Acetals are the most useful protecting groups for carbonyl functional groups. In addition to the role of protection, they can also be used as synthons of carbonyl functions. Previously, we developed a chemoselective deprotection and nucleophilic substitution of acetals from aldehydes in the presence of ketals. This article describes the highly discriminative and chemoselective transformations of acetals bearing different substitution patterns, different types of acetals, as well as mixed acetals. These reactions can achieve the transformations that cannot be attained by conventional methods, and their results strongly suggest the combination of RSiOTf/2,4,6-collidine to promote such unprecedented phenomena.

Herein, we report a ring-opening 1,3-arylboration of aryl cyclopropanes using BCl3 in the presence of arene nucleophiles.

The intermediates formed during the Beckmann fragmentation of α-alkoxy and α-alkoxy-α-alkyl oxime acetates have been successfully trapped as phosphonium salts, which were subsequently reacted with a variety of Grignard reagents to give the corresponding substituted products in good yields. Notably, this reaction proceeded smoothly even from α-alkoxy-α-alkyl oxime acetates.Key words Beckmann fragmentation; phosphonium salt intermediate; Grignard reagent; carbon-carbon bond formationWe recently found that the treatment of acetals with a combination of trifluoromethanesulfonic acid trialkylsilyl ester (R 3 SiOTf) and base (e.g., pyridinium-type bases or triarylphosphines) gave the corresponding electrophilic pyridinium or phosphonium salts as stable intermediates. We subsequently showed that these salts can be used as stable synthetic equivalents of oxonium ions from acetals, and went on to develop a series of acetal substitution reactions using a variety of different nucleophiles (Eq. 1).1-10) One of the main advantages of our new methods is that the nucleophiles can be added to the reaction mixture long after the formation of the intermediate salts. Furthermore, these methods are compatible with a wide variety of nucleophiles, including acid and base labile nucleophiles. It is noteworthy that acid labile nucleophiles cannot normally be used in the reactions of oxonium ions, because oxonium ions are usually formed under Lewis acidic conditions in the presence of a nucleophile, which would preclude the use of an acid labile nucleophile. 11) We subsequently investigated the application of this salt chemistry to another reaction involving sequential Beckmann fragmentation/carbon-carbon (C-C) bond forming reactions, which has been previously reported by our group using organoaluminum reagents.12-15) The treatment of α-alkoxy oxime acetates with trifluoromethanesulfonic acid trimethylsilyl ester (TMSOTf) and 2,4,6-collidine produced the corresponding collidinium salt intermediates, which were successfully subjected to a C-C bond forming reaction with a variety of different Gilman reagents (Eq. 2).16) Gilman reagents were found to be the best type of organometallic carbon nucleophile for this reaction, with several other more popular organometallic reagents, including organolithium and Grignard reagents, failing to provide good results. However, we previously found that the phosphonium salts derived from O,O-acetals and (o-tol) 3 P reacted smoothly with Grignard reagents. [8][9][10] We therefore envisioned that the formation of a phosphonium salt instead of a collidinium salt would make it possible to use Grignard reagents in this reaction, which would represent a significant improvement over our previous method in terms of its scope and convenience.

Migratory cycloisomerization using transition metal catalyst is useful for synthesizing substituted heterocyclic compounds. We achieved palladium-catalyzed migratory cycloisomerization of 3-o-alkynylphenoxy acrylic acid ester derivatives to give 2,3-disubstituted benzofurans. Although there are several reports of benzofuran synthesis with palladium-catalyzed migratory cycloisomerization, migratory groups are limited to allyl and propargyl groups. This report is the first example of benzofuran synthesis with palladium-catalyzed cycloisomerization of C(sp 2 )-O bond cleavage.