Communications
nization and haloetherification reaction with established proto-
citrate buffer (pH 5) with 160 mm KBr and 100 nm CiVCPO. Reac-
[
2b]
tions were started by the addition of 100 mm of H O and stirred
cols
demonstrates its potential environmental benefits
2
2
by a magnetic bar at 500 rpm for 24 h. The reaction mixtures were
extracted with ethyl acetate (1 mL; containing 5 mm acetophenone
(Table 3). The mass intensities of the chemical and chemoenzy-
matic reactions are comparable. However, the quality of the re-
as an internal standard), dried over anhydrous MgSO , and ana-
4
lyzed by GC (Shimadzu; see Table S1).
Table 3. Semiquantitative comparison of the mass intensity of the chemi-
cal and the chemoenzymatic bromolactonization reaction.
Preparative-scale chloro- and bromolactonization reactions
The reaction was performed in a 100 mL Erlenmeyer flask at room
temperature with stirring. The reaction medium consisted of 0.1m
citrate buffer (pH 5, final volume of 50 mL) with 160 mm of KBr or
KCl, 4-pentenoic acid or 2-methyl-4-pentenoic acid (10 mmol), and
1
1
00 nm CiVCPO. The reaction was started by the addition of
00 mm of H O . After 24 h the reaction mixture was acidified, ex-
Chemical process
Chemoenzymatic process
À1
À1
2
2
[
gg product
]
[gg product
]
tracted with dichloromethane (3ꢂ100 mL), and dried over anhy-
drous Na SO . The combined organic layers were concentrated
under reduced pressure. The chloro- and bromolactone products
were purified by flash column chromatography on silica gel
Solvent
CH
2
Cl
2
29.9
H
2
O
35.2
0.67
2
4
citrate
/KBr
CiVCPO
Reagent
Catalyst
NBS
mol. sieve
1
H
2
O
2
0.12/0.67
0.00016
(EtOAc/hexanes, 1:2 v/v); 0.914, 1.4, and 1.15 g of chloro- and bro-
molactone products were isolated with 70, 80, and 60% yield, re-
spectively, as well as hydroxylactone in 30% yield in the case of
bromolactonization of 2-methyl-4-pentenoic acid and analyzed by
NMR spectroscopy.
0.05
agents and waste products differs significantly. In the case of
chemical synthesis, methylene chloride as solvent is question-
able, especially compared to simple citric acid buffer. Further-
more, stoichiometric amounts of succinimide, the recycling of
which necessitates further down-stream processing (DSP)
steps, is formed as a byproduct in the chemical process,
whereas the chemoenzymatic process yields water (and un-
reacted bromide) as byproduct. Finally, the catalyst consump-
tion of both processes also differs significantly.
Preparative-scale synthesis of 7-(bromomethyl)-4,7-dimeth-
yl-6-oxabicyclo[3.2.1]oct-3-ene (19a)
The reaction was performed in a 100 mL Erlenmeyer flask at room
temperature with stirring. The reaction medium consisted of 0.1m
citrate buffer (pH 5, final volume of 50 mL) with 160 mm of KBr,
1
0 mmol carveol and 100 nm CiVCPO. The reaction was started by
the addition of 100 mm of H O . After 24 h the reaction mixture
2
2
was extracted by ethyl acetate (3ꢂ100 mL), dried over anhydrous
Na SO . The combined organic layers were concentrated under re-
Following the established method, the present procedure
entailed extraction of the products with dichloromethane,
which obviously is questionable from an environmental point-
of-view. Therefore, future efforts will concentrate on the substi-
tution of CH Cl with more acceptable alternatives, such as
2
4
duced pressure. The products was purified by flash column chro-
matography on (silica gel, EtOAc/hexanes, 1:2); 1.38 g of 7-(bromo-
methyl)-4,7-dimethyl-6-oxabicyclo[3.2.1]oct-3-ene (19a) was isolat-
ed with 60% yield and analyzed by NMR spectroscopy.
2
2
[
18]
ethyl acetate. A particular focus will lie on the intensification
of the reaction, that is, increasing the substrate loading (and
consequently also the product concentration). This will reduce
the relatively large E-factor contribution of the solvent.
Overall, we are convinced that the proposed chemoenzy-
matic method for halocyclization represents a promising alter-
native to established chemical procedures. Further upscaling
and characterization of the reaction is currently ongoing in our
laboratory.
Preparative-scale of 2-(2-bromopropan-2-yl)-5-methyloxe-
pane (20a)
The reaction was performed in a 10 mL Erlenmeyer flask at room
temperature with stirring. The reaction medium consisted of 0.1m
citrate buffer (pH 5, final volume of 50 mL) with 160 mm of KBr,
10 mmol (+)-b-citronellol and 100 nm CiVCPO. The reaction was
started by the addition of 100 mm of H O . After 24 h the reaction
2 2
mixture was extracted by ethyl acetate (3ꢂ100 mL), dried over an-
hydrous Na SO . The combined organic layers were concentrated
2
4
under reduced pressure. The products was purified by flash
column chromatography on (silica gel, EtOAc/hexanes, 1:2);
Experimental Section
117 mg of 2-(2-bromopropan-2-yl)-5-methyloxepane (20a) was iso-
A detailed description of the biocatalyst preparation and purifica-
tion as well as a complete description of the experimental and an-
alytical procedures can be found in the Supporting information.
lated with 50% yield and analyzed by NMR spectroscopy.
Acknowledgements
Halocyclization of g,d-unsaturated carboxylic acids and alco-
hols
This work was supported by International Collaboration Base
for Molecular Enzymology and Enzyme Engineering
(
2017A050503001), the 111 Project (B17018). F.H. gratefully ac-
The halocyclization reactions were performed by using 1 mL glass
vials containing 40 mm unsaturated acids, and/or alcohols in 0.1m
knowledges funding by the European Research Commission (ERC
&
ChemSusChem 2019, 12, 1 – 6
4
ꢁ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!