2,6-Bis(trifluoromethyl)phenylboronic Esters as Protective Groups for Diols: A Protection/Deprotection Protocol for Use under Mild Conditions

Adjusting the water-sensitivity of sugar/boronate-based organogels

During the investigation of the water-sensitivity of (arylboronate alkylglucoside)-based organogels, we evaluated a series of twelve potential organogelators. They were synthesised in a single step from the corresponding arylboronic acids and alkylglucosides. Eight of them showed organogelation abilities in three solvents (toluene, cyclohexane, and ethyl myristate). Conformational minimisations of the potential organogelators permitted a clear relationship between the arylboronate orientation and the gelation effectiveness to be established. These gels were characterised by rheometry and SEM which revealed a gel-state originating from the self-assembly of the organogelators into long entangled fibres. SAXS confirmed the mode of packing in a hexagonal phase. Gels in toluene were found to be water-sensitive both after addition of a small amount of water and immersion into water. This study demonstrated that the main parameter impacting the water-sensitivity was the length of the alkyl chain at the anomeric position of the glucoside unit, much more than the functionalisation of an arylboronate moiety.

Boronic Acid-Catalyzed Regioselective Koenigs-Knorr-Type Glycosylation

Boronic acid-catalyzed regioselective Koenigs-Knorr-type glycosylation is presented. The reaction of an unprotected or partially protected glycosyl acceptor with a glycosyl halide donor in the presence of silver oxide and a low catalytic amount of imidazole-containing boronic acid was found to proceed smoothly, which enables construction of a 1,2-trans glycosidic linkage with high regioselectivities. This is the first example of the use of a boronic acid catalyst to initiate regioselective glycosylation via the activation of cis-vicinal diols in glycosyl acceptors.

Concise Syntheses of Violaceoids A and C

The concise syntheses of two alkylated hydroquinone natural products, violaceoids A and C, were accomplished by a protecting-group-free method employing the commercially available 2,5-dihydroxybenzaldehyde as the starting material. The key strategy of the syntheses is the utilization of alkenylboronic acid as both the coupling and temporary protective reagents to efficiently introduce the requisite alkenyl side chain of violaceoid A. Moreover, the synthesis of violaceoid C is reported here for the first time.

Forskolin Editing via Radical Iodo- and Hydroalkylation

The modification of highly oxygenated forskolin as well as manoyl and epi-manoyl oxide, two less functionalized model substrates sharing the same polycyclic skeleton, via intermolecular carbon-centered radical addition to the vinyl moiety has been investigated. Highly regio- and reasonably stereoselective iodine atom transfer radical addition (ATRA) reactions were developed. Unprotected forskolin afforded an unexpected cyclic ether derivative. Protection of the 1,3-diol as an acetonide led the formation of the iodine ATRA product. Interestingly, by changing the mode of initiation of the radical process, in situ protection of the forskolin 1,3-diol moiety as a cyclic boronic ester took place during the iodine ATRA process without disruption of the radical chain process. This very mild radical-mediated in situ protection of 1,3-diol is expected to be of interest for a broad range of radical and non-radical transformations. Finally, by using our recently developed tert-butyl­catechol-mediated hydroalkylation procedure, highly efficient preparation of forskolin derivatives bearing an extra ester or sulfone group was achieved.

The B & B approach: Ball-milling conjugation of dextran with phenylboronic acid (PBA)-functionalized BODIPY

Mechanochemistry is an emerging and reliable alternative to conventional solution (batch) synthesis of complex molecules under green and solvent-free conditions. In this regard, we report here on the conjugation of a dextran polysaccharide with a fluorescent probe, a phenylboronic acid (PBA)-functionalized boron dipyrromethene (BODIPY) applying the ball milling approach. The ball milling formation of boron esters between PBA BODIPY and dextran proved to be more efficient in terms of reaction time, amount of reactants, and labelling degree compared to the corresponding solution-based synthetic route. PBA-BODIPY dextran assembles into nanoparticles of around 200 nm by hydrophobic interactions. The resulting PBA-BODIPY dextran nanoparticles retain an apolar interior as proved by pyrene fluorescence, suitable for the encapsulation of hydrophobic drugs with high biocompatibility while remaining fluorescent.

Total syntheses of seminolipid and its analogues by using 2,6-bis(trifluoromethyl)phenylboronic acid as protective reagent

A concise total synthesis of seminolipid, a sulfoglycolipid, has been achieved; key features include regioselective, tin-free sulfation of allyl β-d-galactopyranoside using 2,6-bis(trifluoromethyl)phenylboronic acid as protective reagent, stereoselective epoxidation, and site-selective acylation. The utility of this divergent synthetic approach to introduce 2,2,2-trichloroethyl-protected sulfate group at an early stage without toxic and environmentally unfavorable tin reagents was demonstrated by the syntheses of three seminolipid analogues with different side-chains from the common intermediate.

Amine-tethered phenylboronic acid-enabling ring-opening strategy for carbon chain elongation from double aldol cyclic hemiacetals

The addition of carbon nucleophiles to cyclic hemiacetal forms of double aldols is a promising approach toward the synthesis of structurally attractive 1,3-polyol derivatives. Cyclic hemiacetals are generally unreactive to carbon nucleophiles under neutral conditions, however, because the electrophilic aldehyde function is masked. Here we developed an amine-tethered phenylboronic acid 7g, which transforms double aldol cyclic hemiacetals to ring-opened linear aldehydes. Combined with the previously-developed copper-catalysed asymmetric double aldol reaction (L. Lin, K. Yamamoto, H. Mitsunuma, Y. Kanzaki, S. Matsunaga and M. Kanai, J. Am. Chem. Soc., 2015, 137, 15418), this method produced synthetically useful chiral building blocks containing a 1,3-di- or tri-ol moiety.