The absolute configuration for inthomycin C: Revision of previously published work with a reinstatement of the (3 R)-configuration for (-)-inthomycin C

Article abstract of DOI:10.1021/ol501484t

Stereochemical evidence is presented to demonstrate that (-)-inthomycin C has (3R)- and not (3S)-stereochemistry. Careful reappraisal of the previously published work2-5 now indicates that the Hatakeyama, Hale, Ryu, and Taylor teams all have synthesized (-)-(3R)-inthomycin C. The newly measured [α] _D of pure (-)-(3R)-inthomycin C (98% ee) is -7.9 (c 0.33, CHCl ₃) and not -41.5 (c 0.1, CHCl₃) as was previously reported in 2012.

Full text of DOI:10.1021/ol501484t

Letter
pubs.acs.org/OrgLett
The Absolute Configuration for Inthomycin C: Revision of Previously
Published Work with a Reinstatement of the (3R)‑Configuration for
(−)-Inthomycin C
Karl J. Hale,*,† Susumi Hatakeyama,*,‡ Fumiya Urabe,‡ Jun Ishihara,‡ Soraya Manaviazar,†
Milosz Grabski,† and Maciej Maczka†
†The School of Chemistry and Chemical Engineering and the Centre for Cancer Research and Cell Biology (CCRCB), The Queen’s
University Belfast, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, United Kingdom
‡The Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki 852-8521, Japan
S
* Supporting Information
ABSTRACT: Stereochemical evidence is presented to demon-
strate that (−)-inthomycin C has (3R)- and not (3S)-stereo-
chemistry. Careful reappraisal of the previously published work2−5
now indicates that the Hatakeyama, Hale, Ryu, and Taylor teams all
have synthesized (−)-(3R)-inthomycin C. The newly measured
[α]D of pure (−)-(3R)-inthomycin C (98% ee) is −7.9 (c 0.33,
CHCl3) and not −41.5 (c 0.1, CHCl3) as was previously reported in
2012.
olecules of the inthomycin/phthoxazolin class1 continue
Scheme 1. 2010 (−)-(3R)-Inthomycin C Synthesis of Ryu et
al.
Mto excite the organic chemistry community due to their
novel oxazolo-triene structures and their diverse biological
actions which, in some instances, are associated with significant
anticancer, insecticidal, and herbicidal effects. So far, the greater
bulk of synthetic effort on this class2−6 has focused on the total
synthesis of (3R)-inthomycin C,2−5 which, despite the small size
and deceptively simple structure, has proven a most challenging
and troublesome molecule for synthesis due to the diverse
collection of unusually interposed functional groups.
The molecular constitution of inthomycin C was first disclosed
by Henkel and Zeeck in 1991,1a who suggested that inthomycin
C possessed (3R)-stereochemistry at the hydroxyl stereocenter.
This followed derivatization as a 3-O-(S)-2-phenylbutanoate
ester and careful 2D-NMR comparisons with other family
members. Unfortunately, the Zeeck group was never able to
measure the [α]D for natural (3R)-inthomycin C, due to their
inability to obtain pure material. Subsequently, a successful total
synthesis of (3R)-inthomycin C by Taylor et al.2 led to an [α]D of
+25.9 (c 0.27, CHCl3) for material of 76% ee, contaminated with
20% tetramethylurea. Thereafter, the natural product was given a
(−)-inthomycin C, basing the assignment solely on analogy with
other asymmetric aldol reactions that were described in their Org.
Lett. paper.3
The issue of inthomycin C stereochemistry took a further turn
in 2012, when the Hatakeyama laboratory reported a second
total synthesis of (−)-(3R)-inthomycin C4 that proceeded by
way of the chiral β-lactone 8 and the (3R)-enynol 9 (Scheme 2).
On this occasion, an [α]D of −41.5 (c 0.1, CHCl3) was recorded
(+)-designation.
That status persisted until 2010, when the Ryu group reported
a new total synthesis of (−)-(3R)-inthomycin C3 that exploited
an asymmetric aldol reaction with the chiral oxazaborolidinium
triflate catalyst 3 to provide the C(3)-hydroxyl stereocenter in
the target (Scheme 1). Significantly, the Ryu team thereafter
recorded an [α]D of −34.33 (c 0.1, CHCl3) for pure (−)-(3R)-
inthomycin C of 93% ee.3 Although this [α]D measurement was
similar in magnitude to the value recorded by Taylor et al. (given
the ee),2 the sign was opposite. Despite the discrepancy, the Ryu
team never went on to confirm their new (3R)-assignment for
Received: May 22, 2014
© XXXX American Chemical Society
A
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Scheme 2. A Summary of the Stereochemically Correct Total
Synthesis of (−)-(3R)-Inthomycin C By the Hatakeyama
Group4 with Its Two Incorrect [α]D Values
Scheme 3. A Summary of the Correct Hale Group Total
Synthesis of (3R)-Inthomycin C With Its [α]D Values5
for (−)-(3R)-inthomycin C, which possessed 98% ee. The
Hatakeyama group’s stereochemical assignment was based upon
the successful total synthesis of (+)-(3R)-inthomycins A and B,
which demonstrated that the (3R)-configuration emerged from
the key TMSQD-catalyzed asymmetric [2 + 2]-ketene aldehyde
cycloaddition that was being used to control C(3)-stereo-
chemistry. Given that only the geometry of the enal was changed
in this total synthesis of (3R)-inthomycin C, the Hatakeyama
team saw no reason why such a change would dramatically alter
the stereochemical outcome of the new cycloaddition to obtain 8.
Thus, after securing a full total synthesis of (−)-(3R)-inthomycin
C via the route summarized in Scheme 2, the Hatakeyama group
did not question the (3R)-assignment that had been made, since
Ryu and co-workers had also obtained an [α]D of −34.33 (c 0.1,
CHCl3) for a product of apparently identical (3R)-configuration
and slightly lower ee (93%).3 The Hatakeyama results were
published, with the belief that the (+) [α]D measurement
recorded by Taylor et al.2 for (3R)-inthomycin C might have
arisen from the 20% tetramethylurea contaminent that was also
present in their sample.
Not long after the Hatakeyama report, another paper appeared
in Org. Lett. from the Hale laboratory,5 where a new total
synthesis of (3R)-inthomycin C was described that intersected
directly with the Ryu and Hatakeyama group syntheses. The
Hale synthesis used a Carreira asymmetric alkynylation to forge
the C(3)-hydroxyl stereochemistry of 10 (Scheme 3) and an O-
directed free radical alkyne hydrostannation reaction with
Ph3SnH/catalytic Et3B to set the target’s trisubstituted olefin
geometry. Significantly, the Hale team provided the first
incontrovertible evidence for the (3R)-hydroxyl configuration
of dienylstannane 4, an intermediate that was ultimately used by
them to complete their total synthesis. In this regard, Hale et al.
reported that 4 had an [α]D of +5.3 (c 1.11, CHCl3).5 The same
(3R)-intermediate was also featured in the synthesis of Ryu et al.
(see Scheme 1), but strikingly, these workers reported an [α]D of
−17.5 (c 0.12, CHCl3) for material of 93% ee.3
In light of these discrepant findings, the Hale group undertook
a synthesis of the (3R)-enynol 9 of the Hatakeyama pathway,5
starting from diol 12 which had previously been featured in the
Hale synthesis of (+)-4. This led Hale et al. to complete a second
formal total synthesis of (3R)-inthomycin C.5 Significantly, the
Hale team now recorded an [α]D of +12.6 (c 0.71, CHCl3) for
their (3R)-enynol 9 of 83% ee. Yet, in the earlier 2012 Org.
Biomol. Chem. report of Hatakeyama et al.,4 the (3R)-alkynol 9
had been indicated to have an [α]D of −15.9 (c 1.04, CHCl3) for
material of 98% ee.
On the basis of the Hatakeyama group’s Org. Biomol. Chem.
paper,4 and the Hale team’s definitive assignment of (3R)-
stereochemistry to the alcohol in (+)-9 (via a second Mosher
ester analysis study), Hale et al. concluded5 that the Hatakeyama
group must have prepared the (3S)-enantiomer of enynol 9.
Moreover, since the Hatakeyama team had also recorded an [α]D
of −41.5 (c 0.1, CHCl3) for inthomycin C,4 which was opposite
in sign to the value of Taylor et al.,2 the Hale team further
concluded5 that the Hatakeyama group must have prepared
(3S)-inthomycin C, analogously to Ryu et al.,3 since the Taylor
team2 had apparently confirmed their (3R)-stereochemistry for
synthetic (+)-inthomycin C by Mosher ester analysis.
Given the Hatakeyama laboratory’s concern that a potential
flaw had been uncovered in their published Org. Biomol. Chem.
work, they immediately decided to recheck their laboratory
records to see if an inadvertent error had been made in their
synthesis, or if there had been a problem in the reporting of their
experimental data. As part of that review the Hatakeyama team
reexamined their [α]D data for the (3R)-enynol 9, and to their
dismay, they found that a serious error had somehow crept into
their published 2012 Org. Biomol. Chem. manuscript.4 Rather
than the Hatakeyama group (3R)-enynol 9 having an [α]D of
−15.9 (c 1.04, CHCl3), as was originally stated (see Scheme 2),
the Hatakeyama laboratory records actually revealed an [α]D of
+15.6 (c 1.04, CHCl3) for 9 (see the Supporting Information
(SI) for this actual measurement). The correct +15.6 [α]D
measurement now suggested that the Hale and Hatakeyama
teams had intersected after all and that both teams had indeed
completed total syntheses of (−)-(3R)-inthomycin C!
Unlike the Ryu team, however, the Hale group had definitively
assigned their absolute stereochemistry for (+)-4 by performing
a Mosher ester analytical study on the Carreira alkynol precursor
10. As a result, their assignment was completely secure. The Hale
data clearly suggested that the Ryu group had synthesized the
enantiomeric (S)-dienylstannane, and consequentially (3S)-
(−)-inthomycin C, a conclusion that became even more credible
when considered alongside the report of Taylor et al. that (3R)-
inthomycin C had an [α]D of +25.9 (c 0.27, CHCl3) for material
of 76% ee contaminated with 20% tetramethylurea.2
The Hatakeyama team subsequently searched through their
compound collection and found that they had a sample of
B
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authentic (3R)-(+)-enynol 9 still available from their 2012 total
synthesis of (−)-(3R)-inthomycin C. They then employed the
Hale et al.5 experimental procedures to convert this sample of 9
into the (R)- and (S)-MTPA Mosher esters. When the NMR
spectra of the resulting esters were compared with those in the
2014 Org. Lett. report of Hale et al.,5 an excellent (almost perfect)
match was found (see the SI). The Hatakeyama team then
measured the [α]D of their original sample of (3R)-9, following a
further purification by SiO2 flash chromatography, and found
that (3R)-9 had an [α]D of +12.2 (c 0.95, CHCl3) which
correlated nicely with the +12.6 value reported by Hale et al.
earlier this year.
Scheme 4. (3R)-Inthomycin C Mosher Ester Analyses
Undertaken by the Hale and Hatakeyama Teams
Hatakeyama next contacted the Hale team to alert them to the
error that had appeared in the 2012 Org. Biomol. Chem. paper.4
The two groups then worked together to resolve the discordant
[α]D values that had been published. Hatakeyama agreed to send
Hale their purified authentic sample of the (3R)-enynol 9 for
independent NMR comparison and verification of its [α]D.
When the Hale group ran the 400 MHz 1H NMR spectra of the
Hatakeyama sample of 9 in CDCl3, they found that it matched
theirs in every respect and was highly pure. The Hale group also
recorded the [α]D of the sample of (3R)-9 obtained from the
Hatakeyama team and independently found it to be +14.4 (c
0.58, CHCl3), which now correlated very well with the +12.6
report for authentic (3R)-9 of 83% ee published by the Hale
group. The two teams now agreed that both had synthesized the
same (3R)-compound, and moreover, they accepted that the
Hatakeyama team had most definitely prepared (3R)-inthomycin
C and not the (3S)-enantiomeric counterpart.
To put this correlation beyond any future doubt, Hale and
Hatakeyama decided that they would both try to prepare the (R)-
and (S)-MTPA esters of their respective (3R)-inthomycin C
samples, to unambiguously establish their absolute configu-
rations by Mosher ester NMR analysis, and provide further
corroboration that both groups had synthesized the same
product. We also wished to make these spectra available to other
groups currently working in the field to assist their future
assignments.
The aforementioned NMR studies on the (3R)-inthomycin C
Mosher ester samples 13 and 14 in CDCl3 revealed some quite
spectacular chemical shift differences for the H(5)-proton of the
trisubstituted alkene that lay adjacent to the C(3)-O-stereo-
center. In the (R)-MTPA ester 13, this proton resonated at δ
5.9611 ppm, which was ∼0.1589 ppm more shielded than the
corresponding resonance for the (S)-MTPA ester 14, which
appeared at δ 6.1200 ppm. The C(4)-Me group for the (R)-
MTPA ester 13 likewise appeared at δ 1.7618 ppm, which is
0.0507 ppm more shielded than that for the (S)-MTPA ester 14,
which resonated at δ 1.8125 ppm. This is what one would
anticipate. Likewise the geminal dimethyl groups of 13 and 14
showed all of the expected shielding−deshielding trends.
When the Hatakeyama team reanalyzed their original 2012
sample of (−)-(3R)-inthomycin C, they found that it had
decomposed. It was therefore agreed that the Hatakeyama group
would convert their remaining sample of the (+)-(3R)-enynol 9
into pure, 98% ee, (3R)-inthomycin C, and that the [α]D of this
material would be measured, to put the issue of the [α]D for
(3R)-inthomycin C beyond all question.
Since the Hatakeyama team was now the only one to have a
totally pure, 98% ee, sample of (3R)-inthomycin C in hand, the
[α]D of their freshly resynthesized material was measured and
found to be −7.9 (c 0.33, CHCl3) (Figure 1). Thus, (3R)-
Fortunately, the Hale group still possessed a sample of
chemically synthesized (3R)-inthomycin C in their compound
collection that had never been exposed to the ravages of CDCl3,
and which had withstood storage in the freezer at −28 °C for
seven months under N2. Proton analysis at 400 MHz of that
sample in CD3OD confirmed the pure condition, consisting of a
5.6:1 mixture of (3R)-inthomycin C and the previously
described5 unknown geometric stereoisomer. The Hale team
therefore converted this (3R)-inthomycin C sample into the (R)-
and (S)-MTPA ester derivatives 13 and 14 by treatment with the
(R)- and (S)-MTPA acids, DCC, and DMAP in CH2Cl2. The key
data that they gathered are presented in Scheme 4. The analysis
unambiguously confirmed that the originally assigned (3R)-
configuration of the Hale sample of inthomycin C was correct.
Concurrent with this, the Hatakeyama group did the same and,
once more, a perfect match was obtained with the Hale data,
confirming that the Hale and Hatakeyama laboratories had both
synthesized (3R)-inthomycin C (see the SI).
Figure 1. Corrected Hatakeyama Team [α]D Data.
inthomycin C does indeed have a negative [α]D when pure in
CHCl3. This value however is much lower in magnitude than the
original [α]D measurement of the 2012 Org. Biomol. Chem.
paper.4 The latter value, which was −41.5 (c 0.1, CHCl3), now
appears to be erroneous. Moreover, the newly recorded [α]D of
−7.9 lies much closer to the Hale group5 [α]D value of −8.4 (c
1.0, CHCl3) reported for the 5.9:1 mixture that they obtained
using the Ryu team’s Stille cross-coupling.3
C
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The newly measured [α]D for (−)-(3R)-inthomycin C is also
significantly lower than the [α]D value of −34.33 (c 0.1, CHCl3)
of Ryu et al.,3 which our two teams now believe to be in error,
certainly as regards the magnitude. However, the fact that our
two groups and the Ryu team each recorded a negative [α]D for
(3R)-inthomycin C very strongly suggests that the original
positive [α]D measured by Taylor et al.2 needs to be reevaluated.
Consequentially, the Hale group now no longer claims a total
synthesis of (+)-(3R)-inthomycin C, as was published in our
2014 Org. Lett. communication,5 but rather a total synthesis
(−)-(3R)-inthomycin C, due to our intersection with the
Hatakeyama laboratory (+)-(3R)-enynol 94 whose [α]D value
has now been corrected here.
Of course, there is still the outstanding issue of the Ryu team
[α]D measurement for 4, which was reported to be −17.5 (c 0.12,
CHCl3),3 and which is different from the Hale group
measurement of [α]D +5.3 (c 1.1, CHCl3)5 for a molecule of
proven (3R)-stereochemistry. Again, we believe that the [α]D
measured for 4 by Ryu et al. needs to be carefully reevaluated,
since there are significant discrepancies between a number of the
[α]D measurements in the Ryu group paper and those reported
by the Hale and Hatakeyama teams, including the trienyl methyl
ester 6. While there is an acceptable agreement between the
Hatakeyama4 and Hale5 team [α]D values for 6 [Hale et al. [α]D
−0.43 (c 0.7, CHCl3) for a 17:1 mixture (83% ee); Hatakeyama
et al. [α]D +0.78 (c 1.39, CHCl3) for pure material of 98% ee], the
values of Taylor et al.2 for 6 [[α]D +5.2 (c 1.55, CHCl3) 76% ee]
and Ryu et al.3 [[α]D +8.48 (c 0.9, CHCl3) 93% ee] both deviate
significantly from ours.
that Ryu et al. have indeed prepared (−)-(3R)-inthomycin C, as
was originally claimed,3 notwithstanding the discrepant [α]D
data that they report for 4, 6, and (−)-(3R)-inthomycin C itself.
The combined new stereochemical and [α]D evidence that we
have gathered with the Hatakeyama group confirms that the Hale
and Hatakeyama teams have both synthesized (−)-(3R)-
inthomycin C by their respective routes.4,5 In the case of the
Hale team, while the depictions of the absolute stereochemistry
in our 2014 Org. Lett. paper5 still remain the same, our previous
assertion that a synthesis of (+)-(3R)-inthomycin C had been
achieved must now be corrected to a claim of a synthesis of
(−)-(3R)-inthomycin C instead.5
We trust that other workers in the field will feel much greater
confidence in venturing toward (−)-(3R)-inthomycin C now
that the absolute configuration of this molecule has been securely
assigned.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details, NMR spectra, and the Hatakeyama
group [α]D measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: k.j.hale@qub.ac.uk.
*E-mail: susumi@nagasaki-u.ac.jp.
Notes
So where exactly does this leave the community with regard to
the status of the various total syntheses that have so far been
accomplished of (3R)-inthomycin C? After carefully reviewing all
of the available published evidence, and our own newly acquired
(R)- and (S)-MTPA ester data on (−)-(3R)-(−)-inthomycin C,
we believe that the Taylor, Ryu, Hatakeyama, and Hale groups
have each prepared (−)-(3R)-inthomycin C.
The authors declare no competing financial interest.
REFERENCES
■
(1) (a) For the isolation and structures of inthomycins A, B, and C,
see: Henkel, T.; Zeeck, A. Liebigs Ann. Chem. 1991, 367. (b) For the
first inthomycin ((+)-phthoxazolin/(+)-inthomycin A) to be discov-
ered, see: Omura, S.; Tanaka, Y.; Kanaya, I.; Shinose, M.; Takahashi, Y.
J. Antibiot. 1990, 43, 1034.
A fresh reappraisal of the syntheses of Taylor et al.2 and Ryu et
al.3 now very strongly suggests that each of them have
synthesized (−)-(3R)-inthomycin C, despite their discrepant
[α]D values. In the case of Taylor et al., we believe that their team
has achieved this target based on their correct total synthesis of
(+)-(3R)-inthomycin B and the Mosher ester data that they have
reported for their methyl ester precursor of (+)-inthomycin B.
The latter very clearly shows the same sort of chemical shift
trends that we observed for the (R)- and (S)-MTPA esters of
(−)-(3R)-inthomycin C, which is precisely what one would
expect. Also, the Kiyooka asymmetric aldol process used by
Taylor et al. would be expected to give rise to the (3R)-product,
and it did successfully provide (+)-(3R)-inthomycin B, whose
structure is not in doubt. Taken together, the combined weight of
evidence suggests that the Taylor team indeed synthesized (3R)-
inthomycin C, but there is an anomaly in their [α]D
measurement in CHCl3, possibly due to the 20% tetramethylurea
contaminent that was present.2,7
(2) Webb, M. R.; Addie, M. S.; Crawforth, C. M.; Dale, J. W.; Franci, X.;
Pizzonero, M.; Donald, C.; Taylor, R. J. K. Tetrahedron 2008, 64, 4778.
(3) Senapati, B. K.; Gao, L.; Lee, S. I.; Hwang, G.-S.; Ryu, D. H. Org.
Lett. 2010, 12, 5088.
(4) Yoshino, M.; Eto, K.; Takahashi, K.; Ishihara, J.; Hatakeyama, S.
Org. Biomol. Chem. 2012, 10, 8164.
(5) Hale, K. J.; Grabski, M.; Manaviazar, S.; Maczka, M. Org. Lett. 2014,
16, 1164.
(6) (a) For Whiting’s route to ( )-inthomycin A, see: Hen
Whiting, A. Org. Lett. 1999, 1, 1137. (b) For a recent elegant
( )-inthomycin C formal total synthesis, see: Souris, C.; Frebault, F.;
Patel, A.; Audisio, D.; Houk, K. N.; Maulide, N. Org. Lett. 2013, 15, 3242.
(7) For a previous report on how tetramethylurea can change the [α]D
value of a molecule, see: Cheetham, N. W. H.; Tao, L. Carbohydr. Polym.
1998, 35, 287.
́
aff, N.;
́
As for the Ryu group, like the Hatakeyama group, they report a
negative [α]D for their synthetic (3R)-inthomycin C.3 Their
chiral oxazaborolidinium triflate catalyst 3 also gave rise to the
very same stereochemical outcome in other aldol reactions
reported in their paper, reactions where the aldol adduct
configuration was independently proven. Mechanistically, as
well, the Kiyooka asymmetric aldol process used by the Taylor
team very likely proceeds by a similar transition state to that
mediated by the Ryu catalyst 3. So, on balance, we now believe
D
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