Procedures for selective installation of acyl, silyl ether and para-methoxybenzyl (PMB) ether groups to glycoside substrates have been developed, employing phenylboronic esters as protected intermediates. The sequence of boronic ester formation, functionalization and deprotection can be accomplished with only a single purification step, and the boronic acid component can be recovered and reused after deprotection. Key advances include the identification of reaction conditions for installation of PMB groups in the presence of boronic esters, and the use of the 'phase switching' protocol developed by Hall and co-workers as an efficient method for boronic ester cleavage. The relatively mild conditions for boronate deprotection are tolerant of several functional groups, including esters, silyl ethers, ketals and thioglycosides.

Processes for site-selective, sequential functionalizations of carbohydrate derivatives are described. In these processes, a tricoordinate boronic ester initially serves as a protective group for a sugar-derived 1,2- or 1,3-diol motif, permitting functionalization of free OH groups. In a second step, addition of a Lewis base generates a tetracoordinate adduct, which serves as an activating group, enabling functionalization of one of the boron-bound oxygen atoms by a second electrophile. By combining an initial acylation, alkylation, or glycosylation step with an amine-mediated glycosylation of the boronic ester, a variety of selectively protected di- and trisaccharide derivatives can be accessed in an operationally simple fashion without purification of intermediates. This Lewis base-triggered switching of behavior from "latent" to "active" nucleophile is a unique feature of boronic esters relative to other protective groups for diol moieties in carbohydrate chemistry.

Cyclopropylamines are prevalent motifs in pharmaceuticals and agrochemicals. Herein, we report the synthesis of trans-2-substituted-cyclopropylamines in high diastereoselectivity from readily available α-chloroaldehydes. The reaction proceeds via trapping of an electrophilic zinc homoenolate with an amine followed by ring-closure to generate the cyclopropylamine product. We have also observed that cis/trans-isomerization of the cyclopropylamine can occur in the presence of zinc halide salts and that this process can be turned off by the addition of a polar aprotic co-solvent to the reaction. File list (2) download file view on ChemRxiv Rousseaux-Cyclopropylamines-Manuscript ChemRxiv... (392.68 KiB) download file view on ChemRxiv Rousseaux-Cyclopropylamines-SI ChemRxiv.pdf (3.16 MiB)