Diol formation, stereochemistry

Acid-catalyzed hydrolysis of indene oxides transition-state effects on stereochemistry of diol formation 266... 

ACID-CATALYZED HYDROLYSIS OF INDENE OXIDES TRANSITION-STATE EFFECTS ON STEREOCHEMISTRY OF DIOL FORMATION... 

Disulfonate esters of vicinal diols sometimes undergo reductive elimination on treatment with sodium iodide in acetone at elevated temperature and pressure (usually l(X)-200°). This reaction derived from sugar chemistry has been used occasionally with steroids, principally in the elimination of 2,3-dihy-droxysapogenin mesylates. The stereochemistry of the substituents and ring junction is important, as illustrated in the formation of the A -olefins (133) and (134). 

Both the regiochemistry and stereochemistry of Wacker oxidation can be influenced by substituents that engage in chelation with Pd. Whereas a single y-alkoxy function leads to a mixture of aldehyde and ketone, more highly oxygenated systems such as the acetonide or carbonate of the diol 1 lead to dominant aldehyde formation.107 The diol itself gives only ketone, which perhaps indicates that steric factors are also important. 

So we are still left with two models of the stereochemistry of DNA alkylated by a PAH diol epoxide the PAH either lies in a groove of DNA or else tries to intercalate between the bass of DNA. Since it is covalently bonded to a base it must cause considerable distortion if it tries to lie between the bases. However, the stacking observed in the crystalline state seems to argue for partial intercalation. We will need crystal structures of at least one appropriately alkylated polynucleotide before this problem can be resolved. And when this is done it will be just the beginning of the answer to the problem of alkylation of DNA by activated carcinogens. The subsequent question is, what is the lesion in DNA that is important in carcinogenesis, and then what does it cause to happen so that tumor formation is initiated ... 

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. 

The ion-molecule reactions of Me3Si+ have been employed to distinguish between the cis- and trans- isomers of 1,2-cyclopentanediol145. The formation of a [Me3SiOH2]+ cation (equation 26, top) was found to be indicative for a cA-structure of the reacted diol 37. Thus the formation of the [Me3SiOH2]+ cation from the adduct 38 proceeds without a barrier, whereas the production of this ion from the traws-diol 39 via adduct 40 (equation 26, bottom) was shown to be an endothermic process which possesses a translational energy onset. These studies are the first to use the reactivity of the Me3Si+ ion as a probe of the stereochemistry of the reacted neutrals in a mass spectrometer. 

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