Asymmetric total synthesis of a pentacyclic lycopodium alkaloid: Huperzine-Q

Article abstract of DOI:10.1002/anie.201103550

Right on Q: The first asymmetric total synthesis of (-)-huperzine-Q, which possesses six stereogenic centers and a spiroaminal moiety, has been achieved in 19 steps and 16.4 % overall yield. This synthesis involved a novel stereoselective Pauson-Khand reaction, a vinyl Claisen rearrangement, and a biomimetic spiroaminal formation. TBDPS=tert-butyldiphenylsilyl.

Full text of DOI:10.1002/anie.201103550

DOI: 10.1002/anie.201103550
Natural Products
Asymmetric Total Synthesis of a Pentacyclic Lycopodium Alkaloid:
Huperzine-Q**
Atsushi Nakayama, Noriyuki Kogure, Mariko Kitajima, and Hiromitsu Takayama*
Lycopodium alkaloids have unique skeletal characteristics[1]
and a variety of biological activities, such as acetylcholine
esterase (AChE) inhibition[2] and neurite outgrowth promo-
tion,[3] which have sustained the interest of many researchers
of natural product chemistry, synthetic chemistry, and medi-
cinal chemistry.
In particular, the structural diversity of fawcettimine-type
Lycopodium alkaloids has attracted the attention of several
groups as targets for total synthesis.[4]
Huperzine-Q (1; Figure 1), which was isolated from
Huperzia serrata by Zhu and co-workers in 2002,[5] consists
of a unique pentacyclic skeleton possessing a spiroaminal
moiety and six stereogenic centers, including a quaternary
carbon center. Although its structure and relative stereo-
chemistry were determined by spectroscopic and single-
crystal X-ray diffraction analysis, its absolute configuration
Scheme 1. Retrosynthetic analysis of huperzine-Q (1). Ns=2-nitroben-
zenesulfonyl, TBDPS=tert-butyldiphenylsilyl.
and biological activities have not been reported thus far. To
develop an efficient synthetic route to 1 and to confirm its
absolute configuration, we embarked on the asymmetric total
synthesis of huperzine-Q (1).
Our synthetic plan is shown in Scheme 1. Biogenetically, 1
would be derived from the fawcettimine derivative 2 by
intramolecular spiroaminal formation between a primary
alcohol at C16 and a secondary amine. We anticipated an
efficient synthesis of 2 to arise from azonane ring formation
by utilizing the intramolecular Mitsunobu reaction and
subsequent functional group transformations of 3, which
could be a key intermediate to several fawcettimine-type
Lycopodium alkaloids such as lycoposerramine-A[6]
(Figure 1). We envisioned that successive chiral centers (C5,
C4, and C12) in 3 could be constructed from the bicyclic
cyclopentenone 4 by means of a vinyl Claisen rearrangement,
and the subsequent hydroboration/oxidation process. Bicyclic
compound 4 was expected to be elaborated from the chiral
diol 5 through the novel stereoselective Pauson–Khand
reaction (PKR; Scheme 1).
Our synthesis commenced with the coupling between the
acyl chloride 6 and alkyne 7 to afford ketone 8,[7] which was
transformed into the optically active lactone 9 in a one-pot
operation involving the Noyori reduction[8] and successive
treatment with PPTS. The enantiomeric excess was deter-
mined to be 83% by HPLC analysis using a chiral stationary
phase[9] (the enantiomeric excess of the product was finally
raised to 99% ee during conversion into 15. See below). Then,
an allyl unit was introduced to the a position of the carbonyl
group in 9 to furnish 10 and 11 in quantitative yields in a ratio
of 2.3:1.[10] The conversion of 10 into 11, having the desired
stereochemistry at C15, was successfully achieved by treat-
ment with LHMDS and subsequent addition of a hindered
acid (BHT), thus giving the kinetically controlled product 11
with excellent selectivity (10/11 = 1:16.5). The reduction of
lactone 11 afforded diol 5 for the PKR (Scheme 2).
Figure 1. Structures of huperzine-Q (1), fawcettimine, and lycoposerr-
amine-A.
[*] A. Nakayama, Dr. N. Kogure, Dr. M. Kitajima, Prof. Dr. H. Takayama
Graduate School of Pharmaceutical Sciences, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
E-mail: htakayam@p.chiba-u.ac.jp
Homepage: http://www.p.chiba-u.ac.jp/lab/seitai/index.html
[**] This work was supported by a Grant-in-Aid for Scientific Research
from the Japan Society for the Promotion of Science and the Takeda
Science Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103550.
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Initial attempts to perform the PKR with 5 gave, however,
compound 13 gave the desired compound 14 in 57% yield
under conventional PKR conditions.[11] NOE experiments
showed that C7 in 14 had the desired stereochemistry
(Scheme 3). Next, we optimized the reaction conditions to
develop the one-pot operation to transform 5 into 4. First, we
prepared the silyl-tethered compound 13, to which
[Co2(CO)8] was added, thus affording the alkyne cobalt
complex of 13. Then, we diluted the reaction mixture with
toluene and heated it under CO atmosphere to give the
bicyclic compound 14, which in turn was directly treated with
concentrated hydrochloric acid in MeOH to give the desily-
lated compound 4 in 92% yield from 5. To the best of our
knowledge, this is a first example of a stereoselective PKR
that utilizes a seven-membered silyl-tethered compound.
With the PKR product 4 in hand, we next focused on the
construction of the quaternary carbon center C12 (Scheme 4).
MOM groups were introduced to the two hydroxy groups in 4
and then the enone was reduced with the (R)-Me-CBS
reagent[12] to furnish allyl alcohol 15 with good stereoselec-
tivity. The stereochemistry at C5 was determined by NOE
experiments, and at this point, the enantiomeric excess of 15
was determined by HPLC analysis to be 99% ee.[13]
After conversion of 15 into sulfoxide 16,[14] 16 was heated
at 1708C in 1,2-dichlorobenzene to afford the desired
aldehyde 17 in excellent yield. Treatment of aldehyde 17
with the Wittig reagent gave the diene compound 18.
Our next task was the construction of an azonane ring
using the intramolecular Mitsunobu reaction (Scheme 5). We
prepared substrate 19 for the Mitsunobu reaction from diene
18 by using a sequential reaction that involved a simultaneous
hydroboration/oxidation[15] at two positions (C4–C5 and C9–
C10), the introduction of a nitrogen functional group to C9,
and the subsequent removal of a TBDPS group. With the Ns-
protected derivative 19 in hand, we
12, the undesired C7 epimer as the major product. Mecha-
nistic considerations indicated that a reaction intermediate
such as 5i would have a chairlike conformation with an
equatorial side chain at C15, and therefore control the
stereochemistry at C7. On this basis, we devised the silyl-
tethered compound 13, which would alter the conformation of
the reaction intermediate to yield a bicyclic product having
the desired C7 stereochemistry. Actually, the silyl-tethered
Scheme 2. Synthesis of the chiral diol 5. Reagents and conditions:
a) nBuLi, ZnCl2, THF, À788C, 91%; b) [Ru{(R,R)-Tsdpen}(p-cymene)],
iPrOH, 288C and then PPTS, toluene, 808C, 68%; c) Allylbromide,
LHMDS, HMPA, THF, À788C, quant, 10/11=2.3:1; d) LHMDS, THF,
À788C and then BHT, 86%, 10/11=1:16.5; e) LiBH4, THF, RT, 95%.
BHT=2,6-di-tbutylated-hydroxytoluene, dpen=1,2-diphenylethylenedi-
amine, HMPA=hexamethylphosphoramide, LHMDS=lithium bis(tri-
methylsilyl)amide, PPTS=pyridinium p-toluenesulfonate, THF=tetra-
hydrofuran.
tried to construct the azonane ring.
After several screening steps, we
found that the treatment of 19 with
diethyl azodicarboxylate (DEAD) in
the presence of PPh3 in toluene at
708C afforded 20 in excellent
yield.[4e,16] At this stage, the X-ray
crystallographic analysis of 20 was
performed, and enabled us to confirm
the absolute configuration of all the
chiral centers.[17]
For the completion of the total
synthesis of 1, we converted 20 into
the fawcettimine derivative 2 as fol-
lows (Scheme 6). The removal of the
two MOM groups with trimethylsilyl
bromide gave the corresponding diol
21 in quantitative yield.[18] Then, the
selective acetylation of the primary
alcohol[19] at C16 and the subsequent
Dess–Martin oxidation of the secon-
Scheme 3. Stereoselective synthesis of bicyclic compounds 12 or 14 and one-pot operation to give
4. Reagents and conditions: a) [Co2(CO)8] toluene, RT to 1008C under CO atmosphere, 44% of 12
and 3% of 4; b) SiMe2Cl2, Et3N, DMAP, CH2Cl2, RT, 85%; c) [Co2(CO)8], toluene, RT to 1008C
under CO atmosphere, 57%; d) 1. SiMe2Cl2, Et3N, DMAP, (CH2Cl)2, RT, 2. [Co2(CO)8], toluene, RT
dary alcohol at C13 were carried out
in a one-pot operation to afford 22.
The successive removal of the Ns
to 1008C under CO atmosphere, 3. HCl, MeOH, 08C, 92%. DMAP=N,N-4-dimethylaminopyridine. group and diacetyl groups was also
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Scheme 6. Completion of total synthesis of (À)-huperzine-Q (1).
Reagents and conditions: a) TMSBr, CH2Cl2, 08C, quant; b) AcCl, 2,6-
lutidine, CH2Cl2, À788C and then Dess–Martin periodinane, RT, 96%;
c) PhSH, K2CO3, MeCN, 308C and then MeOH, K2CO3, 308C, 98%;
d) CSA, toluene, reflux, 86%. CSA=(+)-camphorsulfonic acid,
TMS=trimethylsilyl.
Scheme 4. Synthesis of the diene 18. Reagents and conditions:
a) MOMCl, DIPEA, TBAI, CH2Cl2, RT, 84%; b) (R)-Me-CBS, BH3·THF,
THF, RT, 88% (d.r. 9.8:1); c) Phenylvinylsulfoxide, KH, NaH, THF, RT,
98%; d) NaHCO3, 1,2-dichlorobenzene, 1708C, 89%; e) nBuLi,
PPh3CH3Br, THF, RT, 95%. (R)-Me-CBS=(R)-methyloxazaborolidine,
DIPEA=diisopropylethylamine, MOM=methoxymethyl, TBAI=tetra-
n-butylammonium iodide.
silyl-tethered substrate, the construction of a quaternary
carbon center through a vinyl Claisen rearrangement, and a
biomimetic spiroaminal formation. This strategy is an effec-
tive approach to use towards other fawcettimine-type Lyco-
podium alkaloids.
Received: May 24, 2011
Published online: July 12, 2011
Scheme 5. Synthesis of the azonane compound 20. Reagents and
conditions: a) BH3·SMe2, THF, 08C; BH3·THF, 08C; then
NaBO3·4H2O, RT, 67%; b) 1. MsCl, Et3N, CH2Cl2, 08C then Ac2O,
DMAP, pyridine; 2. NH2Ns, K2CO3, DMF, 808C, 97%; c) TBAF, AcOH,
THF, RT, quant; d) DEAD, PPh3, toluene, 708C, 94%. DEAD=diethyl
azodicarboxylate, DMF=N,N-dimethylformamide, Ms=methanesul-
fonyl, Ns=2-nitrobenzenesulfonyl, TBAF=tetra-n-butylammonium
fluoride.
Keywords: alkaloids · natural products · Pauson–
Khand reaction · rearrangement · total synthesis
.
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