Catalytic asymmetric synthesis of hexahydro-furofuran-3-ol and its pyran derivatives

Article abstract of DOI:10.1021/acs.orglett.1c00981

The catalytic asymmetric synthesis of hexahydro-furofuran-3-ol, a key fragment of HIV protease inhibitors, is reported. A signature event is represented by the sequential metal catalysis that combines the Pd-catalyzed asymmetric hydroalkoxylation of ene-alkoxyallene and ring-closing metathesis (RCM). Notably, this unprecedented and highly chemoselective approach allows for a unified access to pyranofuranol and furopyranol derivatives.

Full text of DOI:10.1021/acs.orglett.1c00981

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Letter
Catalytic Asymmetric Synthesis of Hexahydro-furofuran-3-ol and Its
Pyran Derivatives
Mijin Kim and Young Ho Rhee*
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ABSTRACT: The catalytic asymmetric synthesis of hexahydro-furofuran-3-ol, a key fragment of HIV protease inhibitors, is
reported. A signature event is represented by the sequential metal catalysis that combines the Pd-catalyzed asymmetric
hydroalkoxylation of ene-alkoxyallene and ring-closing metathesis (RCM). Notably, this unprecedented and highly chemoselective
approach allows for a unified access to pyranofuranol and furopyranol derivatives.
from the synthetic community. Most studies aimed at the
synthesis of the bis-THF ring rely on chiral pool approaches,
which generally require lengthy sequences and the extensive
use of protective groups.4 In addition, the preparation of
analogs often requires a redesign of the synthetic scheme.
Alternative approaches starting from achiral substrates mostly
proceed in a racemic manner and thus require enzymatic
kinetic resolution for the synthesis of enantioenriched
products.5 Here, we report a catalytic asymmetric synthesis
of hexahydro-furofuranol (Scheme 1). In this new approach,
enantioenriched cyclic acetal generated in an early stage is
exploited as the directing group for all the other stereocenters.
This unique strategy enables the development of a protective
group-free protocol with minimal functional group trans-
formations and thus provides the target compounds in a highly
efficient manner. The key cyclic allylic acetal moiety can be
prepared by the chemoselective Pd-catalyzed asymmetric
coupling reaction of commercial bromo-olefinic alcohols and
readily available ene-alkoxyallene in combination with the ring-
closing-metathesis (RCM) reaction.6−9 This approach also
gives access to derivatives containing a tetrahydropyran ring
with a comparable efficiency to that of bis-THF simply by
changing the structure of the commercial achiral starting
materials.
IV-1 protease inhibitors (PIs) block viral replication by
1
Hinhibiting viral enzymes. Moreover, the combination of
HIV-1 PIs with reverse transcriptase inhibitors proved to be an
effective treatment against AIDS. However, the emergence of
drug-resistant HIV-1 variants cause serious concerns for HIV-
1-infectious patients. Therefore, the development of various
novel PIs that show activity against drug-resistant HIV-1
strains is critical. A number of multidrug-resistant PIs, such as
Darunavir, a new front-line therapy for HIV/AIDS, possess a
unique (3R,3aS,6aR)-hexahydro-furo-[2,3-b]furan-3-ol (bis-
THF) fragment (structure A, Figure 1).2 SAR (Structure−
activity relationship) studies revealed that synthetic analogs
possessing THF-THP (structure B, Figure 1) or a THP-THF
bicyclic ring (structure C, Figure 1) also exhibited interesting
inhibitory effects.3
The unique structure of these bicyclic compounds and their
excellent biological activities have drawn significant attention
Received: March 22, 2021
Published: April 19, 2021
Figure 1. Structures of HIV protease inhibitors.
https://doi.org/10.1021/acs.orglett.1c00981
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3584−3587
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Letter
whose ee was determined to be a moderate 62% by the HPLC
measurement. Notably, switching to the ligand L2 significantly
improved the enantioselectivity (88% ee) of 3 without harming
the yield of the reaction (Table 1, entry 2). Little change of the
result occurred when using CH2Cl2 as the solvent (Table 1,
entry 3). In this case, lowering the temperature to 0 °C led to a
further increase of the ee to 96% (Table 1, entry 4).
Scheme 1. Synthetic Strategies Towards Hexahydro-
furfuranol and Its Pyran Derivatives
Having obtained the key intermediate 4 in an enantioen-
riched manner, we then investigated the synthesis of the target
compound 8. We first anticipated that the radical-mediated 5-
exo cyclization of 4 should generate the intermediate 6 in a
straightforward manner. However, initial attempts to employ
Bu3SnH under thermal conditions led to the extensive
decomposition of 4.12 Using triethylborane as the initiator at
rt minimized this undesired event, affording 6 in a 90% yield.13
The subsequent ozonolysis of this compound generated the
bicyclic ketone 7 in an 87% yield, which was reduced with
NaBH4 to the target alcohol 8 in an 80% yield with complete
diastereoselectivity. The optical purity of this compound was
determined to be 96% by the conversion into the
corresponding benzoate 9. This result confirms that the
enantiopurity of 4 does not erode significantly during the
sequence described in Scheme 2.
Scheme 2. Synthesis of bis-Hexahydro-furofuranol 8
At the outset of the study, we examined the Pd-catalyzed
coupling reaction of readily available allene 110 with
commercially available 2-bromo allylic alcohol 2. As shown
in Table 1, the reaction employing Pd2(dba)3 (2.5 mol %),
Table 1. Optimization of the Monocyclic Acetal Synthesis
As described in Scheme 3, the synthetic scheme developed
above was successfully expanded to the synthesis of the THP-
Scheme 3. Synthetic Route to the THF-THP Ligand
a
b
entry ligand
solvent
temperature
time
yield (%)
ee (%)
1
2
3
4
L1
L2
L2
L2
toluene
toluene
CH2Cl2
CH2Cl2
rt
rt
rt
5 min
5 min
5 min
2 h
89
94
95
94
62
88
88
96
0 °C
a
b
Isolated yield. The ee of compound 5.
chiral ligand L1 (5 mol %), and triethylamine (0.1 equiv) in
toluene immediately generated the adduct 3 in a 89% yield.
The subsequent RCM reaction using 10 mol % first generation
Grubbs catalyst proceeded smoothly to give the cyclic acetal
product 4 in an 84% yield.11 It should be noted that the vinyl
bromide moiety proved to be compatible with the sequential
metal catalysis. Because the measurement of the ee of 4 was
troublesome, we converted this compound to bis-benzoate 5,
THF derivative depicted in Figure 1.14 Performing the Pd-
catalyzed hydroalkoxylation reaction of 2 with allene 1010
under the optimized conditions in Table 1 gave the desired
acetal intermediate 11 in a 91% yield, which was smoothly
converted into the cyclic acetal intermediate 12 by the RCM
reaction in a 99% yield. The subsequent radical-mediated
cyclization, combined with ozonolysis, gave the bicyclic ketone
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Experimental details, spectral data, and 1H and 13C
NMR spectra of all new compounds (PDF)
FAIR data, including the primary NMR FID files, for
compounds 3−9, 11−16, and 18−23 (ZIP)
14 in a 72% combined yield over two steps. The reduction of
this ketone 14 with NaBH4 provided the target molecule 15 in
a 71% yield, again with complete diastereoselectivity. The
enantiopurity of this compound was determined to be 90% by
the conversion into the benzoate derivate 16.
Our final efforts were directed at the synthesis of the THP-
THF core using an analogous strategy.15 Compared with the
previous examples, this route turned out to be more
troublesome because of the poor outcome of the 6-exo radical
cyclization of the vinyl halide 19. As depicted in Scheme 4, the
AUTHOR INFORMATION
Corresponding Author
■
Young Ho Rhee − Department of Chemistry, Pohang
University of Science and Technology (POSTECH), Pohang,
Gyeongbuk, Republic of Korea 37673; orcid.org/0000-
0002-2094-4426; Email: yhrhee@postech.ac.kr
Scheme 4. Synthetic Route to THP-THF Ligand
Author
Mijin Kim − Department of Chemistry, Pohang University of
Science and Technology (POSTECH), Pohang, Gyeongbuk,
Republic of Korea 37673
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00981
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korean
Government (MSIT) (NRF-2018R1A4A1024713 and NRF-
2020R1A3B2079988).
REFERENCES
■
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
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