Separation selectivity methylene

Steroidal agents such as 5a-androstan-17P-ol-3-one (Fig. 4.5,1) and selected analogues (Fig. 4.5, 2-4) dissociate efficiently upon ESI and CID as shown exemplarily for 5a-androstan-17P-ol-3-one (Fig. 4.6a). Commonly, the saturated scaffold of cyclopenta[a]phenanthrene-derived compounds results in product ion mass spectra predominantly containing ions of low specificity, which are separated by methylene units (14 Da). Although a,P-unsaturated 3-keto-steroids also yield such product ions, the presence of double bonds commonly directs the dissociation pathway to specific and more characteristic fragments vide infra). 

PPL and Hpase from Pseudomonas sp. catalyze enantioselective hydrolysis of sulfinylalkanoates. For example, methyl sulfinylacetate (46) was resolved by Pseudomonas sp. Hpase in good yield and excellent selectivity (62). This procedure was suitable for the preparation of sulfinylalkanoates where the ester and sulfoxide groups are separated by one or two methylene units. Compounds with three methylene groups were not substrates for the Hpase (65). 

The homologues of the methylated non-ionic EO/PO surfactant blend were ionised as [M + NH4]+ ions. A mixture of these isomeric compounds, which could not be defined by their structure because separation was impossible, was ionised with its [M + NH4]+ ion at m/z 568. The mixture of different ions hidden behind this defined m/z ratio was submitted to fragmentation by the application of APCI—FIA—MS— MS(+). The product ion spectrum of the selected isomer as shown with its structure in Fig. 2.9.23 is presented together with the interpretation of the fragmentation behaviour of the isomer. One of the main difficulties that complicated the determination of the structure was that one EO unit in the ethoxylate chain in combination with an additional methylene group in the alkyl chain is equivalent to one PO unit in the ethoxylate chain (cf. table of structural combinations). The overview spectrum of the blend was complex because of this variation in homologues and isomers. The product ion spectrum was also complex, because product ions obtained by FIA from isomers with different EO/PO sequences could be observed complicating the spectrum. The statistical variations of the EO and PO units in the ethoxylate chain of the parent ions of isomers with m/z 568 under CID... 

An interesting reaction that has been developed over the past decade is the application of (2 + 2)-cycloaddition reactions to the synthesis of cyclophanes49. One of the earliest examples of this is the selective conversion of the bis(arylalkenes) 112 into the adducts 113. The yield of product is dependent to some extent on the chain length separating the aryl groups and the best yield of 41% is obtained when the separation includes four methylene units (n = 4). Lower yields are recorded with the other derivatives. Mixtures of products are formed when the m-isomers 114 are used. This affords 115 and 116. The yields of these are better than those obtained from the p-isomers 11250 51. Nishimura and coworkers52 have examined the ease with which such cyclobutanes, e.g. 115, n = 2,... 

Temperature variation was more influential on methylene selectivity for the mixtures with high percentages of acetonitrile, while the reverse was true for methanol/C02 mixtures. Like in nonaqueous reversed-phase HPLC, a temperature increase lowered the methylene selectivity. The main conclusion from this work is that acetonitrile/C02 mixtures would be preferred over methanol/C02 mixtures as a flrst attempt to separate homologs. 

The diastereomers 251/ewf-251 and 252/ent-252 could be separated and were decom-plexed separately. From the fraction of 251/ewt-251,253 was obtained with 85% ee (e.r. = 92.5 7.5), and the fraction of 252/ent-252 yielded ewt-253 with 88% ee (e.r. = 6 94). A similar situation results from the reaction with tributyltin chloride or alkylation reagents, but the diastereomeric ratio is strongly dependent on the electrophile. The following conclusion is drawn from these and further experiments The enantiomeric ratio is determined by a selection of the chiral base between the diastereotopic methylene groups, since the benzylic carbanionic centres are labile, whereas the diastereomeric ratio results from the relative rate of the electrophile approach syn or anti with respect to the A-methyl group. One question remains—why are opposite d.r. values formed in the alkylation by methyl iodide and ethyl iodide ... 

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