Convergent syntheses of the WXYZ ring of maitotoxin and the HIJK ring of brevisulcenal-F

Article abstract of DOI:10.1246/cl.171056

A convergent method to construct the 6/7/6/6-tetracyclic ether system possessing contiguous angular methyl groups was developed. The key steps of the synthesis involve coupling of a lithium acetylide and an aldehyde, cyclodehydration of a hydroxy ketone to form a dihydropyran, ring expansion of a six-membered ring ketone into a seven-membered one, and methylation of a mixed-thioacetal. Based on this strategy, syntheses of the WXYZ ring of maitotoxin and the HIJK ring of brevisulcenal-F were achieved, and the stereochemistry of the HIJK ring of brevisulcenal-F was confirmed.

Full text of DOI:10.1246/cl.171056

Advance Publication Cover Page
Convergent Syntheses of the WXYZ Ring of Maitotoxin and the HIJK Ring of Brevisulcenal-F
Naoya Osato, Hisaaki Onoue, Yoshiki Toma, Kohei Torikai, Makoto Ebine,
Masayuki Satake, and Tohru Oishi*
Advance Publication on the web December 15, 2017
doi:10.1246/cl.171056
© 2017 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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1
Convergent Syntheses of the WXYZ Ring of Maitotoxin and the HIJK Ring of Brevisulcenal-F
Naoya Osato,1 Hisaaki Onoue,1 Yoshiki Toma,1 Kohei Torikai,1 Makoto Ebine,1 Masayuki Satake,2 and Tohru Oishi*1
1Department of Chemistry, Faculty and Graduate School of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
2Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
E-mail: oishi@chem.kyushu-univ.jp
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9
A convergent method to construct the 6/7/6/6-
37
However, there are no reports on the synthesis of the
tetracyclic ether system possessing contiguous angular
methyl groups was developed. The key steps of the
synthesis involve coupling of a lithium acetylide and an
aldehyde, cyclodehydration of a hydroxy ketone to form a
dihydropyran, ring expansion of a six-membered ring
ketone into a seven-membered one, and methylation of a
mixed-thioacetal. Based on this strategy, syntheses of the
WXYZ ring of maitotoxin and the HIJK ring of
38 HIJK ring system of KBT-F possessing a β-hydroxy group
39 on the seven-membered ring. Herein, we describe an
40 alternative convergent strategy via two-rings construction4c
41 for synthesizing the 6/7/6/6-tetracyclic ether system
42 possessing contiguous angular methyl groups corresponding
43 not only to the WXYZ ring system of MTX (1a) but also the
44 HIJK ring system of KBT-F (1b).
10 brevisulcenal-F were achieved, and the stereochemistry of
11 the HIJK ring of brevisulcenal-F was confirmed.
45
Our synthetic strategy is shown in Scheme 1. The
46 6/7/6/6-tetracyclic ether system A corresponding to the
47 WXYZ ring of MTX is to be derived from C through the
48 construction of the tetrahydropyran ring via methylation of a
49 mixed-thioacetal reported by Nicolaou10 in an analogous
50 manner as previously reported.7 The seven-membered ring
51 ketone C could be derived from six-membered D by the
52 ring-expansion reaction reported by Mori,11 and the cyclic
53 ketone D can be traced back to an acyclic saturated ketone E
54 via cyclodehydration giving a dihydropyran followed by
55 hydroboration as reported by Crimmins12 and Nakata.13
56 Although a related synthetic method of the ketone E was
57 reported by Crimmins via Horner-Wadsworth-Emmons
58 reaction followed by 1,4-reduction,12 we envisaged that the
59 ketone E would be obtained by coupling of a terminal
60 alkyne F and a carbonyl compound G followed by
61 hydrogenation of the alkyne. The 6/7/6/6-tetracyclic ether
62 system B corresponding to the HIJK ring of KBT-F is to be
12
Epiphytic dinoflagellates produce highly toxic
13 secondary metabolites, so called ladder-shaped polyethers.1
14 Maitotoxin (MTX)2 and brevisulcenal-F (KBT-F),3 in
15 particular, are outstanding in their large molecular weights.
16 Their unique structures and potent biological activities have
17 attracted much interest in the synthetic community.4 Despite
18 development of a number of convergent methods to
19 synthesize ladder-shaped polyethers, it is a daunting task to
20 construct polycyclic ether frameworks possessing
21 contiguous angular methyl groups such as the WXYZ ring
22 system of MTX and the HIJK ring system of KBT-F (Figure
23 1) because of the severe steric repulsion existing within
24 them. Thus, there are only three precedents to construct the
25 polycyclic ether system containing the WXYZ ring system
26 of MTX. The WXYZA’ ring of MTX was synthesized by
27 Nakata5 in a linear manner via SmI2-induced reductive
28 cyclization and 6-endo cyclization of a vinyl epoxide,
29 whereas a convergent synthesis of the WXYZA’ ring of
30 MTX was reported by Nicolaou6 via Takai olefination and
31 ring-closing metathesis. We reported a highly convergent
32 synthesis of the WXYZA’B’C’ ring7 of MTX based on
33 the-cyano ether method8 developed in our laboratory as
34 part of a structure-activity relationship study.9
63 synthesized in
a similar manner from the common
64 intermediate C, where regio- and stereoselective installation
65 of the hydroxy group of the seven-membered ring is
66 required.
67 Scheme 1. Synthetic strategy for synthesizing the 6/7/6/6-tetracyclic
68 ethers (P = protecting group).
69
70
Synthesis of the 6/7/6/6-tetracyclic ether 1a via seven-
35 Figure 1. Partial structures of the WXYZ ring of MTX, the HIJK ring
36 of KBT-F, and structures of the 6/7/6/6-tetracyclic ethers (1a and 1b).
71 membered ring ketone 13 is shown in Scheme 2.

2
Scheme 2. Synthesis of the 6/7/6/6-tetracyclic ether 1a. (a) O3, CH2Cl2, MeOH, −78 °C, 3.5 h, then PPh3, −78 °C to rt, 3.6 h; (b)
MeCOCN2PO(OMe)2, Cs2CO3, MeOH, rt, 1.4 h, 83% (two steps); (c) NaClO2, NaH2PO4, 2-methyl-2-butene, t-BuOH/H2O, rt, 1 h, 90%; (d)
Me(OMe)NH·HCl, DCC, DMAP, Et3N, CH2Cl2, rt, 25.5 h, 97%; (e) 3, n-BuLi, THF, 0 °C, 1 h; then 4, THF, −78 °C, 1 h, 88%; (f) 3, n-BuLi, THF,
0 °C, 1 h; then 5, THF, 0 °C to rt, 2 h, 49%; (g) PtO2, H2, EtOAc, rt, 7.7 h; (h) Dess-Martin periodinane, CH2Cl2, rt, 1.1 h, 87% (two steps); (i)
[CuH(PPh3)]6, phenylsilane, H2O, benzene, 0 °C, 2 h, 76%; (j) TBAF, THF, rt, 1 h, 9 (54%), recovery of 8 (40%); (k) Nafion NR-50, MS4A, toluene,
reflux, 30 min, 76%; (l) BH3·SMe2, THF, 0 °C, then H2O2, NaOH rt, 1.5 h, 98%; (m) TPAP, NMO, MS4A, CH2Cl2, rt, 1.4 h, 97%; (n) TMSCHN2,
BF3·OEt2, CH2Cl2, −78 °C, 1.9 h; (o) p-TsOH·H2O, CH2Cl2/MeOH, rt, 21.2 h, 63% (two steps); (p) TIPSOTf, 2,6-lutidine, CH2Cl2, rt, 14.5 h, 13
(64%), 14 (34%); (q) TBAF, THF, −20 °C, 4 min, 86%; (r) DDQ, pH7 buffer, CH2Cl2, 0 °C to rt, 50 min, 85%; (s) EtSH, TfOH, MS4A, CH2Cl2, 0 °C,
1.6 h; (t) TIPSOTf, 2,6-lutidine, CH2Cl2, 30 min, 74% (two steps); (u) MCPBA, CH2Cl2, −78 °C to −40 °C, 1 h; then Me3Al, −40 °C to 0 °C, 1.5 h,
1
2
3
4
5
6
7
8
9
Terminal alkyne 3 was prepared from known alkene 214 by
ozonolysis followed by treatment with Ohira-Bestmann
reagent15 in the presence of Cs2CO3 in 83% yield for two
steps.
Alkyne 3 was treated with n-BuLi and the resulting lithium
acetylide was reacted with known aldehyde 47 to furnish
propargylic alcohol 6 in 88% yield as a mixture of
diastereomers in a 2.5 : 1 ratio. Hydrogenation of alkyne 6
with PtO2 under a hydrogen atmosphere, followed by Dess-
29 was carried out as reported by Mori.10 Thus, treatment of 12
30 with TMSCHN2 in the presence of BF3·OEt2 resulted in the
31 formation of a seven-membered ring ketone as a mixture
32 with -TMS ketone, which was subjected to hydrolytic
33 conditions with p-TsOH in CH2Cl2/MeOH resulting in
34 concomitant removal of the TMS group and benzylidene
35 acetal to afford the diol in 60% yield for two steps.
36 Protection of the resulting diol as TIPS ethers gave 13
37 (64%) with concomitant formation of silyl enol ether 14
38 (34%), which was recovered to form 13 (86%) by treatment
39 with TBAF at −20 °C for 4 min.
10 Martin oxidation of the resulting saturated alcohol gave
11 ketone 8 in 87% yield over two steps. Although the ketone 8
12 was alternatively obtained from the Weinreb amide 5
13 derived from 4 (87%, two steps) via coupling with 3 (49%)
14 and 1,4-reduction of the resulting ynone 7 with Stryker
15 reagent16 (76%), the former route (77% for three steps from
16 4) was more practical than the latter one (33% for four steps
17 from 4) both in yield and reagent economy. Removal of the
18 TBS group of 8 with TBAF resulted in the formation of
19 hydroxy ketone 9 in 54% yield as a mixture of its
20 hemiacetal in a 1:1 ratio with 40% recovery of 8 (prolonged
21 reaction time resulted in low yield of 9). Treatment of 9
22 with Nafion NR-5017 in toluene under reflux afforded
23 dihydropyran derivative 10 in 76% yield. Hydroboration of
24 10 with BH3·SMe2 proceeded stereoselectively to yield
25 secondary alcohol 11 in 98% yield as a single isomer. Ley
26 oxidation of alcohol 11 with TPAP and NMO17 furnished
27 ketone 11 in 97% yield. The next task, ring-expansion of the
28 six-membered ring ketone into the seven-membered one,
40
Having the key intermediate 13 in hand, the WXYZ
41 ring of MTX was synthesized as previously reported but
42 with modification of the mixed-thioacetal formation.7 After
43 removal of the NAP group of 13 with DDQ giving hydroxy
44 ketone 15 in 85% yield, the mixed-thioacetal formation was
45 examined. As reported,7 treatment of the hydroxy ketone
46 with EtSH in the presence of Zn(OTf)2 afforded the desired
47 mixed-thioacetal 16 (42%) with concomitant formation of
48 dithioacetal (8%). Recovery of the starting material (42%),
49 and recycling of the recovered 15 gave 16 in 64% total yield.
50 After considerable experimentation with various Lewis
51 acids, such as Sc(OTf)3 and In(OTf)3, we finally found that
52 not metal triflates but TfOH in the presence of MS4A gave
53 better results, and 16 was obtained in 74% yield as a
54 mixture of diastereomers (α:β = 4.8:1) after protection of the
55 partially desilylated products. Next, introduction of the
56 angular methyl group was achieved by treatment with

3
Scheme 3. Syntheses of the 6/7/6/6-tetracyclic ether 1b and 1c. (a) LHMDS, TMSCl, Et3N, THF, −78 °C, 1 h; (b) Pd(OAc)2, p-benzoquinone, CH3CN,
rt, 15.5 h, 95% (two steps); (c) B2(pin)2, CuCl, n-Bu3P, DMF, rt, 4.5 h; H2O2, NaOH, rt, 30 min, 84%; (d) DDQ, pH7 buffer, CH2Cl2, 0 °C, 2 h, 93%;
(e) EtSH, TfOH, MS4A, CH2Cl2, 0 °C, 4 h, 74% (α:β = 2.9:1); (f) MCPBA, CH2Cl2, −78 °C to −40 °C, 1 h, then Me3Al, −40 °C to 0 °C, 1 h, 42%; (g)
TBAF, THF, rt, 2.5 h, 87%; (h) NaBH4, CeCl3·7H2O, CH2Cl2, MeOH, 0 °C, 20 min, 81%; (i) MCPBA, CH2Cl2, rt, 2 h, 87%; (j) TPAP, NMO, MS4A,
CH2Cl2, rt, 30 min, 95%; (k) Na[PhSeB(OEt)3], AcOH, EtOH, rt, 1.5 h, 90%; (l) DDQ, pH 7 buffer, CH2Cl2, rt, 1 h, 89%; (m) EtSH, TfOH, MS4A,
CH2Cl2, 0 °C, 4 h, 69% (30% recovery); (n) TIPSOTf, 2,6-lutidine, CH2Cl2, rt, 6.5 h, 91%; (o) MCPBA, CH2Cl2, −78 °C to −40 °C, 1 h; then Me3Al,
−40 °C to 0 °C, 3 h, 94%; (p) TBAF, THF, rt, 27 h, 84%.
1
2
3
4
5
6
7
8
9
MCPBA followed by Me3Al to afford 17 in 73% yield.
Although direct methylation of the mixed thioacetal 16 was
carried out with Me2Zn and Zn(OTf)2 as reported by
Kadota,19 no reaction occurred in CH2Cl2 at room
temperature, and that in ClCH2CH2Cl under reflux afforded
17 with concomitant formation of a enol ether formed via
elimination of the ethylthio group as an inseparable mixture
in a 1.2 : 1 ratio. Removal of the TIPS group with TBAF
furnished the 6/7/6/6-tetracyclic ether 1a corresponding to
37 ethylthio group (38%). Although the low yield of 21 was
38 considered to be due to the presence of the free hydroxy
39 group, and the reactions were carried out after protecting the
40 alcohol as a TIPS ether, the desired product was obtained as
41 an inseparable mixture with unidentified byproducts.
42 Inversion of the secondary alcohol of 21 was unsuccessful,
43 oxidation of 21 and reduction of the resulting ketone gave
44 21 as a single isomer, and Mitsunobu inversion of 21 did not
45 occur. Finally, removal of the TIPS groups of 21 with
46 TBAF afforded 1c in 87% yield.
10 the WXYZ ring of MTX in 87% yield.
11 Next, we moved on to the synthesis of the HIJK ring of
47
Since direct introduction of the β-hydroxy group to the
12 KBT-F as shown in Scheme 3. The seven-membered ring
13 ketone 13 was converted to ,-unsaturated ketone 18 in
14 95% yield for two steps via silyl enol ether formation by
15 successive treatment with LHMDS, TMSCl, and Et3N,20
16 followed by the Ito-Saegusa reaction with Pd(OAc)2 and p-
17 benzoquinone.21 The next crucial step, regio- and
18 stereoselective introduction of the -hydroxy group at the
19 C36 position of KBT-F to form the secondary alcohol 19,
48 enone 18 and inversion of the secondary alcohol of 21 were
49 unsuccessful, we next examined the method applied for the
50 synthesis of gymnocin-A reported by Sasaki.25 Luche
51 reduction26 of the enone 18 resulted in the formation of an
52 allylic alcohol as a single isomer in 81% yield, which was
53 oxidized with MCPBA to afford β-epoxide 22 in 87% yield
54 as a single isomer. Ley oxidation of the alcohol 22 with
55 TPAP/NMO furnished an epoxy ketone, which was treated
56 with Na[PhSeB(OEt)3] developed by Miyashita-
57 Yoshikoshi27 to afford β-hydroxy ketone 23 in 90% yield.
58 Formation of the six-membered ring was achieved in an
59 analogous sequence: 1) removal of the NAP group with
60 DDQ (89%) giving a hydroxyl ketone; 2) mixed-thioacetal
61 formation with EtSH and TfOH to give 24 (69%, 30%
62 recovery of the hydroxy ketone); 3) protection of the
63 hydroxyl group as TIPS ether 25 (91%); 4) introduction of
64 the angular methyl group to afford 26 (94%);23 and 5)
65 removal of the all TIPS groups with TBAF to provide 1b
66 corresponding to the HIJK ring of KBT-F (84%).
20 was examined using
a copper-catalyzed borylation-
21 oxidation sequence reported by Hosomi.22 Thus, treatment
22 of the enone 18 with bis-pinacolborane in the presence of
23 CuCl and n-Bu3P followed by oxidative work-up resulted in
24 the formation of 19 in 84% yield as a single isomer.
25 Unfortunately, the stereochemistry of 19 turned out to be
26 opposite to that of the HIJK ring system of KBT-F.23
27 Although attempts to invert the secondary alcohol by
28 Mitsunobu reaction modified by Tsunoda24 were
29 unsuccessful, producing 18 via β-elimination, we proceeded
30 to the next transformation with the expectation of inverting
31 the stereochemistry after constructing the tetracyclic system.
32 Thus, 19 was converted to 1c in an analogous sequence with
33 1a: 1) removal of the NAP group (93%); 2) mixed-
34 thioacetal formation giving 20 (74%, α:β = 2.9:1); and 3)
35 introduction of the angular methyl group to afford 21
36 (42%)23 with a byproduct formed via β-elimination of
67
The NMR spectra of 1b and 1c were compared with
68 those of KBT-F. The differences in the chemical shifts
69 between KBT-F, 1b and 1c are shown in Figure 2: (a) H
1
70 NMR (600 MHz, C5D5N), (b) 13C NMR (150 MHz, C5D5N).
71 The x- and y-axes represent carbon number and  ( =
72 KBT-F – synthetic 1b and 1c in ppm), respectively (The

4
1
2
3
4
5
6
7
8
9
carbon numbering of KBT-F is shown in Figure 1). For both
diastereomers, 1H NMR chemical shifts at both termini
deviated because the structures are different from the natural
product. However, for other parts, chemical shifts of 1b are
identical with those of KBT-F, while those of 1c at C33 and
C38 deviate by 1.03 ppm and 0.68 ppm, respectively.
Analogously, 13C NMR chemical shifts at both termini
deviate for both diastereomers, and chemical shifts of 1b for
other parts are identical with those of KBT-F, while those of
44
45 References and Notes
46
47
48
49
50
51
52
53
54
55
56
57
58
59
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1
2
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3
4
10 1c at C33, C34, C104, and C38 deviate by more than 2.0
11 ppm. Therefore, the structure of the HIJK ring system of
12 KBT-F has been confirmed, whereas previously its absolute
13 configuration was unknown.3
5
6
7
8
9
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C
16 NMR (150 MHz, C5D5N). The x- and y-axes represent carbon number
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18 1b and 1c, respectively.
82 12 a) M. T. Crimmins, P. J. McDougall, K. A. Emmitte, Org. Lett.
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21 the 6/7/6/6-tetracyclic ether system was developed via
22 acetylide-aldehyde coupling, dehydrative cyclization, and
23 ring-expansion as key steps. The advantages of this strategy
24 are its applicability to ring systems possessing contiguous
25 angular methyl groups, and the use of the highly
26 nucleophilic acetylide as a coupling partner, which can be
27 easily recovered after the coupling reaction if an excess
28 amount of the acetylide was used. Based on this strategy,
29 stereoselective syntheses of the WXYZ ring of MTX (1a),
30 the HIJK ring of KBT-F (1b), and its C36 epimer (1c), have
31 been achieved from the key intermediate 13, and the
32 stereochemistry of the HIJK ring of KBT-F was confirmed.
33 Applications of this method to synthesize other ladder-
34 shaped polyethers are currently underway in our laboratory.
35
87 14 a) K. C. Nicolaou, D. A. Nugiel, E. Couladouros, C.-K. Hwang,
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36 Acknowledgments
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37 This work was supported by JSPS KAKENHI Grant
38 Numbers JP24245009, JP16H04112, JP16J01199, and
39 JP16H01159 in Middle Molecular Strategy, and the Asahi
40 Glass Foundation.
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