An oxidative prins-pinacol tandem process and its application to the synthesis of (-)-platensimycin

Article abstract of DOI:10.1002/chem.201001813

Center stage: An oxidative Prins-pinacol tandem process mediated by a hypervalent iodine reagent has been accomplished. The strategy allows rapid access to highly functionalized spirocyclic cores (see scheme) present in many natural products. A first application to the formal synthesis of (?)-platensimycin has been achieved.

Full text of DOI:10.1002/chem.201001813

DOI: 10.1002/chem.201001813
An Oxidative Prins–Pinacol Tandem Process and its Application to the
synthesis of (À)-Platensimycin
Marc-Andrꢀ Beaulieu, Cyrille Sabot, Nabil Achache, Kimiaka C. Guꢀrard, and
Sylvain Canesi*[a]
Cationic molecular rearrangements[1] provide an appeal-
ing avenue to complex molecular architectures. Noteworthy
examples are apparent from the work of Overman, who har-
nessed the power of tandem Prins–pinacol rearrangement
sequences in a number of brilliant syntheses.[2] As shown in
Scheme 1, this process starts with the ionization of, for ex-
Such a reversal of reactivity is tantamount to an “aromatic
ring umpolung.”[3,7] To illustrate, oxidation of phenolic com-
pound
5 would produce the electrophile rendered in
Scheme 2 as phenoxonium ion 6. Quideau,[5d] Nicolaou,[8]
Scheme 1. The Prins–pinacol process.
Scheme 2. The oxidative Prins–pinacol transformation. P=protecting
group.
ample, a ketal or thioketal (e.g. 1, Z=OR, SR) under the
influence of an appropriate promoter (e.g., Lewis acid, etc.).
Nascent onium intermediate 2 is then intercepted by an ole-
finic p bond, which is part of an allylic alcohol or ether. This
triggers a pinacol-like transposition leading to 4, and thence
to various possible products (Scheme 1).
their co-workers, and us[3c,e] have shown that such reactive
agents can be intercepted by allylsilanes. If the internal
olefin were to add to the electrophilic carbon in 6, then cat-
ionic intermediate 7 would be primed to undergo a pinacol
rearrangement to, ultimately, carbonyl compound 8, argua-
bly through a cyclic, chair-like transition state that should
permit control of the configuration of the emerging quater-
nary carbon.[9] Overall, a stereocontrolled “oxidative Prins–
pinacol” tandem process would result. Herein, we describe
methodology inducing such a transformation.
Our interest[3] in the oxidative dearomatization of elec-
tron-rich aromatics[4] mediated by hypervalent iodine re-
agents[5,6] led us to question whether an analogous process
could be initiated by the oxidative activation of a phenol.
Although phenols normally react as nucleophiles, oxidative
activation converts them into reactive electrophiles, which
may then be intercepted by appropriate nucleophilic traps.
The feasibility of the tandem Prins–pinacol process was
explored by using phenolic substrates 5a–d (Table 1).[10]
These compounds were exposed to the action of iodoben-
zene diacetate (DIB), an environmentally benign and inex-
pensive oxidant, in a mixture of dichloromethane and hexa-
fluoroisopropanol (HFIP). The choice of HFIP was suggest-
ed by the noteworthy work of Kita and co-workers,[11] who
have shown it to be especially well suited for many reactions
involving hypervalent iodine reagents. Pleasingly, the antici-
pated ketones 8a–d emerged in 58–68% yield. The reaction
[a] M.-A. Beaulieu, Dr. C. Sabot, N. Achache, K. C. Guꢀrard,
Prof. Dr. S. Canesi
Dꢀpartement de chimie, Universitꢀ du Quꢀbec ꢁ Montrꢀal
Laboratoire de Mꢀthodologie et Synthꢂse de Produits Naturels
C.P. 8888, Succ. Centre-Ville
Montrꢀal, H3C 3P8, Quꢀbec (Canada)
Fax : (+1)514-987-4054
E-mail: canesi.sylvain@uqam.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001813.
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COMMUNICATION
Table 1. The oxidative Prins–pinacol tandem reaction of olefinic sub-
Table 2. The oxidative Prins–pinacol tandem reaction of acetylenic sub-
strates 12.
strates 5.[a]
Entry
Reagent
R
R’
Yield [%]
Entry
Reagent
R
R’
Yield [%]
1
2
3
4
5a
5b
5c
5d
CH3
CH=CH2
Ph
H
H
CH3
CH3
64
60
58
68
1
2
3
4
12a
12b
12c
12d
CH3
CH3
CH3
H
51
48
55
50
nBu
n-dec[a]
n-dec[a]
CH3
[a] TBDMS=tert-butyldimethylsilyl.
[a] dec=decyl.
simultaneously produces two quaternary carbons, one of
which is also a spiro center. Furthermore, products 8a and d
incorporate a spiroACHTUNGTRENNUNG[4.5]decanyl system of the type found in
natural products such as anhydro-b-rotunol 10[12] and scopa-
dulcic acid 11 (Bz=benzoyl).[13] It should be noted that the
Scheme 4. The formation of spiroACTHNUTRGNEUGN[5.5]undecanyl byproduct 15.
Clues to the stereochemical course of the reaction
emerged from experiments involving phenol 16 (Scheme 5).
Treatment of a mixture of diastereomers of 16 with DIB in
oxidative Prins–pinacol reaction of 8d proceeded with for-
mation of byproduct 9 (Scheme 3; 5–10% yield). The forma-
tion of such a spiroACHTUNGTRENNUNG[5.5]undecanyl system is rationalized by
invoking methyl migration from intermediate 7d, occurring
in competition with, but to a minor extent relative to, ring
contraction.
Scheme 5.
HFIP/DCM afforded cis-ketone 18[14] via intermediate 17 in
a low yield of 30%, about one-half of the yield recorded
earlier for substrates 5 (Table 1). We attribute such poor ef-
ficiency to the fact that only one of the two diastereomers
of 16 is a suitable substrate for the oxidative Prins–pinacol
sequence. To illustrate (Scheme 6), the conformation of dia-
stereomer 16a, conducive to the occurrence of the reaction,
is such that the OP group experiences an A1,3 interaction
with the “inside” vinylic hydrogen.[15] In diastereomer 16b,
Scheme 3. The formation of spiroACTHNUGRTNEUNG[5.5]undecanyl byproduct 9.
As delineated in Table 2, the transformation was readily
extended to acetylenic substrates. The expected enones 14
were thus obtained in 48–55% yield, probably via a half-
chair transition state such as 13. It should be noted that ter-
minal and internal alkynes undergo the reaction in almost
identical yield (cf. 14a vs. 14b–d). Enones 14 are sometimes
accompanied by a small amount of byproduct 15 (5–10%
yield, Scheme 4). As outlined earlier, its formation is proba-
bly a consequence of migration of the R group occurring in
competition with the ring contraction, which remains the
major reaction pathway.
Scheme 6.
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S. Canesi et al.
the analogous allylic interaction is considerably more severe
in that it involves compression of a more sterically demand-
ing Me group against the same vinylic hydrogen. In all like-
lihood, this retards the Prins reaction step and diverts the
reactive electrophile created upon umpolung activation of
the phenol towards other reaction pathways. In either case,
the product of the reaction is cis-ketone 18.[14]
Consistent with the aforementioned rationale, tetrahydro-
furan 20 reacted with DIB, via intermediates 21 and 22, to
furnish compound 23 in considerably higher yield (70%,
Scheme 7). Compound 20 emerged in 88% yield as a single
Scheme 8. The structure of (À)-platensimycin and retrosynthetic logic.
PCC=pyridinium chlorochromate.
droperoxyketal 25 in 64% overall yield. This material was
immediately treated with FeSO4 and CuACTHUNTRGNEU(GN OAc)2 in MeOH to
give 26.
Scheme 9.
Scheme 7. TFA=trifluoroacetic acid; DMP=Dess–Martin periodinane.
Addition of K2CO3 to the reaction mixture induced con-
version into 27 in 44% overall yield from 25. Compound 27
was obtained as a 3:1 mixture of exocyclic (major) and en-
docyclic alkenes. As first demonstrated by Nicolaou and co-
workers,[8] both isomers converge to the tetracyclic core of
platensimycin (32). Accordingly, no separation was required.
The mixture of alcohols was thus advanced to a mixture of
the corresponding aldehydes (28), in 74% yield, by PCC ox-
idation. Aldehydes 28 are known synthetic intermediates for
(À)-platensimycin.[8] Hence, they were subjected to the
action of Kaganꢄs reagent (SmI2),[19] whereupon stereoselec-
tive cyclization to alcohols 29 occurred (Scheme 10). Finally,
the olefin regioisomers of 29 converged to the same final
product 31 upon treatment with TFA. The elaboration of 31
into (À)-platensimycin is well documented in the litera-
ture.[8] Therefore, the synthesis of 31 represents a formal
total synthesis of (À)-platensimycin.
diastereomer (presumably the thermodynamically more fa-
vorable one) upon acid treatment of triols 19, which, in
turn, were assembled by using Evans asymmetric alkylation
technology.[10] We found it convenient to subject crude 23 to
Dess–Martinꢄs oxidation[16] and isolate and characterize ke-
toaldehyde 24, which emerged in 60% overall yield as a
single diastereomer. As anticipated, based upon our mecha-
nistic hypothesis, compound 24 displayed a cis relative con-
figuration of the carbonyl branches. The reaction had creat-
ed contiguous tertiary and quaternary carbon centers with
complete stereocontrol.
As an initial application of the oxidative Prins–pinacol
tandem sequence, we now describe a formal total synthesis
of (À)-platensimycin, 32 (Scheme 8). This substance is an
exciting experimental antibiotic that is believed to act as a
FabF inhibitor.[17a] Its unusual structure and potent bioactiv-
ity have elicited enormous interest within the synthetic com-
munity.[17] Our approach to 32 relies upon the assumption
that one may reach hydroperoxide 25 by intercepting oxoni-
um ion 22 with H2O2. Subsequent Schreiber–Fenton frag-
mentation[18] would provide alkenes 27, which may be elabo-
rated into advanced intermediate 31 as described by Nico-
laou and co-workers.[8]
In conclusion, our continuing investigations centering
upon the concept of aromatic ring umpolung have now pro-
duced an unprecedented oxidative Prins–Pinacol tandem
process. This method allows rapid, stereocontrolled elabora-
The hypothesis was put into practice as shown in
Scheme 9. Oxidation of compound 20 with DIB, as detailed
earlier, followed by the addition of 30% hydrogen peroxide,
afforded a 3:2 mixture of unassigned diastereomers of hy-
Scheme 10. HMPA=hexamethylphosphoramide.
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The Oxidative Prins–Pinacol Tandem Process
COMMUNICATION
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core of natural products will be disclosed in due course.
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Acknowledgements
Acknowledgements are made to Leanne Racicot and to the Donors of
the American Chemical Society Petroleum Research Fund (ACS PRF)
for support of this research. We are also very grateful to the Natural Sci-
ences and Engineering Research Council of Canada (NSERC), the
Canada Foundation for Innovation (CFI), the provincial government of
Quebec (FQRNT and CCVC), and to Boehringer Ingelheim (Canada)
Ltd. for their precious financial support.
Keywords: hypervalent compounds
· iodine · natural
products · oxidation · Prins–pinacol reaction · umpolung
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Received: June 27, 2010
Published online: August 25, 2010
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