Alkylphenyl ketones

Kumada et al. (32) had shown earlier that these same catalysts add methyldichlorosilane to alkylphenyl ketones to give optically active al-kylphenyl carbinols. 

Recently, however, examples of intramolecular triplet sensitization have been described. Alkyl aryl ketones with pendant azido groups in the alkyl moiety were irradiated into the benzoyl chromophore. The excited alkylphenyl ketones undergo intersystem crossing to their triplet states within picoseconds and they can then transfer their triplet energy to the nearby alkyl azido group. This process... 

Figure 4.58 Dissociation of alkylphenyl ketones following intramolecular hydrogen atom abstraction...

Synthese der homologen Reihe der Alkylphenyl-ketone und der d,l-Alkyl-... 

For some time the synthetic potential of this reaction as a source of Diels-Alder adducts underwent considerable study. One outcome of these studies was the realization that of the four possible o-xylylenol isomers, only ones with the enolic OH group pointed out (the F-photoenol ) reacted with dienophiles [23]. Mechanistic studies picked up in the 1970s, after Matsuura and Kitaura reported that, in the absence of dienophiles, benzocyclobutenols are formed from 2,6-dialkylphenyl ketones but not from simple o-alkylphenyl ketones [24]. Previously the absence of cyclobutenol products had been quite puzzling and led to suggestions that they were formed from the initial biradical but underwent rapid electrocyclic opening to the o-xylylenols. 

Finally, the o-xylylenols formed from o-alkylphenyl ketones may undergo photo-induced electrocyclic reactions, one example having been discovered recently [64]. 

The cyanohydrin synthesis of a-hydroxy acids is very often carried out without isolation or purification of the cyanohydrins. The various techniques for the preparation of the cyanohydrins are discussed elsewhere (method 390). Hydrolysis to the a-hydroxy acids is usually effected by heating with concentrated hydrochloric acid. Excellent directions are given for mandelic acid (52% over-all from benzaldehyde), a-methyl-a-hydroxybutyric acid (65% from methyl ethyl ketone), and eighteen dialkyl- and alkylphenyl-glycolic acids (60-80%). Sodium hydroxide solution is used in the preparation of /S-hydroxypropionic acid from the /S-hydroxy nitrile (80%). ... 

A case in point is the photoenolization reaction of o-alkylphenyl ketones (Section 6.3.6) and related compounds. Detailed studies of the temperature dependence of both the primary photoreaction386 3X8 and of the back transfer reaction in the ground state389 have provided convincing evidence for tunnelling contributions in these reactions. 

Table 2 Retention indices as constants of some organic compounds in reversed phase HPLC with -alkylphenyl ketones as reference compounds.

Using a similar system, Johnson et al. have been able to effect the overall result of the Wittig reaction but using sulphur ylides. Thus, an alkylphenyl sulphoxide was converted into the sulphoximine with sodium azide, the imine group was methylated with formic acid-formaldehyde, and the N-methylimine was converted into the ylide (157) with n-butyl-lithium in THF. The ylide reacted with aldehydes or ketones by adding to the carbonyl group to produce the hydroxy sulphoximine (158). The sulphur group could... 

SCHEME 7 Intermediates and alcohol products formed from o-alkylphenyl ketones. 

Eventually it was found that many o-alkylphenyl ketones not only undergo photocyclization to benzocyclobutenols but do so stereoselectively a few early examples are shown in Structure 40. Acids quench the photocyclization, indicating that the o-xylylenols are the precursors to the cyclobutenols and form them by thermal electrocyclization. The reason for their not having been discovered earher is their thermal instability above 60 to 80°C. 

As already mentioned, some byproducts are formed from these o-alkoxyphenyl ketones, one of which resembles the rearrangement that the o-t-alkylphenyl ketones undergo. The scheme below depicts these byproducts for o-ethoxy ketones the benzophenone undergoes only cyclization, whereas the acetophenone rearranges to a hemiketal and to a vinyloxybenzyl alcohol. 

As described earlier, o-alkylphenyl ketones undergo 7-hydrogen abstraction, and the o-xylylenols so formed are highly reactive towards dienophiles. Quite a few tricyclic structures have been synthesized by such photoinduced intramolecular Diels-Alder reactions. Below is an example provided by Kraus and... 

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