Neighboring-Group Participation Involving the Oxygen Atom of the O,O- or O,N-acetal Functional Groups

The application of the neighboring-group participation involving the oxygen atom of O,O- or O,N-acetals can be very fruitful. For instance, the naphthoate ester derivatives of (Z)-hex-3-ene-1,5-diyne were used to generate biradicals via -oxo ketene acetal intermediates, and a synthesis of tricyclic 9-crown-3 ethers bearing a chiral oxathiane ring was achieved by utilizing nucleophilic displacement of a triflic ester leaving-group assisted by neighbouring-group participation of a 1,3-dioxolane function. In a different field, the reaction between o-(hydroxymethyl)phenoxyacetaldehyde dimethyl acetals, or (/-)-3-methoxy-2,3-dihydro-5H-1,4-benzodioxepins with 5-fluorouracil was studied. The intramolecular cyclization may be explained through a neighbouring-group attack to give a 2-(5- fluorouracil-1-yl)oxyranium ion that can be attacked by the silylated benzylic hydroxyl group to yield the benzannelated sevenmembered O,N-acetals. Before carrying out the synthesis of 7- or 9-substituted (/-)-2-methoxy-2,3-dihydro-5H-1,4-benzodioxepins it is necessary to protect the phenolic hydroxy group of the 2-hydroxybenzyl alcohol. Among other functionalities, the 2- methoxyethoxylmethyl (MEM) group was developed as a protective group of alcohols and phenols. Accordingly, it was decided to use the MEM group for the preparation of 7- or 9-substituted (/-)-2-methoxy-2,3-dihydro-5H-1,4-benzodioxepins and their ten-membered benzo-fused analogs. The importance of the ten-membered O,O-acetals sheds light on the mechanism of reaction in which the neighbouring- group participation plays a pivotal role. Transannular versions of the reaction result in the facile ring contraction of 12-membered intermediates to the 10- and to 7-membered benzene-fused O,O-acetals. Acetals with several oxygen atoms in their side-chains have been used in the preparation of highly oxygenated cyclic ether compounds.