of 2-nmino-2-deoxy-aldonic acid B. scission of sugar derivative epoxide with ammonia C. interconversion of hexosamine series D. hemihydrogenation of a-amino nitrile466 E. rearrangement of ketosyl-amine F. Removal of last carbon atom of hexosamine G. Hydrazinolysis (with inversion) of 2-0-tosyl-pentose. 6 Physical constants taken from this reference. c Derivatives (only) isolated. 

A relevant extension of the ring opening of epoxides with a titanium salt can be seen in the concomitant scission of the C-C bond of cyclobutane illustrated in Eqs (285) and (286) [648]. Choice of each diastereoisomer enables highly selective construction of the trisubstituted double bond which finds application in terpene synthesis. The tandem cleavage of the C-O and C-C bonds was similarly feasible in the combination of oxetane and cyclobutane as illustrated in the stereoselective synthesis of a homoallyl alcohol (Eq. 287) [648]. 

Only the second step, the catalytic cleavage of DSA or methyl dihydroxystearate (MDS), was carried out using oxygen when two-stage processes for the synthesis of AA or MMA from fatty acids or esters were developed (Scheme 20.3). The first step, a direct dihydroxylation or an epoxidation/hydrolytic ring scission of an epoxide formed as intermediate, may be performed using H2O2 or performic acid as oxidant, for example. 

The diaxial trans-g yco structure in a-muricholic acid was substantiated by partial synthesis. a-Muricholic acid could be derived from either the 6a, 7a-epoxide XVI [Hsia et al. (39)] or the 6,9,7/9-epoxide XV [Hsia et al. (41)] (Fig. 4) by scission of the epoxide rings and hydrolysis of the acetates thus formed. In accordance to the Fiirst and Plattner rule [Fiirst and Plattner (42)], ionic opening of an ethylene oxide results in the diaxial trans-g yco. The structure of a-muricholic acid therefore should be 3a,6,9,7a-tri-hydroxy-5/9-cholanic acid (XVII). The orientation of the epoxide ring in XV and that... 

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