Room temperature intramolecular hydro-O-alkylation of aldehydes: sp3 C-H functionalization via a Lewis acid catalyzed tandem 1,5-hydride transfer/cyclization.

[reaction: see text] The scope and limitations of intramolecular hydro-O-alkylation of aldehyde substrates leading to spiroketals and bicyclic ketals and aminals is reported. The direct transformation of tertiary and sterically hindered secondary sp(3) C-H bonds into C-O bonds under the action of a catalytic amount of a variety of Lewis acids is described. The mechanism of these transformations is proposed to involve a tandem hydride transfer/cyclization sequence.