Synthesis of (-)-okilactomycin by a prins-type fragment-assembly strategy

Article abstract of DOI:10.1002/anie.201102037

All things converge: A highly convergent synthesis of (-)-okilactomycin utilizes a Prins-type Maitland-Japp cyclization for the fragment assembly of two complex intermediates (see scheme). The synthesis also employs a highly diastereoselective Lewis acid promoted Diels-Alder reaction and an olefin ring-closing metathesis to close a strained 11-membered macrocycle.

Full text of DOI:10.1002/anie.201102037

Communications
DOI: 10.1002/anie.201102037
Natural Product Synthesis
Synthesis of (ꢀ)-Okilactomycin by a Prins-Type Fragment-Assembly
Strategy**
Jason M. Tenenbaum, William J. Morris, Daniel W. Custar, and Karl A. Scheidt*
Okilactomycin (1a) is a structurally interesting antitumor
antibiotic that was isolated from Streptomyces griseoflavus in
1987.[1] In vitro studies have demonstrated that 1a exhibits
significant antitumor and antiproliferative activity against
both lymphoid leukemia L1210 cells and P388 leukemia cells
with IC50 values of 216 nm and 89 nm, respectively.[1b]
A
closely related compound, chrolactomycin (1b), differs only
in structure by the replacement of a methyl group with a
methoxy moiety at the pyranone/lactone ring fusion and
displays promising telomerase inhibition.[1c,d] In addition to
their potent biological activity, these compounds posses a
compact and intriguing architecture. The tricyclic core is
characterized by a unique 6,5-fused tetrahydropyranone g-
lactone bicycle with a spiro fusion to a highly substituted
cyclohexene. A strained deoxygenated dipropionate segment
spans this unusual tricycle to generate a highly rigid tetracy-
clic topology. Despite the biological activity and structural
complexity, there have been only limited reports on the
synthesis of okilactomycin (1a) over the last two decades,
namely from the laboratories of Takeda, Paquette, and
Smith.[2] These synthetic efforts culminated in a total synthesis
of unnatural enantiomer (ꢀ)-1a and determination of the
absolute configuration of the natural product by Smith et al.
in 2007.[2d,e] There are no syntheses of chrolactomycin (1b)
reported to date. We disclose herein a convergent synthesis of
(ꢀ)-1a utilizing a Prins-type Maitland–Japp cyclization strat-
egy of two advanced fragments.
Scheme 1. Retrosynthetic strategy. TBDPS=tert-butyldiphenylsilyl.
(RCM) to install the 11-membered macrocycle. The tetrahy-
dropyranone heart of the molecule would be formed from a
convergent union of the corresponding a-hydroxy aldehyde 3
and b-hydroxy dioxinone 4 through a Prins cyclization.[3] The
cyclohexenyl aldehyde could be accessed using an asymmetric
Diels–Alder reaction in conjunction with functional group
manipulation and Rubottom oxidation. The b-hydroxy dioxi-
none motif could be constructed using a vinylogous aldol
reaction of an acetoacetate equivalent.
Our retrosynthetic plan is outlined in Scheme 1. Given the
electrophilic nature of the exomethylene unit, we elected for a
late-stage installation of this moiety. We envisioned that the
key tetracyclic precursor could be accessed from the lactoni-
zation of seco-ester 2, and subsequent ring-closing metathesis
[*] Dr. J. M. Tenenbaum, W. J. Morris, D. W. Custar,
Prof. Dr. K. A. Scheidt
The synthesis of aldehyde 3 centered on an endo-selective
Diels–Alder reaction to install the necessary substitution
pattern. The requisite diene for this [4+2] strategy was
constructed starting with the hydrozirconation/iodination
Department of Chemistry, Center for Molecular Innovation and
Drug Discovery, Chemistry of Life Processes Institute, Silverman
Hall, Northwestern University, Evanston, IL 60208 (USA)
Fax: (+1)847-467-2184
reaction of benzyl-protected alkyne 5 (Scheme 2).[4]
A
lithium–halogen exchange of vinyl iodide 6 with nBuLi and
subsequent treatment of the resulting vinyllithium species
with Weinreb amide 7 afforded the desired enone in 65%
yield as a > 20:1 mixture of E/Z isomers. A selective Wittig
olefination with ethyltriphenylphosphonium bromide pro-
vided diene 8 with > 20:1 E/Z selectivity.[5,6] In the first key
step of the synthesis, the core cyclohexene was formed in 86%
yield with 20:1 diastereoselectivity for the endo product
through the Diels–Alder reaction of diene 8 (1 equiv) with
acrylimide 9 (1.1 equiv).[7,8] Early attempts using dialkyl
aluminum halides or alternative Lewis acids for this cyclo-
addition with realistic levels of diene (i.e. < 10 equiv) resulted
E-mail: scheidt@northwestern.edu
Homepage: http://chemgroups.northwestern.edu/scheidt/
[**] Financial support for this work was provided by the NCI
(R01 CA126827), the American Cancer Society (Research Scholar
Grant), the Elsa Pardee Foundation, Amgen, and GlaxoSmithKline.
Marianne Lalonde provided assistance with X-ray crystallographic
analysis. W.J.M. thanks Abbott Laboratories for a graduate fellow-
ship. We thank FMCLithium and Wacker Chemical for providing
reagents, Dr. Chris Holmquist for assistance with chromatography,
and Astellas Pharma for providing the spectroscopic data of natural
(+)-okilactomycin.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102037.
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Angew. Chem. Int. Ed. 2011, 50, 5892 –5895

Scheme 3. Synthesis of intermediates 4 and 17. Reagents and condi-
tions: a) Cu(OTf)2, (R)-tol-binap, TBAT, THF, ꢀ508C, 70%, 10:1 d.r.
b) TBSOTf, 2,6-lutidine, CH2Cl2, 08C, 92%. c) NH4F, MeOH, 408C,
92%. d) 1. o-nitrophenyl selenocyanate, PBu3, THF; 2. H2O2, THF,
76%. e) HF·py, THF, 93%. f) KOEt, CH2Cl2, 88%. g) HF·py, THF 94%.
py=pyridine, TBAT=tetrabutylammonium difluorotriphenylsilicate,
tol-binap=2,2’-Bis(di-p-tolylphosphino)-1,1’-binaphthyl. OTf=trifluoro-
methanesulfononate.
Scheme 2. Synthesis of cyclohexene 3. Reagents and conditions:
a) Cp2Zr(H)Cl, NIS, THF, 84% b) nBuLi, THF, ꢀ788C; then 7, 238C,
65%. c) KHMDS, ethyltriphenylphosphonium bromide, THF, ꢀ78 to
08C, 84%. d) Et2AlCl, AgPF6, CH2Cl2, ꢀ788C, 86%, >20:1 d.r.
e) LiBH4, MeOH, Et2O, 08C, 83%. f) Dess–Martin periodinane,
CH2Cl2, 90%. g) TBSOTf, NEt3, CH2Cl2. h) DMDO, CH2Cl2, 08C; then
acidic work-up, 88% over two steps. Bn=benzyl, Cp=cyclopenta-
dienyl, DMDO=dimethyldioxirane, HMSD=hexamethyldisilazide,
NIS=N-iodosuccinimide, TBS=tert-butyldimethylsilyl, THF=tetrahy-
drofuran.
TBS group with HF·py afforded the requisite b-hydroxy
dioxinone 4 in 93% yield.
With efficient routes to access both a-hydroxy aldehyde 3
and b-hydroxy dioxinone 4 in multigram quantities, we
proceeded to investigate conditions for the key Prins-type
fragment assembly reaction. Employing the Lewis acid
conditions previously developed in our laboratory with
scandium(III),[14] the condensation/Prins cyclization reaction
of 3 and 4 was examined. Unfortunately, we found that b-
hydroxy dioxinone 4 was unreactive with a-hydroxy aldehyde
3, and no cyclization took place. We decided to convert the
dioxinone moiety into the corresponding ketoester based on
reports by Clarke et al.[15a–c] indicating that a d-hydroxy-b-
ketoester could undergo a modified Maitland–Japp reac-
tion[15d] with aldehydes to afford 2,6-cis-tetrahydropyran-4-
ones. Thus, treatment of dioxinone 16 with KOEt smoothly
provided a b-ketoester, where the protecting group was
removed with HF·py to afford d-hydroxy b-ketoester 17
without any observed lactonization (Scheme 3). With the
fragments to pursue this modified approach in hand, the
Lewis acid mediated coupling of ketoester 17 with a-hydroxy
aldehyde 3 furnished trioxabicyclo[3.2.1]octane 18 in 35%
yield instead of the desired tetrahydropyran (Scheme 4).[16]
The proposed pathway that leads to this product is shown
in Scheme 4. The condensation of d-hydroxy-b-ketoester 17
with aldehyde 3 forms oxocarbenium ion 19. Instead of enol
cyclization by C2, O cyclization occurs to afford a second
oxocarbenium ion 20. Trapping of the oxocarbenium ion with
the tertiary hydroxy group affords trioxabicyclo[3.2.1]octane
18. The formation of this product is intriguing and suggests
that the mechanism for the modified Maitland–Japp reaction
under these conditions is similar to the condensation/Prins-
type cyclization pathway found with a b-hydroxy dioxinone
in poor conversion owing to rapid decomposition of acryl-
amide 9. Ultimately, we found that AgPF6 was a crucial
additive to achieve high yields of the desired product (10)
when employing practical quantities and ratios of both
reacting partners (ca. 1 equiv each). Studies are underway
to understand and expand this practical improvement. The
reductive cleavage of the Evans oxazolidinone auxiliary with
LiBH4 and MeOH generated the alcohol in 83% yield. The
oxidation of the primary alcohol with Dess–Martin period-
inane provided aldehyde 11 (90% yield). Lastly, the installa-
tion of the a-hydroxyl group through a Rubottom oxidation
produced a-hydroxy aldehyde 3 in 88% yield and 13:1
diastereoselectivity favoring the desired stereoisomer.[9]
The corresponding b-hydroxy dioxinone fragment was
assembled beginning with known aldehyde 13[10,2d,e] (derived
in six steps from pseudoephedrine propionamide and
TBDPS-protected iodoethanol). Using Krꢀger and Carreiraꢁs
copper-catalyzed vinylogous aldol reaction conditions with
silyloxy diene 14 and aldehyde 13, b-hydroxy dioxinone 15
was formed in 70% yield and 10:1 diastereomeric ratio
favoring the desired anti adduct (Scheme 3).[11] The resulting
secondary alcohol was protected as a TBS ether, and the
primary TBDPS group was selectively removed upon treat-
ment with NH4F to afford the primary alcohol.[12] In
anticipation of the ring-closing metathesis later in the route,
the alcohol was converted into terminal olefin 16 with a
Grieco elimination sequence.[13] The removal of the secondary
instead of
a Knoevenagel/oxo-conjugate addition reac-
tion.[15a,d]
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Communications
Scheme 4. Proposed pathway for the formation of 18. M.S.=molecular
sieves.
Scheme 5. Completion of the synthesis of (ꢀ)-okilactomycin. Reagents
and conditions: a) aq HF, CH3CN. b) KOtBu, CH2Cl2. c) K2CO3, MeI,
CH3CN, 708C, 58% over three steps). d) TBDPSCl, imidazole, CH2Cl2,
90%. e) DDQ (30 equiv), CH2Cl2, 61%. f) 1. o-nitrophenyl-selenocya-
nate, PBu3, THF; 2. H2O2, THF, 60%. g) Grubbs second-generation
catalyst (40 mol%), CH2Cl2, 408C. h) H2, PtO2, EtOAc, 65% over two
steps. i) HF·py, THF, >99% j) LiHMDS, dimethylmethylideneammo-
nium iodide, THF 95%. k) Dess–Martin periodinane, CH2Cl2, 83%
l) NaClO2, NaH2PO4, 2-methyl-2-butene, tBuOH/THF/H2O (4:4:1),
50%. DDQ=2,3-dichloro-5,6-dicyanobenzoquinone.
Despite the unexpected formation of tricycle 18, we were
encouraged that d-hydroxy-b-ketoester 17 was a competent
coupling partner for the sterically demanding a-hydroxy
aldehyde 3. A revised strategy was pursued involving the
protection of the tertiary alcohol in 3 to facilitate the desired
cyclization pathway. After extensive experimentation with
various protecting groups, only TBS ether 21 survived the
Lewis acid mediated conditions. Ultimately, the optimal
conditions for the stereoselective coupling of 21 and 17
were TMSOTf in CH2Cl2, and led to the desired tetrahydro-
pyranone 22 in a 60% yield and as a 13:1 mixture of
diastereomers favoring the desired 2,6-cis isomer [Eq. (1)].
alcohol 23 was reprotected as the TBDPS ether and the
benzyl ether was cleaved under oxidative conditions with
DDQ.[17] The resultant primary alcohol was converted into
the terminal olefin without interference from the
additional double bond in the molecule by using the
Grieco protocol.[13]
At this stage, we pursued the planned ring-closing
metathesis of 24 to form the macrocycle and complete
the core of (ꢀ)-okilactomycin. This approach was
originally proposed by Paquette[2b,c] and subsequently
realized in the Smith synthesis.[2d] However, unique to
our system was the presence of different functionality,
most notably the olefin already installed in the
cyclohexene ring. This internal alkene ultimately
facilitates smooth installation of the a,b-unsaturated
acid of the natural product (see below) and could
have potentially compromised a metathesis approach.
Pleasingly, the exposure of bis(olefin) 24 to Grubbs second-
generation catalyst and subsequent hydrogenation furnished
the tetracycle 25 in 65% yield over the two steps.[18] For our
end-game approach, we were concerned that the exomethy-
lene unit might be incompatible with conditions used to
remove the silyl group, so we first removed the silyl unit and
attempted to install the exocyclic olefin in the presence of the
free allylic alcohol. Fortunately, silyl ether 25 was treated with
HF·pyr to yield the allylic alcohol, and treatment with
LiHMDS and dimethylmethylideneammonium iodide
(Eschenmoserꢁs salt) cleanly installed the exocyclic olefin.[19]
Finally, an oxidation to enal 26 with Dess–Martin periodinane
(83% yield) followed by a Pinnick oxidation[20] afforded (ꢀ)-
ꢀ
The union of these two fragments establishes the key C O
ꢀ
and C C bonds of the central 6,5-fused pyranone/lactone in a
single operation. In addition, this process relays the config-
uration of b-ketoester 17 to the new stereogenic center at C7
with high level of fidelity and constructs the full carbon
skeleton of the natural product.
The global removal of the silyl groups of pyranone 22 was
accomplished with aqueous HF and set the stage for
completion of the synthesis. A lactonization between the
unmasked hindered tertiary alcohol and pendant ethyl ester
with KOtBu and subsequent stereoselective C methylation
produced the key tricycle 23 in a 58% yield with only one
purification over the three-step sequence (Scheme 5). Allylic
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okilactomycin, which possessed identical characterization
data (1H NMR, 13C NMR, HRMS, IR spectra) to the natural
20
material.[21] The ½aꢁD observed was ꢀ20 degcm3 gꢀ1 dmꢀ1
(MeOH, c = 0.04 gcmꢀ3) which is opposite in sign compared
to isolated (+)-okilactomycin (+ 34 degcm3 gꢀ1 dmꢀ1, c =
1.0 gcmꢀ3, MeOH).[2b,22]
In summary, the total synthesis of (ꢀ)-okilactomycin (1a)
has been achieved in 1.0% overall yield over 26 steps as the
longest linear sequence (via 17). Stereoselective alkylation
and Diels–Alder routes facilitated quick access to the d-
hydroxy b-ketoester and a-silyloxy aldehyde fragments,
respectfully. A Lewis acid promoted Maitland–Japp reaction
established the full carbon core with a high degree of
diastereoselectivity for the 2,6-cis tetrahydropyrans core.
This Prin-type transformation is one of the most advanced
to date in terms of size and functionality of the reactants and
further defines the potential of this approach for late-state
unions of complex intermediates. The installation of the
exocyclic olefin at the end of the synthesis and convergent
nature of this route makes this synthesis amenable to the
production of analogues and structure-activity relationship
studies. The synthesis of these related compounds, which are
intended for biological investigations, based on our complex
fragment assembly Prins/Maitland–Japp route described here
are ongoing in our laboratory.
[14] W. J. Morris, D. W. Custar, K. A. Scheidt, Org. Lett. 2005, 7,
1113 – 1116.
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A. J. Blake, Chem. Commun. 2005, 1061 – 1063; c) P. A. Clarke,
W. H. C. Martin, J. M. Hargreaves, C. Wilson, A. J. Blake, Org.
Biomol. Chem. 2005, 3, 3551 – 3563; d) F. R. Japp, W. Maitland, J.
Chem. Soc. Trans. 1904, 85, 1473 – 1489.
[16] The configuration was assigned by an nOe interactions.
[17] N. Ikemoto, S. L. Schreiber, J. Am. Chem. Soc. 1992, 114, 2524 –
2536.
Received: March 22, 2011
Published online: May 10, 2011
Keywords: Maitland–Japp reaction · natural products ·
.
Prins reaction · tetrahydropyrans · total synthesis
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[21] We thank Astellas Pharma for providing spectroscopic data of
(+)-okilactomycin for comparison.
[22] With regard to this synthesis of the unnatural enantiomer of 1a,
(ꢀ)-okilactomycin, since our intermediates up to and including
25 possessed (+) rotation values, we anticipated that the route
would produce the natural (+) enantiomer. Comparing our
structures to the previous synthesis by Smith (Ref. [2f]) was
complicated since the absolute structure of (+)-1a in Figure 1 of
Ref. [2f] should be the enantiomer. Importantly, all other
structures and assignments in Ref. [2f] are correct (Prof. Amos
Smith, private communication). Interestingly, both natural
chrolactomycin and the recently isolated congeners of okilacto-
mycin (okilactomycin A–D) have negative optical rotation
values. Efforts are underway in our laboratory to produce the
natural enantiomer along with structure analogues for our
biological investigations.
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