A platform for the discovery of new macrolide antibiotics

Article abstract of DOI:10.1038/nature17967

The chemical modification of structurally complex fermentation products, a process known as semisynthesis, has been an important tool in the discovery and manufacture of antibiotics for the treatment of various infectious diseases. However, many of the therapeutics obtained in this way are no longer effective, because bacterial resistance to these compounds has developed. Here we present a practical, fully synthetic route to macrolide antibiotics by the convergent assembly of simple chemical building blocks, enabling the synthesis of diverse structures not accessible by traditional semisynthetic approaches. More than 300 new macrolide antibiotic candidates, as well as the clinical candidate solithromycin, have been synthesized using our convergent approach. Evaluation of these compounds against a panel of pathogenic bacteria revealed that the majority of these structures had antibiotic activity, some efficacious against strains resistant to macrolides in current use. The chemistry we describe here provides a platform for the discovery of new macrolide antibiotics and may also serve as the basis for their manufacture.

Full text of DOI:10.1038/nature17967

ARTicLE
doi:10.1038/nature17967
A platform for the discovery of new
macrolide antibiotics
ian B. Seiple1*†, Ziyang Zhang1*, Pavol Jakubec1, Audrey Langlois-mercier1†, Peter m. Wright1†, Daniel T. Hog1†, Kazuo yabu1†,
Senkara Rao Allu1, Takehiro Fukuzaki1, Peter N. carlsen1, yoshiaki Kitamura1†, Xiang Zhou1, matthew L. condakes1†,
Filip T. Szczypiński1†, William D. Green1† & Andrew G. myers1
The chemical modification of structurally complex fermentation products, a process known as semisynthesis, has been an
important tool in the discovery and manufacture of antibiotics for the treatment of various infectious diseases. However,
many of the therapeutics obtained in this way are no longer effective, because bacterial resistance to these compounds
has developed. Here we present a practical, fully synthetic route to macrolide antibiotics by the convergent assembly of
simple chemical building blocks, enabling the synthesis of diverse structures not accessible by traditional semisynthetic
approaches. More than 300 new macrolide antibiotic candidates, as well as the clinical candidate solithromycin, have
been synthesized using our convergent approach. Evaluation of these compounds against a panel of pathogenic bacteria
revealed that the majority of these structures had antibiotic activity, some efficacious against strains resistant to
macrolides in current use. The chemistry we describe here provides a platform for the discovery of new macrolide
antibiotics and may also serve as the basis for their manufacture.
Natural products have provided critical starting points for the develop- hospitals and the community18–20. Here we present a practical, fully
ment of a majority of the antibacterial drugs listed as essential medicines synthetic platform for the preparation of macrolide antibiotics, provid-
by the World Health Organization. During the period from about 1940 ing both a discovery engine for structurally novel antibiotic candidates
to 1960, sometimes described as the golden age of antibiotics research, that would be difficult or impossible to obtain from erythromycin, as
academic and industrial laboratories identified the natural products well as a basis for their eventual manufacture. Using simple building
that went on to define many of the major classes of modern antibiotics1. blocks and a highly convergent assembly process, we have prepared
Few if any natural products prove to be optimal for safety, efficacy or more than 300 structurally diverse macrolide antibiotic candidates,
oral use in humans, however, for these were probably not evolutionary as well as the approved drug telithromycin and the clinical candidate
pressures for the microbes in which they developed. For six decades a solithromycin. We have identified molecules with diverse macrocyclic
primary tool by which new antibiotics have been discovered and manu- scaffolds that exhibit potent activities against bacterial strains resistant
factured is semisynthesis, or the chemical modification of natural prod
-
to erythromycin, azithromycin and other current antibiotics of different
ucts derived from fermentation. Semisynthesis is inherently limited classes.
because it is challenging to modify structurally complex natural prod-
ucts selectively, and typically few positions of any given scaffold can Synthesis of 14-membered azaketolides
be modified effectively2,3. Macrolide antibiotics (macrocyclic lactones To illustrate our approach to the synthesis of macrolide antibiotics, we
with one or more pendant glycosidic residues), which have proven begin by detailing a route to new and highly active hybrid antibiotics,
to be safe and effective for use in treating human infectious diseases 14-membered azaketolides (Fig. 2), and then show how the approach
such as community-acquired bacterial pneumonia, gonorrhoea and can be easily expanded to subsume many other active macrolide scaf-
others, provide a compelling case in point. Since the discovery of eryth- folds, with broad latitude for substitutional variation. Ketolides replace
romycin from a Philippine soil sample in 19494, in spite of extraor- the C3-cladinose carbohydrate residue of erythromycin with a carbonyl
dinary efforts leading to fully synthetic5–8 and modified biosynthetic group, a chemical modification that allows them to evade certain inducible
routes9–12 to macrolide antibiotics, all members of this class approved forms of macrolide resistance21,22, whereas the azalide azithromycin
or in clinical development for use in humans have been manufactured incorporates a nitrogen atom within an expanded (15-membered)
by chemically modifying erythromycin (erythromycin itself is unstable macrolactone ring, features that have been proposed to impart favour-
in the stomach, and rearranges to form a product with gastrointestinal able pharmacological properties, including increased efficacy against
side-effects)13. Thus, azithromycin is prepared from erythromycin certain Gram-negative pathogens23,24. The hybrid ‘azaketolides’, or
in four steps14, clarithromycin requires six steps15, and the advanced macrolides that contain both a C3 carbonyl group and a nitrogen atom
clinical candidate solithromycin is currently produced from eryth- within the macrolactone ring, have been little explored; one such com-
romycin by a 16-step linear sequence (Fig. 1)16,17. New macrolides pound has been reported and was found to be essentially inactive25. We
are badly needed, for resistance to approved macrolide antibiotics were particularly interested in 14-membered azaketolides that would
such as clarithromycin and azithromycin is now widespread in both arise from formal replacement of the carbonyl group at position C9
1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA. †Present addresses: Department of Pharmaceutical Chemistry, University of
California, San Francisco, California 94158, USA (I.B.S.); Novartis Pharma AG, Chemical and Analytical Development, CH-4002 Basel, Switzerland (A.L.-M.); McKinsey and Company, 55 East
52nd Street, 21st Floor, New York, New York 10022, USA (P.M.W.); Bayer Pharma AG, Medicinal Chemistry, Müllerstrasse 178, 13353 Berlin, Germany (D.T.H.); Medicinal Chemistry Research
Laboratories, Daiichi Sankyo Co., Ltd, Shinagawa R&D Center, 1-2-58 Hiromachi, Shinagawa, Tokyo 140-8710, Japan (K.Y.); Department of Chemistry and Biomolecular Science, Gifu University,
1-1 Yanagido, Gifu 501-1193, Japan (Y.K.); Department of Chemistry, University of California, Berkeley, California 94720, USA (M.L.C.); Department of Chemistry, University of Cambridge, Lensfield
Road, Cambridge CB2 1EW, UK (F.T.S.); Trinity College, University of Cambridge, Cambridge CB2 1TQ, UK (W.D.G.).
*These authors contributed equally to this work.
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Article reSeArcH
H3C
O
O
9
N
H3C
HO
CH3
OH
H3C
HO
CH3
H3C
HO
CH3
OH
9
9
OCH3
H3C
H3C
OH
H3C
CH3
H3C
OH
H3C
CH3
HO
H3C
OH
H3C
CH3
12
6
N(CH3
)
12
6
12
6
N(CH3)
2
N(CH3
)
2
2
H3C
H3C
HO
HO
O
CH3
O
CH3
O
CH3
O
O
O
O
O
O
1
1
1
O
O
O
O
OCH3
CH3
OH
O
OCH3
O
OCH3
CH3
OH
CH3
CH3
CH3
CH3
OH
O
O
O
CH3
CH3
CH3
Erythromycin
Clarithromycin
Azithromycin
Fermentation product
1952
6 steps from erythromycin
1991
4 steps from erythromycin
1991
US FDA approval:
NH2
N
N
N
N
N
N
N
O
9
O
O
H3C
H3C
H3C
CH3
CH3
CH3
O
O
H
O
9
9
N
N
N
OCH3
O
OCH3
O
O
O
CH3
CH3
CH3
6
6
12
12
6
N(CH3
)
N(CH3
)
12
N(CH3
)
2
H3C
H3C
H3C
H3C
2
2
H3C
H3C
H3C
H3C
H3C
HO
HO
HO
O
CH3
O
CH3
O
CH3
O
O
O
O
O
O
3
3
3
O
O
O
O
O
O
H3C
F
CH3
CH3
Telithromycin
12 steps from erythromycin
2004
Cethromycin
9 steps from erythromycin
Clinical candidate
Solithromycin
16 steps from erythromycin
Clinical candidate
US FDA approval:
Figure 1 | Summary of macrolide antibiotic development by
azithromycin and telithromycin along with the dates of their FDA approval
and the number of steps for their synthesis from erythromycin. The
previous ketolide clinical candidate cethromycin and the current clinical
candidate solithromycin are also depicted. It is evident that increasingly
lengthy sequences are being employed in macrolide discovery efforts.
semisynthesis. To date, all macrolide antibiotics are produced by
chemical modification (semisynthesis) of erythromycin, a natural product
produced on the ton scale by fermentation. Depicted are erythromycin
and the approved semisynthetic macrolide antibiotics clarithromycin,
of ketolides with an amino group, a transformation that would not be is by design a convergent coupling reaction, and permits optimal late-
feasible by semisynthesis. stage diversification of a key side-chain residue that is known to occupy
An overview is helpful, as it emphasizes our synthetic strategy. Eight a novel pocket in the bacterial ribosome30–34
simple building blocks (1–8), two of which are commercially available Aspects of certain steps before the macrocyclization reaction merit brief
.
and six of which are synthesized in sequences of 3–6 steps in 30–150-g discussion (see Fig. 2). The key aldol coupling reaction that unites building
amounts, are assembled in a highly convergent manner (Fig. 2). blocks 135 and 236, initiating the synthesis of the left-half intermediate,
As depicted, building blocks 1–3 are transformed by a seven-step sequence involves new methodology that was developed specifically for the present
featuring two convergent coupling reactions to provide a key left-hand purpose (though it proves to be a general transformation) and proceeds
intermediate (15, an amine) in 57% yield, while building blocks 4–7 in 98% yield on a 33-g scale to form a single, crystalline diastereomer36.
are transformed in a seven-step sequence featuring three convergent In a subsequent step, which transforms the pseudoephenamine amide
coupling reactions to form a key right-hand intermediate (22, a ketone) function into a methyl ketone in one operation, the pseudoephenamine
in 58% yield. (Here left and right refer to the position in the graphi- auxiliary is recovered by a simple aqueous extraction procedure in 92%
cal representation of the target molecule, Fig. 2a.) The left- and right- yield. The convergent coupling reaction that unites building blocks
hand intermediates are then joined in a highly stereoselective and 4 and 5 to begin the construction of the right-half intermediate is
efficient reductive coupling reaction (82% yield, diastereomeric ratio (d.r.) based on a palladium-catalysed ene reaction developed by Mikami and
>20:1) that also serves to remove the benzoate protective group within co-workers, and proceeds in 93% yield and 92% enantiomeric excess
the desosamine residue. The next step is the key macrocyclization reac- (e.e.)37. Two convergent coupling reactions follow three linear functional
tion, for which we employ a method developed by Boeckman et al. in group interconversions, the first coupling being a Mukaiyama38 aldol
the context of their approaches towards the natural products diplodi- reaction (to incorporate building block 639) whose diastereoselectivity
alide A (a 10-membered macrolactone), kromycin (a 14-membered was observed to be dependent upon the reaction conditions. The condi-
macrolactone) and ikarugamycin (a 17-membered macrolactam)26,27
.
tions employed here (magnesium bromide etherate as Lewis acid, trieth-
We found that thermolysis of intermediate 23 (1mM in chlorobenzene) ylamine as base, dichloromethane as solvent, and a reaction temperature
at 132°C afforded the macrolactone 24 in 78% yield (1-g scale). There of −78°C) provided maximum selectivity (d.r. >20:1, 91% yield) for
are a number of notable features of this transformation, not least its the desired syn-aldol product. The next convergent coupling reaction
efficacy and, as we will show, generality. Many pioneering efforts to introduced the protected desosamine carbohydrate residue and for this
synthesize macrolide antibiotics failed at the stage of macrocyclization; purpose we employed the thioglycoside 740,41, a donor very similar to
successful macrocyclizations were shown to require carefully designed, that employed by Woodward et al. in their synthesis of erythromycin5.
rigidifying protective group strategies28. We believe that the success of In the present case, we found that the use of a benzoyl group to shield the
the present cyclization (which we note occurs in the presence of a free secondary alcohol on the glycosidic donor in lieu of methoxycarbonyl
secondary amine as well as another secondary alcohol) is attributable improved the yield and diastereoselectivity of the glycosidation reaction.
in part to the rigid structuring induced by the cyclic carbamate func- The overall yield of the fully synthetic azaketolide antibiotic candidate 25
tion on the left-hand side of the molecule, which we suspect brings the was 32% from initial building blocks 1 and 2 (a 10-step sequence) or 32%
putative acyl ketene function (produced from the dioxolenone function from initial building blocks 4 and 5 (also a 10-step sequence).
upon thermolysis) and the reactive secondary alcohol into favourable
proximity. A single operation then transforms the product of macro- Synthesis of 15-membered azaketolides
cyclization into a fully synthetic antibiotic candidate (25) while incor- By design, the convergent building block strategy we employ enables
porating the final building block using Sharpless’ optimized conditions rapid access to structurally related scaffolds; for example, synthesis
for azide–alkyne dipolar cycloaddition29 (8, 86% yield). This final step of a 15-membered hybrid azaketolide scaffold comprising structural
1 9 m A y 2 0 1 6
| V O L 5 3 3 | N A T U R E | 3 3 9
© 2016 Macmillan Publishers Limited. All rights reserved

reSeArcH Article
OTIPS
CH3
O
a
EtO
CH3
Target topology
O
H3
C
CH3
4
5
O
O
Ph
O
O
H3
C
Ph
NH2
aR
93%
OTMS
N
H3
C
CH3
OTBDPS
CH3
OH
CH3
dR
cR
97%
bR
92%
96%
1
2
6
19
18
17
16
N(CH3
)
2
eR
91%
aL
98%
BzO
PymS
OTf
O
CH3
Br
bL
cL
76%
9
10
11
3
7
20
fR
dL
92%
81%
eL
fL
gL
gR
96%
95%
92%
100%
12
13
14
15
22
21
h
82%
i
H2N
From 1 and 2: 10 steps, 32% yield
From 4 and 5: 10 steps, 32% yield
Modular building blocks: 8
78%
8
24
23
Convergent coupling reactions: 7
j
86%
25
N3
N3
N3
b
CH3 OH
3
H3
C
O
N
CH3
H3
C
NH2
N
O
O
CH3
NH2
H
O
O
O
O
Ph
aL
bL,cL
dL,eL
fL
gL
N
1
N
O
N
HO
H3
O
C
Ph
O
C
O
C
98%
76%
(2 steps)
88%
(2 steps)
95%
92%
C
NH2
H3
C
H3
C
H3
C
C
H3
H3C
H3
H3
C
H3
H3
OH
OTBDPS
OTBDPS
OTBDPS
OTBDPS
2
15
14
9
11
13
O
O
CH3
O
CH3
CH3
O
O
O
O
OCH3
OCH3
OCH3
6
7
CH3
CH3
CH3
BzO
O
CH3
BzO
O
CH3
N(CH3
)
N(CH3)
2
2
O
O
a
R,bR
c
R,dR
93%
eR
fR
gR
OH
OCH3
4
5
H3C
H3C
H3C
H3C
O
CH3
O
CH3
CH3
CH3
OH
86%
(2 steps)
91%
81%
100%
(2 steps)
H3C
H3C
O
EtO
O
H
O
O
O
O
CH3
CH3
21
CH3
CH3
CH3
O
O
O
17
19
O
O
CH3
N
N
N
20
22
N3
N3
NH2
H
H3C
N
CH3
O
H
H
N
N
10
8
H3C
H3C
OCH3
N
CH3
OCH3
CH3
HO
CH3
OCH3
CH3
HO
O
O
O
C
8
N
N
CH3
HO
12
N(CH3
)
6
2
H3
O
h
j
i
O
H3C
H3C
H3
C
H3C
N(CH3
)
2
N(CH3
)
2
H3C
H3C
O
CH3
O
O
4
H3C
H3C
86%
78%
82%
O
CH3
O
CH3
2
O
O
O
OH
O
O
H3C
CH3
O
O
O
CH3
CH3
CH3
O
O
25
24
23
Figure 2 | A convergent, fully synthetic route to the 14-membered
76% over 2 steps, 92% of recovered (R,R)-pseudoephenamine;
(dL) NaHMDS; (eL) NaN3, 88% over 2 steps; (fL) NH3, Ti(OiPr)4, NaBH4,
95%; (gL) Bu4NF, 92%; (aR) Pd[(S)-SegPhos]Cl2, AgSbF6, 93%, 92% e.e.;
(bR) ethylene glycol, PPTS, 92%; (cR) KH, MeI, 97%; (dR) (iBu)2AlH, 96%;
(eR) Et3N, MgBr2•OEt2, 91%; (fR) AgOTf, 81%, 16:1 β:α; (gR) HCl, 100%;
(h) NaCNBH3, 82%; (i) 132 °C, 1 mM, PhCl, 78%; (j) CuSO4, sodium
l-ascorbate, 86%.
azaketolide 25. a, Graphical representation of the convergent synthesis
of azaketolide 25 from eight variable building blocks (represented by
coloured spheres). Downward, ‘Y’-shaped arrows signify convergent
coupling reactions. b, Synthesis of azaketolide 25, reagents and conditions
(subscripts L and R indicate left and right halves, respectively) as follows:
(aL) LiHMDS, LiCl, 98%; (bL) EtiPr2N, COCl2; (cL) iPrMgCl, CH3Li,
features of both the azalide antibiotic azithromycin and the ketolide clin- blocks. The first coupling, which unites building blocks 2642 and
ical candidate solithromycin (Fig. 3) required modification of just three 2743,44 to form the tertiary alcohol 29, emerged only after extensive
of the eight building blocks that were used to assemble 14-membered experimentation, in which we discovered that incorporating the large
azaketolides (Fig. 2). The left-hand amine intermediate (15) was the tert-butyldiphenylsilyl protective group within component 27 was key
same in both 14- and 15-membered azaketolide scaffold syntheses and to achieve high diastereoselectivity. After two subsequent functional
thus construction of the 15-membered azaketolides simply required group interconversions, building blocks 6 (as before) and 28 (the orig-
access to the homologated right-hand intermediate, aldehyde 35. inal glycosyl donor of Woodward et al.5, which proved to be effective
Here, too, we employed a seven-step sequence (38% yield) featuring in this scaffold synthesis) were incorporated by successive convergent
three convergent coupling reactions to assemble four simple building coupling reactions. Two linear steps then provided the right-hand
3 4 0
| N A T U R E | V O L 5 3 3 | 1 9 m A y 2 0 1 6
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Article reSeArcH
O
a
CH3
CH3
TBDPSO
Target topology
O
O
I
H3
C
H3
C
CH3
O
CH3
26
27
O
Ph
O
O
H3C
Ph
NH2
aR
81%
OTMS
N
H3C
CH3
OTBDPS
CH3
OH
CH3
cR
99%
bR
99%
6
31
30
29
1
2
N(CH3
)
2
dR
91%
aL
98%
McO
PymS
OTf
O
CH3
Br
bL
cL
76%
9
10
11
3
28
32
dL
92%
eR
70%
eL
fL
gL
gR
92%
fR
96%
95%
92%
95%
12
13
14
15
35
34
33
h
86%
i
From 1 and 2: 10 steps, 43% yield
From 26 and 27: 10 steps, 33% yield
Modular building blocks: 8
H2N
94%
Convergent coupling reactions: 7
8
37
36
j
93%
38
OTBDPS
CH3
OCH3
CH3
McO
O
OTBDPS
CH3
OCH3
CH3
b
26
27
OTBDPS
CH3
H
CH3
OH
OTBDPS
CH3
OCH3
CH3
OCH3
6
28
eR
CH3
N(CH3
)
N(CH3
)
2
2
aR
b
R,cR
dR
f
R,gR
CH3
McO
H3C
H3C
H3C
H3C
O
CH3
O
CH3
O
O
OH
81%
98%
(2 steps)
91%
70%
87%
(2 steps)
H3C
O
O
H3C
O
O
O
O
O
H
O
H3C
CH3
CH3
CH3
CH3
CH3
CH3
29
O
O
O
O
CH3
31
N
32
33
35
N
N
N3
N3
N3
NH2
H3C
NH2
O
HN
H3C
N
H
CH3
N
O
HN
H3C
N
H3C
N
10
8
CH3
CH3
O
O
OCH3
O
N
8
H3C
H3C
O
OCH3
OCH3
CH3
HO
O
12
6
N(CH3)
2
H3C
H3C
O
H3C
H3C
j
i
94%
CH3
McO
O
CH3
McO
O
N(CH3
)
N(CH3
)
OH
2
H3C
2
2
H3C
H3C
O
CH3
H3C
O
O
H3C
15
4
93%
O
CH3
O
CH3
O
OH
O
H3C
O
h
86%
O
O
O
CH3
CH3
CH3
CH3
37
O
O
36
38
Figure 3 | A convergent, fully synthetic route to the 15-membered
76% over 2 steps, 92% of recovered (R,R)-pseudoephenamine; (dL) NaHMDS;
(eL) NaN3, 88% over 2 steps; (fL) NH3, Ti(OiPr)4, NaBH4, 95%;
(gL) Bu4NF, 92%; (aR) tBuLi, MgBr2, 81%; (bR) KH, MeI, 99%; (cR) H5IO6,
99%; (dR) ZnCl2, 91%; (eR) AgOTf, 70%; (fR) HF (aq.); (gR) Dess–Martin
periodinane, 87% over 2 steps; (h) NaCNBH3, 86%; (i) 132 °C, 1 mM, PhCl,
94%; (j) CuSO4, sodium l-ascorbate, 93%.
azaketolide 38. a, Graphical representation of the convergent synthesis
of azaketolide 38 from eight variable building blocks (represented by
coloured spheres). Downward, ‘Y’-shaped arrows signify convergent
coupling reactions. b, Synthesis of azaketolide 38, reagents and conditions
(subscripts L and R indicate left and right halves, respectively):
(aL) LiHMDS, LiCl, 98%; (bL) EtiPr2N, COCl2; (cL) iPrMgCl, CH3Li,
intermediate, aldehyde 35. Reductive coupling of the left-hand amine solithromycin as well as the approved drug telithromycin, with the goal
(15) and the right-hand aldehyde (35) in the presence of sodium of preparing diverse structural analogues. Towards this end we envi-
cyanoborohydride proceeded without detectable epimerization to sioned coupling of the same right-hand intermediate employed in the
afford the macrocyclization precursor 36 in 86% yield. Thermolysis synthesis of 15-membered azaketolides (aldehyde 35) with a Grignard
of 36 at 132 °C in chlorobenzene then afforded the 15-membered reagent derived from hydromagnesiation of the acetylenic alcohol 42
macrolide 37 in 94% yield on a 1.9-g scale. Incorporation of the final (Fig. 4). Intermediate 42 was obtained in one step (14:1 d.r., 85% yield)
building block (8, as before) and cleavage of the methoxycarbonyl by the addition of 1-lithiopropyne (39) to ketone 4035 in the presence
protective group were achieved in a single, final step to provide the of lithium (1S,2R)-1-phenyl-2-(pyrrolidin-1-yl)-1-propanolate45.
azaketolide antibiotic candidate 38. The overall yield of the fully syn- Hydromagnesiation of 42 with cyclopentylmagnesium bromide and
thetic macrolide 38 (FSM-20707, see below) was 43% from initial bis-cyclopentadienyltitanium dichloride46 proceeded regioselectively to
building blocks 1 and 2 (a 10-step linear sequence) or 33% from initial furnish a vinyl Grignard reagent that was captured in situ with aldehyde 35.
building blocks 26 and 27 (also a 10-step linear sequence).
The resulting diastereomeric mixture of allylic alcohols (obtained in
80% yield on a 3-g scale) was oxidized directly (Dess–Martin periodi-
nane, 97%) and the α,β-unsaturated ketone produced was desilylated
Synthesis of 14-membered ketolides
We next adapted our convergent assembly strategy to synthesize (tetrabutylammonium fluoride (Bu4NF), 95%) to afford the keto alcohol
14-membered ketolides, a class that includes the clinical candidate 45. Macrocyclization, as before, then provided the 14-membered
1 9 m A y 2 0 1 6
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reSeArcH Article
O
a
CH3
CH3
TBDPSO
O
O
Target topology
I
H3C
H3C
O
CH3
O
CH3
26
27
H3C
aR
OTMS
81%
CH3
cR
99%
bR
99%
6
31
30
29
O
N(CH3
)
2
dR
91%
H3C
CH3
McO
PymS
O
CH3
Li
CH3
OTBS
40
39
28
32
H2N
aL
eR
70%
N3
85%
NH2
gR
fR
92%
95%
41
8
42
35
34
33
bL
h
80%
95%
m
l
k
j
i
Modular building blocks: 8
99%
85%
66%
95%
97%
Convergent coupling reactions: 7
49
48
47
46
45
44
43
From 26 and 27: 14 steps, 16% yield
n
87%
Solithromycin
40
39
aL
85%
b
CH3
O
OH
O
HO
CH3
H3C
CH3
OCH3
CH3
McO
H3C
HO
CH3
H3C
H3C
H
OCH3
CH3
McO
OCH3
HO
OTBS
CH3
McO
O
H3C
H3C
H3C
H3C
N(CH3
)
N(CH3
)
N(CH3)
2
2
2
H3C
H3C
i, j
92%
42
H3C
H3C
O
CH3
O
CH3
O
CH3
O
O
OH
OTBS
h
H3C
H3C
O
O
O
80%
CH3
CH3
CH3
CH3
O
O
O
O
O
O
CH3
CH3
43
45
35
N
N
N
bL
95%
8
k
66%
H2N
N
NH2
N
N
49
41
NH2
O
O
O
H3C
N
O
CH3
H3C
HO
CH3
OCH3
CH3
McO
H3C
CH3
O
10
8
OCH3
HO
OCH3
CH3
HO
O
H3C
H3C
H3C
H3C
CH3
McO
O
N(CH3
)
N(CH3)
2
N(CH3
)
2
2
H3C
H3C
12
6
m, n
87%
l
H3C
H3C
H3C
O
CH3
O
CH3
O
CH3
O
O
O
O
4
85%
2
O
O
O
O
O
O
F
CH3
F
CH3
47
CH3
46
Solithromycin
Figure 4 | A convergent, fully synthetic route to solithromycin.
respectively): (aL) lithium (1S,2R)-1-phenyl-2-(pyrrolidin-1-yl)-1-propanolate,
85%; (bL) CuSO4, sodium l-ascorbate, 95%; (aR) tBuLi, MgBr2, 81%;
(bR) KH, MeI, 99%; (cR) H5IO6, 99%; (dR) ZnCl2, 91%; (eR) AgOTf, 70%;
(fR) HF (aq.), 95%; (gR) Dess–Martin periodinane, 92%; (h) Cp2TiCl2,
cyclopentylmagnesium bromide, 80%; (i) Dess–Martin periodinane, 97%;
a, Graphical representation of the convergent synthesis of solithromycin,
which was previously only accessible by semisynthesis. This route has been
adapted for the synthesis of >30 novel ketolide antibiotic candidates, as
well as the FDA-approved ketolide telithromycin. Downward, ‘Y’-shaped
arrows signify convergent coupling reactions. b, Synthesis of solithromycin, (j) Bu4NF, 95%; (k) 132 °C, 0.5 mM, PhCl, 66%; (l) KOtBu, FN(SO2Ph)2,
reagents and conditions (subscripts L and R indicate left and right portions,
85%; (m) Im2CO, DBU; (n) imidazole hydrochloride, 60°C, 87% over 2 steps.
macrolactone 46 in 66% yield (1.7-g scale). Fluorination at position C2 The present route to solithromycin proceeds in 14 steps and 16% yield
proceeded in 85% yield47. Transformation of the C12 tertiary alcohol from building blocks 26 and 27, and differs from the semisynthetic
within the latter intermediate (47) to the corresponding acyl imida- route (16 steps from erythromycin, yields not reported16) in that the
zolide (carbonyl diimidazole (Im2CO), 1,8-diazabicyclo[5.4.0]undec- linker and its attached side-chain heterocycle are introduced in the final
7-ene (DBU)) and trapping with amine 49 then afforded fully synthetic step of our synthesis and thus modification of these residues is quite
solithromycin in one operation (87% yield, the methoxycarbonyl facile, whereas the linker residue is introduced 6 steps before the penul-
protective group was cleaved concomitantly under these conditions). timate step in the published semisynthetic route to solithromycin16.
3 4 2
| N A T U R E | V O L 5 3 3 | 1 9 m A y 2 0 1 6
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Article reSeArcH
F
NH2
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NH2
O
O
O
O
H3
C
H3C
CH3
OCH3
CH3
HO
CH3
H3
C
O
H3
C
O
CH3
CH3
OCH3
CH3
HO
O
O
N
N
OCH3
N
N
OCH3
CH3
O
C
O
C
O
O
C
CH3
N(CH3
)
N(CH3)
2
H3
C
2
H3
N(CH3
)
H3
C
2
N(CH3
)
2
H3
H3C
H3C
H3
H3C
H3C
HO
H3
O
C
H3C
HO
O
CH3
O
CH3
H3C
O
O
O
O
O
CH3
O
CH3
O
O
O
O
O
O
O
O
O
O
O
H3C
F
H3
C
F
H3C
F
H3C
F
FSM-100573
FSM-100563
FSM-100490
FSM-11563
NH2
NH2
NH2
N
N
N
N
N
N
N
N
N
N
H
H3
C
N
CH3
O
O
H
H
O
HN
H3C
N
H
N
H3
C
N
CH3
CH3
OCH3
CH3
HO
O
C
H3
C
O
CH3
O
O
CH3
HO
N(CH3
)
2
N
H3
N
N
OCH3
CH3
HO
OCH3
H3
C
H3
C
O
C
O
C
O
O
CH3
O
O
N(CH3
)
2
N(CH3
)
H3
CH3
H3
2
N(CH3
)
H3
C
2
H3
C
H3C
H3
C
H3
C
H3C
H3C
HO
O
CH3
O
CH3
O
O
O
O
O
O
CH3
O
O
CH3
O
O
O
O
O
O
CH3
CH3
H3C
F
FSM-20707
FSM-22391
FSM-21397
FSM-20919
NH2
NH2
NH2
NH2
N
S
N
N
N
N
N
N
N
N
N
N
N
N
H
H
O
H
H
O
H3
C
N
N
N
H3
C
N
CH3
H3
C
CH3
OCH3
CH3
HO
CH3
CH3
OCH3
CH3
HO
O
O
O
O
N
N
N
O
N
OCH3
OCH3
O
C
O
C
O
C
O
C
CH3
CH3
N(CH3)
2
H3
N(CH3
)
2
O
N(CH3
)
N(CH3)
2
H3
H3
2
H3
H3C
H3
C
C
H3
C
H3
C
H3C
H3
C
HO
H3C
HO
O
CH3
O
CH3
O
O
O
CH3
O
F
O
O
O
O
N
H
N
O
O
O
O
O
O
O
H3
F
CH3
CH3
CH3
FSM-21760
FSM-11453
FSM-11044
FSM-21887
Species
Strain description
Erythro Azithro Telithro Solithro 100573 100563 100490 11563 20707 22391 21397 20919 11453 11044 21887 21760
S. aureus
S. aureus
S. aureus
S. aureus
ATCC 29213
BAA-977; iErmA
MP513; MRSA; cErmA
NRS384; MRSA; MsrA
0.5
>256
>256
64
0.03
>256
≤0.03
1
1
>256
>256
128
0.06
>256
≤0.03
4
0.125
0.06
256
0.125
≤0.03
0.125
≤0.03
≤0.03
16
0.125
≤0.03
>64
0.25
≤0.03
0.25
≤0.03
≤0.03
32
0.06
0.06
16
0.06
≤0.03
≤0.03
≤0.03
0.03
1
≤0.03
0.06
16
0.125
≤0.03
≤0.03
≤0.03
0.03
2
≤0.03
0.03
64
0.06
≤0.03
≤0.03
≤0.03
0.03
2
0.06
0.06
64
0.125
≤0.03
≤0.03
≤0.03
≤0.03
4
0.5
0.5
>64
1
≤0.03
2
≤0.03
0.125
>64
2
0.25
0.5
64
4
4
64
8
0.06
8
0.06
0.5
64
8
0.25
0.5
64
0.5
≤0.03
0.5
≤0.03
0.25
>64
4
1
1
>64
2
≤0.03
2
≤0.03
0.125
>64
4
1
1
>64
2
≤0.03
8
≤0.03
0.125
64
8
8
>64
16
0.06
1
0.06
0.5
>64
16
0.5
1
64
1
4
S. pneumoniae ATCC 49619
S. pneumoniae UNT-042; ErmB/MefA
≤0.03
0.125
≤0.03
0.06
32
4
4
4
≤0.03
1
≤0.03
0.06
>64
4
S. pyogenes
E. faecalis
E. faecalis
H. influenzae ATCC 49247
A. baumannii ATCC 19606
K. pneumoniae ATCC 10031
ATCC 19615
ATCC 29212
UNT-047; VRE; ErmB
>256
4
16
>256
2
32
2
4
4
4
16
4
2
2
2
2
8
8
2
8
4
2
4
4
8
32
8
4
2
16
8
16
16
4
2
32
8
32
4
4
2
E. coli
ATCC 25922
64
64
4
64
16
64
32
64
8
16
16
32
16
64
16
32
4
64
8
64
32
64
4
64
8
>64
64
>64
16
>64
8
64
P. aeruginosa ATCC 27853
MIC colour scale (μg ml–1
)
<0.03
0.03
0.06
0.125
0.25
0.5
1
2
4
8
16
32
64
128
256
>256
Figure 5 | Structures of selected fully synthetic macrolide antibiotics,
and their MICs compared to those of natural and semisynthetic
macrolide antibiotics. Top three rows, structural formulae. Bottom row,
table showing MICs (values in μg ml−1 colour coded, key at bottom)
of current macrolide antibiotics and of selected analogues against
9 Gram-positive and 5 Gram-negative microorganisms. iErmA, inducible
erythromycin ribosome methyltransferase A; cErmA, constitutive
erythromycin ribosome methyltransferase A; MsrA, macrolide
streptogramin resistance efflux pump A; MefA, macrolide efflux
protein A; ErmB, erythromycin ribosome methyltransferase B; Erythro,
erythromycin; Azithro, azithromycin; Telithro, telithromycin; Solithro,
solithromycin.
We prepared a number of fully synthetic 14-membered ketolides by Information for a complete list of structures synthesized and Extended
taking advantage of this feature of our synthesis to simultaneously vary Data Figs 1–10 for exemplary schemes for their preparation). This strat-
the linker and side-chain heterocycle, and many of these products show egy not only provided novel scaffolds to explore, but also permitted
potencies that are superior to solithromycin in microbiological assays deep-seated variations of positions within these scaffolds, thus enabling
(see below).
access to molecules that could not be prepared using semisynthetic
methods.
Construction of a library of macrolide antibiotics
We prepared an initial library of >300 fully synthetic macrolide (FSM) Microbiological testing
antibiotic candidates by varying the building blocks in concert with To evaluate our FSMs for antibiotic activity, we screened 305 of them
modifying readily diversifiable elements (for example, an azido group, against a panel of pathogens comprising standardized Gram-positive
an amino group, a β-keto lactone function, an allyl group) that we and Gram-negative strains (see Supplementary Information). For
introduced into and around the macrolide ring, a powerful tactical the most promising antibiotic candidates, the panel was expanded
combination. In addition to members of the three primary macrocy- to include bacteria with genetically characterized resistance mech-
clic scaffolds discussed in detail above, we prepared a number of other anisms to erythromycin as well as other antibiotics. These include
unique scaffolds by modifying the principal coupling components three different strains of Staphylococcus aureus, two of them clinically
(left- and right-halves) and their modes of coupling using straight- isolated methicillin-resistant (methicillin-resistant Staphylococcus
forward alternative chemical transformations (see Supplementary aureus, MRSA) pathogens, a clinically isolated strain of Streptococcus
1 9 m A y 2 0 1 6
| V O L 5 3 3 | N A T U R E | 3 4 3
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reSeArcH Article
9. Khosla, C. Harnessing the biosynthetic potential of modular polyketide
synthases. Chem. Rev. 97, 2577–2590 (1997).
pneumoniae with both ribosome-modifying methyltransferase (ermB)
and efflux (mefA) genes, and finally a clinically isolated strain of
Enterococcus faecalis with an ermB gene, a strain resistant to all approved
macrolides as well as vancomycin (vancomycin-resistant Enterococcus,
VRE) (Fig. 5). The data show that a majority of compounds in the
library exhibit demonstrable antibiotic activity. For example, 83%
of compounds in the collection displayed a minimum inhibitory
concentration (MIC) ≤4μgml−1 against wild-type S. pneumoniae,
which is known to be highly susceptible to inhibition by macrolides
(see Supplementary Information). Among the most promising com-
pounds (Fig. 5) are represented variously 14-, 15- and 16-membered
azaketolide scaffolds (for example, FSM-22391, FSM-20707 and FSM-
21397, respectively), a 15-membered azacethromycin hybrid anti-
biotic (FSM-20919), and a number of fully synthetic 14-membered
ketolides (most notably, FSM-100573 and FSM-100563). The last
two substances display superior potencies relative to any macrolide
in current clinical use in the following extremely challenging strains:
an S. pneumoniae with both ermB and mefA genes (MIC for both FSM-
100573 and FSM-100563 ≤0.03μgml−1), a VRE with an ermB gene
(MICs 1 and 2μgml−1, respectively), an MRSA with a constitutively-
expressed erythromycin ribosome methyltransferase (c-erm) gene
(MICs 16 μg ml−1), and Pseudomonas aeruginosa (MICs 16 and
32μgml−1, respectively). Clearly, further work will be required to drive
potencies in the latter two strains to clinically efficacious levels, but we
believe that demonstration of even modest activity in these challenging
strains by a macrolide is significant. It is interesting and encouraging
to note that FSM-100573 also displays improved Gram-negative activ-
ity relative to many other macrolides; further advances in this regard
would address an important area of unmet medical need48.
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Conclusion
Employing as a design strategy the multiply convergent assembly
of simple chemical building blocks, we have developed a platform
of unprecedented versatility for the discovery and practical synthesis of
novel macrolide antibiotics. Varying the building blocks as well as (at a
later stage) diversifiable elements incorporated within them permits an
almost exponential expansion of variability within any given scaffold.
In addition, our work shows that variant scaffolds can be obtained by
straightforward perturbations of our generalized assembly process.
As with our earlier convergent synthesis of tetracycline antibiotics49,50
,
we anticipate that many thousands of novel macrolide structures can
be prepared for evaluation as potential antibiotics using the present
synthetic platform. In light of this, it seems logical to conclude that
developing similar convergent routes to other naturally occurring anti-
biotic families may accelerate the discovery of new therapeutic agents
for human infectious diseases.
Online Content Methods, along with any additional Extended Data display items and
Source Data, are available in the online version of the paper; references unique to
these sections appear only in the online paper.
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Supplementary Information is available in the online version of the paper.
Acknowledgements We acknowledge funding from Alistair and Celine
Mactaggart, from the Gustavus and Louise Pfeiffer Research Foundation,
and from the Blavatnik Biomedical Accelerator Program at Harvard
University. We thank NERCE (NIH project number U54 AI057159), W. Weiss
(Univ. North Texas), and R. Alm and S. Lahiri (Macrolide Pharmaceuticals)
for measuring MIC values, R. Alm and T. Dougherty (Harvard Medical School)
for genetic characterization of a microbial resistance gene, and S.-L. Zheng
(Harvard University) for conducting X-ray crystallographic analyses. I.B.S.
acknowledges postdoctoral fellowship support from the National Institutes of
Health (F32GM099233); Z.Z. is a Howard Hughes Medical Institute International
Student Research fellow; A.L.-M. acknowledges postdoctoral fellowship
support from the Swiss National Science Foundation (PBGEPE2-139864)
and the Novartis Foundation; D.T.H. is indebted to the Deutsche
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Author Contributions I.B.S., Z.Z. and A.G.M. identified the targets and designed
the synthetic routes; I.B.S. and Z.Z. executed and optimized the synthetic routes
described in the main text; I.B.S., Z.Z., P.J., P.M.W., A.L.-M. and D.T.H. executed
the synthetic routes shown in the Extended Data; I.B.S., Z.Z., P.J., P.M.W., A.L.-M.,
D.T.H., K.Y., T.F., P.N.C., X.Z., M.L.C., F.T.S. and W.D.G. synthesized individual
macrolide analogues; I.B.S., Z.Z., Y.K. and S.R.A. synthesized and scaled the
building blocks. I.B.S., Z.Z. and A.G.M. wrote the paper. All authors discussed the
results and commented on the manuscript.
Author Information Atomic coordinates and structure factors for the crystal
structure reported have been deposited with the Cambridge Crystallographic
Database under accession number CCDC 1440650. Reprints and permissions
information is available at www.nature.com/reprints. The authors declare
competing financial interests: details are available in the online version of
the paper. Readers are welcome to comment on the online version of the
paper. Correspondence and requests for materials should be addressed to
A.G.M. (myers@chemistry.harvard.edu).
1 9 m A y 2 0 1 6
| V O L 5 3 3 | N A T U R E | 3 4 5
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reSeArcH Article
Extended Data Figure 1 | Synthesis of a C2-fluoro 14-membered azaketolide by a late-stage fluorination reaction. Subjection of β-keto lactone 25
(FSM-22367) to potassium tert-butoxide (1.0 equiv.) at −78°C followed by N-fluorobenzenesulfonimide (1.0 equiv.) afforded 50 (FSM-22391) in 43% yield.
© 2016 Macmillan Publishers Limited. All rights reserved

Article reSeArcH
Extended Data Figure 2 | Synthesis of a 15-membered azaketolide
with a modified C2-substituent. Thermolysis of a β-keto tert-butyl ester
substrate (55) proceeded at a lower temperature (80 °C) than that for
dioxolenone substrates (132 °C) and afforded a 15-membered macrocycle
without substitution at C2 (56). (Here and elsewhere we omit mention
of the intermediate compounds for clarity.) This macrocycle served as
a nearly ideal intermediate for preparation of macrolides with diverse
C2-substitutions. For example, an allyl group was introduced at C2 by
treatment of 56 with sodium tert-butoxide (1.1 equiv.) and allyl iodide
(1.1 equiv.) at −40 °C followed by warming the reaction solution to 23 °C.
The product 57 (obtained in 62% yield) was then transformed to 59
(FSM-56156) in two steps (via 58; 72% yield).
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reSeArcH Article
Extended Data Figure 3 | Synthesis of a 15-membered azacethromycin
hybrid. Macrolactone 63 was prepared from amine 60 and aldehyde
61 in two steps (via 62) by a reductive amination–macrocyclization
sequence. Treatment of 63 with paraformaldehyde (6.0 equiv.) and acetic
acid (10.0 equiv.) furnished adduct 64 as a crystalline solid (84% yield;
X-ray structure shown). The imidazolidine group within 64 served to
protect both the secondary amine and the cyclic carbamate functions, and
permitted the introduction of a quinoline heterocycle via a Heck reaction.
Methanolysis (TFA, CH3OH) cleaved the imidazolidine group, affording
65 (FSM-20919; 29%, two-step yield).
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Article reSeArcH
Extended Data Figure 4 | Synthesis of a 15-membered azaketolide with
a modified C10-substituent. N-tert-butylsulfinyl imine 68 (prepared in
five steps via 66 and 67 from amide 10) allowed for the stereocontrolled
introduction of various C10-substituents. For example, addition of
allylmagnesium bromide proceeded with >20:1 stereoselectively to
establish the stereocentre at C10; subsequent cleavage of the sulfinyl
(HCl, CH3OH) and tert-butyldiphenylsilyl (Bu4NF) groups within the
adduct then furnished left-hand intermediate 69 (81% yield). Amine
69 and aldehyde 35 were coupled by a reductive amination reaction
(NaBH3CN, 60%–75% yield). The product (70) was then transformed to 73
(FSM-11044) in a three-step sequence that consisted of a macrocyclization
reaction (giving 71; 72% yield), a methanolysis reaction (giving 72 in
quantitative yield) and lastly a [3 +2] dipolar cycloaddition reaction
(giving 73 in 92% yield).
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reSeArcH Article
Extended Data Figure 5 | Synthesis of a 15-membered azaketolide with
a modified C13-substituent. Modification of position C13 was achieved
by modification of a single component, in this case the ketone building
block 74 depicted above. Reductive coupling of 78 and 35 united the
left- and right-halves to give 79; subsequent thermal macrocyclization
provided macrolactone 80. The allyl group within intermediate 80 was
cleaved upon ozonolysis (O3, trifluoroacetic acid); reductive workup
with sodium cyanoborohydride afforded alcohol 81. Subjection of 81 to
bis(2-methoxyethyl)aminosulfur trifluoride afforded the fluoroethyl-
substituted macrocycle 82 (30%, two-step yield), which was transformed
to 83 (FSM-11453) by a [3 +2] dipolar cycloaddition reaction.
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Article reSeArcH
Extended Data Figure 6 | Synthesis of a 15-membered azaketolide
with a modified desosamine sugar residue. The 15-membered
macrolactone 90 was synthesized using thioglycoside 84 and alkyne
89 as building blocks (in lieu of building blocks 28 and 8 used for the
synthesis of 15-membered azaketolide FSM-20707). Treatment of
90 with tributyltin hydride (2.0 equiv.), acetic acid (5.0 equiv.), and
tetrakis(triphenylphosphine)palladium (2 mol%) led to cleavage of the
allyloxycarbonyl protective group, giving rise to amine 91 (92% yield).
The latter intermediate has been transformed into a number of fully
synthetic macrolides with modified desosamine sugar residues. For
example, acylation of the primary amino group of intermediate
91 with pyridine-2-carbonyl chloride (2.0 equiv.) in the presence
of trimethylamine (3.0 equiv.) followed by removal of the
tert-butoxycarbonyl group afforded 92 (FSM-21887, 86%, two-step yield).
© 2016 Macmillan Publishers Limited. All rights reserved

reSeArcH Article
Extended Data Figure 7 | Synthesis of a 16-membered azaketolide.
Homologation of aldehyde 35 was achieved by a Wittig olefination
reaction (CH3OCH3PPh3+ Cl−, NaHMDS) followed by hydrolysis of the
resulting enol ether to afford aldehyde 93 in 65% yield. Reductive coupling
of amine 15 and aldehyde 93 furnished macrocyclization precursor 94
(73% yield). The 16-membered macrolactone 95 was obtained in 78% yield
upon thermolysis of 94 (1 mM, 132 °C). Two additional steps transformed
95 via 96 to the 16-membered azaketolide 97 (FSM-21397).
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Article reSeArcH
Extended Data Figure 8 | Synthesis of a 14-membered macrolide with
a trans-olefin linkage. Mesylate 98 was prepared in quantitative yield by
treatment of alcohol 34 with methanesulfonyl chloride (1.50 equiv.) and
triethylamine (2.0 equiv.). Displacement of the mesylate group in 98 with
and sulfone 100 were coupled in a Julia–Kocienski olefination reaction
(NaHMDS, −78 → 23 °C) to provide a 4.8:1 mixture of E- and Z-olefin
isomers. The E-isomer 102 was isolated and desilylated (Bu4NF, 79%).
Thermolysis of the product 103 (1 mM, 132 °C) furnished the
sodium 1-phenyl-1H-tetrazole-5-thiolate (2.0 equiv.) followed by oxidation 14-membered macrocycle 104 in 83% yield. 104 was then transformed
of the resulting thioether with ammonium molybdate (0.20 equiv.)–hydrogen to 106 (FSM-21079) in two additional steps (methanolysis and [3 +2]
peroxide (100 equiv.) afforded sulfone 100 in 70% yield. Aldehyde 101
dipolar cycloaddition) via 105.
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reSeArcH Article
Extended Data Figure 9 | Synthesis of a 15-membered macrolide with
an amide linkage (C9-N9a). Oxidation of aldehyde 35 with sodium
chlorite (10.0 equiv.) in the presence of sodium dihydrogen phosphate
(10.0 equiv.) and 2-methyl-2-butene (100 equiv.) afforded carboxylic acid
107 in 70% yield. Acid 107 and amine 15 were coupled in the presence
of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (2.00 equiv.) to
provide amide 108. Macrocyclization of 108 (1 mM, 132 °C) proceeded
in 81% yield to afford macrolactam 109. Methanolysis and [3 +2] dipolar
cycloaddition then transformed 109 to 111 (FSM-21344) in two steps
via 110.
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Article reSeArcH
Extended Data Figure 10 | Synthesis of a 15-membered macrolide with
an amide linkage (C10-N9a). Amine 112 was prepared in 70% yield by
displacement of the mesylate group in 98 with sodium azide followed
by reduction of the resulting alkyl azide (H2, Pd). The coupling of amine
112 and acid 113 proceeded in 54% yield. The product, amide 114,
was desilylated (Bu4NF, 80%) to afford the macrocyclization precursor
115. Thermal macrocyclization of 115 followed by cleavage of the
methoxycarbonyl protective group afforded lactam 116 in 80% yield.
Copper-catalysed [3 +2] dipolar cycloaddition then provided 117
(FSM-21473) in 64% yield.
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