Diastereoisomerism, fragments

The imidazopyridine 123 was synthesized by treating amino acid 122 with p-toluenesulfonyl cyanide and DCC. The yield of 123 was lower (38%) when the mtinchnone was generated with mesyl chloride (72). The minor amide product 124 is proposed to arise from the diastereoisomeric mtinchnone adduct that undergoes fragmentation rather than decarboxylation. 

In some cases, it appears that the mechanism of the fragmentation is E2, since an anti elimination has been observed [6]. A systematic investigation has been made on the behavior of the diastereoisomeric monotosylates VIII/5, VIII/6, VIII/7, and VIII/8 under fragmentation conditions (KOtBu, HOtBu, lh, 40°) [7], Scheme VIII/2. The results demonstrate the importance of the geometry of the substrate. In VIII/6 and VIII/7, the bonds marked as a and b are antiperiplanar to each other with an angle of 180°. The reactions yielded the... 

Now, permutation of N and P donors yields stereoisomers, while the (5) configuration of the [Fe(S4)] fragment remains unaltered as before. As above, the difference between 34 and 35 can be expressed with respect to thioether and thiolate atoms standing cis or trans to the NH group. The underlying cause for the diastereoisomerism of 34 and 35 (or 30 and 31), however, is the stereo-genicity of the Fe centers. 

Numerous examples of stereospecific reactions in the gas phase are reported in the mass spectrometric literature [1,2]. Many if not most of them, however, deal with relatively rigid systems, e.g., 1, or polyfunctional molecules such as derivatives of tartaric acid [6] the latter gave rise to the first chirality effect observed in mass spectrometry [7]. For stereogenic centers linked by flexible alkyl chains, however, diastereoisomeric differentiation in ion fragmentation is often poor. Two epimers of the aminoalkanol 2, for example, show quite small differences in their mass spectra whereas these differences increase if the two centers are linked by cyclization upon formation of 3 as indicated in Scheme 2 with the epimeric center being marked by an asterisk [8]. 

The highest mass peak in the mass spectra of these dimers corresponds to M /2, even at 120 °C and 15 eV. The fragmentation pattern for presecamine (138) is identical to that of compound (136) which has been named secodine. This indicates the occurrence of a facile retro-Diels-Alder reaction such a process can be carried out preparatively at 175°C/0.2mm to give secodine (136) from presecamine itself, and dihydrosecodine (137) from tetrahydropresecamine. The product (137) has been identified by direct comparison with synthetic material. On standing without solvent, (136) and (137) dimerise to give diastereoisomeric mixtures of the corresponding presecamines. 

When chiral induction is achieved with amines or aldehydes bearing an asymmetric center in the preparation of the imine fragment, a mixture of diastereoisomeric trans-and c/i-cycloadducts 45 and 46 are obtained in which the tranj-isomers 45 are the major... 

The methodology has been extended to C-C coupling reactions of alkenes and S-dicarbonyl compounds with air as the oxidant and with CeCl3 7H20 as the catalyst (Christoffers et al., 2003b Rdssle et al., 2005). The reactions yield diastereoisomeric mixtures of hydroxyperox-ides, which disproportionate in presence of acetyl chloride/pyridine by Kornblum-DeLaMare fragmentation into dioxo esters (scheme 62). These dioxo derivatives have a 1,4-dicarbonyl structural motif. This reaction has been explored with styrene and substituted alkenes as the alkene substrate. 

Steric interactions in the cyclic transition structures for a hydrogen transfer result in different abundance of the butene radical cation fragment, obtained from the diastereoisomeric unsaturated alcohols erythro-10 and threo-10. The higher energy of the transition state of erythro-10 due to the steric interaction of the methyl and R groups, results in a lower abundance of the fragment, as compared with the stereoisomeric threo-9 (see Scheme 9). 

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