Approaches to Synthesis

Most gas-phase reactions of interest, however, have a complex product distribution, can be sequential, and are to some extent catalyzed by many different catalysts examples are the partial oxidation reactions. For this class of reactions, a parallel testing would be much better than cocktail screening, as in this case promising catalysts could be directly identified. The drawback of parallel screening, however, is the more complex reactor design and the fact that a parallel analysis or, at least, a relatively fast sequential analysis is necessary. 

Using sputtering or evaporation techniques will normally result in materials which are quite different from most conventional catalysts in that they expose only their geometric surface area and usually will not have any porosity. Therefore, synthesis approaches have been developed which allow the preparation of materials which are closer to a conven- 

Schiith, Hoffmann, Wolf Schunk, Stichert and Brenner 

More demanding are hydrothermal reactions, for example to synthesize zeolites, though these can also be extensively parallelized. The first publication concerned a semiparallel zeolite synthesis procedure, where the gel was loaded into a relatively small Teflon vessel, of which 96 were arranged on one block [15]. As in most zeolite syntheses the hydrothermal treatment is the most time-consuming step, parallelizing this step immediately saves a considerable amount of time. The next important step was reported recently by W.F. Maier and co-workers, who used a spatially resolving X-ray diffractometer by AXS to analyze automatically a library of possible zeolites in which the individual members were present in only minute amounts, i.e., on the milligram scale [16]. 

In our own laboratory, a multipurpose synthesis system was developed which allows all types of impregnation techniques to be carried out, as well as ion exchange and precipita- 

---

This approach to synthesis is one of making a series of best local decisions. Equipment is added only if it can be justified economically on the basis of the information available, albeit an incomplete picture. This keeps the structure irreducible, and features which are technically or economically redundant are not included. 

The role of the 1,1-bielectrophile in ring closures of this type is to provide a one-carbon unit (or heteroatbm) to close the cycle. Thus, the synthesis of the four-atom precursor with two nucleophilic centers 1,4 to each other is an appreciable challenge, especially to obtain a heterocycle at the desired oxidation level. The examples below illustrate the way this approach to synthesis may be gainfully utilized. 

In this Section, 1 shall briefly exemplify some topics that illustrate how the needs of materials science and engineering have shaped chemists approaches to synthesis and processing. 

Umeda T, Harada T and Shiroko K (1979) A Thermodynamic Approach to Synthesis of Heat Integration Systems in Chemical Processes, Comp Chem Eng, 3 273. 

A Multivalued Logic Predicate Calculus Approach to Synthesis Pla ining... 

WIPKE AND DOLATA A Predicate Calculus Approach to Synthesis Planning 189... 

The great advantage of this approach to synthesis is, of course, speed. Making the 200 compounds in the library just discussed by traditional methods would take a very long time, considerable money, and much labor. In fact, costs and time constraints would probably make the process prohibitive by traditional techniques. In combinatorial chemistry, however, the 200 compounds in the library can be made all at once with relatively little cost and expenditure of time and money. The only problem (and a significant problem it is ) is to find out which of the 200 compounds (if any) meet some predetermined criterion, such as biological potency. 

In the spontaneous self assembly approach to synthesis, the inherent kinetic lability of the Fe/Mo/S system allows for the fcnmation of various thermodynamically stable, heteronuclear products that, as of now, do not include one with the desir le Fe/Mo/S atom ratio of 6-7 1 6-8. 

Khunt RC, Akbari JD, Manvar AT, Tala SD, Dhaduk MF, Joshi HS, Shah A (2008) Green chemistry approach to synthesis of some new trifluoromethyl containing tetrahydropyrimi-dines under solvent-free conditions. Arkivoc 277-U8... 

A summary of the fragment condensation approach to synthesis of the coiled-coil library is shown in Scheme 10. The library was synthesized in two fragments using Fmoc chemistry the C-terminal fragment (residues 11-27) housed the library positions (labeled X in Scheme 10) and the C-terminal lactam the N-terminal fragment (residues 1-10) contained the Cys-Gly-Gly linker and N-terminal lactam bridge. 

The most general approach to synthesis of five-membered heterocyclic compounds involves cycloaddition of a 1,3-dipole to an appropriate unsaturated substrate, the dipolarophile. Intermolecular cycloadditions result in the formation of one new ring only. When the 1,3-dipole and the substrate are part of the same molecule, cycloaddition is intramolecular and leads to a new bicyclic system. Thus, intramolecular... 

A much more successful approach to synthesis of poly(p-phenylene) from dihalo-benzenes is via the decomposition of the corresponding Grignard reagent ... 

Unfortunately, these six initially proposed compounds have so far resisted all logical approaches to synthesis, and are at present still unknown. What has been successfully achieved, the building up of a big bulky hydrocarbon glob at these positions, has rather unexpectedly led to a remarkable enhancement of potency. As with all true exploration into areas of the unknown, the deeper you get. the less you understand. 

A new strategy for the synthesis of erythromycin A and closely related mac-rolide antibiotics was elaborated in our laboratory (88). This new approach to synthesis is based on the knowledge that stereoelectronic effects control the conformation of acetals. The strategy is based on the 1,7-dioxaspiro-[5.5]undecane system which was found to be conformationally rigid, existing in conformation J50 only (see Chapter 2). This is so because in this confor-mation, steric effects are at their minimum and the acetal function has... 

The cleavage of dodecamethylcyclohexasilane by phosphorus penta-chloride in sym-tetrachloroethane (66), anhydrous hydrogen chloride or tert-butyl chloride (56) (see Section II, C, l,c) provides a convenient approach to synthesis of certain a,tu-dichloro permethylated polysilanes. 

An alternative approach to synthesis of this ring system involves insertion of CO into the brominated secondary base (43), by treatment with carbon monoxide, lead tetra-acetate, and triphenylphosphine in tributylamine, the product being the amide (44), reduction of which affords the amine.81 Govadine (42 R1 = R4 = OMe, R2 = R5 = OH, R3 = H) has been synthesized by the conventional ring-closure, with formaldehyde, of the diphenolic base (45), or of its dibenzyl ether, in acid solution.82 Tetrahydroberberines, together with N-benzyltetrahydroisoquinolines, have also been obtained by the electrolytic reduction of 3,4-dihydroisoquinolines of structure (46).83... 

---