ChemComm
Cite this: Chem. Commun., 2012, 48, 6127–6129
COMMUNICATION
Unique biocatalytic resolution of racemic tetrahydroberberrubine via
kinetic glycosylation and enantio-selective sulfationw
ac
a
Hai-Xia Ge,z Jian Zhang,z Ying Dong,d Kai Cuib and Bo-Yang Yu*b
Received 25th March 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc32175k
In this communication, we document a facile kinetic glycosylation
resolution of racemic tetrahydroberberrubine. We also demonstrate
that the enantiomeric excess of the resolved products is increased via
a second resolution of the minor product of the first glycosylation
resolution. This provides a rare example of tandem kinetic resolution
of racemates.
We therefore hypothesized that Gliocladium deliquescens NRRL
1086 would be a good organism to selectively glycosylate alkaloids,
vide infra.
Using a whole cell approach, a near-perfect resolution of
tetrahydroberberrubine (ꢀ)ꢁ1 was achieved via a tandem
kinetic resolution (see Scheme 1). A tandem kinetic resolution
is expected to give almost enantiomerically pure compounds if
the first kinetic resolution step produces enantio-enriched
product and the second resolution is selective for the minor
enantiomer. This way, the second resolution depletes the minor
enantiomer (obtained during the first resolution) and thereby
increases the enantiomeric excess of the major product from the
first resolution step.
Glycosylations of both macromolecules and small molecules are
critical for cellular functions.1 Glycoconjugates play structural
roles in the cell, are involved in cell-to-cell recognition2 and
communication,3 and intra- and extra-cellular signalling.4
Glycosylated small molecules, such as heparin, amikacin, and
cytarabine, have been shown to be clinically useful for the
treatment of a variety of diseases, including bacterial and fungal
infections, cancer, and other human diseases.5 Consequently
there has been tremendous interest in the identification, synthesis
or manipulation of glycosylated small molecules for potential
biomedical applications. It has been demonstrated that for most
of the alkaloidal glycosides found in plants, in particular
benzyltetrahydroisoquinoline alkaloids, removal of the carbo-
hydrate units lead to aglycone units that are marginally active
or less specific,6 underscoring the importance of the glycosyl units
for biological activity. Consequently, methods (both chemical
and enzymatic) that make possible the facile introduction
of glycosyl units onto alkaloids in a regio-specific manner are
needed. Thus far, only a handful of strategies have been reported
for the regio-specific glycosylation of natural products.7 Herein
we describe, to the best of our knowledge, the first example of
kinetic resolution via glycosylation of racemic substrates, using a
whole cell approach (fermentation). Gliocladium deliquescens
NRRL 1086 has been shown to facilely glycosylate ruscogenin.8
Treating tetrahydroberberrubine (ꢀ)ꢁ1 with Gliocladium
deliquescens NRRL 1086 gave glycosylated products M1 and
M2 as colorless crystals in a 15 : 1 ratio, after silica gel
chromatographic separation. The identities of M1 and M2
were confirmed via HR-ESI-MS, 13C-NMR, 1H-NMR,
HMBC, HSQC and DEPT. These analyses revealed that M1
and M2 were both glycosylated products of 1. X-ray crystallo-
graphic analysis of M1 (Fig. 1) provided further evidence for the
assigned structure of M1 and M2. The absolute configurations of
M1 and M2 were assigned as 14S and 14R respectively. These
absolute configurations were assigned by comparing the CD of
M1 and M2 with those of 14S and 14R protoberberines.9 M1
exhibited negative Cotton effects around 210 nm (similar to 14S
protoberberine) while M2 showed positive Cotton effects around
210 nm (similar to that seen with 14R protoberberine).
With our aim to increase the enantiomeric excess of the
major isomer, M1, we proceeded to monitor the reaction
profile of the kinetic resolution of tetrahydroberberrubine
(ꢀ)ꢁ1 with Gliocladium deliquescens NRRL 1086 at different
times, using HPLC monitoring. Our aim was to identify a time
point whereby the ratio of M1 to M2 would be maximized.
Interestingly, HPLC analysis of the progress of the reaction
indicated that the S-isomer, M1, formed faster than the
R-isomer, M2. Interestingly, we also observed that within
the first 12 h of the reaction, the concentration of M2 initially
increased but then decreased. Curiously, the relative amount
of another product, denoted M3, increased as the reaction
time increased (see Fig. 2).
a State Key Laboratory of Natural Medicines, China Pharmaceutical
University, 24# Tong Jia Xiang St, Nanjing, 21009, P. R. China.
Fax: +86 25 86185158; Tel: +86 25 86185157
b Department of Complex Prescription of TCM, China
Pharmaceutical University, 639# Long Mian Avenue, Nanjing,
211198, P. R. China. E-mail: boyangyu59@163.com
c Department of chemistry, Huzhou Teachers College,
1# Xue Shi Road, Huzhou, Zhejiang Province, 313000, P. R. China
d Department of organic chemistry, China Pharmaceutical University,
639# Long Mian Avenue, Nanjing, 211198, P. R. China
w Electronic supplementary information (ESI) available: The details of
isolation and identification of metabolites, NMR spectra. CCDC
851213. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc32175k
Therefore, it appears that the enantiomeric excess of the
major enantiomer, M1, could be increased to over 99% by leaving
the reaction to go on for longer, contradicting the recommended
z These authors contributed equally to the work.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6127–6129 6127
Ge, Hai-Xia
