Scalable Sn-Catalyzed Regioselective Allylation of 1-Methyl-l-α-rhamnopyranoside

Abstract

A robust selective allylation of 1-methyl-l-α-rhamnose was developed using di-n-butyltin oxide (n-Bu2SnO) as the catalyst. Proton sponge was found to be the optimal base for high regioselective control. The optimized condition afforded the 3-O-allylated rhamnose in excellent regioselectivity (>20:1) and 82% isolated yield on a 50 g scale. A scalable isolation/purification process was developed which afforded the desired product in 72% yield with 1530 ppm of Sn. The residual Sn level can be further reduced to <20 ppm by treatment with scavengers. This catalytic system also showed outstanding potential in the selective benzylation of the same substrate.

A highly enantioselective approach towards optically active γ-amino alcohols by tin-catalyzed kinetic resolution of 1,3-amino alcohols

“…Finally, based on the results stated above and previous reports, 12,16 a possible mechanism was proposed (Scheme 5). First, the chiral organotin catalyst C6 and 1,3-amino alcohol 1 bond covalently to form the diastereomers I-1 and I-2 .…”

Solvent-Free Approaches in Carbohydrate Synthetic Chemistry: Role of Catalysis in Reactivity and Selectivity

“…It is pertinent to specify at this stage that some tin-catalyzed approaches in solution have appeared after the described report on the solvent-free conditions, with considerable simplification over the traditional stepwise stoichiometric method [48][49][50][51][52]. It is also worth observing that solvent-free benzylation protocols described herein have already been incorporated into a variety of synthetic projects [53][54][55][56], to testify their practical scope.…”

Crystal structures of the flavonoid Oroxylin A and the regioisomers Negletein and Wogonin

“…Wogonin 3 was prepared starting from Chrysin, its parent flavone, following a synthetic route described previously by Li et al (2017). In our hands, the literature approach of crystallization of the monohydrate 3a by slow evaporation from EtOAc (Zhong et al, 2006) Table 1 Experimental details.…”

“…1) with alternative positions of substitution for hydroxy and methoxy substituents in the 6-, 7-and 8-positions of the flavone A-ring. They may either be found together in natural extracts, or microbiological transformation, or in synthetic routes to these potential pharmaceuticals (Huang et al, 2003;Kostrzewa-Susłow et al, 2007;Pham et al, 2012;Tan et al, 2015;Li et al, 2017;Fujita et al, 2018;Majeed et al, 2018). The similarities of the structures and compositions, especially in the case of Oroxylin A and Negletein, often result in confusion in the assignment of the structures of the isolated products (Huang et al, 2003) and only recently have their structural assignments been reliably determined (Panhekar et al, 2015;He et al, 2016;Hemantha et al, 2019).…”

“…However, its convenient and pure extraction is hampered by the presence of the regioisomer Oroxylin A, which possesses similar solubility. We have worked on both separation approaches from natural extracts and new routes to a more efficient synthesis of Wogonin since existing methods all have drawbacks (Huang et al, 2003;Li et al, 2017). The need for authentication of the regioisomer Oroxylin A, which is also attracting attention for medicinal use, led us to prepare a pure sample for comparison, as well as a further regioisomer, Negletein, which has in the past sometimes been confused with Oroxylin A (Huang et al, 2003).…”