Quantum chemical modeling of chiral catalysis. Part 14. On the coordination of carbonyl compounds to diaza-, oxaza- and dioxaluminolidines of potential use as chiral controllers in organic synthetic chemistry

Conformers of 4-coordinate adducts of carbonyl compounds and diaza-, oxaza- and dioxaluminolidines were investigated by means of ab initio MO methods (RHF). Formaldehyde was used as a model of carbonyl compounds. Relative stabilities of the conformers indicate formation of syn adducts of carbonyl compounds and aluminolidines (C_C=O and aluminolidine ring syn about the Al---O_C=O bond) to be favoured over that of the corresponding anti ones (all syn/anti ratios higher than 99:1, 6–31G//6–31G). The energetic preference for the formation of syn adducts of oxazaaluminolidines was about twice as high as that of diaza- or dioxaluminolidines of which the syn/anti selectivities were found to be practically equal.     

A reduction technique for weighted grouping problems

Weighted grouping problems are shown to have an equivalent reduced form, which is often considerably smaller than the original problem. Although the reduction may be small for randomly generated problems, real-life problems often contain non-random properties that greatly increase the effect of reduction. We give an efficient algorithm to build the reduced problem instance, and analyse the expected amount of reduction for certain statistical distributions and real-life data. In addition, we briefly discuss the effect of reduction on traditional solving methods of the grouping problem. The results show clearly the usefulness of problem reduction: it is computationally cheap to apply and may make the reduced problem solvable in a practical time whilst the original one is not. The method is readily applicable to the job grouping problem of printed circuit board (PCB) printing industry.     

Electron impact induced fragmentations of some 2,2-disubstituted 1,3-dithiolanes

Electron impact induced fragmentations of the title compounds were studied by exact mass measurement and metastable ion analysis. Sulphur atoms within the ring effectively stabilize the positive charge. In most cases the loss of the larger substituent, and not the other, methyl group, gives rise to the base peak in the spectrum. Examination of competing metastable transitions shows that generally this is also the lowest activation energy primary process. In general primary ring cleavage reactions are not important unless there is a heteroatom in the substituent that can assist this cleavage.     

Total synthesis of thiostrepton. Retrosynthetic analysis and construction of key building blocks

The first phase of the total synthesis of thiostrepton (1), a highly complex thiopeptide antibiotic, is described. After a brief introduction to the target molecule and its structural motifs, it is shown that retrosynthetic analysis of thiostrepton reveals compounds 23, 24, 26, 28, and 29 as potential key building blocks for the projected total synthesis. Concise and stereoselective constructions of all these intermediates are then described. The synthesis of the dehydropiperidine core 28 was based on a biosynthetically inspired aza-Diels-Alder dimerization of an appropriate azadiene system, an approach that was initially plagued with several problems which were, however, resolved satisfactorily by systematic investigations. The quinaldic acid fragment 24 and the thiazoline-thiazole segment 26 were synthesized by a series of reactions that included asymmetric and other stereoselective processes. The dehydroalanine tail precursor 23 and the alanine equivalent 29 were also prepared from the appropriate amino acids. Finally, a method was developed for the direct coupling of the labile dehydropiperidine key building block 28 to the more advanced and stable peptide intermediate 27 through capture with the highly reactive alanine equivalent 67 under conditions that avoided the initially encountered destructive ring contraction process.     

Synthesis of Fmoc-protected trans-4-methylproline

Fmoc-protected trans-4-methylproline was synthesized starting from D-serine. The chiral scaffold of serine in the form of olefinated Garner's aldehyde 3 was used to control the diastereoselective formation of the new stereocenter on the hydrogenation of allylic alcohol 4. The diastereoselectivity (syn/anti ratio) of the process was 86:14, attained with Raney nickel. Hydrogen migration seems not to be the sole factor lowering the diastereoselectivity, as nickel is known not to promote double-bond migration. Instead, the moderate stereocontrol is attributed to the mobility of the side chain of 4, which allows the attack of hydrogen on both faces of the olefin (open transition state). A series of transformations led to ring precursor 8, which after recrystallization afforded the syn diastereoisomer in dr = 95:5. Protected trans-4-methylproline 11 was obtained from 8 in a straightforward fashion.     

Computer-assisted identification of multitrace electrophoretic patterns in differential display experiments

Modern multicapillary devices allow researchers to address increasingly complex biological questions involving comparisons of gene expression patterns across electrophoretic samples under various experimental conditions. As labor-intensive visual evaluation of the electrophoretic results is often the bottleneck of large-scale differential display (DD) studies, one way to further streamline this process is to focus only on a highly compressed list of the most potential patterns that are likely to provide reliable findings. To enable the identification of such candidate patterns, we present a computer-assisted method for objective ranking of multitrace peak patterns in DD experiments. The fundamental component of the multitrace pattern ranking method (MRANK) is the multiple alignment algorithm that allows for discovery of patterns involving sets of peak complexes from various electrophoretic samples. A score value is attached to each detected pattern which characterizes how accurately the pattern resembles the desired pattern query, freely defined by the researcher. The ranked pattern list produced by MRANK is validated against visual evaluation in terms of detecting and ranking a group of relevant patterns in a DD analysis of T-helper cell differentiation. We demonstrate high enrichment of the desired patterns on top of the score-ranked list (e.g., 90% of the visually selected patterns are discovered by looking through the first 3% of patterns in the ranked list of all patterns). The results suggest that a substantial amount of manual labor can be saved without compromising the accuracy of the findings by prioritizing the patterns according to MRANK output in the visual confirmation phase.