Chains---Examples of Synthesis

Fig. 36 Example of synthesis of an imino sugars by insertion of the nitrogen on the sugar chain and subsequent cyclization at the anomeric centre.

In this chapter benzisothiazolones and benzisothiazolone S,S-dioxides are separately treated. According to the large amount of literature already available on the second group of compounds (saccharin derivatives), most of all for their interest as sweetener, here only some examples of synthesis or reactivity that appear particularly appealing are considered. Regarding the benzisothiazolones, not oxidised at the sulphur atom, several synthetic methods were described and substitutions with different alkyl or heteroalkyl chain at the nitrogen atom considered. In many cases competitive N- and 0-alkylation was observed. 3-Oxoisothiazolo[5,4-fo]pyridines were also considered. 

Many examples of stereospecific allylation consistent with the above mechanism have been reported. As one example, the regioselective and highly diastereoselective allylation of the lactone 17 with the optically active allylic phosphate 16 proceeded with no appreciable racemization of the allylic part to give the lactones l8 and 19, and the reaction has been used for the synthesis of a polypropionate chain[26]. 

One of the virtues of the Fischer indole synthesis is that it can frequently be used to prepare indoles having functionalized substituents. This versatility extends beyond the range of very stable substituents such as alkoxy and halogens and includes esters, amides and hydroxy substituents. Table 7.3 gives some examples. These include cases of introduction of 3-acetic acid, 3-acetamide, 3-(2-aminoethyl)- and 3-(2-hydroxyethyl)- side-chains, all of which are of special importance in the preparation of biologically active indole derivatives. Entry 11 is an efficient synthesis of the non-steroidal anti-inflammatory drug indomethacin. A noteworthy feature of the reaction is the... 

You will recognize the side chain oxidation of p xylene to terephthahc acid as a reaction type discussed previously (Section 11 13) Examples of other reactions encoun tered earlier that can be applied to the synthesis of carboxylic acids are collected m Table 19 4... 

It is possible to prepare very heavy elements in thermonuclear explosions, owing to the very intense, although brief (order of a microsecond), neutron flux furnished by the explosion (3,13). Einsteinium and fermium were first produced in this way they were discovered in the fallout materials from the first thermonuclear explosion (the "Mike" shot) staged in the Pacific in November 1952. It is possible that elements having atomic numbers greater than 100 would have been found had the debris been examined very soon after the explosion. The preparative process involved is multiple neutron capture in the uranium in the device, which is followed by a sequence of beta decays. Eor example, the synthesis of EM in the Mike explosion was via the production of from followed by a long chain of short-Hved beta decays,... 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

As an example of the chemical signihcance of the process technology, the products of die Fischer-Tropsch synthesis, in which a signihcant amount of gas phase polymerization occurs vary markedly from hxed bed operation to the fluidized bed. The hxed bed product contains a higher proportion of straight chain hydrocarbons, and the huidized bed produces a larger proportion of branched chain compounds. 

A detailed discussion of individual halides is given under the chemistry of each particular element. This section deals with more general aspects of the halides as a class of compound and will consider, in turn, general preparative routes, structure and bonding. For reasons outlined on p. 805, fluorides tend to differ from the other halides either in their method of synthesis, their structure or their bond-type. For example, the fluoride ion is the smallest and least polarizable of all anions and fluorides frequently adopt 3D ionic structures typical of oxides. By contrast, chlorides, bromides and iodides are larger and more polarizable and frequently adopt mutually similar layer-lattices or chain structures (cf. sulfides). Numerous examples of this dichotomy can be found in other chapters and in several general references.Because of this it is convenient to discuss fluorides as a group first, and then the other halides. 

Fischer s original method for conversion of the nitrile into an aldehyde involved hydrolysis to a carboxylic acid, ring closure to a cyclic ester (lactone), and subsequent reduction. A modern improvement is to reduce the nitrile over a palladium catalyst, yielding an imine intermediate that is hydrolyzed to an aldehyde. Note that the cyanohydrin is formed as a mixture of stereoisomers at the new chirality center, so two new aldoses, differing only in their stereochemistry at C2, Tesult from Kiliani-Fischer synthesis. Chain extension of D-arabinose, for example, yields a mixture of D-glucose and o-mannose. 

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