Acetylene groups

In addition to inorganic radicals, which profoundly modify the properties of a paraflSn hydrocarbon residue, there is a whole series of organic groupings which are distinguished by exceptional reactivity, for example, the ethylene and acetylene groupings, and the phenyl and naphthyl radicals. Thus the characterisation of unsaturated hydrocarbons and their derivatives, e.g., the aromatic compounds, becomes possible. 

Conjugation with olefinic or acetylenic groups lowers the frequency and raises the intensity. Conjugation with carbonyl groups usually has little effect on the position of absorption. 

Similarly, the acetylenic group, —C=C—, shows an intense absorption system at about... 

After reaction for 15 min, the band near 3295 cm (not shown in Fig. 13) decreased significantly in intensity, indicating that the mono-substituted acetylene groups were reacting. New bands also appeared near 1539 and 1512 cm . After reaction for 30 min, several additional bands appeared near 1011, 1030, 1085, 1232, 1320, 1430, and 1515 cm. The bands near 1011, 1030, 1085, 1232, 1320, and 1430 cm were clearly related to the benzothiazole sulfenamide fragment of DCBS while the band near 1539 cm was related to zinc stearate. 

Poly(phenylquinoxalines) containing acetylene groups in main chains of macromolecules 99MI28. 

There are slight differences in the reactivity of the acetylenic groups depending on their position in the pyrazole ring. This can be followed using py-razole diethynyl derivatives as an example. In the case of 4,5-diethynyl- and... 

In recent years, trimethylsilyl protection has often been used for the methine proton of the acetylenic group because of the mild reaction conditions for desilylation. As a rule, the starting pyrazole trimethylsilyl derivative is mixed up, at room temperature, with a 2 A aqueous solution of NaOH, potash, or methanol solution in ammonia. 

It is known that the ability of nitrotolane to cyclize depends on electronic factors (69MI2) hence l,3-dimethyl-4-nitro-5-phenylethynylpyrazole, whose acetylene group is in the most electron-accepting position of the pyrazole ring, i.e., favorable for nucleophilic addition, was introduced into the reaction of cyclization. Thus,... 

Nevertheless, the adjacent position of the amide and acetylenic groups was used in another type of heterocyclization. The nitrogen atom in the amide group is a weak nucleophile. Therefore, the N anion should be generated by potassium ethoxide. There are two possible variants of nucleophilic addition to the triple bond. Only one takes place, i.e., the formation of y-lactam. After 7 h of heating in EtOH in the presence of KOH, amide 72 isomerized into the known isoindoline 73 in 80% yield (Scheme 128). 

When additional substituents are introduced in the 2- and/or 4-position, the thermal rearrangement gives products in which the substituents that were originally located in the 1- and 5-position of the quadricyclane are then located at C4 and C5 of the oxepin 7.30,123 In order to trap intermediates of this rearrangement reaction by intramolecular cycloaddition vinyl and acetylene groups were linked with different spacer groups to C2 of quadricyclane.123 In this manner two different intramolecular cycloadducts were isolated in addition to oxepin derivatives.123... 

This procedure gives a product free of acetylenic groups. It illustrates the synthesis of an olefinic compound by cracking a cyclobutane into two fragments. 

Fig. 48. Core expansion using a pendant acetylene group.

The facility with which the transfer of acetylenic groups occurs is associated with the relative stability of the ip-hybridized carbon. This reaction is an alternative to the more common addition of magnesium or lithium salts of acetylides to aldehydes. 

The fact that the cyclization is directed toward an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide could lead to a more complex product mixture because the cyclized product could undergo iodine atom transfer and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive and the reaction product is unreactive. Owing to the potential... 

A vinyl substituent deshields both primary (CF2H) and secondary (CF2) groups by between 6 and 10 ppm. There are little available data on the impact of an acetylenic group, but it seems to have slightly... 

Bicyclization of 1 (n = 3 or 5) by this method is also successful, but yields are lower (60%, 27%). The reaction fails when the acetylene groups are separated by six methylene groups. 

The final steps in a total synthesis of (+ )-gloeosporone (3, a natural germination inhibitor of a fungus) required oxidation of the acetylene group of 1 to a diketone group. The oxidation was carried out in 74% yield by the catalytic Ru02 procedure of Sharpless. On liberation (pyridine-HF) of the C7-hydroxyl group, the hydroxy... 

FIGURE 4.75 Oxidation of the acetylenic group of ethinyl estradiol with rearrangement leading to ring expansion. 

There are not many drugs that are alkynes however, one good example is ethinyl estradiol (Fig. 4.6). Even though ethinyl estradiol is not an aryl alkyne, the acetylenic group is attached to a tertiary carbon and not adjacent to an a-carbon-hydrogen bond. 

Thus, it would be reasonable to expect that the acetylenic group would be metabolized to a carboxylic acid. While the acid metabolite has not been detected, ethinyl estradiol is a suicide substrate inhibitor of CYP2B6 consistent with a reactive ketene being formed as an intermediate (142). In addition, there is a rearrangement product (4) shown in Figure 4.75. 

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