Chiral compounds synthetic utility

The mechanism of 1,3-dipolar cycloaddition can be found in Ref. 63 and the references within. The reaction of nitrone with 1,2-disubstituted alkenes creates three contiguous asymmetric centers, in which the geometric relationship of the substituents of alkenes is retained. The synthetic utility of nitrone adducts is mainly due to their conversion into various important compounds. For instance, P-amino alcohols can be obtained from isoxazolidines by reduction with H2-Pd or Raney Ni with retention of configuration at the chiral center (Eq. 8.44). 

The synthetic utility of chiral epoxy alcohol synthons produced by the Sharpless asymmetric epoxidation has been demonstrated in enantioselective syntheses of many important compounds. 

Alkylation of a-amino esters with 9-bromo-9-phenylf uorene serves as the principal step in the preparation of N-(9-phenylfluoren-9-yl)-a-amino carbonyl compounds which are useful chiral educts for asymmetric synthesis. A discussion of the synthetic utility of N-9-phenylfluoren-9-yl derivatives of amino adds and amino acid esters appears in the procedure following. 

In summary, the chemistry of the donor/acceptor-substituted carbenoids represents a new avenue of research for metal-catalyzed decomposition of diazo compounds. The resulting carbenoids are more chemoselective than the conventional carbenoids, which allows reactions to be achieved that were previously inaccessible. The discovery of pan-tolactone as an effective chiral auxiliary, and rhodium prolinates as exceptional chiral catalysts for this class of rhodium-carbenoid intermediate, broadens the synthetic utility of this chemistry. The successful development of the asymmetric intermolecular C-H activation process underscores the potential of this class of carbenoids for organic synthesis. 

Enzymes as chiral catalysts play a role in all three methods. In nature enzymes catalyse all production of chiral compounds. In the laboratory enzymes can catalyse asymmetric synthesis, as well as resolve racemates. Which of the three methods is chosen in different cases depends on several factors, like price of starting materials, number of synthetic steps, available production technology and know-how etc. There is at present a constant ongoing development of synthetic methods and biotransformation is one field. Utilization of method i) requires knowledge of classical organic synthesis, enzymes have already played their role. Enzymes may play a part both in asymmetric synthesis and resolution. 

The high synthetic utility of alcohols 38 stems from the fact that terminal alkynes are among the most versatile functional groups for the further elaboration of a carbon skeleton. Asymmetric synthesis of alcohols 38 from aldehydes with the concurrent formation of the two stereogenic C atoms has been accomplished mainly by two methods. The first features synthesis of chiral nonracemic allenylmetal compounds from the corresponding chiral nonracemic propargyl alcohols and addition of the former to aldehydes [26] and the second method in-... 

The next example makes more involved use of these [2,3]-sigmatropic allylic sulfoxide-allylic alcohol rearrangements. It comes from the work of Evans (he of the chiral auxiliary) who, in the early 1970s, first demonstrated the synthetic utility of allylic sulfoxides. Here he is using this chemistry to make precursors of the prostaglandins, a family of compounds that modulate hormone activity within the body. 

The synthetic utility of this procedure was illustrated by the practical synthesis of the versatile intermediate y-lactone (i )-5-methyltetrahydrofuran-2-one (Fransson et al, 2005). Chiral y-lactones are important structural syn-thons for the synthesis of natural products and biologically active compounds (Benincori et al, 2004). 

The synthetic utility of (5S)- and (5R)-menthyloxy-2(5H)-furanones has been extensively explored and is well documented. The ready availability of these enantiomerically pure butenolides via the singlet photooxygenation of furfuraF and their wide range of reactivity contribute to their popularity as chiral building blocks. The present procedure for the preparation of the title compound and the useful epimerization step are modifications of methods originally reported by Feringa and co-workers. ... 

Optically active organometallic compounds, especially pseudotetrahedral half-sandwich complexes of the type Cp(OC)(Ph3P)Fe-R with iron as the center of chirality have been extensively investigated in the past. However, synthetic utilization of stereocontrol by the chiral iron has been limited to systems with a-bonded carbon ligands [2]. In contrast, silicon-iron complexes have not yet found analogous application. In context with our studies concerning metallo-silanols we have established simple routes to isolate diastereomerically pure derivatives with a chiral iron fragment. 

Topics which have formed the subjects of reviews this year include theoretical studies of the photochemistry of thiiranes, photoaddition of amines to aryl olefins and arenes, the synthesis of heterocyclic compounds, photoamination directed towards the synthesis of heterocycles, selective addition of organic dichalcogenides to carbon-carbon unsaturated bonds, photocyclisation mechanisms of c/5-stilbene analogues, synthetic utility of the photocyclisation of aryl-and heteroarylpropenoic acids, photochromic diarylethenes, spiropyrans, cy-clophanes, and polycondensed aromatics," photochromic organic media, photophysics and photochemistry of P-carbolines, and the photochemical synthesis of macrocycles.Chirality switching by light has also been described. ... 

Utilization of enzymes in organic synthesis to prepare chiral compounds of synthetic value is well documented.For instance, porcine pancreatic hpase (PPL, E.C. 3.1.1.3.) which is an inexpensive commercially available enzyme, specifically catalyzes the hydrolysis of esters of racemic alcohols and wc50-diols. Thus, on a preparative scale (0.25 mol) dimethyl 2-methyl-butanedioate, on treatment with PPL in buffered water at pH 7.2, underwent regio- and en-antioselective hydrolysis. Extraction with diethyl ether gave the unhydrolyzed dimethyl (/ )-2-methylbutanedioate (93%), while acidification of the aqueous phase provided the (5)-half ester,which on esterification with methanol (acidified by thionyl chloride) gave the dimethyl (5 )-2-methylbutanedioate (76%). 

Enantiomerically pure homoallylic amines are very important chiral building blocks for the synthesis of pharmacologically important molecules and natural products. The enantioselective synthesis of these compounds initially involved the chiral auxiUary-based asymmetric allylation of imines [41a, 4lb, 41c], and it is just recently that a few enantioselective variants have been reported. Although still in the regime of stoichiometric asymmetric synthesis, the first methods described below merit discussion for their synthetic utility and for establishing the groundwork for future development. 

Davis oxaziridine reagents such as 1 have exhibited ample synthetic utility as oxidizing agents for the hydroxylation of enolates to provide a-hydroxy carbonyl compounds, such as 2 with superb yield. When the oxaziridine is chiral and nonracemic, the hydroxylation has been shown to proceed with high stereoselectivity.1... 

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