Degradation by /3-elimination

Further examples of modified Smith degradation, in which the polyalcohol is methylated prior to hydrolysis under mild conditions, uronic acid degradation, and /3-elimination preceded by oxidation have been reported in connection with structural studies on the Klebsiella type 28 (Ref. 182), 57 (Ref. 183), 59 (Ref. 142), and 81 (Ref. 184) capsular polysaccharides. 

XLIX) was degraded by Hofmann eliminations (Fig. 6) to a mixture of dienes which w as oxidized to X( — )-3-methoxyadipic acid. The absolute configuration of this acid was knowui from earlier work in which it had been a key intermediate in establishing the absolute configuration of sterols 20) and led to the 3 R configuration for j8-erythroidine. 

Substance (h), arising from this ionized enediol by /3-elimination, has never been isolated, but it is known as the enol form of a 3-deoxyglycosulose and is extremely important in the formation of these compoimds and in the acid-catalyzed degradation to 5-(hydroxymethyl)-2-furaldehyde. Indeed, in strong acidic media, the chief product is 5-(hydroxymethyl)-2-furaldehyde, formed by steps (h) and (i), but compound (j) has also been... 

Experiments have been carried out to compare nanoscale catalysts composed of Fe-, Ni-, and Co-complexes of several porphyrins or cyanocobalamin (Dror et al. 2005). A cobalt-porphyrin complex and cyanocobalamin in the presence of Ti(III)citrate reduced the initial concentrations of tetrachloromethane and tetrachloroethene by —99.5%, and the porphyrin was equally effective with trichloroethene. The advantage of using heterogeneous catalysts was shown by experiments in repetitive cycling of tetrachloromethane. Zero-valent metals degrade vicinal dichlorides such as tetrachloroethene by a-elimination to produce dichloroacetylene and hnally acetylene (Roberts et al. 1996). 

This and other work indicates that HC1 is largely undissociated in nitromethane for [HC1]>- 0.015 M and that there is little association either. There is evidence that a corresponding addition occurs to olefins in theimally degraded PVC. Results carried out in a variety of solvents (26) are consistent with elimination of HC1 occurring by a/3 -elimination of the Ex type favored by polar solvents. The same authors showed that at least in nitrobenzene containing dissolved HC1, the reverse reaction, i.e. addition of HC1, takes place. The fact that this may be interpreted as a retardation of the degradation process may have contributed to the confusion which has arisen and emphasizes the care which must be taken to disentangle the possible catalytic effect of HC1 when concurrent addition of HC1 to the polyenes is possible. 

---