Total synthesis of phoslactomycin B and its biosynthetic deamino precursor

Article abstract of DOI:10.1002/anie.200600458

(Chemical Equation Presented) The implicated intermediate in the biosynthesis of phospholine (phoslactomycin B, see picture), namely its deamino precursor, as well as phospholine itself are synthesized. The phoslactomycins show antitumor, antibacterial, a

Full text of DOI:10.1002/anie.200600458

Communications
Natural Product Synthesis
We envisioned that compound 3 would be transformed
into 1 by reduction of the triple bond and introduction of the
amino and phosphate groups (Scheme 1). To construct 3, we
chose a strategy using chelation-controlled addition of a vinyl
anion to ketone 4, followed by Evans aldol, lactone forma-
DOI: 10.1002/anie.200600458
Total Synthesis of Phoslactomycin B and Its
Biosynthetic Deamino Precursor**
Yong-Gang Wang, Ryuichi Takeyama, and
Yuichi Kobayashi*
Phoslactomycins A–F and I are a unique class of compounds
that contain a phosphate group and an amino group.[1–3]
Similar structures are also seen in phosphazomycins C1 and
[5]
C2,[4] leustroducsins A–Cand H,
and fostriecin.[6] Antitu-
mor, antibacterial, and antifungal activities have been shown
for the phoslactomycins.[1a,2a] Inhibitory activity to human
protein phosphatase 2A (PP2A) was later disclosed,[7] and the
binding site that interacts with the phoslactomycins through
the phosphate group was identified.[8] The carboxylate residue
on the cyclohexane ring does not affect the activities, whereas
the effect of other parts of the molecule on the activities has
not been established. Herein, we describe the synthesis of
phoslactomycin B (phospholine; 1) and its deamino precursor
2.[9]
Scheme 1. Retrosynthetic analysis of phoslactomycin B (1). PMB=p-
methoxybenzyl.
tion[10] and Sonogashira coupling with vinyl iodide 5. Thus, the
protecting group for the hydroxy group at C9 should be an
ether to attain high selectivity in the chelation-controlled
reaction and, furthermore, to be discriminated from the other
hydroxy groups so as to introduce a phosphate group on it.
Among possible candidates, we selected the p-methoxybenzyl
(PMB) group.[12g]
The synthesis of leustroducsin B was recently reported by
Fukuyama and co-workers,[10] who introduced the specific p-
(TBSO)C6H4CHO acetal protecting group (TBS = tert-butyl-
dimethylsilyl), which was removed during the last stage of the
synthesis, for the C8,9-diol unit. On the other hand, the
synthesis of fostriecin has been demonstrated by several
groups.[11,12] Thus, we initially attempted the synthesis of 1 by
applying the transformations developed for the synthesis of
fostriecin. However, such efforts were in vain owing to the
inefficiency of the transformations[13] and the unexpected
instability of the product.[14] We then examined another
method that furnished 1 and 2 efficiently, as outlined below.
The ketone synthon 4 was obtained in the form of
compound 12, and a Sharpless asymmetric epoxidation[15a,c]
was used to install conveniently the hydroxy groups at C9 and
C11 (Scheme 2). Thus, kinetic resolution of racemic alcohol
rac-6 by the asymmetric epoxidation produced (R)-6 (> 97%
ee by HPLCanalysis of the derived benzoate using a chiral
column) in 44% yield after easy separation of the epoxy
alcohol coproduct (structure not shown). The hydroxy group
of (R)-6 was protected to afford the PMB ether, which was
transformed into unsaturated ester 8 through a conventional
sequence of reactions. Reduction of 8 with DIBAL-H gave
the allylic alcohol, which was subjected to the asymmetric
epoxidation[15b,c] to give epoxy alcohol 9 as a single diaste-
reomer in 93% yield. The protocol of Yadav et al.[16] was
applied to 9, and subsequent ozonolysis of the resulting
compound 10 produced an aldehyde, which upon aldol
reaction with LiCH2CO2Et followed by reduction with
LiAlH4 afforded diol 11 in 64% yield. Finally, selective
protection of the primary alcohol as the TBDPS ether
followed by Swern oxidation furnished the key ketone 12.[17]
[*] Y.-G. Wang, R. Takeyama, Prof. Y. Kobayashi
Department of Biomolecular Engineering
Tokyo Institute of Technology
Box B52, 4259 Nagatsuta-cho, Midori-ku
Yokohama 226-8501 (Japan)
Fax: (+81)45-924-5789
E-mail: ykobayas@bio.titech.ac.jp
[**] We thank Professor H. Osada at RIKEN, Japan, for the generous
supply of phoslactomycin B. This work was supported in part by a
Grant-in-Aid for Young Scientists from the JSPS.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Next, zinc-mediated reduction[24] of the triol 22 provided
Z,Z-diene 23 with perfect cis selectivity according to 1H NMR
spectroscopic analysis. The remaining part of the synthesis
involved integration of the amino and phosphate groups on
the molecule. As removal of the TES protecting group of the
hydroxy group at C8 at the last stage under various conditions
was unsuccessful,[14] the more labile TMS group was next
studied as the hydroxy-protecting group. Selective protection
of the primary and secondary alcohols with TESCl afforded
24 in 85% yield. The remaining tertiary hydroxy group at C8
in 24 was then protected to afford TMS ether 25, which was
exposed to PPTS in THF/MeOH to yield 26 in 53% yield,
along with recovered 25 in 38% yield (86% yield of 26 based
on recovered 25). Introduction of the amino group was
successfully carried out with HN(CO2-allyl)2 under Mitsu-
nobu conditions (DIAD, PPh3)[25] to furnish 27 in 73% yield.
To install the phosphate group, the PMB group was removed
with DDQ, which proceeded cleanly; subsequent phosphor-
[26,12f]
ylation of the resulting alcohol 28 with (iPr)2NP(O-allyl)2
Scheme 2. Synthesis of ketone 12 (synthon 4 in Scheme 1). Reagents
and conditions: a) tBuOOH (1 equiv), Ti(OiPr)4 (0.25 equiv), l-(+)-
and 1H-tetrazole (then 35% H2O2) provided phosphate 29 in
good yield. Deprotection of the silyl group (AcCl, CH2Cl2/
THF/MeOH 5:5:1) produced diol 30 in 95% yield. Finally, all
the allyl groups on the nitrogen and phosphorous atoms were
removed cleanly by palladium-catalyzed reaction[27,28] with
Bu3SnH and H2O at 08Cfor 1.5 h to afford 1 in 88% yield
after preparative TLC(silica gel, normal phase, MeOH/
H2O 8:1). The overall yield of 1 from rac-6 in 38 steps was
0.9% (2% from (R)-6). Spectral data (1H NMR (500 MHz),
13CNMR (75 MHz), IR), optical rotation ([ a]2D8 = + 80 (c =
0.03, MeOH)), and mobility on TLCof synthetic phoslacto-
mycin B were identical in all respects to those reported for the
natural material ([a]2D1 = + 81 (c = 1.0, MeOH)).[1b,2]
Next, we turned our attention to the synthesis of the
deamino precursor 2, which is the implicated intermediate in
the biosynthesis of 1.[9] Furthermore, stronger PP2A inhib-
itory activity was expected from the inhibitory study of the
biotin-containing amide derivative at the amino group of 1.[8]
The synthesis started with selective removal of the PMB
group of 25 by DDQ followed by assembly of the phosphate
group to furnish phosphate 31 in 64% yield (Scheme 4).
Desilylation of 31 under acidic conditions provided triol
phosphate 32, which upon palladium-catalyzed deallylation
with Bu3SnH and H2O at 08Cfor 1 h furnished the second
target 2 in 77% yield after chromatography on silica gel.
In summary, a highly stereocontrolled and efficient total
synthesis of phoslactomycin B (1) and its biosynthetic dea-
mino precursor 2 was accomplished by selective protection of
the hydroxy group at C9 using the PMB group, which not only
provided high selectivity in the construction of the C8
stereocenter but also enabled easy installation of the phos-
phate group. Furthermore, the successful differentiation of
the three other hydroxy groups was demonstrated, thus
allowing modification of the functional groups and attach-
ment of another functional group for the study of the
structure–activity relationship of 1 and its derivatives and
for the execution of chemical biology in this field.
=
DIPT (0.30 equiv), 4Š M.S., ꢀ208C, 30 h, 44%; b)PMBOC( NH)CCl3
(2 equiv), CSA (3 mol%); c) LiAlH4 (0.7 equiv), 87% for 2 steps;
d)(COCl) 2 (1.2 equiv), DMSO (3 equiv), Et3N (4 equiv);
e)(EtO) 2P(O)CH2CO2Et (1.2 equiv), DBU (1.25 equiv), LiCl (1.3 equiv),
MeCN, 83% for 2 steps; f)DIBAL-H (2.2 equiv), THF, 96%;
g) tBuOOH (1.5 equiv), Ti(OiPr)4 (0.30 equiv), d-(ꢀ)-DIPT
(0.36 equiv), 4-Š M.S. ꢀ208C, 93%; h)PPh 3 (1.2 equiv), NaHCO3
(0.22 equiv), CCl4, 88%; i) nBuLi (3.14 equiv), ꢀ788C, THF; j)TBSCl
(1.2 equiv), imidazole (2 equiv), 91% for 2 steps; k) O3, 2,6-lutidine
(1.5 equiv), MeOH; l) LiCH2CO2Et from LDA (2.1 equiv)and EtOAc
(2.1 equiv), ꢀ788C, THF; m)LiAlH 4 (1.5 equiv), THF, 64% for 3 steps;
n)TBDPSCl (1.2 equiv), imidazole (2 equiv); o)(COCl)
(1.2 equiv),
2
DMSO (3 equiv), Et3N (5 equiv). DIPT=diisopropyl tartrate;
M.S.=molecular sieves; CSA=(+)-10-camphorsulfonic acid;
DMSO=dimethyl sulfoxide; DBU=1,8-diazabicyclo[5.4.0]undec-7-ene;
DIBAL-H=diisobutylaluminum hydride; TBS=tert-butyldimethylsilyl;
LDA=lithium diisopropylamide; TBDPS=tert-butyldiphenylsilyl.
Construction of the triol 22, which corresponds to the
proposed key intermediate 3 in Scheme 1, was accomplished
by the method summarized in Scheme 3. Addition of CH2
=
CHMgBr to ketone 12 proceeded cleanly to furnish 13 as the
sole product in 87% yield from diol 11.[18,19] The stereochem-
istry at C8 was tentatively assigned as depicted, by analogy,[20]
and was proved by completion of the synthesis. After
protection of the OH group at C8 with TESOTf and 2,6-
lutidine, the resulting compound 14 was subjected to ozonol-
ysis in the presence of 2,6-lutidine to produce an aldehyde,
which upon Horner–Wadsworth–Emmons reaction furnished
a,b-unsaturated ester 15. Thus, the stage was set for coupling
with the remaining partners at the two ends to construct the
full carbon skeleton. Installation of the fragment on the right
at the acetylene carbon center by Sonogashira reaction[21] with
Z-vinyl iodide 5 provided enyne 16 in 87% yield. Attachment
of the lactone on the left was accomplished by using Evans
aldol.[10,22] Thus, ester 16 was reduced to an alcohol and
reoxidized to aldehyde 17. Aldol reaction[23] of 17 with the
boron enolate derived from 18 proceeded stereoselectively to
afford an aldol, which was converted to lactone 22 by routine
reactions through 19–21 (see Scheme 3).
Received: February 3, 2006
Published online: April 18, 2006
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Communications
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Scheme 3. Synthesis of phoslactomycin B (1)through the key intermediate 22 (compare 3 in Scheme 1). Reagents and conditions: a) CH2
CHMgBr (2 equiv), ꢀ788C, THF, 87% from diol 11; b) TESOTf (1.1 equiv), 2,6-lutidine (2 equiv), 91%; c) O3, 2,6-lutidine (2 equiv), MeOH/iPrOH
(1:1); d) (EtO) 2P(O)CH2CO2Et (2.1 equiv), NaH (2 equiv), THF, 48% for 2 steps; e) 5 (1.2 equiv), [Pd(PPh3)4] (5 mol%), CuI (15 mol%), tBuNH2
(10 equiv), 87%; f) DIBAL-H (2.5 equiv), THF, 97%; g) SO3·C5H5N (3 equiv), DMSO (30 equiv), Et3N (10 equiv), 97%; h) 18 (1.55 equiv),
Bu2BOTf (1.5 equiv), (iPr)2NEt (2.2 equiv), ꢀ788C to RT, CH2Cl2; i)TESCl (2 equiv), C 5H5N (15 equiv), 95% for 2 steps; j) EtSLi from EtSH
(5 equiv)and nBuLi (3 equiv), 08C, 90%; k)DIBAL-H (1.3 equiv), ꢀ788C, toluene; l)(PhO) 2P(O)CH2CO2Et (1.8 equiv), Bu4NOH (1.75 equiv),
THF, 86% for 2 steps; m) PPTS (3 mol%), MeOH/THF (3:1), 86%; n) Ti(OiPr)4 (0.2 equiv), 808C, benzene, 94%; o)Bu 4NF (4 equiv), 88%; p) Zn
activated with BrCH2CH2Br, LiCuBr2, EtOH, reflux, 83%; q)TESCl (4 equiv), C 5H5N (30 equiv), 08C, 85%; r)TMSOTf (4 equiv), 2,6-lutidine
(30 equiv); s) PPTS (0.1 equiv), THF/MeOH (1:1), 86% for 2 steps based on recovered 25 (38%); t) DIAD (2.6 equiv), PPh 3 (2.4 equiv), HN(CO2-
allyl)2 (1.3 equiv), toluene, ꢀ788C to 08C, 73%; u)DDQ (1.5 equiv), CH 2Cl2/H2O (19:1), 85%; v) (iPr)2NP(O-allyl)2 (1.4 equiv), 1H-tetrazole
(2 equiv)then 35% H 2O2 (5 equiv), 85%; w) AcCl (0.6 equiv), 08C, CH2Cl2/THF/MeOH (5:5:1), 95%; x) [PdCl2(PPh3)2] (5 mol%), Bu3SnH
(5 equiv), H2O (50 equiv), 08C, 1.5 h, CH2Cl2, 88%. Tf=trifluomethanesulfonyl; TES=triethylsilyl; Bn=benzyl; PPTS=pyridinium p-toluenesulfo-
nate; TMS=trimethylsilyl; DIAD=diisopropyl azodicarboxylate; DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
Scheme 4. Synthesis of biosynthetic precursor 2. Reagents and conditions:
a)DDQ (1.2 equiv), CH 2Cl2/H2O (19:1), 87%; b) (iPr)2NP(O-allyl)2
(1.4 equiv), 1H-tetrazole (2 equiv)then 35% H 2O2 (4 equiv), 73%; c) AcCl
(0.6 equiv), CH2Cl2/THF/MeOH (5:5:1), 08C, 92%; d)[PdCl 2(PPh3)2]
(5 mol%), Bu3SnH (2.5 equiv), H2O (30 equiv), CH2Cl2, 08C, 1 h, 77%.
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Keywords: asymmetric synthesis · natural products ·
.
phoslactomycins · phospholine · total synthesis
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=
=
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=
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