Synthesis of (S)- and (R)-sporochnol by using the allylic substitution of the secondary allylic picolinate

Article abstract of DOI:10.3987/COM-15-S(T)11

The allylic substitution of secondary allylic picolinates and copper reagents for the construction of a quaternary carbon was applied to synthesis of sporochnol. The enantiomerically enriched allylic picolinate (R)-5 was synthesized through the asymmetric hydrogen transfer of acetylene ketone 11 and the Pd-catalyzed methylation of the iodoallylic alcohol 16a. The key allylic substitution of the allylic picolinate (R)-5 with 4-MeOC₆H₄MgBr/Cu(acac)₂ (2:1) proceeded with 95% chirality transfer with 98% regioselectivity to afford anti S_N2' product 6 in 89% yield, which was converted to the methyl ether of unnatural (R)-sporochnol. Similarly, the methyl ether of (S)-sporochnol (the natural form) was synthesized.

Full text of DOI:10.3987/COM-15-S(T)11

HETEROCYCLES, Vol. 93, No. 1, 2016
47
HETEROCYCLES, Vol. 93, No. 1, 2016, pp. 47 - 54. © 2016 The Japan Institute of Heterocyclic Chemistry
Received, 28th July, 2015, Accepted, 21st December, 2015, Published online, 8th January, 2016
DOI: 10.3987/COM-15-S(T)11
SYNTHESIS OF (S)- AND (R)-SPOROCHNOL BY USING THE ALLYLIC
SUBSTITUTION OF THE SECONDARY ALLYLIC PICOLINATE†
Yuichi Kobayashi,* Yusuke Sugihara, Toshifumi Tojo, and Takuri Ozaki
Department of Bioengineering, Tokyo Institute of Technology, Box B-52,
Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8501, Japan. e-mail:
ykobayas@bio.titech.ac.jp
Abstract – The allylic substitution of secondary allylic picolinates and copper
reagents for the construction of a quaternary carbon was applied to synthesis of
sporochnol. The enantiomerically enriched allylic picolinate (R)-5 was
synthesized through the asymmetric hydrogen transfer of acetylene ketone 11 and
the Pd-catalyzed methylation of the iodoallylic alcohol 16a. The key allylic
substitution of the allylic picolinate (R)-5 with 4-MeOC6H4MgBr/Cu(acac)2 (2:1)
proceeded with 95% chirality transfer with 98% regioselectivity to afford anti SN2'
product 6 in 89% yield, which was converted to the methyl ether of unnatural
(R)-sporochnol. Similarly, the methyl ether of (S)-sporochnol (the natural form)
was synthesized.
(S)-Sporochnol, isolated from the Caribbean marine alga Sporochnus bolleanus, is an effective chemical
defense against marine herbivores.1 However, the defense mechanism including the structure-activity
relationship is not clarified. Up to date, several constructions of a quaternary carbon have been applied to
synthesis of (S)- as well as (R)-sporochnol (Figure 1).2 The asymmetric copper-assisted allylic
substitution of the primary allylic alcohol derivatives reported by the two groups2q,r is among the most
convenient strategies. This allylic substitution proceeds with high regioselectivity at the γ position (SN2’),
but suffers from the somewhat low asymmetric induction of 82% ee and 57% ee. In contrast, use of
secondary allylic esters in the allylic substitution have benefit of attaining high enantiopurity as
delineated in Scheme 1, in which the picolinoxy leaving group on allylic picolinates 3 and the copper
reagents derived from ArMgBr and Cu(acac)2 are responsible for the observed high stereo- and
regioselection.3,4 Herein, we present synthesis of sporochnol using this method. We first studied synthesis
† This paper is dedicated to Professor Dr. Lutz F. Tietze on the occasion of his 75th birthday.

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HETEROCYCLES, Vol. 93, No. 1, 2016
of allylic picolinate (R)-5 to discuss efficiency with the previous method3 and (R)-5 was then transformed
to (R)-1 (Scheme 2). With the established method in hand synthesis of (S)-1 was also accomplished.
OR
OR
natural form (S)-1 (R = H)
methyl ether (S)-2 (R = Me)
unnatural form (R)-1 (R = H)
methyl ether (R)-2 (R = Me)
Figure 1. Sporochnol and its methyl ether
O
N
R2
O
ArMgBr / Cu(acac)2
anti SN2'
R1 R2
Ar
R3
R3
R1
*
*
3
>97% chirality transfer
>92% regioselectivity
4
Scheme 1. Construction of a quaternary carbon by the allylic substitution
O
N
O
4-MeOC6H4MgBr / Cu(acac)2
(R)-1
OMe
(R)-5
6
Scheme 2. A strategy for synthesis of (R)-sporochnol
In our previous construction of quaternary carbons, alcohol (R)-8, the precursor of picolinate (R)-5, was
synthesized with 82% ee starting from geraniol (7) via Corey–Bakshi–Shibata reduction5 (CBS reduction)
of the corresponding ketone with (S)-MeCBS. In the present study the kinetic resolution of racemic
alcohol rac-8 was examined according to the literature procedure6 using the Sharpless asymmetric
epoxidation7 (the upper equation of Scheme 3), producing (R)-8 (>99% ee by chiral HPLC) and epoxide 9
in 29% and 53% yields, respectively, after chromatography on silica gel. This yield of (R)-8 was among
the level similar to that reported,6 and epoxide 9 was a roughly 1:1 mixture of the diastereomers by 1H and
13C–APT NMR spectroscopy,8 though the single diastereomer for the epoxide is drawn in the reported
supporting information.6 Since the yield of (R)-8 with high % ee was low, epoxidation of the Et and i-Pr
homologues (derived from 7 with EtMgBr and i-PrMgBr) was also attempted. However, the allylic
alcohols were isolated in 21% and 28% yields with 90 and 77% ee, respectively. Finally, alcohol (R)-8
was converted to picolinate (R)-5 in 92% yield by the standard method.
As an alternative synthesis of (R)-5, a sequence of reactions shown in the lower part of Scheme 3 was
envisioned. Among the steps involved in, slow methylation of 16(a,b) was anticipated because of the cis
stereochemistry regarding to the halogen atom (X) and the CH(OH)Me group on the olefin moiety. Thus,

HETEROCYCLES, Vol. 93, No. 1, 2016
49
the 1,3-allylic strain on the olefin was expected to place the Me and OH groups out of the allylic plane,
blocking the methylation. Probably, because of this steric reason, the previous methylation of a similar
vinyl iodide was achieved with Me2Zn and Pd(t-Bu3P)2.9 To avoid the use of such an unstable catalyst, the
methylation of 16(a,b) was studied as described below.
(1) via kinetic resolution by asymmetric epoxidation
OH
OR
OH
OH
t-BuOOH
L-(+)-DIPT, Ti(OPr)4
1) NMO, TPAP, MS4A
CH2Cl2, rt
+
O
CH2Cl2, –25 °C
2) MeMgBr, THF
86%
9, 53%
(R)-8, R = H
29%, >99% ee
(R)-5, R = 2-PyCO
7
rac-8
2-PyCO2H
DCC, DMAP
92%
(2) via Ru-catalyzed asymmetric hydrogentransfer
1) TESCl, DMAP, Et3N
CH2Cl2, rt
(R,R)-Ru cat.,a i-PrOH, rt
TESO
TESO
OR
O
HO
2) n-BuLi, Ac2O
THF, –78 °C to rt
86%
11
10
TBDPSCl, DMAP
Et3N, CH2Cl2
90%
12, R = H, 97% ee
13, R = TBDPS
82%
1) PPTS
1) [Ph3PCHMe2]+ I–, n-BuLi
THF, –78 °C to rt
OHC
EtOH, CH2Cl2, rt
OTBDPS
OH
2) NMO, TPAP, MS4A
CH2Cl2, rt
2) TBAF, THF, rt
77%
15
14
78%
OR
X
OH
Red-Al, THF, reflux, 2 h
Table 1
86%
then NIS, rt, 1.5 h
quant.
16a (X = I)
(R)-8, R = H
(R)-5, R = 2-PyCO
2-PyCO2H
DCC, DMAP
cf. 16b (X = Br)
88%
Scheme 3. Synthesis of the allylic picolinate by two methods
a Ru[(1R,2R)-TsDPEN](p-cymene).
Acetylene 10 was first converted to ynone 11 in two steps. The asymmetric hydrogen transfer reaction of
11 with Ru[(1R,2R)-TsDPEN](p-cymene)10 produced alcohol 12 in 86% yield with 97% ee (as
determined by chiral HPLC analysis of the derived benzoate), and the alcohol was converted to the
TBDPS ether 13 in 90% yield. The TES group was removed and the resulting alcohol was oxidized to
afford aldehyde 14 in 78% yield. Wittig reaction of 14 with the ylide prepared from [Ph3PCHMe2]+ I– and
n-BuLi followed by deprotection of the TBDPS group produced alcohol 15 in 77% yield.
Hydroalumination with Red-Al and subsequent reaction with NIS gave vinyl iodide 16a quantitatively.
Similarly, bromide 16b was synthesized by adding NBS after the Red-Al reaction. Both of the halides
16a,b were used for the next methylation.

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HETEROCYCLES, Vol. 93, No. 1, 2016
Table 1. Methylation of the model and the real vinyl halides
Sub-
Entry strate
Additive
(equiv)
Time Producta
Temp. (h) Me : H : SC (%)
Yieldb
Me–M (equiv)
Cat. (equiv)
X
OH
OH
OH
Me–M, cat., additive
THF, temp., time
starting compound (SC)
17a or 17b
+
+
C6H13
C6H13
C6H13
17a, X = I
17b, X = Br
19 (H product)
18 (Me product)
1
2
3
4
5
6
7
8
9
17b
17a
17a
17a
17a
17a
17a
17a
17b
MeMgBr (4)
Ni(acac)2 (0.2)
Ni(acac)2 (0.2)
Fe(acac)3 (0.5)
Fe(acac)3 (0.5)
Fe(acac)3 (0.5)
–
–
rt
13
13
0.5
2
81 : 10 : 9
nd
MeMgBr (4)
rt
70 : 30 : <1 nd
16 : 16 : 68 nd
57 : 43 : <1 nd
27 : 20 : 53 nd
MeMgBr (6)
NMP (9)
0 °C
rt
MeMgBr (6)
NMP (9)
NMP (9)
dppf (0.1)
dppf (0.1)
dppf (0.1)
dppf (0.1)
MeMgCl (6)
0 °C
rt
2
MeLi (3), ZnCl2 (3)
Pd2(dba)3·CHCl3 (0.1)
21
21
20
22
92 : 8 : <1
90 : 6 : 4
94 : 3 : 3
8 : 2 : 90
nd
nd
nd
nd
MeMgBr (3), ZnCl2 (3) Pd2(dba)3·CHCl3 (0.1)
rt
MeLi (3), ZnI2 (3)
MeLi (3), ZnCl2 (3)
Pd2(dba)3·CHCl3 (0.1)
Pd2(dba)3·CHCl3 (0.1)
rt
rt
X
OH
Me–M, cat., additive
THF, temp., time
OH
+
+
starting compound (SC)
(R)-8
(Me product)
16a or 16b
16a, X = I
20 (H product)
16b, X = Br
10
11
12
16a
16a
16b
MeLi (3), ZnI2 (3)
MeLi (3), ZnI2 (3)
MeMgBr (6)
Pd2(dba)3·CHCl3 (0.1)
Pd2(dba)3·CHCl3 (0.15) dppf (0.15) rt
Ni(acac)2 (0.2)
50c
dppf (0.1)
rt
25
37
13
86 : 9 : 5
93 : 7 : <1
86 : 11 : 3
nd
86
55
–
a Determine by 1H NMR using the following absorbance for one proton (δ): 17a, 5.59 (d, J = 7.2 Hz); 17b, 5.74 (d, J = 7.5 Hz);
18, 5.20 (d, J = 8.4 Hz); 19, 5.50 (dd, J = 15.3 Hz, 6.3 Hz); 16a, 5.59 (d, J = 7.2 Hz); 16b, 5.74 (d, J = 7.5 Hz); (R)-8, 5.22 (d,
b
J = 8.4 Hz); 20, 5.53 (dd, J = 15.3 Hz, 6.6 Hz). nd: Not determined because of the model study (entries 1–9) or the low
product selectivity (entry 10). c Reaction at rt for 14 h gave a 74 : 6 : 20 mixture.
On the basis of the previous study on the Ni-catalyzed arylation of the cis bromo-allylic alcohol
derivatives with arylborates,11 methylation of a model bromide 17b with MeMgBr was first attempted in
the presence of Ni(acac)2 to produce a 81:10:9 mixture of 18, the deiodinated compound 19, and the
starting compound (abbreviated as Me, H, and SC in Table 1) (entry 1). Iodide 17a was more reactive
than the bromide, but less product selective (70%) (entry 2). Based on the observed reactivity, iodide 17a
was used for further study. Fe-Catalyzed methylation with MeMgX (X = Br, Cl) was attempted in the
presence of NMP (N-methyl-2-pyrrolidone) according to the literature procedure for alkylation of alkenyl
and aryl halides.12 However, product selectivity under the conditions of entries 3–5 was quite low. We
then investigated Pd-catalyzed reaction under the Negishi conditions.13 As shown in entry 6, methylation
of iodide 17a with MeZnCl·LiCl (3 equiv) in the presence of Pd2(dba)3·CHCl3 (0.1 equiv) and dppf (0.1
equiv) gave 18 with 92% product-selectivity over 19 and 17a. A similar ratio was observed with
MeMgBr-based zinc reagent (entry 7). Further investigation found an excellent product-selectivity with

HETEROCYCLES, Vol. 93, No. 1, 2016
51
ZnI2 (entry 8, cf. entry 6). The excellent conditions of entry 6 was applied to bromide 17b, which showed
little reactivity (entry 9).
The procedure used in entry 8 was applied to the real iodide 16a to obtain a somewhat lower ratio of
86:9:5 for (R)-8, 20, and 16a (abbreviated as Me, H, and/or SC as well) (entry 10). Fortunately, the use of
the slightly larger quantity of the Pd catalyst resulted in 93% product-selectivity of (R)-8, which was
isolated in 86% yield after chromatography (entry 11). The Ni-catalyzed methylation with MeMgBr
under the conditions of entry 1 was also attempted to give (R)-8 in 55% yield with 86% product-
selectivity (entry 12). Finally, alcohol (R)-8 was converted to picolinate (R)-5.
Overall yield of (R)-5 from acetylene alcohol 10 was 29% in 11 steps, whereas that of the former from
geraniol (7) was 23% in 4 steps. Taking the overall yields, the total steps, and almost equal ease of access
to the (S)-isomer into consideration, the former synthesis would be suitable for synthesis of natural
sporochnol (S)-5 and the enantiomer (R)-5, while the latter would be useful for synthesis of analogues of
sporochnol.
O
4-MeOC6H4MgBr (3.0 equiv)
Cu(acac)2 (1.5 equiv)
N
m-CPBA, CH2Cl2
O
THF, –40 to –10 °C, 1 h
–5 °C to rt, 1 h
91%
OMe
OMe
O
(R)-5
89% yield
95% chirality transfer
98% regioselectivity
6
21
[Ph3PMe]+ I–
NaHMDS
1) O3, CH2Cl2
then Me2S
OHC
2) Zn, NaI
AcOH, rt, 1.5 h
THF, 0 °C, 1 h
90%
OMe
OR
22
(R)-2, R = Me
(R)-1, R = H
lit.
2b,c,h,q
43%
Scheme 4. Synthesis of (R)-sporochnol
The last part of the synthesis is delineated in Scheme 4. The required copper reagent (1.5 equiv), prepared
from 4-MeOC6H4MgBr and Cu(acac)2 in the 2:1 ratio, was subjected to the allylic substitution of (R)-5
(>99% ee, 1 equiv). The reaction proceeded smoothly to afford 6 in 89% yield with 95% chirality transfer
(CT) and 98% regioselectivity. The absolute configuration of the product was assigned later by
comparing the optical rotations of (R)-2 synthesized herein and reported in the literature. Epoxidation of 6
with m-CPBA proceeded regioselectively to afford 21 (1:1 mixture of the diastereomers) in 91% yield.
The olefin part of 21 was cleaved by ozonolysis, and the epoxide moiety was reduced with Zn, NaI, and
AcOH according to the literature procedure14 to furnish aldehyde 22 in 43% yield. Finally, Wittig reaction
of the aldehyde with Ph3P=CH2 gave the known methyl ether (R)-2 in 90% yield: 95% ee by chiral HPLC

52
HETEROCYCLES, Vol. 93, No. 1, 2016
19
20
analysis; [α]D –3.0 (c 0.35, CHCl3); cf. reported2c [α]D –3.7 (c 1, CHCl3). The methyl ether can be
converted to (R)-sporochnol ((R)-1) as described in the literature.2b,c,h,q
In a similar way, (S)-5 (98% ee) was synthesized via the Sharpless asymmetric epoxidation of rac-8 with
(D)-(–)-DIPT (Scheme 5). Allylic substitution of (S)-5 proceeded with 95% CT as well and the further
20
transformation afforded (S)-2: 93% ee by chiral HPLC; [α]D +2.7 (c 0.12, CHCl3); cf. reported2r [α]D
20
+3.0 (c 1.1, CHCl3).
O
N
OH
t-BuOOH
D-(–)-DIPT, Ti(OPr)4
O
2-PyCO2H
rac-8
DCC, DMAP
80%
CH2Cl2, –25 °C, 21 h
26% yield
OR
(racemic)
(S)-5
(S)-2, R = Me
(S)-1, R = H
(S)-8, 98% ee
lit. 2a,b,h,r
Scheme 5. Synthesis of (S)-sporochnol
In summary, we established a new method for synthesis of sporochnol with high ee. The preparation of
the allylic picolinate 5 through the Sharpless asymmetric epoxidation is straightforward for synthesis of
sporochnol as such, whereas the method starting from acetylene 10 (the second method in Scheme 3)
would be flexible for study of the structure-activity relationship of sporochnol.
ACKNOWLEDGEMENTS
This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (KAKEN, 26410111).
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