Syntheses of cyclopentyl nucleoside (?)-neplanocin A through tetrazole-fragmentation from cyanophosphates

Article abstract of DOI:10.1016/j.tet.2018.03.020

We recently reported a novel synthetic method for five-membered unsaturated cyclic compounds from ketones involving cyanophosphates (CPs) under neutral conditions, in which alkylidene carbenes generated through tetrazole-fragmentation undergo [1,5]-C–H in

Full text of DOI:10.1016/j.tet.2018.03.020

Accepted Manuscript
_
Syntheses of cyclopentyl nucleoside ( )-neplanocin a through tetrazole-fragmentation
from cyanophosphates
Hiroki Yoneyama, Kenji Uemura, Yoshihide Usami, Shinya Harusawa
PII:
S0040-4020(18)30263-1
10.1016/j.tet.2018.03.020
TET 29360
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 February 2018
Revised Date: 7 March 2018
Accepted Date: 9 March 2018
Please cite this article as: Yoneyama H, Uemura K, Usami Y, Harusawa S, Syntheses of cyclopentyl
_
nucleoside ( )-neplanocin a through tetrazole-fragmentation from cyanophosphates, Tetrahedron (2018),
doi: 10.1016/j.tet.2018.03.020.
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ACCEPTED MANUSCRIPT
_
Syntheses
of
cyclopentyl
nucleoside
( )-neplanocin
A
through
tetrazole-fragmentation from cyanophosphates
Hiroki Yoneyama, Kenji Uemura, Yoshihide Usami, Shinya Harusawa*
1

ACCEPTED MANUSCRIPT
_
Syntheses of cyclopentyl nucleoside ( )-neplanocin A through
tetrazole-fragmentation from cyanophosphates
Hiroki Yoneyama, Kenji Uemura, Yoshihide Usami, Shinya Harusawa*
Osaka University of Pharmaceutical Sciences, 4-20-1 Nasahara, Osaka 569-1094, Japan
ABSTACT
We recently reported a novel synthetic method for five-membered unsaturated cyclic
compounds from ketones involving cyanophosphates (CPs) under neutral conditions, in
which alkylidene carbenes generated through tetrazole-fragmentation undergo
_
[
1,5]-C H insertions to produce the target compounds. The present paper describes the
use of the tetrazole-fragmentation from CPs for the efficient and practical syntheses of
_
(
)-neplanocin A and a protected tetrol, the latter of which is an important synthetic
_
precursor of both ( )-neplanocin A and its analogues. Furthermore, formation of an
unusual dihydropyran derivative was observed during the synthetic study of the tetrol.
Keywords:
Neplanocin A
Carbocyclic nucleoside
Cyanophosphate
Neutral condition
_
[
1,5]-C H insertion
Alkylidene carbene
2

ACCEPTED MANUSCRIPT
_
(
)-Neplanocin A (NPA: 1) is a naturally occurring carbocyclic nucleoside that was first
1
isolated from the culture filtrate of the soil fungus Ampullariella regularis in 1981.
NPA and other natural analogues have received great attention owing to their interesting
2
biological properties, such as their potent antiviral and antitumor properties (Fig. 1).
_
(
)-NPA, one of the most potent S-adenosylhomocysteine hydrolase inhibitors, has
3
broad-spectrum antiviral activity. However, NPA itself is apparently cytotoxic to host
2
c,4
cells
and is also rapidly deaminated by adenosine deaminase to a
5
chemotherapeutically inactive inosine congener, which may account for its reduced
therapeutic potency. Therefore, the chemical synthesis and structural modification of
2
c,6
NPA have been investigated extensively,
and several total syntheses have been
achieved. In these approaches, the key steps were palladium-catalyzed rearrangement of
7
the acetate moiety, the construction of a carbocyclic ring through intramolecular
_
_
8
9
Horner Wadsworth Emmons or Wittig reactions, an intramolecular aldol reaction, or a
1
0
ring-closing metathesis reaction using Grubbs catalysis. Other approaches for the
preparation of 1 have included a palladium-catalyzed desymmetrization of
11
cyclopentenes with 6-chloropurine, chemoenzymatic desymmetrization of bicyclic
1
2
13
Diels-Alder adducts, intramolecular nitrone cycloaddition, zirconocene-mediated
1
4
15
ring constraction, and an intramolecular Baylis-Hillman reaction.
_
Figure 1. ( )-Neplanocin A (1) and important synthetic precursor 2α for neplanocin
derivatives.
Protected cyclopentenyl tetrol 2 has been widely used as a synthetic precursor not only
_
6-15
for ( )-1, but also its analogues.
The shortest method thus far was reported by Ohira
_
and co-workers, who synthesized of ( )-1 from 2α [PG = triphenylmethyl (Tr)] by
_
16
employing a C
–
H insertion reaction of alkylidene-carbene 4 (Scheme 1 A) . With this
derived from -ribose is reacted with lithium
method, ketone
3
D
3

ACCEPTED MANUSCRIPT
17
trimethylsilyldiazomethane [TMSC(Li)N ], affording cyclopentene derivative 5αβ in
2
1
6
5
5−65% yield (5α/5β = 1:2.7). Removal of the tert-butyldimethylsilyl (TBDMS)
group of 5αβ with tetrabutylammonium fluoride (TBAF) furnished protected tetrol
2
2
αβ (69%). Subsequent pyridinium dichromate (PDC) oxidation (80%) of
αβ followed by lithium aluminum hydride (LAH) reduction (87%) provided desired
tetrol 2α as a single stereoisomer. Then, Mitsunobu reaction of 2α with adenine using
1
8
_
the Nokami procedure followed by deprotection of the hydroxy groups afforded ( )-1.
On the basis of the Ohira method, Liao and co-workers also described the syntheses of
1
9
2
’-β-C-methyl-neplanocin derivatives.
Alternatively, Matsuda and co-workers employed ketone 6 derived from adenosine
for the C H insertion reaction of alkylidene-carbene 7 using TMSC(Li)N (Scheme
–
2
_
20
1
B). Reaction of ketone 6 with TMSC(Li)N afforded a 1:4 epimeric mixture (8αβ)
2
of 8α and 8β at the 1’-position in only 21% yield. Generated carbene 7 has two
conformers, 7A and 7B. Steric repulsion between the isopropylidene and adenine
groups in conformer 7A could lead preferentially to the adoption of conformer 7B,
_
which would afford desired β-diastereomer 8β (Scheme 1 B). However, desired isomer
8
β was unfortunately not isolated from epimeric mixture 8αβ, presumably owing to the
difficulty of separation through column chromatography on silica gel. As a result, this
second synthetic study employing a C H insertion reaction of an alkylidene-carbene
–
2
0
could not be accessed.
α-Cyanophosphates (CPs) have been widely utilized as synthetic intermediates in
2
1
organic synthesis. In continuation of our program on the utilization of the CPs, we
recently reported a novel synthetic method for five-membered unsaturated cyclic
compounds from ketones based on the reactions of CPs with trimethylsilylazide
2
2
(
TMSN ) in the presence of Bu SnO as a catalyst, as shown in Scheme 2. In this
3 2
two-step transformation, CPs 10 may form tetrazolylphosphates 11, which subsequently
2
3
undergo successive fragmentation to generate alkylidene carbenes 13, which undergo
_
22
[
1,5]-C H insertions to produce five-membered cyclic compounds 14. The scope of
these reactions that occur under neutral conditions could be extended towards a variety
of cyclopentenes and heterocyclic products that are not usually accessible from the
corresponding carbonyl compounds through the TMSC(Li)N procedure, which requires
2
1
7,22
basic conditions.
In addition, Postel and co-workers have reported the preparation of
cyclopentene derivatives as the intermediates for the syntheses of NPA stereoisomers
_
from
D
-mannose using similar [1,5]-C H insertions of alkylidene carbenes generated
2
4
from α-cyanomesylates.
4

ACCEPTED MANUSCRIPT
Scheme 1. (A) Synthesis of neplanocin A (1) from synthetic precursor 2α using the
Ohira method, (B) Synthesis of epimeric mixture 8αβ at the 1’-position using the
Matsuda method
5

ACCEPTED MANUSCRIPT
Scheme 2. Synthesis of five-membered unsaturated cyclic compounds 14 from ketones
9
through CPs 10.
On the basis of these reports, to demonstrate the synthetic utility of the alkylidene
2
2
carbenes generated under neutral condition from CPs (CP method), we attempted to
apply the CP method to ketones 3 and 6 employed by Ohira and Matsuda,
1
6,20
respectively.
tetrol 2α and ( )-1 using CPs 16 and 21, respectively, in which the intramolecular
1,5]-C insertion reactions of the alkylidene carbenes generated via
Herein, we report the efficient and practical syntheses of protected
_
[
–H
tetrazole-fragmentation were the key synthetic transformations. Furthermore, in the
synthesis of tetrol 2α, formation of an unusual dihydropyran derivative 17 was newly
2
3
observed.
_
1
. Synthesis of ( )-neplanocin A from CP 16
2
5
Ketone 3, which was prepared via triphenylmethyl (Tr) ether 15 from
D
-ribose in five
2
6
steps (71% overall yield), was subjected to the CP method, as illustrated in Scheme 3.
21
Reaction of ketone 3 with diethyl phosphorocyanidate (DEPC, 3.0 equiv.) in the
2
7
presence of LiCN (3.0 equiv.) easily afforded CP 16 in 95% yield. Reaction of CP 16
with TMSN (3.0 equiv.) in the presence of Bu SnO (0.3 equiv.) in refluxing toluene for
3
2
22
2
4 h afforded an inseparable mixture of epimeric cyclopentenes 5αβ, which was the
result of the C
–
H insertion reaction of alkylidene-carbene 4, along with unexpected
1
compound 17. The ratio of 5αβ to 17 was 3:1 according to H-NMR (TLC on silica gel:
Rf = 0.7, 20% EtOAc in hexane). After the mixture was subsequently treated with
TBAF to remove the TBDMS group, deprotected cyclopentenes 2αβ (2α/2β = 1/3) and
6

ACCEPTED MANUSCRIPT
compound 17 were separated using column chromatography on silica gel and obtained
in 65% and 20% yields, respectively, from CP 16. Compound 17 was determined to be a
dihydropyran derivative and the stereochemical assignment was concluded using
_
NOESY analysis, as illustrated in Figure 2 A. The unexpected formation of 17 may
_
have been due to [1,6]-O Si bond insertion of the carbene 4a or 1,2-TBDMS shift of
_
28a
_
ylide 4b (Figure 2 B), as opposed to common [1,5]-O Si insertions that generate
2
3,28b
dihydrofuran derivatives.
Ohira and co-workers reported only the formation of 5αβ in the reaction of ketone 3
with TMSC(Li)N and did not mention the production of dihydropyran 17, as shown in
2
_
16
_
Scheme 1 A The unusually broad yield range (55 65%) of 5αβ reported by Ohira may
be a combined yield of 5αβ and 17 owing to the difficulty of their separation. Indeed,
when we attempted to reproduce Ohira’s result [reaction of ketone 3 with TMSC(Li)N2
o
16
(
3 equiv) in THF for 1 h at 0 C], an inseparable mixture of 5αβ (5α / 5β = 1/3) and
1
7 was produced in 61% combined yield (see experimental section).
Transformation of 2αβ into desired cyclopentenol 2α using Dess-Martin periodinane
(
DMP) oxidation (93%) and subsequent LAH reduction (98%) was more effective than
_
the corresponding reactions in the Ohira method (Scheme 1 A). Cyclopentenol 2α was
obtained in 49.5% overall yield in eight steps from protected
D
-ribose 15. This result is
_
superior to that achieved by Ohira method (18 23% overall yield of 2α from 15): the
_
overall yield was more than two times higher. Alchohol 2α can be converted to to ( )-1
through efficient Mitsunobu coupling with N-6 amino bis-Boc-protected adenine using
2
9
the Schneller procedure followed by deprotection under acidic condition.
7

ACCEPTED MANUSCRIPT
_
Scheme 3. Synthesis of ( )-neplanocin A from CP 16
8

ACCEPTED MANUSCRIPT
Figure 2. (A) NOESY analysis of 17, (B) Formation of 17 through two possible
pathways
_
2
. Synthesis of ( )-neplanocin A from CP 21
_
We next investigated the synthesis of ( )-1 via CP 21 from adenine-containing ketone 6,
starting from 2’,3’-O-isopropylidene-adenosine (18) using Matsuda’s approach (Scheme
4
). Matsuda used the reductive tetrahydrofuran-ring cleavage reaction reported by
3
0
Maki. Treatment of 18 with diisobutyl aluminum hydride (DIBAL) in THF afforded
3
1
acyclic nucleoside 19 in 57% yield. Meanwhile, we found that the use of cyclopentyl
methyl ether (CPME)-toluene (1:1, v/v) solvent system for the ring-cleavage reaction of
1
8 led to improve yield of 19 to 85% owing to much easier extraction. After selective
protection (90%) of the primary hydroxyl group of 19 with TBDMSCl, we oxidized
(
95%) secondary alcohol 20 with o-iodoxybenzoic acid (IBX) to furnish ketone 6. The
2
0a
DMP oxidation of 20 reported by Matsuda gave a lower yield (71%).
2
7
Cyanophosphorylation (quant) of 6 with DEPC catalyzed by LiCN followed by
treatment of CP 21 with TMSN (6 eq) catalyzed by Bu SnO in refluxing toluene for 24
3
2
_
h afforded an epimeric mixture (8αβ) of carbocyclic nucleosides through a [1,5]-C H
2
3
insertion reaction of alkylidene carbene 7. Although desired product 8β was not
2
0b
isolated successfully from mixture 8αβ in the Matsuda approach,
it was
3
2
chromatographed over spherical silica gel [MeOH-EtOAc (1:99, v/v)] and obtained in
3
6% yield in this study, with 25% yield of 8α. Deprotection of thus obtained β-epimer
_
8β under acidic conditions afforded ( )-1 with a high purity in an overall yield of 25.6%
8
i
in six steps from protected adenosine 18. The physical and spectral properties of 1
_
_
were essentially identical to literature data {1: [α] 156.0 (c 0.5, H O) and mp 220 221
D
2
9

ACCEPTED MANUSCRIPT
o
8i
_
_
o
C; lit. [α] 156.6 (c 0.5, H O) and mp 220 222 C}.
D
2
_
Scheme 4. Synthesis of ( )-neplanicin A from CP 21
10

ACCEPTED MANUSCRIPT
In conclusion, the efficient and practical syntheses of neplanocin A (1) and widely
applicable intermediate 2α were achieved using CPs through tetrazole-fragmentations
under neutral conditions. The CP method gave 2α in 49.5% overall yield in eight steps
from protected
D
-ribose 15, this yield was more than two times higher than that
_
(18 23%) achieved by Ohira’s approach. Furthermore, formation of the unusual
dihydropyran derivative 17 was newly observed through two possible pathways in this
study. In addition, the CP method was applied to the efficient synthesis of natural
neplanocin A in 25.6% overall yield in six steps from protected adenosine 18 following
successful isolation of cyclopentyl nucleoside 8β. In contrast, the synthesis using the
Matsuda approach was unfinished. We hope that the lessons learned in this undertaking
can be applied fruitfully to other synthetic targets. In addition, the present study
2
1
demonstrated the applicability of CPs as intermediates in organic synthesis.
3
. Experimental
3
.1 General information
All reactions were carried out under an inert argon atmosphere. Anhydrous solvents
THF, toluene, CH Cl , DMF, CPME and MeCN) were purchased from Wako Chemical
(
2
2
Company. During organic workup, solvent extracts were dried over Na SO and
2
4
subsequently removed in a rotary evaporator under reduced pressure. Fuji Silysia
FL-60D silica gel was used for flash column chromatography. 8α and 8β were
chromatographed over spherical silica gel (Fuji Silysia PSQ 100B silica gel). TLC was
1
performed using precoated plates (Wako silica gel 70 F254). H-NMR spectra were
recorded on a Varian Mercury-300 or an Agilent 400-MR-DD2 spectrometer in CDCl3
with tetramethylsilane (TMS) as an internal standard or CD OD with chemical shifts (δ)
3
given relative to from CD OD (3.3 ppm). Coupling constants (J) are reported in Hertz
3
(
Hz). For multiplicities the following abbreviations were used: s, singlet; d, doublet; t,
1
3
triplet; m, multiplet; br, broad. C-NMR spectra were recorded on a Varian
Mercury-300 or an Agilent 400-MR-DD2 spectrometer in CDCl . Chemical shifts (δ)
3
are given relative to CDCl (77.0 ppm) or CD OD (49.0 ppm). High-resolution mass
3
3
spectra were obtained using a JMS-700(2) double-focusing magnetic sector mass
spectrometer (Jeol Ltd., Tokyo, Japan) operating in positive-ion mode, with
3
3
3
-nitrobenzyl alcohol (NBA)-NaCl or triethanolamine (TEOA)-NaCl as matrices.
11

ACCEPTED MANUSCRIPT
3
.2. Synthesis of (-)-neplanocin A from CP16
.2.1. (2S,3S)-[4-(tert-Butyldimethylsilyloxymethyl)-2,3-isopropylidenedioxy-1-
3
cyanobutyl diethyl phosphate (16)
DEPC (831 mg, 5.1 mmol) and LiCN (168 mg, 5.1 mmol) were added to a solution
of ketone 3 (944 mg, 1.7 mmol) in THF (20 mL) at rt. After it was stirred for 30 min,
the reaction mixture was treated with water (60 mL), and then extracted with
EtOAc-hexane (1:1, 100 mL). The organic layer was washed with brine, dried over
Na SO , filtered, and concentrated. Purification via silica gel column chromatography
2
4
(
EtOAc-hexane, 1:4) afforded CP 16 (1165 mg, 95%, oil).
1
H-NMR (300 MHz, CDCl ): δ 0.09 (s, 3H), 0.10 (s, 1.5H), 0.11 (s, 1.5H), 0.91 (s,
3
4
.5H), 0.91 (s, 4.5H), 1.22-1.33 (m, 10.5H), 1.43 (s, 1.5H), 3.60 (d, 0.5H, J = 8.4 Hz),
_
_
3
.61 (d, 0.5H, J = 10.2 Hz), 3.76 (d, 0.5H, J = 8.4 Hz), 3.89 4.27 (m, 6.5H), 4.40 4.48
(
m, 1H), 4.59 (d, 0.5H, J = 6.3 Hz), 4.71 (dd, 0.5Hz, J = 6.3, 4.5 Hz), 7.24-7.34 (m, 9H),
13
7
2
7
1
.45-7.49 (m, 6H); C-NMR (75.5 MHz, CDCl ): δ -5.2, -5.0, 15.9, 16.0, 18.4, 24.9,
3
5.1, 25.9, 26.4, 26.8, 61.4, 61.5, 63.1, 63.9, 64.4, 64.5, 64.6, 64.7, 64.8, 64.9, 75.5,
5.6, 75.9, 76.0, 79.1, 79.3, 87.5, 87.7, 109.2, 109.4, 115.7, 115.8, 116.0, 127.3, 127.8,
+
27.9, 128.7, 128.8, 142.8, 142.9; HRMS (FAB+NaCl): m/z [M+Na] calcd for
C H NO PSiNa: 732.3098; found: 732.3099.
38
52
8
3
.2.2. (4R,5S)-4,5-O,O-Isopropylidene-3-(trityloxymethyl)-2-cyclopenten-1-ol (2αβ);
(3aS,7aR)-2,2-dimethyl-7-(trityloxymethyl)-6-tert-butyldimethylsilyl-3a,7a-dihydro-4H-
[
1,3]dioxolo[4,5-c]pyran (17)
TMSN (0.20 mL, 1.50 mmol) and Bu SnO (37 mg, 0.15 mmol) were added to a
3
2
solution of CP 16 (355 mg, 0.50 mmol) in toluene. After it was refluxed for 24 h, the
reaction mixture was concentrated to give a residue, which was purified using silica gel
column chromatography to give an inseparable 3:1 mixture of 5αβ (5α/5β = 1/3) and 17
(210 mg). To a solution of the mixture in THF (5 mL), 1 M solution of TBAF in THF
(
1.2 mL, 1.20 mmol) was added. After 1 h, saturated aqueous NH Cl was added to the
4
reaction mixture to quench it. After the mixture was extracted with EtOAc (30 mL), the
organic layer was washed with H O and then brine, dried over Na SO , filtrated, and
2
2
4
concentrated to give a residue, which was purified using silica gel column
chromatography (EtOAc–hexane, 1:4) to give 2αβ (139 mg, 65%, yellow amorphous)
and 17 (54 mg, 20%, oil). In addition, 2β could be partially resolved by use of the above
solvent system.
1
2
β: H-NMR (300 MHz, CDCl ): δ 1.30 (s, 3H), 1.33 (s, 3H), 2.06 (br, 1H), 3.70 (d, 1H,
3
12

ACCEPTED MANUSCRIPT
J = 15.0 Hz), 3.89 (d, 1H, J = 15.0 Hz), 4.51 (d, 1H, J = 5.7 Hz), 4.78 (br, 1H), 5.08 (d,
1
3
1
H, J = 5.7 Hz), 6.00 (br, 1H), 7.20–7.32 (m, 9H), 7.45–7.48 (m, 6H); C-NMR (75.5
MHz, CDCl ): δ 26.1, 27.4, 61.3, 76.6, 79.9, 83.7, 86.4, 111.8, 127.0, 127.8, 128.5,
3
+
1
1
3
1
5
43.8, 147.4; HRMS (EI): m/z [M ] calcd for C H O : 428.1988; found: 428.1990.
28
28
4
1
7: H-NMR (400 MHz, CDCl ): δ -0.44 (s, 3H), -0.16 (S, 3H), 0.67 (s, 9H), 1.48 (s,
3
H), 1.55 (s, 3H), 3.20 (dd, 1H, J = 10.0, 10.0 Hz), 3.52 (d, 1H, J = 10.0 Hz), 3.98 (dd,
H, J = 10.0, 4.8 Hz), 3.99 (d, 1H, J = 10.0 Hz), 4.24 (ddd. 1H, J = 10.0, 6.0, 4.8 Hz),
1
3
.00 (d, 1H, J = 6.0 Hz), 7.21–7.31 (m, 9H), 7.46–7.50 (m, 6H); C-NMR (100 MHz,
CDCl ): δ -5.8, -5.4, 17.2, 26.0, 26.5, 28.5, 61.5, 64.9, 68.3, 70.4, 86.4, 107.7, 120.6,
3
+
1
26.9, 127.6, 129.0, 143.9, 161.2; HRMS (FAB+NaCl): m/z [M+Na] calcd for
C H O SiNa: 565.2750; found: 565.2753.
34
42
4
3
.2.3. Conversion of 2αβ to 2α
DMP (166 mg, 0.39 mmol) was added to a solution of 2αβ (112 mg, 0.26 mmol) in
o
CH Cl at 0 C. After the reaction mixture was stirred at rt for 1 h, saturated aqueous
2
2
NaHCO and aqueous Na S O were added to quench it. The mixture was extracted
3
2
2
3
with methyl tert-butyl ether (30 mL). The organic layer was washed with H O and then
2
brine, dried over Na SO , filtered, and concentrated to give a residue, which was
2
4
purified using silica gel column chromatography (EtOAc–hexane, 1:4) to give
cyclopentenone derivative (22: 103 mg, 93%, white solid).
3
.2.4. (4R,5R)-4,5-O,O-Isopropylidene-3-(trityloxymethyl)-2-cyclopentenone (22)
1
H-NMR (300 MHz, CDCl ): δ 1.34 (s, 6H), 3.94 (dd, 1H, J = 18.3, 1.2 Hz), 4.25 (dd,
3
1
1
6
H, J = 18.3, 1.8 Hz), 4.47 (d, 1H, J = 5.7 Hz), 4.94 (d, 1H, J = 5.7 Hz), 6.44–6.46 (m,
1
3
H), 7.23–7.36 (m, 9H), 7.40–7.47 (m, 6H); C-NMR (75.5 MHz, CDCl ): δ 26.3, 27.5,
3
2.5, 77.7, 78.1, 87.4, 115.4, 127.4, 128.0, 128.2, 128.4, 143.3, 174.6, 201.9; HRMS
+
(EI+): m/z [M] calcd for C H O : 426.1831; found: 426.1828.
28
26
4
3
.2.5. (1S,4R,5S)-4,5-O,O-Isopropylidene-3-(trityloxymethyl)-2-cyclopenten-1-ol (2α)
A solution of 22 (81mg, 0.19 mmol) in THF (2 mL) was added dropwise to a
o
suspension of LAH (36 mg, 0.96 mmol) in THF (2 mL) at 0 C. After the reaction was
stirred at rt for 2 h, H O (1 mL) was added to quench it. After the mixture was stirred at
2
rt for 1 h, MgSO was added, and the resulting mixture was filtered through Celite and
4
concentrated to give a residue, which was purified using silica gel column
1
6
chromatography (EtOAc–hexane 1:3) to give 2α (79 mg, 98%, white solid).
1
2
α: H-NMR (300 MHz, CDCl ): δ 1.36 (s, 3H), 1.37 (s, 3H), 2.74 (d, 1H, J = 10.2 Hz),
3
13

ACCEPTED MANUSCRIPT
3
.65 (dt, 1H, J = 14.4, 1.8 Hz), 3.88 (d, 1H, J = 14.4 Hz), 4.59 (br, 1H), 4.75 (t, 1H, J =
.4 Hz), 4.88 (d, 1H, J = 5.4 Hz), 6.00 (br, 1H), 7.20–7.32 (m, 9H), 7.44–7.48 (m, 6H).
5
3
.2.6. Reaction of ketone 3 with TMSC(Li)N2
A 1.6M solution of n-BuLi in hexane (1.1 mL, 1.71 mmol) was added dropwise to a
o
solution of diisopropylamine (0.29 mL, 1.71 mmol) in THF (3mL) at -78 C. After the
o
mixture was stirred at -78 C for 10 min, 0.6 M solution of TMSCHN in hexane (2.8
2
o
mL, 1.71 mmol) was added dropwise. After 30 min at -78 C, a solution of ketone 3
o
(
315mg, 0.57 mmol) in THF (3 mL) was added dropwise at -78 C. After the reaction
o
mixture was stirred at 0 C for 1 h, H O was added to quench it. The mixture was
2
extracted twice with Et O (50 mL), and the combined organic layers were washed with
2
H O and then brine, dried over Na SO , filtered, and concentrated to give a residue,
2
2
4
which was purified by silica gel column chromatography (EtOAc–hexane 1:19) to give
an inseparable mixture (188 mg, 61%) of 5αβ (5α/β = 1/3) and 17.
3
.3. Synthesis of (–)-neplanocin A from CP 21
3
.3.1. 9-[(2S,3R,4R)-(5,4-Dihydroxy-2,3-isopropylidenedioxy)pentyl]adenine (19)
A 1M solution of DIBAL-H in toluene (82 mL, 81.5 mmol) was added dropwise to a
solution of 2',3'-O-isopropylideneadenosine 18 (5.0 g, 16.3 mmol) in CPME (80 mL) at
o
0
C. After the reaction was stirred at rt for 24 h, aqueous potassium sodium tartrate (20
o
g/100 mL) was added at 0 C to quench it. The mixture was stirred at rt for 24 h. After
the mixture was extracted three times with BuOH (150 mL), the combined organic
layers were concentrated to give a residue, which was purified using silica gel column
chromatography (MeOH–CH Cl 1:4) to give 19 (4.3 g, 85%, amorphous).
2
2
1
H-NMR (400 MHz, CDCl ): δ 1.26 (s, 3H), 1.48 (s, 3H), 3.60–3.84 (m, 3H), 4.22 (dd,
3
1
H, J = 9.2, 6.0 Hz), 4.30 (dd, 1H, J = 14.0, 10.8 Hz), 4.56–4.62 (m, 1H), 4.75 (dd, 1H,
1
3
J = 14.0, 2.4 Hz), 8.12 (s, 1H), 8.19 (s, 1H); C-NMR (100 MHz, CDCl ): δ 25.7, 28.4,
3
4
5.6, 65.3, 70.9, 76.9, 77.4, 110.6, 119.7, 143.4, 150.7, 153.6, 157.2.
3
.3.2. 9-[(2S,3R,4R)-5-(tert-Butyldimethylsilyloxy-4-hydroxy-2,3-
isopropylidenedioxy)-pentyl]adenine (20)
TBDMSCl (2.11 g, 14.0 mmol) and imidazole (1.08 g, 15.9 mmol) were added to a
solution of 19 (2.89 g, 9.35 mmol) in DMF (20mL). After the reaction mixture was
stirred at rt for 15 h, EtOAc (200 mL) was added. The organic layer was washed twice
with saturated aqueous NaHCO and then brine, dried over Na SO , filtered, and
3
2
4
14

ACCEPTED MANUSCRIPT
concentrated to give a residue, which was purified using silica gel column
chromatography (MeOH–EtOAc 1:9) to give 20 (3.56 g, 90%, oil).
1
H-NMR (400 MHz, CDCl ): δ 0.11 (s, 6H), 0.92 (s, 9H), 1.29 (s, 3H), 1.50 (s, 3H),
3
3
.29 (d, 1H, J = 6.0 Hz), 3.72–3.81 (m, 2H), 3.88 (dd, 1H, J = 9.6, 2.4 Hz), 4.14–4.21
(
m, 2H), 4.57 (ddd, 1H, J = 10.0, 6.0, 2.4 Hz), 4.92 (dd, 1H, J = 14.4, 2.4 Hz), 6.06 (brs,
1
3
2
2
1
H), 7.97 (s, 1H), 8.38 (s, 1H); C-NMR (100 MHz, CDCl ): δ -5.5, -5.4, 18.3, 25.3,
3
5.8, 28.1, 44.2, 64.3, 69.1, 75.8 (75.76), 75.8 (75.82), 109.4, 119.3, 141.5, 150.1, 152.9,
55.5.
3
.3.3. 9-[(2S,3S)-(5-tert-Butyldimethylsilyloxy-2,3-isopropylidenedioxy-4-
pentanone)-1-yl]adenine (6)
A solution of 20 (1.00 g, 2.36 mmol) in MeCN (20 mL) was added to a suspension of
IBX (2.00 g, 7.08 mmol) in MeCN (70 mL). After it was refluxed for 12 h, the reaction
mixture was filtered through Celite. The filtrate was concentrated to give a residue,
which was purified using silica gel column chromatography (EtOAc) to give 6 (0.94 g,
9
5%, oil).
1
H-NMR (400 MHz, CDCl ): δ 0.14 (s, 3H), 0.15 (s, 3H), 0.94 (s, 9H), 1.34 (s, 3H),
3
1
1
1
.64 (s, 3H), 3.90 (dd, 1H, J = 14.0, 9.6 Hz), 4.45 (d, 1H, J = 19.2 Hz), 4.56 (d, 1H, J =
9.2 Hz), 4.57 (dd, 1H, J = 14.0, 2.4 Hz), 4.83 (ddd, 1H, J = 9.6, 7.6, 2.4 Hz), 4.98 (d,
1
3
H, J = 7.6 Hz), 6.47 (brs, 1H), 7.91 (s, 1H), 8.33 (s, 1H); C-NMR (100 MHz,
CDCl ): δ -5.6, -5.5, 18.3, 24.7, 25.7, 25.8, 27.2, 44.3, 68.7, 75.1, 79.7, 110.6, 119.0,
3
1
41.3, 149.8, 152.4, 155.3, 206.6.
3
.3.4. (2S,3S)-{4-(6-Amino-9H-purin-9-yl)-2-(tert-butyldimethylsilyloxymethyl)-2,3-
isopropylidenedioxy-1-cyanobutyl diethyl phosphate (21)
DEPC (20 mg, 0.12 mmol) and LiCN (2 mg, 0.06 mmol) were added to a solution of
ketone 6 (42 mg, 0.10 mmol) in THF (1 mL) at rt. After it was stirred for 30 min, the
reaction mixture was treated with EtOAc-hexane (1:1, 50 mL). The organic layer was
washed with water and then brine, dried over Na SO , filtered, and concentrated.
2
4
Purification using silica gel column chromatography (MeOH-EtOAc, 1:9) afforded CP
2
1 (59 mg, quant).
1
H-NMR (400 MHz, CDCl ): δ 0.13–0.17 (m, 6H), 0.92–0.95 (m, 9H), 1.26 (s, 1H),
3
1
.26 (s, 2H), 1.34–1.44 (m, 6H), 1.50 (s, 2H), 1.62 (s, 1H), 4.04–4.14 (m, 1H), 4.20–
.35 (m, 5H), 4.54–4.82 (m, 3H), 4.93 (dd, 0.3H, J = 14.0, 2.0 Hz), 5.05 (dd, 0.7H, J =
4.0, 2.0 Hz), 6.00 (brs, 2H), 7.90 (s, 0.7H), 7.98 (s, 0.3H), 8.35 (s, 0.3H), 8.36 (s,
4
1
0
1
3
.7H); C-NMR (100 MHz, CDCl ): δ -5.7, -5.6, -5.5 (-5.52), -5.5 (-5.46), 16.0 (15.98),
3
15

ACCEPTED MANUSCRIPT
1
6.0 (16.03), 16.1, 18.2, 18.3, 24.7, 25.1, 25.7, 26.5, 26.8, 43.2 (43.17), 43.2 (43.23),
3.1, 64.3, 64.8, 64.9, 65.0 (64.96), 65.0 (65.02), 65.1 (65.09), 65.1 (65.14), 65.4
6
(65.37), 65.4 (65.42), 74.2, 74.3, 75.0, 75.7, 75.8, 75.9, 76.0, 76.2, 77.2 (77.15), 77.2
(
77.19), 77.5, 110.0, 110.3, 115.8, 115.9, 119.4 (119.40), 119.4 (119.41), 141.5, 141.9,
+
1
49.8, 149.9, 152.6, 152.7, 155.2, 155.3; HRMS (FAB): m/z [M+H] calcd for
C H N O PSi: 585.2621; found: 585.2618.
24
42
6
7
3
.3.5. (1R,4R,5S)-9-N-[3-(tert-Butyldimethylsilyloxymethyl)-4,5-O,O-
isoprppylidene-2-cyclopenten-1-yl]adenine (8
TMSN (0.16 mL, 1.2 mmol) and Bu SnO (30 mg, 0.12 mmol) were added to a
β
)
3
2
solution of 21 (116 mg, 0.2 mmol) in toluene (10 mL). After it was refluxed for 24 h,
the reaction mixture was concentrated to give a residue, which was purified using silica
gel column chromatography (PSQ100B silica gel, MeOH-EtOAc, 1:99) to give 8β (30
2
0b
mg, 36%) and 8α (21 mg, 25%).
1
8
β: H-NMR (300 MHz, CDCl ): δ 0.10 (s, 6H), 0.92 (s, 9H), 1.35 (s, 3H), 1.48 (s,
3
3
H), 4.42 (brs, 2H), 4.71 (d, 1H, J = 5.7 Hz), 5.30 (d, 1H, J = 5.7 Hz), 5.60 (brs, 1H),
5
.78-5.90 (brm, 3H), 7.67 (s, 1H), 8.38 (s, 1H).
3
.3.6. (-)-Neplanocin A: (1S,2R,5R)-5-(6-Amino-9H-purin-9-yl)-3-
(hydroxymethyl)cyclopent-3-ene-1,2-diol (1)
Trifluoroacetic acid (TFA) (0.5 mL, 6.8 mmol) was added to a solution of carbocyclic
nucleoside 8β (73 mg, 0.17 mmol) in ClCH CH Cl/MeOH (1/1; 4 mL). After it was
2
2
stirred for 18 h, the reaction mixture was concentrated to give a residue, which was
purified using silica gel column chromatography (MeOH-EtOAc-H 0, 1:8:1) to give 1
2
(44 mg, 98%, white solid). Compound 1 thus obtained was recrystallized from MeOH
to give colorless prisms.
o
1
[
α] -156.0 (c 0.50, H O); m.p. 220–221 C; H-NMR (300 MHz, CD OD): δ 4.31 (brs,
D 2 3
2
H), 4.38 (dd, 1H, J = 5.4, 5.4 Hz), 4.62 (d, 1H, J = 5.4 Hz), 5.45–5.55 (m, 1H), 5.91 (d,
H, J = 1.8 Hz), 8.10 (s, 1H), 8.18 (s, 1H).
1
Acknowledgment
We are grateful to Dr. K. Yaguchi and Mr. H. Sachi at Fuji Silysia Chemical Ltd. for
information on the spherical silica gel. This work was financially supported by research
funds from Osaka University of Pharmaceutical Sciences.
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16

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1
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(
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prepared
the
single
epimer
8β
from
2
2
2
2
(
2
(
Heterocycles 2018; 96: 106.
2
3. For recent reviews on alkylidene carbene chemistry, see: (a) Grainger RS, Munro
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18

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3
2
2
6. Ohira and co-workers prepared ketone 3 from protected
D-ribose 15 in 73%
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2
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(
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1. Matsuda and co-workers did not show the yield of 19 in their report (see ref 20a)
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Chemical Ltd.
3
3
3. Fujitake M, Harusawa S, Araki L, et al, Tetrahedron, 2005; 61: 4689.
19