Diastereoselective Michael-Claisen cyclizations of γ-oxa-α,β-unsaturated ketones en route to 5-oxatetracyclines

Article abstract of DOI:10.1021/acs.orglett.6b03491

5-Oxatetracyclines were synthesized from D-arabinose using sequential Michael-Claisen cyclization reactions via a 5-oxa-AB enone substrate. The 5-oxatetracyclines were found to have poor stability in aqueous buffer (pH 7.4, 37°C) and showed little to no inhibition of bacterial growth (S. aureus, E. coli).

Full text of DOI:10.1021/acs.orglett.6b03491

Letter
pubs.acs.org/OrgLett
Diastereoselective Michael−Claisen Cyclizations of γ‑Oxa-α,β-
unsaturated Ketones en Route to 5‑Oxatetracyclines
Fan Liu, Peter M. Wright, and Andrew G. Myers*
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
S
* Supporting Information
ABSTRACT: 5-Oxatetracyclines were synthesized from D-arabinose using sequential Michael−Claisen cyclization reactions via a
5-oxa-AB enone substrate. The 5-oxatetracyclines were found to have poor stability in aqueous buffer (pH 7.4, 37 °C) and
showed little to no inhibition of bacterial growth (S. aureus, E. coli).
oupling of various o-toluate phenyl esters (D-ring
precursors) with enone 1 (AB enone)1 by a highly
Scheme 1. Retrosynthetic Disconnection of 5-Oxatetracycline
Analogue 2 by Sequential MC Cyclizations
C
diastereoselective Michael−Claisen (MC) cyclization reaction2
provides the key step in a short and general route to tetracycline
antibiotics, broadly defined (Figure 1).3,4 As part of an
coupling of 3 and 4 was recognized to be potentially quite
different from cyclization of the 5-carba-AB enone 1 in terms of
both its feasibility and the stereochemical outcome, and it was
therefore of interest to us to explore this transformation from a
basic chemistry perspective. We imagined further simplification
of subtarget 4 by a second MC transform, depicted in Scheme 1,
giving rise to the chiral masked pyrone equivalent 6 and the
known isoxazole phenyl ester 5 as proposed reactants.7 The
acetonide group within 6 was considered to provide a strong
biasing element for trans nucleophilic addition (γ-pyrone itself,
of course, would provide no stereochemical biasing element).
Dihydropyrone 6 is available in optically pure form in three
steps from D-arabinose.8,9 Addition of the masked pyrone B-ring
precursor 6 (1 equiv) to the sodium enolate of isoxazole ester 5
(2.00 equiv, formed using sodium hexamethyldisilazide as the
base) at −78 °C followed by warming to −15 °C afforded the
cyclized product 7 in 40% yield after purification by flash column
chromatography (>20:1 dr; Scheme 2A). The use of phenyl ester
5 as the Claisen condensation partner was crucial to the success
Figure 1. Michael−Claisen cyclization reaction of D-ring precursors and
the AB enone (1) provides fully synthetic tetracycline antibiotic
candidates with diverse substitution patterns; most are molecules that
would be inaccessible through semisynthesis.
exploration of modifications of C5 within the B ring of
tetracyclines, we were led to consider replacing the C5 methylene
group with an oxygen atom. This was of interest because it would
give rise to a molecule of nearly equal molecular weight but
greater polar surface area (PSA = 161.4 Å2 (5 = CH2) vs PSA =
170.6 Å2 (5 = O)), which has been suggested to correlate with
improved antibiotic activity against infections caused by Gram-
negative bacteria, an area of current unmet medical need.5,6
The 7-fluoro-5-oxatetracycline analogue 2 was identified as an
interesting specific target for initial consideration in light of the
favorable effects of 7-fluoro substitution upon antibiotic activity
within fully synthetic (5-carba)tetracyclines.3d Applying a
retrosynthetic disconnection paralleling that of Figure 1 to the
novel target 2 gave rise to the known D-ring precursor 3 and the
novel 5-oxa-AB enone 4 (Scheme 1). The proposed MC
Received: November 22, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.6b03491
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Scheme 2. MC Cyclization Reactions of 6
Scheme 3. (A) C12a Oxygenation of MC Adduct 11; (B)
Synthesis of Oxa-AB Enone Precursor 14 from MC Adduct 7
of the reaction; when the corresponding methyl ester 8 was used
instead, the noncyclized fragmentation product 9 (Scheme 2B;
stereochemistry not determined) was observed to be the primary
adduct (37% yield). A much more efficient MC cyclization
reaction occurred when the masked pyrone 6 (7.00 g, 41.1 mmol,
1 equiv) was added to the sodium enolate of N-Boc phenyl ester
109 (2.00 equiv, formed using sodium hexamethyldisilazide as
the base) at −78 °C followed by warming after 1.5 h to −25 °C,
affording the product 11 in 92% yield (19.5 g, >20:1 dr) after an
aqueous workup and purification by flash column chromatog-
raphy. Evidently, the tert-butyl carbamate function of the
modified isoxazole coupling partner 10 greatly improved the
efficiency of the cyclization reaction. This observation is
consistent with our prior finding that o-toluates that bear
anion-stabilizing groups typically undergo MC cyclization
reactions more efficiently than those that do not.3b,10 The
observed facial selectivity is consistent with nucleophilic addition
trans to the sterically demanding isopropylidene substituent. The
adjacent stereogenic centers at C4 and C4a were formed with
complete diastereocontrol, but the N,N-dimethylamino group
was epimeric relative to natural tetracyclines, necessitating the
development of an epimerization protocol (vide infra).
We first used to advantage the improper stereochemistry at C4
to direct oxygenation at C12a with natural stereochemistry.
Thus, addition of a catalytic amount of lithium tert-butoxide
(0.20 equiv) to a solution of 3-(4-nitrophenyl)-2-
(phenylsulfonyl)oxaziridine (1.30 equiv) and adduct 11 (1.98
g, 3.85 mmol, 1 equiv) at −40 °C followed by warming to 0 °C
afforded amino alcohol 12 as the only observable product in 83%
yield after purification by flash column chromatography (Scheme
3A).7,11 Oxygenation at C12a of the MC adduct 7 could also be
carried out with similar efficiency to give compound 13, which
upon treatment with trimethylsilylimidazole (5.00 equiv)
afforded trimethylsilyl ether 14 in 43% yield over two steps.
The intermediate amino alcohol 13 could be recrystallized from
ethyl acetate−hexanes, providing crystals suitable for X-ray
analysis. The solid-state structure we obtained (Scheme 3B)
confirmed the stereochemical outcomes of both the MC
cyclization and C12a-oxygenation reactions.
mild conditions.12,7 In all examples of which we are aware,
tetracyclines and their analogues with the natural C4-S
configuration are thermodynamically more stable than the C4-
R epimers (tetracycline itself exists as a 1.5:1 mixture of 4S and
4R epimers, respectively, at equilibrium in 1 M aqueous sodium
dihydrogen phosphate/methanol at 25 °C12a,b). When we
attempted epimerization of intermediate 14 under similar
conditions, however, we obtained solely quinone 15 (70%
yield; Scheme 4A). Use of N-Boc-methylamine 12 as the
substrate likewise led to hydroquinone 16 (50% yield).
Inspection of the X-ray structure of intermediate 13 (depicted
in Scheme 3B) reveals that the molecule adopts a conformation
in which the C4a−O and C4−H bonds are antiperiplanar, an
alignment that potentially facilitates an eliminative fragmentation
that is not possible in the 5-carba analogues; this is likely the case
for its trimethylsilyl ether 14 and N-Boc methylamine 12 as well,
since comparison of coupling constants from 1H NMR spectra of
13, 14, and 12 suggest that they adopt similar conformations. Of
equal or greater concern is the fact that computational analyses of
13 and 14 and their “natural” C4-S epimers suggested that we
may have been attempting a contrathermodynamic epimeriza-
tion; i.e., 13 was calculated to be more stable than its 4S epimer
(ΔGcalc = +0.6 kcal/mol).9 This is in contrast to 5-carba-AB
enone 1, which was computed and observed to be significantly
more stable than its 4R epimer (the observed equilibrium ratio of
des-TBS-(4S)-1 and its 4R epimer is 13:1, respectively, in 4 M
NaH2PO4/methanol/THF solution at 60 °C;7 ΔGcalc = −2.9
kcal/mol9). Therefore, to effect the desired epimerization, we
developed an alternative strategy, which is depicted in Scheme
4B. Treatment of N-Boc methylamine 12 with trifluoroacetic
acid at 0 °C afforded amino alcohol 17 in 72% yield. Heating of a
solution of 17 in benzene at 45 °C in the presence of
benzaldehyde (10.0 equiv) and acetic acid (2.00 equiv) afforded
three products, which were separated by flash column
chromatography: recovered 17 (38%), its 4S epimer 18 (31%),
and benzylidene aminal 19 (∼17%, impure). Thus, the ratio of
the desired 4S epimer 18 to the undesired 4R epimer 17 was
1:1.2, which we believe to be the equilibrium distribution of
products. Separately, benzylidene aminal 19 could be hydrolyzed
to give a 10:1 mixture of 18 and 17, respectively (81% combined
isolated yield). A simpler and improved epimerization protocol
was developed, employing a larger excess of acetic acid (20.0
To complete the synthesis of 5-oxa-AB enone 4, we next
investigated epimerizations of the C4 stereogenic center within
intermediates 12 and 14. Prior studies have shown that the 4-
position of tetracyclines is prone to epimerization, often under
B
DOI: 10.1021/acs.orglett.6b03491
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Scheme 4. (A) Attempted Epimerizations of 14 and 12 Led to Fragmentation To Give Quinone 15 and Hydroquinone 16,
Respectively; (B) Epimerization of the C4 Stereogenic Center under Mildly Acidic Conditions in the Presence of Benzaldehyde;
(C) Epimerization of 17 on a Gram Scale and Triethylsilyl Ether Protection of the Tertiary Hydroxyl Group
equiv) to give a 1:1.5:1.5 mixture of 17, 18, and 19 (Scheme 4C).
This was directly concentrated and subjected to purification by
column chromatography (leading to lysis of 19 in situ), providing
18 in 47% yield (20:1 mixture with 17, 3.70 g scale). The trace
amount of 17 was removed in the next step, where only the
desired 4S epimer 18 underwent triethylsilylation (TESOTf, 2,6-
lutidine) to afford pure triethylsilyl ether 20 in 73% yield.
Triethylsilyl ether 20 was transformed into 5-oxa-AB enone 4
in four steps (Scheme 5). N-Methylation was achieved upon
protective group was cleaved while preserving the triethylsilyl
ether function, affording separately, after purification by flash
column chromatography, hemiacetal 22 (37%) and its
dehydration product 23 (22%). Both products were transformed
into the common enol triflate 24 in the presence of triflic
anhydride (3.50 equiv for 22 and 2.00 equiv for 23) and an
appropriate base (5.00 equiv of 2,6-lutidine for 22 and 4.00 equiv
of 2,6-di-tert-butyl-4-methylpyridine for 23) (Scheme 5). Enol
triflate 24 was reduced directly, without purification, using
borane−dimethylamine complex13 (5.00 equiv) in the presence
of palladium acetate (0.80 equiv) to afford 5-oxa-AB enone 4
(23% yield from hemiacetal 22 and 21% yield from enol 23).
We next investigated construction of the C ring of 5-
oxatetracyclines by the proposed MC cyclization reaction
(Scheme 6). Deprotonation of the D-ring precursor 3 (2.10
Scheme 5. Synthesis of 5-Oxa-AB Enone 4
Scheme 6. MC Cyclizations of 4 and 25 with D-Ring Precursor
3 To Afford 5-Oxatetracycline Derivatives 26 and 27
equiv) with lithium tetramethylpiperidide (2.40 equiv) in the
presence of N,N,N′,N′-tetramethylethylenediamine (7.00 equiv)
and triethylamine hydrochloride (0.03 equiv)3d at −78 °C
produced a bright-red solution. Addition of enone 4 at −78 °C
followed by warming to −25 °C over 1.5 h provided the desired
cyclization product 26 as a single diastereomer in 43% yield. The
analogous transformation was also achieved using 5-oxa-AB
enone 259 as the substrate to afford the cyclization product 27
(59%). Both cyclization products were stereochemically
homologous with natural tetracyclines, as indicated by NOE
subjection of a solution of 20 in dichloromethane to aqueous
formaldehyde (4.00 equiv) and sodium triacetoxyborohydride
(12.0 equiv) at 0 °C to afford the (4S)-N,N-dimethylamino
product 21, contaminated with ∼6% of its 4R epimer (not
shown). The mixture was treated with boron trichloride (1.50
equiv) in dichloromethane at 0 °C for 0.5 h. The acetonide
C
DOI: 10.1021/acs.orglett.6b03491
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analysis. This stereochemistry arises from addition of the o-
toluate anion along a pseudoaxial trajectory, opposite the bulky
triethylsilyl ether substituent, and is the same stereochemical
outcome we had observed in the earlier “5-carba” series.
After considerable experimentation, a workable if modest-
yielding sequence for deprotection of the 5-oxatetracycline
precursors 26 and 27 was found (Scheme 7A). Hydrogenolysis
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03491.
■
S
Procedures and spectroscopic data (PDF)
AUTHOR INFORMATION
■
Scheme 7. (A) Synthesis of 5-Oxatetracyclines 2 and 28; (B)
Debenzylation and B-Ring Fragmentation of 27
Corresponding Author
*myers@chemistry.harvard.edu
ORCID
Andrew G. Myers: 0000-0001-9602-6915
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSF for predoctoral graduate fellowship support
(F.L.), Dr. Shao-Liang Zhang (Harvard University) for X-ray
data collection, and Dr. Janine May and Dr. Yuan Qiao (Harvard
University) for guidance on MIC studies.
REFERENCES
■
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of 26 and 27 in ethyl acetate containing acetic acid (20.0 equiv)
in the presence of palladium hydroxide (4.00 equiv) and
hydrogen gas (1 atm) at 23 °C for 10 min afforded the
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an apparent eliminative fragmentation reaction (Scheme 7B).
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fragmentation under basic conditions. This enabled a stereo-
selective synthesis of 5-oxatetracyclines, which were found to
have poor stability under physiological conditions.
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D
DOI: 10.1021/acs.orglett.6b03491
Org. Lett. XXXX, XXX, XXX−XXX