Synthetic routes, devising

Methyl jasmonate (218) has a //arris-locked, 2,3-disubstituted cyclopentanone structure.297 The synthetic route devised by Sisido and co-workers, outlined in Scheme 34, is direct and requires no isomer separation.298) Reaction of the pyrrolidine enamine of methyl 2-oxycyclopent-l-yl acetate (214) with 3-bromo-2-penta-none in toluene gave 215 after hydrolysis. An intramolecular aldol condensation afforded 216 (note double bond migration) whose epoxidation followed by treat-... 

Crystal syntheses do not differ, in their essence, from classical chemical experiments in which molecules are modelled, synthetic routes devised, products characterized and their properties evaluated. However, this typical chemical approach needs, in a sense, to be repeated twice first, in order to prepare the building blocks (whether molecules or ions), and then to arrange the building blocks in a desired way via nucleation, precipitation and crystallization to attain and/or control crystal properties. Obviously, crystallization invariably implies the need to characterize the solid product, often in the form of a polycrystalline powder, for which routine analytical and spectroscopic laboratory tools are much less useful than in the case of solution chemistry. 

Ai,A/-bis(hydroxymethyl) formamide [6921-98-8] (21), which in solution is in equiUbrium with the monomethylol derivative [13052-19-2] and formaldehyde. With ben2aldehyde in the presence of pyridine, formamide condenses to yield ben2yhdene bisformamide [14328-12-2]. Similar reactions occur with ketones, which, however, requite more drastic reaction conditions. Formamide is a valuable reagent in the synthesis of heterocycHc compounds. Synthetic routes to various types of compounds like imida2oles, oxa2oles, pyrimidines, tria2ines, xanthines, and even complex purine alkaloids, eg, theophylline [58-55-9] theobromine [83-67-0], and caffeine [58-08-2], have been devised (22). 

It is a common experience in synthetic chemistry that a truly optimal ordering of a synthetic route may not be possible in the planning stage, but may have to determined experimentally. The precise information necessary for the complete and unambiguous evaluation of each step in a possible synthesis is hardly ever available. Nonetheless it is clearly wise to try to optimize a synthetic plan on the basis of available information before the experimental approach begins. Such an effort may suggest certain preliminary or "model" experiments that can be helpful in the choice or refinement of a synthetic plan. It is also obviously desirable to devise and consider alternate or bypass paths for each problematic step of a synthetic sequence. 

It is clear that, if synthetic routes could be devised which would mechanistically hinder disproportionation, such compounds might be preparable. Although univalent compounds of the Group 2 metals have not yet been isolated, there is some evidence for the formation of Mg species during electrolysis with Mg electrodes. Thus H2 is evolved at the anode when an aqueous solution of NaCl is electrolysed and the amount of Mg lost from the anode corresponds to an oxidation state of 1.3. Similarly, when aqueous Na2S04 is electrolysed, the amount of H2 evolved corresponds to the oxidation by water of Mg ions having an average oxidation state of 1.4 ... 

X = alkyl, H, halogen, etc. They are usually colourless, crystalline compounds with mp in the range 0-100° for X = H and 50-200° for X = halogen. Synthetic routes, and factors affecting the stability of the adducts have already been discussed (p. 165 and p. 198). In cases where diborane undergoes unsymmetrical cleavage (e.g. with NH3) alternative routes must be devised ... 

Many variations on these synthetic routes have been devised and the field is still being actively developed. The reactions of NO coordinated to transition metals have been extensively reviewed. ... 

Similar considerations have been invaluable in devising synthetic routes to numerous other isomeric complexes of Pt" but. as can be seen in Hg. A, other considerations such as the relative stabilities of the different Pt-ligand bonds arc also involved. 

Some advances have been made in the Paal-Knorr synthesis of pyrroles by the condensation of primary amines with 1,4-dicarbonyl species. For instance, a new synthetic route to monosubstituted succinaldehydes allows for the facile preparation of 3-substituted pyrroles <96TL4099>. Additionally, a general method for the synthesis of 1-aminopyiroles has been devised by the condensation of commercially available 2,2,2-trichloroethyl- or 2-(tri-methylsilyl)ethylhydrazine with 1,4-dicarbonyl compounds <96JOCl 180>. A related route to such compounds involves the reaction of a-halohydrazones with p-dicarbonyl compounds <96H(43)1447>. Finally, hexamethyldisilazane (HMDS) can be utilized as the amine component in the Paal-Knorr synthesis in the presence of alumina, and this modification has been employed in the synthesis of tm azaprostacyclin analog <96S1336>. 

A large number of conducting polymers can be synthesised via the use of catalysts [28-30], but generally little control can be exercised over the morphology of the product and purification of the material obtained can be problematical. In recent years however, a number of alternative synthetic routes have been devised which involve soluble precursor polymers which can be more easily purified and cast onto substrates, with subsequent conversion (usually by heating) to the desired product, and... 

In terms of economical synthetic approaches to indoles, the synthesis of this heterocycle from anilines and trialkylammonium chlorides was effected in an aqueous medium (H20-dioxane) at 180°C in the presence of a catalytic amount of ruthenium(III) chloride hydrate and triphenylphosphine together with tin(II)chloride <00TL1811>. Muchowski devised a novel synthetic route to indole-4-carboxaldehydes and 4-acetylindoles 86 via hydrolytic cleavage of W-alkyl-5-aminoisoquinolinium salts 85 to homophthaldehyde derivatives upon heating in a two phase alkyl acetate-water system containing an excess of a 2 1 sodium bisulfite-sodium sulfite mixture <00JHC1293>. 

Nakagawa devised a concise synthetic route to physostigimine (169) where the key step involves the alkylative cyclization of 1,3-dimethylindole (167) with (Z)-aziridine catalyzed by Sc(OTf)3 and TMSC1 to give 168, which, in turn, can be converted into 169 . A similar asymmetric approach to this natural product was also developed by these authors via treatment of tryptophan carbamates with the Corey-Kim reagent so as to induce intramolecular cyclization to the pyrrolo-indole skeleton . 

To confirm this structure, we devised a synthetic route to CNS 2103 which has the virtue of being easily modified to give access to a variety of unnatural analogs of the spider neurotoxin as well (31). 

With your knowledge of organic reactions, functional groups and functional group interconversions, you are now in a position to apply this knowledge in devising a synthetic route from a given reactant to a final product. 

H II H 1 You must be able to devise a synthetic route, with no... 

An alternative strategy in devising a synthetic route to a target molecule is known as retrosynthesis. To find out more about this, visit www.brightredbooks.net... 

Chemical substitution, where chemists devise synthetic routes to make commercial production of the chemical economical— vanillin is one example. 

Chemical synthesis of labeled compounds suffers from some limitations and problems, though. One limitation concerns the amount and cost of the radioactive starting material. This factor necessitates devising synthetic routes to the desired compounds in which the radiolabel can be introduced near the end of the sequence of reactions, so as to secure as high an overall yield of labeled material as possible. At present, numerous labeled compounds are available commercially as starting materials for syntheses. Still, in planning a new synthetic route, it is necessary to consider its compatibility with the specific stalling material available. 

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