Minimal fluorous tagging strategy that enables the synthesis of the complete stereoisomer library of SCH725674 macrolactones

Article abstract of DOI:10.1021/ja302260d

Four mixtures of four fluorous-tagged quasiisomers have been synthesized, demixed, and detagged to make all 16 stereoisomers of the macrocyclic lactone natural product Sch725674. A new bare-minimum tagging pattern needs only two tags-one fluorous and one nonfluorous-to encode four isomers. The structure of Sch725674 is assigned as (5R,6S,8R,14R,E)-5,6,8-trihydroxy-14- pentyloxacyclotetradec-3-en-2-one. Various comparisons of spectra of 32 lactones (16 with tags, 16 without) and 16 ester precursors (8 with tags, 8 without) provide insights into when and why related compounds have the same or different spectra.

Full text of DOI:10.1021/ja302260d

Article
pubs.acs.org/JACS
Minimal Fluorous Tagging Strategy that Enables the Synthesis of the
Complete Stereoisomer Library of SCH725674 Macrolactones
Jared D. Moretti,† Xiao Wang,† and Dennis P. Curran*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
S
* Supporting Information
ABSTRACT: Four mixtures of four fluorous-tagged quasiisomers
have been synthesized, demixed, and detagged to make all 16
stereoisomers of the macrocyclic lactone natural product
Sch725674. A new bare-minimum tagging pattern needs only two
tagsone fluorous and one nonfluorousto encode four isomers.
The structure of Sch725674 is assigned as (5R,6S,8R,14R,E)-5,6,8-
trihydroxy-14-pentyloxacyclotetradec-3-en-2-one. Various compari-
sons of spectra of 32 lactones (16 with tags, 16 without) and 16
ester precursors (8 with tags, 8 without) provide insights into when
and why related compounds have the same or different spectra.
numbering system of the isolation paper, which puts stereo-
centers at C4, C5, C7, and C13, as shown in Figure 1.
INTRODUCTION
■
Fluorous mixture synthesis (FMS) is proving to be a powerful
tool to make stereoisomer libraries of natural products.1 In a
simple view of this process, molecules are labeled instead of
vials or flasks. Different fluorous tags are attached to true
isomers to make quasiisomers2 (the labeled molecules) with
encoded stereochemical information.3 The quasiisomers can
then be mixed and carried through multiple steps until a final
demixing, which is also controlled by the fluorous tags.
Three of the most important themes in fluorous mixture
synthesis have been tagging strategies,4 expedited natural
product structure assignment,5 and comparison of spectra and
properties of stereoisomer libraries.6 Here we report a total
synthesis of the complete stereoisomer library of the macro-
lactone Sch725674 that advances all three of these themes. We
introduce a new tagging strategy that uses fewer total fluorine
atoms than any other to date; we confirm the two-dimensional
structure of Sch725674 and assign the three-dimensional
structure; and we derive interesting information from
comparison of spectra of true stereoisomers and quasiisomers
both before and after macrolactonization. This is the first time
that a natural product stereostructure has been assigned from
scratch by making a complete stereoisomer library by FMS.
Macrolactones and related macrolactams are large and
important classes of natural products that are often thought
to strike a balance between preorganization and conformational
flexibility in binding to their biological targets.7 Because of
these features, diversity-oriented synthesis based on macro-
lactone templates is a lively area of research.8
Figure 1. Two-dimensional structure of Sch725674 and complete
structure of gloeosporone.
14-Membered macrolactone templates like 1 that lack methyl
groups on the backbone are rare, with the popular synthetic
target10 gloeosporone11 2 being the closest relative of 1. All
known 14-membered (and related 18-membered ring) macro-
lactone natural products have the (13R)-configuration as
established by Celmer12 and Seebach and Schreiber.11a Those
that have a hydroxyl group at C7 are also usually (7R), again
like gloeosporone.
RESULTS AND DISCUSSION
■
We describe here a second-generation synthesis of the
Sch725674 stereoisomer library. The first-generation synthesis,
described in the thesis of Dr. X. Wang,13 produced eight
isomers (including Sch725674) on a submilligram scale.
Improvements to make the synthesis robust enough to produce
all 16 isomers on a multimilligram scale and model tagging
experiments are discussed in the thesis of Dr. J. Moretti.14 Here
Sch725674 is a macrolactone that was isolated by Yang and
co-workers from a culture of Aspergillus sp.,9 and it displayed
antifungal activity against Saccharomyces cerevisiae and Candida
albicans. The two-dimensional structure was assigned as (E)-
5,6,8-trihydroxy-14-pentyloxacyclotetradec-3-en-2-one 1 based
on a battery of 2D NMR experiments. We follow the
Received: March 7, 2012
Published: April 20, 2012
© 2012 American Chemical Society
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we focus exclusively on the successful second-generation
fluorous mixture synthesis.
Tagging Plan. We set out to make all 16 stereoisomers of
Sch725674 1, and the plan that evolved for the FMS is shown
in Figure 2. Late-stage coupling of a pair of enantiopure
components, having different configuration and constitution
(specifically, fluorine content) from all the other components.
Consider first compound 6 (Figure 3) with two tagged
stereocenters. From the information standpoint, only two tags
(T1 and T2) are needed to uniquely encode four bits of
information, here the four possible configurations of the two
stereocenters. However, summing the tag numbers of the four
different components that are generated with two tags shows
that two of the components will be constitutional isomers (not
quasiisomers). These have the same fluorine content (T1 + T2
= T2 + T1), so demixing (which is based primarily on fluorine
content) will not be reliable.
The use of four different tags (in the sequence T1, T2, T3, T5)
solves this redundancy problem for 6. Less obvious but also
reliable is the use of only three tags.4 Here, the first
stereocenter gets T1 and T2. However, instead of using T3
and T5 to tag the second stereocenter (the four-tag method),
T1 is reused along with a T3. This gives products with different
fluorine count; in other words, all are quasiisomers, and none
are constitutional isomers.
We recognized that the redundancy problem that is
apparently inherent in the two-tag approach could be lifted
by putting one set of tags (here “a”) on once and the other set
of tags (“b”) on twice. This is shown in Figure 3 with a triol 7
as used in the Sch725674 setting.
Stereocenter C7 gets one tag that encodes its absolute
configuration; for example, (7R) gets T1 and (7S) gets T2. In
contrast, stereocenters C4 and C5 receive tags at the same time
and accordingly get the same tag. These tags again encode
absolute configuration, but of only one of the possible pairs of
relative configurations (two cis or two trans).
For example, the two possible trans configurations of 7,
(4R,5R) and (4S,5S), are again encoded with T1 and T2,
respectively. These are the same tags as on C7; however, each
isomer needs two tags at this point, so the fluorine content of
the second set of tags is doubled and the redundancy problem
lifted. Four quasiisomers result. The up side of this strategy is
that four quasiisomers can be tagged with two tags, but there is
also a down side: there are eight total isomers, so the other four
cis-isomers are not provided for at all. However, this fits the
Sch725674 plan well since the trans- and cis-series of precursors
will be made by different chemistry.
Figure 2. Tagging plan calls for one principal fragment 3 as two
enantiomers and the other principal fragment 4 as two mixtures of four
quasiisomers. One of these mixtures is the 4,5-trans series, and the
other is the 4,5-cis series. Coupling of the two enantiomers of 3 with
the two mixtures of 4 gives four mixtures of four tagged quasiisomers
M-5 as penultimate precursors of the 16 true isomers of 1.
fragments (R)-3 and (S)-3 with mixtures M-415 followed by
ring-closing metathesis, hydrogenation, demixing, and detag-
ging would give the final products. In turn, the preparation of
the 4,5-trans series (4R,5R and 4S,5S) and 4,5-cis series (4R,5S
and 4S,5R) isomers needed different chemistry, so two four-
compound mixtures of 4 were prepared.
The tagging pattern for each of the four-compound mixtures
needs to encode two pieces of information (configurations at
C4,5 and at C7) to allow for orderly demixing of the four
components. Information and separation (demixing) aspects of
several possible tagging strategies are shown in Figure 3. Here
the numbers (#) on the tags T# serve as proxies for fluorine
content, so the prospect for demixing of a mixture can be
assessed simply by summing the tag numbers of each
component. After the tagging is complete, each component
of the mixture should be a quasiisomer of all the other
To summarize, the 16-compound library of isomers 1 will be
made from four mixtures of four quasiisomers each. The
absolute configuration at C13 and the relative configuration at
C4/C5 are known by which mixture the sample comes from (in
other words, the labels on the flasks), while the identities of
each of the four quasiisomers that constitute those mixtures are
known by the fluorous tags (in other words, the labels on the
molecules). For the first time, the encoding and separation of
four quasiisomers is enabled with only two tags, and only one
of the two tags needs to be fluorous.
Stereoisomer Library Synthesis. Prior to starting the
fluorous mixture synthesis phase, the needed stereoisomeric
precursors were made as summarized in Schemes 1 and 2. In
the 4,5-trans series (Scheme 1), Sharpless asymmetric
dihydroxylation16 of diene 8 with Admix-α produced diol
(S,S)-9 as a 96/4 ratio of enantiomers as assessed by Mosher
ester analysis17 (see Supporting Information). The (S,S)-
configuration was encoded by attachment of two standard TIPS
groups (triisopropylsilyl, Si(iPr)3, hereafter shown as TH) to
give (S,S)-10 in 100% yield.
Figure 3. Tagging patterns for making four quasiisomers of
compounds with two (6) and three (7) stereocenters. Tags “a” and
“b” encode stereochemical information. The superscripted numbers
are proxies for the fluorine content. Notice the redundancy when two
tags encode two stereocenters that is broken when two tags encode
three stereocenters.
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a
configurations at C7 were encoded with a standard TIPS group
(TH) for (7R) and a fluorous TIPS group bearing a
pentafluoroethyl substituent (Si(iPr)2CH2CH2C2F5, hereafter
FTIPS or simply TF) for (7S). This gives (S,S,R)-13a with zero
fluorines and (S,S,S)-13b with five (suffixes a−d or e−h
indicate the tagging pattern (“a” is all three TH, etc.)).
Scheme 1. Synthesis of trans-Series Quasiisomers 13a−d
Members of the (4R,5R) series isomers were made from 8
and Admix-β, then encoded with the same two fluorous TIPS
groups (TF).19 Brown/Ramachandran allylation and isomer
purification as before provided the RRR and RRS isomers of 12
(not shown), which were again encoded with TIPS for (7R)
F
and TIPS for (7S) to give (R,R,R)-13c (10 F’s) and (R,R,S)-
13d (15 F’s). The resulting four quasiisomers were then mixed
in equimolar amounts to make the first trans-series mixture M-
13a−d.
The 4,5-cis-series quasiisomers were made by a similar series
of reactions starting from both enantiomers of 2-deoxyribose, as
detailed fully in the Supporting Information. The key allylation
results are summarized in Scheme 2.
While better diastereoselectivities (up to 8/1) were achieved
in the Brown/Ramachandran allylations, the minor C7
diastereomers of 12 could not be separated from the major
ones in any case, either before or after silylation. Contrast this
to the trans-series where the minor products for the allylation
were conveniently separated before silylation in every case.
Lacking a better option, we tagged these mixtures to give 13e−
h and moved ahead. The same tagging pattern was used in the
cis-series as in the trans-series (a/e, 0 F; b/f, 5 F; c/g, 10 F; d/h
15 F). The four quasiisomers were mixed in equimolar amounts
to make the first cis-series mixture M-13e−h.
Prior to starting the fluorous mixture syntheses in earnest, we
tested the demixing of M-13a−d and M-13e−h by analytical
fluorous HPLC. The chromatogram resulting from the trans-
series isomers is representative and is shown in Figure 4. The
quasiisomers eluted in order of fluorine content and four well-
separated peaks were observed.
a
Minor products 12 from allylboration were separated prior to
silylation. TH is −Si(iPr)3; TF is −Si(iPr)2CH2CH2C2F5.
a
Scheme 2. Synthesis of cis-Series Quasiisomers 13e−h
Figure 4. Trial demixing of M-13a−d. Compounds elute in order of
fluorine content: 13a (0 F), 13b (5 F), 13c (10 F), 13d (15 F).
FluoroFlash PF-C8 column, 90/10 MeCN/water for 10 min, then
100% MeCN.
a
The minor allylation products could not be separated so the products
12 were silylated directly. Numbers in parentheses are epimer ratios of
the tagged quasiisomers at C7.
The mixture synthesis phase of the work is summarized in
Scheme 3. Again taking the 4,5-trans-series as representative,
the methyl ester of M-13a−d was cleaved with TMSOK,20 and
then the acid was divided in half and each portion coupled with
either (R)-3 or (S)-3 under Yamaguchi conditions21 to give
esters M-(R)-14a−d and M-(S)-14a−d in good yields (80%
and 84%). Ring-closing metathesis with the second-generation
Grubbs catalyst22 followed by hydrogenation with a poisoned
Pd catalyst gave the final tagged quasiisomer mixtures M-(R)-
15a−d and M-(S)-15a−d (88% and 87%). The SrCO3 poison
Standard removal of the PMB group and Swern oxidation
provided aldehyde (S,S)-11, which was divided in half and
exposed to both enantiomers of the Brown/Ramachandran
reagent,18 Ipc2B(allyl). Each reaction produced a major
diastereomer, (SSR)-12 or (SSS)-12, in about 4/1 ratio along
with the C7 epimer. To ensure maximum stereopurity of the
tagged isomers, these mixtures were separated to give the two
pure isomers in 59% and 67% yield, respectively. Then the
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Scheme 3. Fluorous Mixture Synthesis Provides the
Table 2. Summary of Demixing and Detagging of Lactones
Penultimate Precursors 15 of the Final 16 Stereoisomers of 1
M-(R)-15a−h and M-(S)-15a−h to Give the 16 Final
a
a
as Four Mixtures of Four Quasiisomers
Isomers of 1
b
isomer of 1
precursor
amount
yield
a
(R)/(S) represents the configuration at C13; a−d (4,5-trans series)
c
(4S,5S,7R,13R)
(4S,5S,7S,13R)
(4R,5R,7R,13R)
(4R,5R,7S,13R)
(4S,5S,7R,13S)
(4S,5S,7S,13S)
(4R,5R,7R,13S)
(4R,5R,7S,13S)
(4S,5R,7R,13R)
(4S,5R,7S,13R)
(4R,5S,7R,13R)
(4R,5S,7S,13R)
(4S,5R,7R,13S)
(4S,5R,7S,13S)
(4R,5S,7R,13S)
(4R,5S,7S,13S)
(R)-15a
(R)-15b
(R)-15c
(R)-15d
(S)-15a
(S)-15b
(S)-15c
(S)-15d
(R)-15e
(R)-15f
(R)-15g
(R)-15h
(S)-15e
(S)-15f
(S)-15g
(S)-15h
6 mg
19%
or e−h (4,5-cis series) represents the various quasiisomers at C4/C5
and C7. Conditions: (1) TMSOK, Et2O, rt, 16 h; (2) 2,4,6-
trichlorobenzoyl chloride, DMAP, Et3N, toluene, rt, 3 h; (3) 2nd
generation Grubbs catalyst, CH2Cl2, reflux, 48 h; (4) Pd/SrCO3,
hydrogen balloon, EtOH, rt, 1 h.
15 mg
13 mg
8 mg
51%
60%
51%
c
5 mg
15%
17 mg
3 mg
67%
c
14%
was important since other conditions often gave side products
in which both double bonds were saturated.
The same reactions were conducted on the cis-series M-13e−
h with similar yields to provide the other two mixtures of four
quasiisomers, M-(R)-15e−h and M-(S)-15e−h. The final four
mixtures of four quasiisomers were produced in substantial
quantities (>200 mg each).
11 mg
11 mg
12 mg
11 mg
5 mg
29%
c
29%
cd
,
37%
32%
40%
45%
17%
73%
36%
c
c
c
c
c
c
13 mg
6 mg
Demixing and Detagging. The results of the demixing
14 mg
7 mg
and detagging are summarized globally in Tables 1 and 2. In
a
Table 1. Summary of Demixing and Detagging of Open
Esters M-(R)-14a−d and M-(S)-14a−d to Give Eight Open
Esters 16 in the 4,5-trans Series
The assigned structure of Sch725674 is shown as an example.
b
c
Includes demixing, detagging, and flash chromatography. Repurified
d
by chiral chromatography. Contains 6% of the C7 epimer.
(see Figures S1 and S2 in the SI). Four well-separated peaks
were observed in each case, and no cross-contamination of
quasiisomers occurred. The pure quasiisomers were charac-
terized (see below) then desilylated with TBAF in THF. The
crude products were purified by flash chromatography to give
the eight stereoisomeric trihydroxy esters 16 shown in Table 1
in overall yields of 60−88%.
a
The demixings of the four lactone quasiisomer mixtures M-
15 (∼90 mg injections) are summarized in Table 2. These
separations presented solvable problems. Demixing of the two
trans-series isomers M-(R)-15a−d and M-(S)-15a−d and
detagging provided the eight trans triol lactone isomers after
chromatographic purification, but only five of these were pure
(see Figures S3 and S4 in the SI). The two samples containing
three TIPS groups and therefore zero fluorine atoms ((R)-15a
and (S)-15a) were poorly retained by the column. They
contained nonisomeric impurities that we think resulted from
inadequate separation of this first peak from accumulated
nonfluorinated impurities in the solvent front. With 10
fluorines, quasiisomer (S)-15c was the third-eluting component
in its mixture, and it was contaminated with about 25% of the
prior, second-eluting quasiisomer (S)-15b of the mixture.
Of the several ways to solve these problems, we found it
most expeditious just to detag the three mixtures and purify the
final products by preparative chiral chromatography. An (S,S)-
Whelk-O-1 column was used for the triols derived from
detagging of (R)-15a and (S)-15a, and a Chiralcel OD column
was used for the triol from (S)-15c.
isomer of 16
precursor
amount
yield
(4S,5S,7R,13R)
(4S,5S,7S,13R)
(4R,5R,7R,13R)
(4R,5R,7S,13R)
(4S,5S,7R,13S)
(4S,5S,7S,13S)
(4R,5R,7R,13S)
(4R,5R,7S,13S)
(R)-14a
(R)-14b
(R)-14c
(R)-14d
(S)-14a
(S)-14b
(S)-14c
(S)-14d
17 mg
16 mg
26 mg
11 mg
24 mg
30 mg
24 mg
29 mg
65%
60%
69%
63%
67%
85%
77%
88%
a
Includes demixing, detagging, and flash chromatography.
total, 24 individual, pure samples were produced after
detagging. Eight ester stereoisomers 16 in the trans-series
were isolated for comparison of spectra of pairs of ring-open
and ring-closed analogues with the same configurations, and all
16 Sch725674 stereoisomers 1 were isolated to compare with
each other and with the natural product. This necessitated six
preparative demixings (two ester mixtures and four macro-
lactone mixtures), which are detailed in the Supporting
Information.
Briefly, the preparative demixings of two trans-series ester
mixtures M-(R)-14a−d and M-(S)-14a−d were straightforward
In the demixing of the two cis-series quasiisomer mixtures
(SI, Figures S5 and S6), we were more careful to separate the
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quasiisomers with zero fluorines from solvent front, and this
time seven of the eight products were obtained in good quality.
One of the third eluting quasiisomers had some cross
contamination of the prior quasiisomer, and this was simply
demixed a second time to remove the contaminant.
Fuller discussions of spectra comparisons can be found in the
theses of Drs. X. Wang13 and J. Moretti.14 Here we focus on
selected NMR spectral data that are especially informative.
Comparison of the NMR spectra of the tagged isomers 14 and
15 is limited because these spectra depend on the structure of
the tag. Unlike most prior work, the two tags here are not
homologues. However, we can still compare spectra of ester 14
(open) and lactone 15 (closed) analogues that have the same
tags.
Recall that the eight quasiisomers in the cis-series (second
group of eight) are not stereoisomerically pure because we
could not separate the minor products of C7 after the allylation.
However, because the minor product of one sample is the
major product of another sample, it proved easy to analyze all
The CF2 regions of the 19F spectra of two related pairs of
lactones and esters are shown in Figure 5. The compounds
1
eight samples by H NMR spectroscopy. The identity and
amount (10−25%) of each minor isomer was as expected from
the allylboration results in Scheme 2 (see the SI for the actual
ratios).
In preliminary experiments, one of the cis-series isomers was
desilylated slowly with TBAF in THF, so we switched to HF/
CH3CN/H2O for preparative detagging. All eight samples were
reliably desilylated, and chiral HPLC analyses showed one
major and one minor peak for each sample. Finally, preparative
chiral HPLC purification provided the eight final samples, seven
of which were diastereomerically pure. The eighth sample,
(4S,5R,7S,13R)-1, had the misfortune of arising from
quasiisomer (R)-15f with the lowest starting isomer ratio
(75/25) and the tightest separation on chiral HPLC. This was
obtained in an enriched 94/6 ratio of diastereomers and as such
is the only one of the 24 final samples with a detectable
isomeric impurity. Substantial amounts of all 16 of the final
lactone isomers (3−17 mg) were obtained, as shown in Table
2.
Isomer Characterization and Structure Assignment of
Sch725674. The fluorous mixture synthesis and demixing
produced in total 48 individual samples grouped as 24 with tags
and 24 without tags. Within each grouping of 24, there are 8
open chain esters (14 or 16, all isomers in the trans-series) and
16 closed lactones (15 or 1, all isomers in both trans- and cis-
series).
Figure 5. Comparison of the CF2 resonances of two lactones 15 (left)
and esters 14 (right) with three fluorous silyl tags (−Si-
(iPr)2CH2CH2C2F5). The top spectra are (13R) isomers; the bottom
spectra are (13S). The other stereocenters are each (4R,5R,7S).
have the same tags (three FTIPS groups) and the same
configurations at C4, C5, and C7, differing only in the
configuration at C13. In theory, there should be three triplets
because there are three heterotopic CF2 groups (which couple
to the adjacent CH2 groups but not to the adjacent CF3
groups). In the two esters (right-side pair of spectra), the
resonances look like broad quartets and are identical. In the two
lactones (left-side pair), one set of resonances is three clear
triplets, and the other is three overlapping but easily assigned
triplets. The difference in chemical shifts between the two
lactones is remarkable given how remote the CF2 groups are
from the stereocenter at C13 (12 atoms, 14 atoms, and 16
atoms at the shortest counts).
The members of the tagged set of 24 (14 and 15) were
1
characterized by the usual means with 1D H and 13C NMR
spectra and HRMS. Characterization of the final 24 detagged
compounds (16 and 1) depended on enantiomeric series.
Members of one enantiomeric series were characterized by IR,
HRMS, chiral HPLC analysis, optical rotation, 1D NMR
spectra, and a set of full 2D NMR experiments (1H−1H-COSY,
This trend (ester spectra identical, lactone spectra different)
held for the other three pairs of tagged C13 epimers, as
summarized in Figure 6. Also, the 1H and 13C NMR spectra of
the pairs of (R/S)-epimers of C13 of tagged lactones 15 were
different, but the pairs of tagged esters 14 were the same. In
1
1H−13C-HMQC, and H−13C-HMBC). In this way, substan-
tially all the protons and carbon resonances were assigned in all
the final isomers.23
Members of the other enantiomeric series were characterized
1
by chiral HPLC analysis, H NMR spectroscopy, and optical
rotation. This produced a substantial data set (presented as
Tables S1−S3, S5, S6 and copies of spectra in the SI), which
could have multiple uses. Here we use selected data for
comparison with each other and for structure assignment of
Sch725674.
The values of the optical rotations of 1 are shown in the SI,
Table S4 (the concentrations (c) in MeOH were all about 1 g/
100 mL). The magnitudes of the optical rotations of the final
isomers 1 can be loosely characterized as low to moderate,
ranging from about 3 up to 40. The values are spread out
enough to be informative in many cases. In other words, the
structure of an unknown isomer could be reduced to two or
three candidates (sometimes even one) based on optical
rotation alone. The optical rotation of the natural sample of
Sch725674 was not recorded, so comparison is not possible.
Figure 6. Groups of stereocenters (C13 and C4/C5/C7) behave
independently in the spectra of the esters 14 and 16 but are mutually
dependent in the spectra of the lactones 15 and 1.
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addition, the same trend held for all of the compounds after
detagging to make the free alcohols. In other words, all eight
diastereomers of the lactones 1 gave clearly different H and
and 5. In other words, from the chemical shift of any one
resonance selected from any one of these three H’s, the
stereostructure can be uniquely assigned in every case!
Likewise, the alkene protons (H2, H3) exhibited large
differences from isomer to isomer (see SI). In previous
stereoisomer libraries, we have been surprised by the
similarities in the spectra. In this library, we are surprised by
the differences.
There are also differences in the coupling constants from
isomer to isomer, but these seem to represent primarily local
effects of the group of three stereocenters. For example, H4 has
vicinal protons on each side and can have a long-range allylic
coupling as well. When H4 and H5 are trans, these J’s play out
as a broad triplet or a doublet of triplets (the first four spectra,
two larger J’s and one smaller J). When H4 and H5 are cis, then
a narrower peak shape results from three relatively small J’s (the
last four spectra). In other words, the closure of the macrocycle
changes the chemical shifts more than the coupling constants.
This conclusion is reinforced by comparing 1H NMR spectra
of ester and lactone pairs that have the same configurations at
C4, C5, and C7 but differ at C13. There are four sets of these
groupings, one of which is shown in Figure 8. As mentioned
above, the spectra of the two esters are identical to each other,
and the spectra of the two lactones are different from each
other.
1
13C NMR spectra, while the four diastereomeric esters 16
(trans-series only) gave two pairs of identical 1H and 13C NMR
spectra. The pairs differed only in the configuration at C13.
In the open chain esters 14 and 16, the conformation of the
ester C−O bond is trans, so C13 in the ester oxygen substituent
is remote from the other three stereocenters in the carbonyl
substituent. In no case can any spectrum (19F, 1H, or 13C) sense
this difference. In the lactones 15 and 1, the comparable C−O
bond is probably also trans, but the two parts of the molecule
are now connected through the carbon ring backbone as well. A
change in configuration in one part of the 14-membered ring
can alter conformational populations of the ring and is
therefore sensed in the spectral features of the other part of
the ring.
This principlethat spectra of large rings with remote
stereocenters are more sensitive to configuration changes than
spectra of comparable acyclic moleculesseems intuitively
sensible. However, it is nonetheless impressive how the effect
plays out in these libraries. In the open systems, every one of
the 11 pairs of spectra compared was identical to its partner. In
the closed systems, not one of the 22 pairs of spectra compared
was identical.
Accordingly, each one of the final eight lactone diastereomers
1 exhibited a unique 1H and 13C NMR spectrum; no spectrum
was substantially identical to any other spectrum. This makes
assignment of the relative configuration of Sch725674
unambiguous; its published spectra were clearly different
from seven of the diastereomers and matched only those of
(4R,5S,7R,13R)-1 and its enantiomer. Since all such lactone
natural products have the (13R) configuration,11 we can
confidently assign the former structure to 1 as shown in Table
2.
Figure 7 illustrates some of the most obvious differences in
the 1H NMR spectra of 1 with expansions of the regions of the
three carbinol protons, H4, H5, and H7. Not one of these
spectra has one single resonance that overlaps. This can be seen
in every case by eyeball except perhaps that of H5 in spectra 4
Figure 8. Expansions of the carbinol regions of the 1H NMR spectra of
two lactones 1 (top) and two esters (bottom) 16 with the same
configurations at C4, C5, and C7, differing only at C13.
In comparing the chemical shifts of these spectra of the esters
to the lactones, it might be prudent to ignore H7, which is
homoallylic in the esters 16 but not in the lactones 1 (the
alkene is removed after RCM and reduction). No resonance of
H4 or H5 of the lactone matches its ester counterpart, and
while there are some differences in the lactone and ester
coupling constants, it is the differences in chemical shifts that
are more remarkable.
CONCLUSIONS
■
We have described a fluorous mixture synthesis of all 16
stereoisomers of the macrocyclic lactone natural product
Sch725674. The general approach to making four mixtures of
four fluorous-tagged quasiisomers each is by now standard;
however, the tagging pattern has been advanced significantly in
two ways. First, only two tags are used, and only one of these is
fluorous. Second, only 30 fluorine atoms are used in total to
make all four quasiisomers. With the previous tag sets in
comparable libraries, the smallest total number of fluorines was
1
Figure 7. Expansions of the carbinol regions of the H NMR spectra
(700 MHz, CD3OD) of the (13R) series of lactones 1.
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Article
46.4b The new approach is atom economical, and larger,
environmentally persistent groups26 (C6F13, C8F17) are not
used.
AUTHOR INFORMATION
■
Corresponding Author
curran@pitt.edu
After demixing and detagging, the structure of Sch725674
was assigned as (4R,5S,7R,13R)-1 by comparison of the
published spectra to the library spectra. This is the first time
that a complete stereoisomer library has been made by FMS to
assign a natural product of completely unknown configuration.
A substantial amount of data were produced by character-
izing the 16 open esters and the 32 lactones. In each case, half
of the compounds had three silyl tags, and half had three free
hydroxy groups. These data took significant time to collect but
provided substantial information beyond the structure of
Sch725674. For example, compared to previous libraries that
we have made, we were surprised by the large differences
between all of the spectra of the final lactones 1. In the 28
possible comparisons of pairs of spectra, there is little
Author Contributions
†These authors contributed equally to this project.
Notes
The authors declare the following competing financial
interest(s): HPLC columns were purchased from Fluorous
Technologies, Inc. DPC owns an equity interest in this
company. The other authors declare no competing financial
interest.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health, National Institute
of General Medical Sciences, for funding of this work. We
thank Dr. Damodaran Krishnan and Ms. Sage Bowser for help
with the NMR spectra.
1
overlapping at all of the H spectra except in the regions of
the consecutive methylene groups. In contrast to the lactones,
the spectra of the esters came in pairs with two isomers
differing at C13 exhibiting substantially identical spectra.
It was also surprising that the differences in chemical shifts in
REFERENCES
■
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1
the H NMR spectra of the various lactone isomers are more
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C
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using various models of NMR spectra to assign configurations
of macrolactones. The caution is for use of NMR spectra of
model compounds to assign stereochemistry.24 The technique
has great power that is increasing with the size of the databases.
However, at least for 14-membered lactones, a database of
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This provides a present challenge to NMR modelers. Pose
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eight possible diastereomers? In other words, calculate the eight
possible 13C NMR spectra of the diastereomers of 1 and match
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case, then you could have assigned the structure of Sch725674
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ASSOCIATED CONTENT
■
S
* Supporting Information
Contains complete experimental details, tabular NMR data,
supplemental Figures, and copies of NMR spectra of the
stereoisomer library members. This material is available free of
charge via the Internet at http://pubs.acs.org.
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(19) We use the FTIPS abbreviation for consistency, but more
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rather than a triisopropylsilyl group.
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1
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University of Pittsburgh Center for Methodology and Library
Development (CMLD) and are available for biological testing or in
the event that additional spectra are needed.
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