Alkylation and acylation at an oxygen atom

O-Alkylation of 5-hydroxychromones is usually difficult. A study of this reaction, leading to potentially useful pharmaceuticals, was undertaken comparatively by thermal, sonochemical, and PTC procedures (Fig. 21). 

Alkylation occurs with the same efficiency using sonication or PTC, both of these leading to results better than the thermal process. Aliquat was efficient as a PTC in the case of reactive halides (benzyl, allyl), but useless with 1- and 2-butyl bromide. In this case, 18-c-6 must be used, in which case the reaction is more efficient than the thermal or sonochemical counterparts. s 

Further results were described by Mason and Davidson.In the presence of PEGMe or TBAC, phenol imdergoes quantitative ethylation (Fig. 22). 

The protection of hydroxyl groups as methoxymethyl (MOM) ethers was achieved in a simple and convenient solvent-free reaction on alumina under sonication (Eq. 54).The reactions are generally reasonably fast and high-yielding and conditions are mild enough not to induce isomerization of double or triple bonds in allylic and propargylic alcohols. Alumina can be recycled after washing and reactivation. Without sonication, reactions were very slow. 

In the presence of a less frequently used base, silver oxide, the introduction of the 0-benzyl protecting group was described in a paper dedicated to the synthesis of a fragment of aplysiatoxins. Contrary to the silent reaction, the sonochemical reaction is reproducible and constitutes, in the authors opinion, the first example of selective monobenzylation of a 1,2-diol at the primary site (Eq. 55). 

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