Polyunsaturated systems

Boronate esters have been used for the preparation of polyunsaturated systems such as retinoic acid esters. 

Diimide selectively reduces terminal over internal double bonds in polyunsaturated systems.25 There are several methods for generation of diimide and they are illustrated in Scheme 5.3. 

Coming now to conjugation, results like those due to Malrieu et al. play an important role in the discussion of polyunsaturated systems. They offer the theoretical background for the well-known thermochemical stabilization of a chemical bond due to conjugation. This stabilization is not reflected in the aa qk + F part of... 

The addition of hydrogen across multiple bonds is one of the most widely studied of catalytic reactions. Alkenes and alkynes, as well as di- and polyunsaturated systems can all be hydrogenated, provided the suitable experimental conditions are used. Studies on the ways in which these compounds react with hydrogen have revealed very complex reaction patterns. Because of their resonance stabilization, carbocyc-lic aromatic hydrocarbons are more difficult to hydrogenate than are other unsaturated compounds. 

Consequently some of the predicted compounds could not be detected while others were formed which were not expected from direct alkoxy radical cleavage. Furthermore, the amounts of volatiles present varied markedly with heating time. It is not surprising, therefore, that at high temperatures, and particularly with polyunsaturated systems, it has been extremely difficult to establish the nature and extent of preferential hydroperoxide scission. 

Tlie cross-coupling of a terminal alkyne 9 with a 1 -bromoalkync 8 in the presence of an aliphatic amine and a catalytic amount of a Cu(I) salt affords unsymmetrically substituted diynes [10, Eq.(5)]. This useful reaction, discovered by Cadiot and Chodkiewicz [8], can be employed advantageously for the synthesis of several polyunsaturated systems. Generally the bromoalkyne is introduced dropwise to a mixture of the alkyne, ethylamine, and MeOH or EtOH in the presence of a catalytic amount of CuCl, and a small amount of NH OH-HCl. The reducing agent, NHjOH-HCl, is used to reduce the copper(TI) ion. The alkynylcopper(I) is assumed to be the reactive intermediate. The formation of the symmetrical diyne can be suppressed by maintaining the concentration of the bromoalkyne. This side reaction is particularly significant in the case of less acidic alkynes such as alkylalkynes [9J. 

The addition of organometals to non-activated carbon-carbon double or triple bonds, although a breakthrough in polymer chemistry about 70 years ago, has now become an efficient way of creating linear or cyclic substituted structures. The new organometallic thus formed can be quenched by various electrophiles, or can be used, in the case of polyunsaturated systems, to create polycycles via cascade reactions. 

The heterocoupling of a terminal alkyne with a 1-bromoalkyne in the presence of an aliphatic amine and a catalytic amount of a copper(I) salt affords unsymmetrically substituted diynes (2 equation 5). This useful reaction, discovered by Chodkiewicz and Cadiot, can be employed advantageously for the synthesis of several polyunsaturated systems. 

Cyclic vinylmagnesium halides react similarly, but in lower yield Isomerism around the double bond (E-Z) may be partly lost in the formation of the reagent once formed, they are configurationally stable. Occasionally, rearrangements are encountered in polyunsaturated systems, e.g. ... 

Intramolecular insertion has been observed in reactions of homoallylic diazoacetates. With acetylenes, reaction can either consist of an insertion into the C—C bond to the acetylene" or an addition reaction forming a cyclopropene, which spontaneously rear-ranges to a vinyl carbene intermediate (284). In a polyunsaturated system, this carbene can undergo further addition". ... 

The behavior of polyunsaturated systems such as ot,p,y,8-unsaturated ketones as electrophiles in the Michael reaction under PTC conditions has been the subject of a thoughtful study (Scheme 5.13). The conjugate addition of p-ketoesters to this kind of particular conjugated systems has been found to occur with complete 1,6-selectivity, affording a final p,y-unsaturated adduct, which in some exceptional cases can isomerize to the more stable conjugated... 

The light-sensitive compound in rods is called rhodopsin. In 1952, Nobel Laureate George Wald (Harvard University) and his co-workers showed that the chromophore in rhodopsin is the conjugated polyunsaturated system of 11-cw-retinal. Rhodopsin is produced by a chemical reaction between 11 -ch-retinal and a protein called opsin. 

Cycloadditions Several metals are known to trigger stereoselective [2-I-2-I-2] cycloaddition of polyunsaturated systems [3] and this approach has been applied to different types of unsaturated substrates. In this general overview, most cited examples will focus on cobalt, nickel, and rhodium catalysis. 

Figure 2.62. Some long-range couplings in polyunsaturated systems.

Schreiner, P.R., Navarro-Vazques, A. and Prall, M. (2005) Computational studies on the cyclizations of enediynes, enyne-allenes, and related polyunsaturated systems. Accounts of Chemical Research, 38(1), 19-i7. 

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