Ru-catalyzed asymmetric transfer hydrogenation of α-acyl butyrolactone via dynamic kinetic resolution: Asymmetric synthesis of bis-THF alcohol intermediate of darunavir

Article abstract of DOI:10.1016/j.tetlet.2021.152831

The Ru-catalyzed enantio- and diastereoselective dynamic kinetic resolution of α-(benzyloxy/benzoyloxy)acyl-γ-butyrolactones has been examined via transfer hydrogenation. Employing the in situ prepared (R,R)-Ru-FsDPEN catalyst, the transfer hydrogenation of using formic acid/triethylamine at rt gave the corresponding (S)-3-((S)-2-(benzyloxy/benzoyloxy)-1-hydroxyethyl)dihydrofuran-2(3H)-one with good to excellent diastereo- and enantioselectivity. One of the resulting hydrogenation product prepared on gram scales was utilized for the synthesis of (3R,3aS,6aR)-hexahydrofuro[2,3–b]furan-3-ol (1), a key synthetic intermediate of various HIV protease inhibitors such as darunavir with excellent enantio- (95% ee) and diastereoselectivities (dr 95:5).

Full text of DOI:10.1016/j.tetlet.2021.152831

Tetrahedron Letters 66 (2021) 152831
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ru-catalyzed asymmetric transfer hydrogenation of -acyl butyrolactone
a
via dynamic kinetic resolution: Asymmetric synthesis of bis-THF alcohol
intermediate of darunavir
Ganesh V. More a, , Pushpa V. Malekar a,b, Rupali G. Kalshetti a,b, Mahesh H. Shinde a,b
,
⇑
Chepuri V. Ramana a,b,
⇑
a Division of Organic Chemistry, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
b Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The Ru-catalyzed enantio- and diastereoselective dynamic kinetic resolution of
a-(benzyloxy/benzoy-
Received 18 December 2020
Revised 3 January 2021
Accepted 4 January 2021
Available online 27 January 2021
loxy)acyl- -butyrolactones has been examined via transfer hydrogenation. Employing the in situ pre-
c
pared (R,R)-Ru-FsDPEN catalyst, the transfer hydrogenation of using formic acid/triethylamine at rt
gave the corresponding (S)-3-((S)-2-(benzyloxy/benzoyloxy)-1-hydroxyethyl)dihydrofuran-2(3H)-one
with good to excellent diastereo- and enantioselectivity. One of the resulting hydrogenation product pre-
pared on gram scales was utilized for the synthesis of (3R,3aS,6aR)-hexahydrofuro[2,3–b]furan-3-ol (1), a
key synthetic intermediate of various HIV protease inhibitors such as darunavir with excellent enantio-
(95% ee) and diastereoselectivities (dr 95:5).
Keywords:
Bis-THF-OH intermediate
Darunavir
Ó 2021 Elsevier Ltd. All rights reserved.
Ru-catalysis
Dynamic kinetic resolution
Transfer hydrogenation
Introduction
among the Chinese manufacturers who supply to the majority of
the generic retroviral API manufacturers (Scheme 1).
Acquired immunodeficiency syndrome (AIDS) caused by the
human immunodeficiency virus (HIV) damages the immune sys-
tem of human beings which eventually leads to death [1]. For the
last couple of decades, the HIV protease inhibitors have been
employed as effective antiretroviral drugs (ARV) and are important
constituents of combination therapy or highly active anti-retrovi-
ral therapy (HAART) to treat HIV-AIDS [2]. The synthetic interme-
diate (3R,3aS,6aR)-hexahydrofuro[2,3–b]furan-3-ol (1), which is
trivially known as bis-THF alcohol 1 is a key unit in various HIV
protease inhibitors like darunavir and brecanavir (Fig. 1) [3–6].
Due to its significant utility in various HIV PIs, the bis-THF alcohol
unit 1 has gained considerable synthetic attention and numerous
methods have been reported for its synthesis [7–10]. Amongst
Interestingly, the catalytic asymmetric hydrogenation via
dynamic kinetic resolution of related -benzyloxyacyl- -butyro-
a
c
lactones (3) was documented by the Sumitomo Chemical Com-
pany, Limited in 2003 (Scheme 1) [12]. However, this suffered
from drawbacks of low diastereo- or enantioselectivity, the use
of expensive phosphine-based ligands and the conducting of the
hydrogenation at higher pressure and temperature, which seems
to be a limiting factor for its practical applications. Considering
all these drawbacks, we wondered about the catalytic dynamic
kinetic resolution of these lactones 2 and 3 under asymmetric
transfer hydrogenation conditions. The asymmetric transfer hydro-
genation of racemic compounds via dynamic kinetic resolution
(ATH-DKR) is an elegant and powerful method for the preparation
of various optically active compounds containing two stereocen-
ters [13]. ATH-DKR processes have gained a lot of interest over
the past few years because of their operationally simpler approach
than molecular hydrogenation as well as due to high diastereo-
and enantioselectivities notices [14]. Herein, we describe the first
these, the traditional
L-glyceraldehyde based approaches and those
of the enzymatic dynamic kinetic resolution of
a
-benzyloxyacyl-
c-
butyrolactones are of commercial significance [8–10]. Though the
former has stood the test of time, the latter is a relatively new
approach invented by Lonza Ltd. [11] and seems to be popular
enantioselective ATH-DKR of
a-(benzyloxy/benzoyloxy)acyl-c-
butyrolactones, a transformation that (1) uses readily available
and inexpensive starting materials, (2) requires low catalyst load-
ing, (3) has in situ catalyst formation, and (4) after transformation
⇑
Corresponding authors.
E-mail addresses: ganesh555m@gmail.com (G.V. More), vr.chepuri@ncl.res.in
(C.V. Ramana).
https://doi.org/10.1016/j.tetlet.2021.152831
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

G.V. More, P.V. Malekar, R.G. Kalshetti et al.
Tetrahedron Letters 66 (2021) 152831
Table 1
Catalyst and solvent screening on substrate 2.
Fig. 1. Structures of HIV-protease inhibitors with the bis-THF alcohol 1 structural
unit.
entry Catalyst Solvent
Conv. (%)b syn/antib ee (%)b
1
2
3
4
5
6
7
8
A
A
A
A
A
A
A
A
A
A
A
B
C
DCM
ACN
MeOH
EtOAc
MTBE
DMSO
IPA
CHCl3
99
99
89
99
97
99
99
99
99
99
99
99
99
99
99
99
99
99
99
78/22
51/49
62/38
74/26
35/65
24/76
67/33
88/12
70/30
74/26
72/28
71/29
86/14
64/36
81/19
79/ 21
92/08
80/20
86/14
90/10
90/10
86/14
73
67
39
76
54
59
49
64
69
67
68
17
73
1.4
74
79
73
75
78
78
72
67
Scheme 1. Methods reported for DKR of a-acyl-c-butyrolactones 2 and 3.
forms bis-THF alcohol having three contiguous stereo-centers with
up to 95% enantiomeric excess (ee) and 95/05 diastereomer ratio
(dr).
9
EtOAc + DCM
CHCl3 + THF
DCM + THF
DCM
DCM
DCM
DCE
DCM + THF
CHCl3
CHCl3 + THF
t-amyl alcohol
t-amyl alcohol + CHCl3 99
t-amyl alcohol + CHCl3 99
t-amyl alcohol + CHCl3 99
10
11
12
13
14
15
16
17
18
19
20
21
22
Results and discussion
D
C
C
C
C
Having mentioned the importance of the ATH-DKR and to
examine its suitability with lactones 2 and 3, the work in the direc-
tion commenced to identify the optimal substrate/conditions. Our
initial optimization experiments started with the lactone 2 and the
RuCl(p-cymene)[(R,R)-TsDPEN] (A). As shown in Table 1, using a
mixture of HCO2H/Et3N (5:2) as hydrogen source in dichloro-
methane, the hydrogenation proceeded smoothly at rt and pro-
vided the product 4 in 73% ee and 78:22 dr ratio (Table 1, entry
1). In the process of optimizing the reaction conditions, initially,
different solvents as well as a combination of solvents have been
screened employing catalyst A. As shown below, in all solvents,
the reactions ended up with high conversion; however, without
any further improvement in ee/dr (Table 1, entries 2–11). This
prompted us to examine the other complexes (RuCl[(R,R)-
TsDPEN]-(mesitylene)) (B), (RuCl[(R,R)-FsDPEN](p-cymene)) (C),
and (RuCl [(R,R)-Teth-TsDPEN] (D), in this pursuit.
Initial experiments with complexes B, C and D (Table 1, entries
12–14) in dichloromethane as solvent revealed that the catalyst C
was found to improve diastereoselectivity up to 86/14, with no
change in % ee (Table 1, entry 13). For further improvement in %
ee of the desired product, the reaction was examined in different
solvents and solvent combinations using catalyst C (Table 1, entries
15–19). These experiments revealed that the reaction in CHCl3
resulted in high dr ratio (92/08) but lower % ee (73%) (Table 1, entry
17) and the same in t-amyl alcohol as solvent gave high % ee (78%)
and lower dr (86:14) (Table 1, entry 19).
C
C
Cc
Cd
aConditions: 0.1 mmol of 2, 5 mol % of catalyst, and 0.1 mL of FA/TEA (5:2) were
b
added to the solvent (2 mL), and the mixture was stirred at 27 °C for 48 h.
-
Conversions, dr, and ee were calculated from chiral HPLC. cFA/DIPEA (5:2), d FA/DBU
(5:2) used instead of FA/TEA (5:2).
With this clue, we next examined the reaction in a 70:30 mix-
ture of t-amyl alcohol/CHCl3, which resulted in a high dr ratio
(90/10) with good % ee (78%) (Table 1, entry 20) as compared to
other solvents. At this juncture, we screened the mixture of HCO2-
H/ DIPEA (5:2) and HCO2H/ DBU (5:2), which were used as hydro-
gen source to check its effect on % ee and dr ratio but we ended up
with a lower % ee in both the cases (Table 1, entries 21 and 22).
With these results in hand, the next set of reactions have been car-
ried out by varying the percentage from 10 to 50% of CHCl3 in t-
amyl alcohol (Table 2, entries 1–6). At the outset, the solvent com-
bination of t-amyl alcohol and CHCl3 in 1.6:0.4 mL ratio proved to
be the better with improved ee (78%) and dr (91:9) of the desired
product (Table 2, entry 3). From these results, it was evident that
2

G.V. More, P.V. Malekar, R.G. Kalshetti et al.
Tetrahedron Letters 66 (2021) 152831
Table 2
Table 4
DKR of benzyloxylactone 3 under optimized conditions 2.
Screening of t-amyl alcohol:CHCl3 solvent ratio.
Entry
Catalyst loading (mol %)
Conv. (%)b
syn/antib
ee (%)b
Entry t-amyl alcohol (mL) CHCl3 (mL) Conv. (%)b syn/antib ee (%)b
1
2
3
4
5
5
2
1
0.5
0.2
99
99
99
99
60
91/09
95/05
95/05
95/05
94/06
91
94
95
95
95
1
2
3
4
5
6
1.8
1.6
1.4
1.2
1.0
0.8
0.2
0.4
0.6
0.8
1.0
1.2
99
99
99
99
99
99
86/14
89/11
91/09
91/09
88/12
83/16
78
77
78
77
75
72
aConditions: 1 mmol of 3, and 1 mL of FA/TEA (5:2) were added to the solvent.
bConversion, dr ratio, and ee were calculated from chiral HPLC.
aConditions: 0.1 mmol of 2, 5 mol % of catalyst, and 0.1 mL of FA/TEA (5:2) were
added to the solvent (2.0 mL). bConversions, dr and ee were calculated from chiral
HPLC.
Table 3
Screening of in situ complex/varying the catalyst loading.
Entry
Catalyst (mol %)
Conv. (%)b
syn/antib
ee (%)b
1c
2d
3d
4d
5
5
2
1
99
99
99
99
90/10
91/09
88/12
91/09
77
78
78
76
Scheme 2. Synthesis of (3R,3aS,6aR)-Hexahydrofuro[2,3–b]furan-3-ol (1).
further advanced to the benzoate 7 of the key Darunavir interme-
diate 1 by following the established 3 step sequence [7f,9b] and its
absolute configuration/enatioselectivity (ee: 95%; dr: 95/05) has
been established by comparing with the authentic benzoate pre-
pared from the commercial 1. Finally, saponification of 7 with aq.
NaOH in THF provided the bis-THF alcohol 1.
aConditions: 0.5 mmol of 2, and 0.5 mL of FA/TEA (5:2) were added to the solvent
(10.0 mL). bConversions, dr ratio and ee were calculated from chiral HPLC. cLigand:
[Ru]-complex proportion 1:1, dLigand: [Ru]-complex proportion 1:2.
the % ee and dr outcome seems to be dependent on the combina-
tion of solvents and their respective concentrations.
At this stage, to avoid the use of commercial Ru-complexes that
are very expensive, the possibility of employing in situ generated
(RuCl[(R,R)-FsDPEN](p-cymene)) was explored (Table 3, entries
1–4) by varying the catalyst loading from 5 to 1 mol% (Table 3,
entries 1–4). These experiments revealed that 5 mol% catalyst
(Ru: Ligand ratio 1:2) gave the best results with a complete conver-
sion of 2 resulting in the desired product 4 with 78% ee and 91/09
dr ratio (Table 3, entry 2). The decrease in catalyst loading resulted
in a nominal decrease in the % ee and dr ratio (Table 3, entries 3–4).
Based on these detailed experiments with the benzoyl substrate
2, we next moved to examine the compatibility of benzyl lactone 3
under the optimized conditions. As shown in Table 4, the results
are encouraging. With 5 mol% in situ generated catalyst C (Ru:
ligand ratio = 1:2) the complete conversion of the substrate was
noticed within 12 h at room temperature and the corresponding
hydroxyl lactone 5 was obtained with good enantioselectivity
(91% ee) and diastereoselectivity (91/09) (Table 4, entry 1). This
inspired us to further investigate the catalyst loading (Table 4,
entries 2–5). Gratifyingly, even with 0.5 mol% catalyst, the reaction
proceeded smoothly, with increased 95% ee and 95/05 dr ratio
(Table 4, entry 4). Decreasing the catalyst loading to 0.2 mol%
resulted in a lower conversion with no loss of ee and dr ratio
(Table 4, entry 5).
Conclusion
In conclusion, the dynamic kinetic resolution of benzoyloxy/
benzyloxy a-acyl-c-butyrolactone via asymmetric transfer hydro-
genation has been examined for the first time employing chiral
Ruthenium complexes. The best results were obtained with the
benzylated substrate. After substantial optimization, the ATH reac-
tion was successfully carried out employing 0.5 mol% of in situ pre-
pared (R,R)-Ru-FsDPEN complex and using HCO2H/Et3N mixture as
a hydrogen source. With both substrates, the ATH-DKR reactions
have been executed on gram scales and obtained good to excellent
diastereo and enantioselectivities (up to dr = 95:5 and ee = 95%)
and one of the intermediate has been advanced to synthesize bis-
THF alcohol intermediate of darunavir on a multi gram scale.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
The efficiency of this ATH-DKR process developed has been
examined by conducting the reaction on a 10 g scale, under opti-
mized reaction conditions. This gave the desired product 5 with
95% ee and 95/05 dr ratio (Scheme 2). This compound 5 has been
We thank CSIR, New Delhi for funding and for research fellow-
ships to P.V.M, R. G. K and M. H. S; DST-SERB, New Delhi for the
award of a NPDF to G.V.M.
3

G.V. More, P.V. Malekar, R.G. Kalshetti et al.
Tetrahedron Letters 66 (2021) 152831
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152831.
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