Intercepting the Gold-Catalysed Meyer-Schuster Rearrangement by Controlled Protodemetallation: A Regioselective Hydration of Propargylic Alcohols.

The regioselective gold-catalysed hydration of propargylic alcohols to ß-hydroxy ketones can be achieved by diverting the gold-catalysed Meyer-Schuster rearrangement through the addition of a protic additive with a pKa of 7-9 such as p-nitrophenol, boric acid or a boronic acid. This provides an interesting alternative to an aldol reaction when combined with the straightforward addition of an alkyne to an aldehyde or ketone. The gold-catalysed reaction of an electron-deficient, sterically hindered propargylic alcohol with a boronic acid led to the formation of an unusually stable cyclic boron enolate.

Gold- and silver-catalyzed reactions of propargylic alcohols in the presence of protic additives.

A wide range of primary, secondary and tertiary propargylic alcohols undergo a Meyer-Schuster rearrangement to give enones at room temperature in the presence of a gold(I) catalyst and small quantities of MeOH or 4-methoxyphenylboronic acid. The syntheses of the enone natural products isoegomaketone and daphenone were achieved using this reaction as the key step. The rearrangement of primary propargylic alcohols can readily be combined in a one-pot procedure with the addition of a nucleophile to the resulting terminal enone, to give ß-aryl, ß-alkoxy, ß-amino or ß-sulfido ketones. Propargylic alcohols bearing an adjacent electron-rich aryl group can also undergo silver-catalyzed substitution of the alcohol with oxygen, nitrogen and carbon nucleophiles. This latter reaction was initially observed with a batch of gold catalyst that was probably contaminated with small quantities of silver salt.

A general procedure for the synthesis of enones via gold-catalyzed Meyer-Schuster rearrangement of propargylic alcohols at room temperature.

Meyer-Schuster rearrangements of propargylic alcohols take place readily at room temperature in toluene with 1-2 mol % PPh(3)AuNTf(2), in the presence of 0.2 equiv of 4-methoxyphenylboronic acid or 1 equiv of methanol. Good to excellent yields of enones can be obtained from secondary and tertiary alcohols, with high selectivity for the E-alkene in most cases. A one-pot procedure for the conversion of primary propargylic alcohols into ß-arylketones was also developed, via Meyer-Schuster rearrangement followed by Pd-catalayzed addition of a boronic acid.

Gold catalysed synthesis of 3-alkoxyfurans at room temperature.

Synthetically important 3-alkoxyfurans can be prepared efficiently via treatment of acetal-containing propargylic alcohols (obtained from the addition of 3,3-diethoxypropyne to aldehydes) with 2 mol% gold catalyst in an alcohol solvent at room temperature. The resulting furans show useful reactivity in a variety of subsequent transformations.