Tetrahedron 58 (2002) 9903–9923
`
`Synthetic studies of himbacine, a potent antagonist of the
`muscarinic M2 subtype receptor 1. Stereoselective total synthesis
`and antagonistic activity of enantiomeric pairs of himbacine
`
`0S,60R)-diepihimbacine, 4-epihimbacine, and novel
`and (2
`himbacine congeners
`
`Masanori Takadoi,a,* Tadashi Katoh,b Akihiro Ishiwatab,† and Shiro Terashimab
`
`aDiscovery Research Laboratories, Kyorin Pharmaceutical Company Ltd, 2399-1 Nogi, Nogi-machi, Tochigi 329-0114, Japan
`bSagami Chemical Research Center, 2743-1 Hayakawa, Ayase, Kanagawa 252-1193, Japan
`
`Received 25 September 2002; accepted 21 October 2002
`
`Abstract—Total synthesis of an enantiomeric pair of himbacine 1 and ent-1 was achieved in a highly stereoselective manner by employing
`an intermolecular Diels – Alder reaction of tetrahydroisobenzofuran 8 with chiral furan-2(5H)-one (S)-9 and (R)-9, respectively, as a key step.
`0
`0
`R)-diepihimbacine 24 and ent-24, 4-epihimbacine 4-epi-1, and novel himbacine congeners bearing the same
`S,6
`An enantiomeric pair of (2
`tricyclic moiety as that of 1 were also successfully prepared by utilizing the key synthetic intermediates for 1, establishing the convergency
`and flexibility of the explored synthetic route. All of the synthesized compounds used were subjected to muscarinic M2 subtype receptor
`binding affinity assay, disclosing novel aspects of the structure – activity relationships for 1. q 2002 Elsevier Science Ltd. All rights reserved.
`
`1. Introduction
`
`Himbacine 1,
`isolated from the bark of Galbulimima
`baccata,
`is a potent antagonist of the muscarinic M2
`subtype receptor.2 This alkaloid bears a characteristic
`structural
`feature in which the perhydronaphtho[2,3-
`c]furan ring system consisting of cis-fused g-lactone and
`trans-fused decaline moieties is connected with trans-
`disubstituted piperidine via an (E)-double bond (Fig. 1).
`
`is well known that senile dementia associated with
`It
`Alzheimer’s disease is directly correlated with diminished
`levels of synaptic acetylcholine (ACh) in the cortical and
`hippocampal areas of the brain,3 and current forms of
`therapy address this issue, relying on the cholinergic
`hypothesis4 of memory dysfunction. Such therapies com-
`prise the following four approaches to enhance synaptic
`ACh levels, namely, use of an ACh esterase inhibitor,5 a
`choline acetyl transferase (ChAT) synthesis enhancer,6 a
`choline re-uptake enhancer,7 and a muscarinic receptor
`agent.8 It has been reported that enhancement of synaptic
`ACh levels can be achieved by antagonizing presynaptic
`
`Keywords: alkaloids; Diels– Alder reactions; natural products; asymmetric
`synthesis.
`
`* Corresponding author. Tel.: þ81-280-56-2201; fax: þ81-280-57-1293;
`
`e-mail: masanori.takadoi@mb2.kyorin-pharm.co.jp
`† Present address: RIKEN (The Institute of Physical and Chemical
`Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
`
`Figure 1. Structure of natural himbacine 1.
`
`muscarinic M2 receptors which regulate ACh release, the
`latter which acts as an autoreceptor.9 Under
`these
`circumstances, the therapeutic agent requires the high M2/
`M1 selectivity because of the post-synaptic localization of
`the M1 receptor. Himbacine 1 was reported to be a potent
`antagonist of the muscarinic M2 subtype receptor with 10 –
`20-fold selectivity toward the M1 subtype. Therefore, much
`attention has been focused on 1 by medicinal and synthetic
`organic compound synthesis communities.10 Two total
`syntheses of 1 have hitherto been achieved by Hart –
`Kozikowski11 and Chackalamannil12 by employing intra-
`molecular Diels – Alder reaction as their key steps.
`
`In order to disclose novel aspects of the structure – activity
`relationships of 1, and moreover, to explore the promising
`congeners of 1 which may show more improved M2 subtype
`selectivity, an efficient synthetic route to 1 was sought
`
`0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
`PII: S 0 0 4 0 - 4 0 2 0 ( 0 2 ) 0 1 3 5 8 - 3
`
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`M. Takadoi et al. / Tetrahedron 58 (2002) 9903–9923
`
`Scheme 1. Novel synthetic design of himbacine 1 featuring intermolecular Diels – Alder reaction.
`
`which would be more convergent and flexible than those
`that had been previously reported.11,12 We wish to report
`here the novel total synthesis of an enantiomeric pair of
`himbacine 1 and ent-1 accomplished by a method featuring
`a highly stereoselective intermolecular Diels – Alder
`reaction as a key step. This paper is also concerned with
`0
`0
`S,6
`R)-diepihim-
`the synthesis of an enantiomeric pair of (2
`bacine 24 and ent-24, 4-epihimbacine 4-epi-1, and novel
`congeners of 1 prepared by employing the synthetic
`intermediates of 1 along with their M2 antagonistic
`activity.1 The latter studies were carried out in order to
`gain a more thorough understanding of the convergency and
`flexibility of the explored synthetic route and to disclose
`novel aspects of the structure – activity relationships of these
`molecules.
`
`2. Results and discussion
`
`2.1. Synthetic strategy
`
`Our synthetic strategy as regards 1 is outlined in Scheme 1.
`The key step of our synthesis was a highly stereoselective
`intermolecular Diels – Alder reaction between tetrahydro-
`isobenzofuran 8 and chiral furan-2(5H)-one (S)-9. Thus,
`the cycloadduct 7 was
`exo-selective construction of
`envisioned, based on a previously reported result of the
`reaction of 8 with maleic anhydride.13 It was expected that
`the four chiral centers of 7 could be introduced in a highly
`stereoselective manner due to the (S)-chirality of the methyl
`group at the C-5 position of (S)-9. All of the chiral centers of
`tricyclic alcohol 4a bear the natural absolute configuration
`of 1, and 4a was readily derived from 7 by sequential
`hydrogenation to saturated tetracyclic compound 6 from a
`convex face, ring opening of 6, followed by double bond
`isomerization to unsaturated alcohol 5, and hydrogenation.
`Introduction of an arylsulfonylmethyl group into 4 gave
`tricyclic sulfone 2, which was connected to the bottom half
`of 1, the piperidinaldehyde 3, according to Hart’s pro-
`cedure11 featuring the Julia – Lythgoe olefination protocol,
`followed by subsequent deprotection and N-methylation of
`the piperidine moiety.
`It
`is generally accepted that
`
`intermolecular Diels – Alder reactions are more convergent
`and flexible than the corresponding intramolecular reac-
`tions, especially in the synthesis of complex natural
`products. Therefore, our approach was anticipated to be
`not only more advantageous for the total synthesis of 1
`itself, but also more useful for the exploration of novel
`congeners of 1 by changing the starting tetrahydroiso-
`benzofuran derivatives, the chiral furan-2(5H)-one deriva-
`tives, and/or the chiral piperidine units. Actually,
`the
`usefulness of the explored synthetic route was exemplified
`by successful syntheses of enantiomeric ent-1 and some
`novel congeners of 1. These studies were accomplished by
`employing (R)-9 in place of (S)-9 and by constructing novel
`structural features from the synthetic intermediates of 1.
`
`2.2. Synthesis of natural himbacine 1
`
`tetrahydroisobenzofuran 814 and
`The starting materials,
`furan-2(5H)-one (S)-9,15 were synthesized in large quan-
`tities according to the reported procedure with some
`modifications. With 8 and (S)-9 in hand, the intermolecular
`Diels – Alder reaction could then be examined (Scheme 2).
`
`As shown in Table 1, heating of a mixture of 8 and (S)-9 in
`the absence (entry 1) or the presence (entries 2 – 4) of Lewis
`acid16,17 failed to afford 7. The same unsuccessful results
`
`Scheme 2. Intermolecular Diels– Alder reaction of tetrahydroisobenzo-
`furan 8 with (S)-furan-2(5H)-one (S)-9. (a) see Table 1; (b) H2, 10% Pd – C,
`EtOH, rt, 12 h, 96%.
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`
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`
`Table 1. Intermolecular Diels– Alder reaction of tetrahydroisobenzofuran 8
`with furan-2(5H)-one (S)-9
`
`Entrya
`
`Catalyst
`(equiv.)
`
`Solvent
`
`Conditions
`
`Result
`(yield of 7, %)b
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`
`1508C, 65 h
`Decomposition
`None
`None
`rt – 408C, 19 h Decomposition
`Toluene
`SiO2 – Et2AlCl
`rt, 9 h
`Decomposition
`Toluene
`SiO2 – TiCl4
`rt, 48 h
`No reaction
`Toluene
`Florisil
`rt, 48 h
`No reaction
`CH2Cl2
`ZnCl2 (2.0)
`16 (20)c
`(Ph3P)3RhCl (0.1) CF3CH2OH rt, 168 h
`59 (88)c
`rt, 120 h
`ZnI2 (0.5)
`CH2Cl2
`4 M LiNTf2
`Et2O
`rt, 48 h
`No reaction
`Benzene
`rt, 24 h
`Decomposition
`1 M LiBF4
`Et2O
`rt, 168 h
`58
`5 M LiClO4
`
`a Tetrahydroisobenzofuran 8 was allowed to react with furan-2(5H)-one
`(S)-9 (1– 1.5 equiv.).
`b The cycloadduct 7 was obtained as the sole product. No other reaction
`products were detected by 1H NMR analysis of the crude reaction
`mixture.
`c Corrected for the recovery of (S)-9.
`
`19
`
`18 and LiBF4
`were obtained by the reactions using LiNTf2
`(entries 8, 9). On the other hand, upon treatment with
`Wilkinson’s catalyst20 (entry 6), ZnI2 (entry 7), or 5 M
`LiClO4 – Et2O21 (entry 10), the cycloaddition reaction took
`place successfully, giving rise to cycloadduct 7. The
`reaction under Livinghouse’s condition (entry 6) gave 7
`with poor reproducibility. In the case of entry 7,
`the
`isolation of 7 was found to be troublesome due to the
`recovery of unreacted (S)-9. Among these conditions
`examined, use of 5 M LiClO4 – Et2O afforded the best
`results, producing 7 as the sole product in a 58% yield. As
`expected,
`it appeared that
`this Diels – Alder
`reaction
`proceeded in a highly exo-selective manner, affording 7 as
`the sole product. Exclusive formation of 7 was determined
`by the 1H NMR analysis of the crude reaction product. To
`the best of our knowledge, this is the first example of a
`Diels – Alder reaction of furan and furan-2(5H)-one deriva-
`tive.22 Since the structural determination of 7 by the spectral
`data proved difficult, and because 7 was also found to be
`fairly unstable, 7 was directly subjected to the next
`hydrogenation. Thus, hydrogenation of 7 under convention-
`al conditions occurred from the sterically less hindered
`convex face to give the hydrogenated compound 6 as the
`sole product in a 96% yield. The stereochemistry of 6
`
`included a cis – anti – cis ring system, and was confirmed by
`single-crystal X-ray diffraction analysis, as shown in
`Scheme 2.
`
`Next, our synthetic efforts were focused on converting the
`stereochemistries of 6 to those of 1, a cis – trans ring system
`(Scheme 3). Toward this end, we first examined a ring-
`opening reaction of the 7-oxabicyclo[2.2.1]heptane system
`in 6. After conducting the experiments, it was found that this
`reaction proceeded under basic conditions,23 giving rise to
`hydroxyenone 10 in a 92% yield. Treatment of 10 with 1,8-
`diazabicyclo[5.4.0]undec-7-ene efficiently underwent
`double-bond isomerization to afford deconjugated alcohol
`5 as the sole product in an 83% yield. This highly selective
`formation of 5 may be explained by thermodynamic control.
`In order to obtain saturated alcohol 4a bearing the desired
`cis – trans ring system, catalytic hydrogenation of 5 was next
`examined using the various catalytic systems shown in
`Table 2.
`
`Among the catalytic systems tested, hydrogenation of 5 in
`the presence of PtO2 was found to produce the saturated
`alcohol 4a as the sole product in a 96% yield (entry 4). On
`the other hand, employing other catalytic systems for this
`hydrogenation regularly gave a mixture of 4a and its isomer
`4b (C-3a, 9a position,
`the himbacine numbering). The
`structures of 4a and 4b were unambiguously determined by
`single-crystal X-ray diffraction analyses. The ratio of 4a to
`4b definitely depended on the metal species of the catalytic
`systems. Formation of 4b may be explained by the double-
`bond shift from C-8a, 9 to the C-3a, 9a position, followed by
`hydrogenation from the sterically less hindered direction
`opposite of the C-3 methyl group. However, the reason
`remains unclear why such an abnormal hydrogenation took
`place by catalytic systems not employing PtO2. Thus, the
`stereoselective
`construction
`of
`the
`dodecahydro-
`naphtho[2,3-c]furan ring system involved in 1 was readily
`accomplished in four steps from exo-cycloadduct 7,
`produced by the intermolecular Diels – Alder reaction.
`
`Prior to oxidation of the secondary hydroxy group at the C-4
`position of 4a, the lactone moiety was protected in a form of
`acetal by sequential reduction and acetalization, yielding
`a-methyl acetal 11, the thermodynamically more stable
`compound, in a 74% yield from 4a as almost the sole
`product (Scheme 4). The hydroxy group at the C-4 position
`of 11 was oxidized using the tetrapropylammonium
`perruthenate(VII)-4-methylmorpholine N-oxide (TPAP-
`NMO) system,24 producing ketone 12 in a 95% yield.
`Methylenation of 12 by Wittig reaction provided exo-
`methylene compound 13 in an 86% yield. Sequential
`
`Table 2. Catalytic hydrogenation of deconjugated alcohol 5
`
`Entrya
`
`Catalystb
`
`Ratio of 4a/4b (yield, %)c
`
`1
`2
`3
`4
`
`Rh– Al2O3
`10% Pd– C
`Raney Ni
`PtO2
`
`1:1 (92)
`5:1 (65)
`10:1 (76)
`1:0 (96)
`
`Scheme 3. Stereoselective construction of the tricyclic part 4a of 1.
`(a) LiN(TMS)2, THF, 278 to 2408C, 4 h, 92%; (b) DBU, toluene, 1008C,
`5 h, 83%; (c) see Table 2.
`
`a Hydrogenation was carried out at rt in EtOH.
`b Amount of the catalyst corresponding to the 10% weight of 5 was used.
`c The ratio of 4a – 4b was determined by the 1H NMR spectrum of the crude
`reduction product.
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`M. Takadoi et al. / Tetrahedron 58 (2002) 9903–9923
`
`Scheme 4. Synthesis of the sulfone 2, the key intermediate of 1. (a) (i)
`DIBAL-H, Et2O, 2788C, 1 h, (ii) BF3·Et2O, MeOH, CH2Cl2, 260 – rt, 12 h,
`74% (two steps); (b) TPAP, 4-methylmorpholine N-oxide, MS 4 A˚ ,
`CH2Cl2, rt, 1.5 h, 95%; (c) Ph3PCH3I, NaN(TMS)2, Et2O, 08C to rt, 2 h,
`86%; (d) (i) BH3·THF, THF, 2788C to rt, 3 h, (ii) 30% H2O2, 10% NaOH,
`08C, 0.5 h, 14a:14b¼73:8%; (e) methanesulfonyl chloride, 4-(dimethyl-
`amino)pyridine, Et3N, CH2Cl2, 08C to rt, 3 h, 100%; (f) thiophenol, tBuOK,
`DMSO, rt, 3 h, 100%; (g) mCPBA, NaHCO3, CH2Cl2, rt, 2 h, 82%.
`
`hydroboration and oxidation gave rise to 4b-carbinol 14a in
`a 73% yield along with the undesired C-4 epimer 14b in an
`8% yield. When the hydroboration was carried out at 08C
`instead of at 2788C, the ratio of 14a to 14b was reduced to
`5:1. The structure of 14a bearing all of the desired chiral
`centers in the tricyclic part of 1, was rigorously confirmed,
`as shown by single-crystal X-ray diffraction analysis.
`Stereoselective formation of 14a may reflect
`increased
`steric hindrance of the b-face of 13.
`
`With 14a in hand, transformation of 14a into the known
`sulfone 211 was next examined. It was accomplished in an
`82% combined yield by a three-step sequence involving O-
`mesylation of the hydroxy group of 14a, replacement of the
`O-mesyl group in 15 with a phenylsulfide group, and
`oxidation of the resulting sulfide 16. Spectral and physical
`data of 2 were identical to those that have previously been
`reported.11
`
`The remaining task for the total synthesis was conversion of
`2 to 1 according to the Hart – Kozikowski procedure11
`(Scheme 5). Thus, the coupling reaction25,26 of 2 with
`aldehyde 3, prepared according to the reported procedure,27
`and a subsequent elimination reaction of the intermediary b-
`hydroxy sulfone almost exclusively provided olefin 17
`possessing (E)-stereochemistry in a 66% yield. Sequential
`oxidation of the acetal moiety in 17 and deprotection of the
`N-Boc group in 18 afforded the piperidine 19. Finally,
`reductive N-methylation of 19 furnished natural himbacine
`1 in a high yield over a three-step sequence. Physical and
`spectral properties of the synthetic sample of 1 were found
`to be identical to those previously reported.11b As described
`above, our efforts culminated in the successful development
`of a novel synthetic route to 1 in which the intermolecular
`Diels – Alder reaction of 8 with (S)-9 is employed as the key
`stereoselective reaction.
`
`Scheme 5. Completion of the total synthesis of natural himbacine 1.
`(a) nBuLi, 2788C, 1 h, 100% conversion (a mixture of the diastereomers);
`(b) 5% Na – Hg, Na2HPO4, MeOH, rt, 2.5 h, 66%; (c) Jones reagent,
`acetone, rt, 0.5 h, 100%; (d) trifluoroacetic acid, CH2Cl2, rt, 1.5 h, 100%;
`(e) 37% HCHO aq., NaBH3CN, CH3CN, rt, 0.5 h, 91%.
`
`2.3. Synthesis of unnatural ent-himbacine ent-1
`
`To clarify the relationship between the absolute configur-
`ation of 1 and muscarinic M2 subtype antagonistic activity,
`as well as to explore the convergency and flexibility of the
`developed synthetic route to 1, we next planned to
`synthesize the enantiomer of 1, unnatural ent-himbacine
`ent-1, by using (R)-915 instead of (S)-9 as a starting material.
`Synthesis of (R)-9 was accomplished starting with (R)-
`methyl lactate. According to the synthetic scheme to 1
`detailed above, we succeeded in obtaining ent-1 from (R)-9,
`disclosing prominent aspects of the explored synthetic
`route. Spectral data of ent-1 were superimposable on those
`of 1. To avoid confusion, the compounds carrying natural
`configurations are the only compounds depicted in Scheme
`2 – 5.
`
`2.4. Synthesis of an enantiomeric pair of (2
`diepihimbacine
`
`0S,6
`
`0R)-
`
`In order to elucidate the structure – activity relationships
`between the absolute configuration of the tricyclic moiety
`on 1 and the piperidine moiety, we next planned to
`0
`0
`S,6
`R)-diepihimbacine
`synthesize an enantiomeric pair of (2
`24 and ent-24. It was expected that the synthesis of 24 could
`be accomplished by the use of our explored synthetic route,
`employing the sulfone 2 with a natural absolute configur-
`ation and the chiral piperidinaldehyde ent-3 possessing an
`unnatural absolute configuration.
`
`Thus, the Julia – Lythgoe coupling reaction of 2 and ent-3
`was examined to produce 24 (Scheme 6). However, in
`contrast to the cases for the synthesis of 1 and ent-1, the
`starting material 2 was fully recovered when the reaction
`was quenched by adding water. It seems likely that the retro-
`aldol
`type of reaction might have occurred when the
`materials were immersed in excess water, probably due to
`the decreased stability of the in situ formed lithium aldolate
`or an aldol-type product under protic conditions. After much
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`
`9907
`
`Table 3. In vitro binding activity of 1, ent-1, 24, ent-24, and 4-epi-1
`
`Entry
`
`Compound
`
`2log Ki
`
`M1 (cortex)
`
`M2 (brainstem)
`
`1
`2
`3
`4
`5
`6
`
`1
`1a
`ent-1
`24
`ent-24
`4-epi-1
`
`7.1
`7.2
`5.9
`6.5
`6.5
`6.1
`
`a Authentic sample of 1.28
`
`7.9
`7.9
`6.3
`6.7
`6.7
`5.7
`
`against the muscarinic M1 and M2 subtype receptors were
`performed. Results of these assays are shown in Table 3. In
`contrast to 1, all of the tested compounds ent-1, 24, ent-24,
`and 4-epi-1 were found to show very weak comparable
`binding affinity against the muscarinic M1 and M2 subtype
`receptors. These results clearly indicated that the stereo-
`chemistry of both the tricyclic and the piperidine moieties of
`1 play crucial roles in its strong muscarinic M2 antagonistic
`activity.
`
`2.7. Synthesis of some novel congeners of 1
`
`With completion of the total synthesis of enantiomeric pairs
`0
`0
`of natural himbacine 1 and ent-1, (2
`S,6
`R)-diepihimbacine
`24 and ent-24, and 4-epihimbacine 4-epi-1, we next
`examined the synthesis of some novel congeners of 1.
`These studies were performed in order to further examine
`the convergency and flexibility of the explored synthetic
`scheme, and moreover, to disclose novel aspects of the
`structure – activity relationships. Studies of the synthesis and
`muscarinic M2 subtype antagonistic activity of himbacine
`congeners have thus far been reported by Kozikowski et al.10
`and Chackalamannil et al.12c Our novel synthesis and
`activity evaluation of 1, ent-1, 24, ent-24, and 4-epi-1
`disclosed that the absolute configuration plays a pivotal role
`in muscarinic M2 subtype antagonistic activity. Therefore, it
`appeared that novel congeners of himbacine should be
`synthesized in the natural configuration. Taking into
`account these points, we designed some novel congeners
`of 1, shown in Fig. 2. It was anticipated that the chiral
`piperidine ring of 1 could be replaced with various
`substituted heterocycles such as pyridine,
`imidazole,
`morpholine, pyrrolidine, and piperidine derivative. More-
`in 1 was expected to be
`the (E)-olefin present
`over,
`substituted by its bioisosteres such as ether, ester,
`carbamate, and amide bond species. Based on the above
`considerations, the compounds, 26, 28, 30, 32, 35, 37, 39,
`
`Figure 2. Novel himbacine congeners designed as synthetic targets.
`
`0
`0
`R)-diepihimbacine 24. (a) nBuLi, DME,
`Scheme 6. Synthesis of (2
`S,6
`278 to 2208C, 4 h; (b) (i) benzoyl chloride, 2208C to rt,1 h, (ii)
`3-(dimethylamino)propylamine, rt, 1 h; (c) 5% Na – Hg, Na2HPO4, MeOH,
`rt, 1 h, 33% (four steps) (recovery of 2: 51%); (d) Jones reagent, acetone, rt,
`2 h, 68%; (e) trifluoroacetic acid, CH2Cl2, rt, 0.5 h, 91%; (f) 37% HCHO
`aq., NaBH3CN, CH3CN, rt, 3 h, 73%.
`
`experimentation, it was finally found that quenching the
`reaction of 2 and ent-3 by the addition of excess benzoyl
`chloride successfully gave the desired benzoate 20 as a
`diastereomeric mixture after removal of the excess benzoyl
`chloride with 3-(dimethylamino)propylamine. Without
`separation, the reaction mixture was directly subjected to
`reductive elimination, giving rise to (E)-olefin 21 in a 33%
`yield from 2, along with a 51% recovery of 2. In this case as
`well, formation of the (Z)-olefin was not observed by 1H
`NMR analysis of the crude reaction product. According to
`the same synthetic procedure as that used for 1, 21 was
`converted to the target compound 24 in a three-step
`sequence involving oxidation of the hemiacetal moiety,
`deprotection of the N-Boc group, and reductive N-methyl-
`ation. The enantiomer of 24, ent-24, was also synthesized
`employing ent-2 and 3 in the same manner as that described
`for the synthesis of 24.
`
`2.5. Synthesis of 4-epi-1
`
`in order to disclose the relationships between
`Next,
`muscarinic M2 subtype antagonistic activity and C-4
`stereochemistry in the tricyclic moiety of 1, we examined
`the synthesis of 4-epihimbacine 4-epi-1 by employing 4a-
`carbinol 14b as a starting material. Thus, according to our
`methodology featuring the Julia – Lythgoe coupling reaction
`of the tricyclic sulfone and the chiral piperidinaldehyde that
`was established by the synthesis of 24 as well, we have
`readily succeeded in preparing 4-epi-1 from 14b in eight
`steps.
`
`2.6. Muscarinic M2 subtype binding activity of
`enantiomeric pairs of natural himbacine 1 and ent-1 and
`0S,6
`0R)-diepihimbacine, and 4-epihimbacine
`(2
`
`With enantiomeric pairs of natural himbacine 1 and ent-1
`0
`0
`R)-diepihimbacine 24 and ent-24, and 4-epihim-
`S,6
`and (2
`bacine 4-epi-1 in hand, receptor binding affinity assays
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`
`8). Similarly, acylation of 11 with chloroacetyl chloride
`followed by substitution with morpholine and Jones
`oxidation furnished the morpholin-4-yl acetate derivative
`35 by way of 2-chloroacetate 33 and acetal 34. The
`N-(pyridin-2-yl) carbamate derivative 37 was prepared by
`the reaction of 11 with 2-isocyanatopyridine in situ
`generated from 2-picolinic acid and subsequent oxidation
`with Jones reagent. In a similar fashion, 4b-carbinol 14a
`was transformed to the homologous N-(pyridin-2-yl)
`carbamate derivative 39 (Scheme 9). The (S)-1-Benzyl-
`pyrrolidin-3-yl ether derivative 41 was synthesized using
`commercially available (S)-1-benzylpyrrolidin-3-ol by way
`of O-mesylate 15 derived from 14a. The 1-methylpiperidin-
`3-yl ether derivative 44 was derived from 43 prepared by
`etherification of 15 with readily obtainable (S)-1-(tert-
`butoxycarbonyl)piperidin-3-ol29 followed by reduction of
`the N-Boc group to an N-Me group with lithium aluminum
`hydride. Furthermore, 4b-carbinamine 45 was prepared
`from 14a by the Mitsunobu procedure30 and subsequent
`removal of a phthalide moiety. This reaction was coupled
`with 6-methyl-2-picolinic acid by use of EDCI as a coupling
`reagent, affording the 2-methylpyridine-6-carboxamide 47
`by way of acetal 46.
`
`2.8. Muscarinic binding activity of some novel congeners
`of himbacine 1
`
`Using the thus obtained congeners of 1, receptor binding
`affinity assays against the muscarinic M1 and M2 subtype
`receptors were performed. Unfortunately, as shown in Table
`4, all of the compounds tested exhibited poor M2 receptor
`binding affinity and low selectivity compared to 1. In the
`cases of (E)-olefin compounds (entry 2, 3), 2-methyl-
`pyridine and 1-methylimidazole rings were found to be
`unsuitable for binding muscarinic receptors, probably due to
`the aromaticity and planar structures of these rings. The (Z)-
`olefin 30 appeared to be unrewarding in a manner similar to
`that of the (E)-olefin 28. The congeners bearing the
`bioisosteres of the (E)-olefin structure, e.g., ether (entries
`5, 9, 10), ester (entry 6), carbamate (entries 7, 8), and amide
`(entry 11), also showed weak M1 and M2 receptor binding
`affinity. These observations clearly suggest
`that
`the
`conformations of these congeners might differ greatly
`from that of 1.
`
`Scheme 7. Synthesis of himbacine congeners bearing the natural tricyclic
`moiety (I). (a) nBuLi, 6-methylpyridine-2-carboxaldehyde, DME, 2788C,
`1 h, 100%; (b) 5% Na – Hg, Na2HPO4, MeOH, rt, 2 h, 19%; (c) Jones
`reagent, acetone, rt, 60% (for 26), 13% (for 28), 64% (for 30); (d) nBuLi,
`1-methylimidazole-2-carboxaldehyde, DME, 2788C, 1 h, 62%; (e) 5%
`Na – Hg, Na2HPO4, MeOH, rt, 4 h, 42% (for 27), 20% (for 29).
`
`41, 44, and 47 were designed as representative congeners
`of 1, none of which had hitherto been prepared.
`These congeners were readily prepared from the function-
`alized intermediate for the synthesis of 1 such as 2, 11, and
`14a.
`
`Thus, conversions of 2 to the 2-methylpyridine derivative
`26 and 1-methylimidazole derivatives, 28 and 30 were
`readily obtained by employing a Julia – Lythgoe coupling
`reaction followed by elimination, and then a Jones
`oxidation, both in a similar manner to that described for
`the total synthesis of 1 (Scheme 7). Alkylation of 11 with
`commercially available 2-(chloromethyl)pyridine in the
`presence of NaH, and subsequent Jones oxidation clearly
`provided the 2-pyridylmethyl ether derivative 32 (Scheme
`
`Scheme 8. Synthesis of himbacine congeners bearing the natural tricyclic moiety (II). (a) 2-(chloromethyl)pyridine hydrochloride, NaH, DMF, rt, 14 h, 43%;
`(b) Jones reagent, acetone, rt, 37% (for 32), 55% (for 35), 13% (for 37); (c) chloroacetyl chloride, NaH, DMF, rt, 6 h, 83%; (d) morpholine, 1008C, 5 h, 84%;
`(e) 2-picolinic acid, DPPA, Et3N, toluene, 1008C, 6 h, 64%.
`
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`
`9909
`
`Scheme 9. Synthesis of himbacine congeners bearing the natural tricyclic moiety (III). (a) 2-picolinic acid, DPPA, Et3N, toluene, 1008C, 7 h, 92%; (b) Jones
`reagent, acetone, rt, 56% (for 39), 64% (for 41), 21% (for 44), 35% (for 47); (c) methanesulfonyl chloride, Et3N, CH2Cl2, 08C to rt, 2.5 h, 100%; (d) (S)-1-
`benzylpyrrolidin-3-ol, NaH, DMF, 808C, 17 h, 8%; (e) (S)-1-(tert-butoxycarbonyl)piperidin-3-ol, NaH, DMF, 808C, 17 h, 38%; (f) lithium aluminum
`hydride, THF, rt, 1 h, then 808C, 1 h, 100%; (g) phthalimide, diethyl azodicarboxylate, Ph3P, THF, 08C to rt, 7 h, 99%; (h) H2NNH2·H2O, EtOH, rt, 4 h, 52%;
`(i) 6-methyl-2-picolinic acid, EDCI, CH2Cl2, rt, 5 h, 80%.
`
`3. Conclusion
`
`In summary, we have succeeded in the highly stereo-
`selective total synthesis of an enantiomeric pair of
`himbacine 1, a potent antagonist of the muscarinic M2
`subtype receptor, by employing an intermolecular Diels –
`Alder reaction of 8 with (S)-9 as a key step. Our explored
`synthetic route was successfully applied to the total
`synthesis of unnatural himbacine ent-1, an enantiomeric
`0
`0
`R)-diepihimbacine 24 and ent-24, and
`S,6
`pair of
`(2
`4-epihimbacine 4-epi-1. Based on the observed muscarinic
`receptor binding affinity assay of these compounds,
`it
`appears that the absolute configuration of 1 plays a pivotal
`role in its activity. Based on the results obtained, selected
`novel congeners of 1 were further prepared by starting with
`
`the intermediates for the total synthesis of 1, and these
`congeners were then subjected to receptor binding affinity
`assay. However, these latter congeners were unfortunately
`also found to exhibit very weak muscarinic M2 subtype
`receptor binding affinity and low selectivity. Although no
`novel congener with a more prominent profile than 1 was
`found, the successful synthesis of various structural types of
`congeners clearly disclosed the convergency and flexibility
`of the explored synthetic route to 1. Our goal to discover a
`novel drug candidate targeting muscarinic M2 subtype
`receptors for the treatment of Alzheimer’s disease still
`remains to be achieved, and studies along this line are
`ongoing in our laboratories.
`
`4. Experimental
`
`Table 4. In vitro binding activity of some novel congeners of himbacine
`
`Entry
`
`Compound
`
`2log Ki
`
`4.1. General
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`
`1
`26
`28
`30
`32
`35
`37
`39
`41
`44
`47
`
`M1 (cortex)
`
`M2 (brainstem)
`
`7.1
`5.2
`5.2
`,5.1
`!5.1
`!5.1
`!5.1
`!5.1
`5.5
`5.6
`!5.1
`
`7.9
`!6.1
`!6.1
`!6.1
`!6.1
`!6.1
`!6.1
`!6.1
`!5.9
`,5.4
`!6.1
`
`All melting points were determined with a Yamato MP-500
`melting point apparatus, a Yamaco MP-3 micro melting
`point apparatus, or a Yanaco MP-500D micro melting point
`apparatus, and are uncorrected. Measurements of optical
`rotations were carried out using a Horiba SEPA-200
`automatic digital polarimeter, a JASCO DIP-360 automatic
`digital polarimeter, or a P-1020 automatic digital polari-
`meter. Infrared (IR) spectra were recorded with a JASCO
`FT/IR-5300 spectrometer. 1H NMR spectra were measured
`with a JEOL JNM-EX-400 (400 MHz) spectrometer or an
`Avance 500 (500 MHz) spectrometer. 13C NMR spectra
`were taken with a JEOL JNM-EX-400 (100 MHz)
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`
`spectrometer or an Avance 500 (125 MHz) spectrometer.
`The chemical shifts are expressed in parts per million (d
`value) downfield from tetramethylsilane, using tetramethyl-
`
`silane (d¼0) and/or residual solvents such as chloroform
`(d¼7.26) as an internal standard. Splitting patterns are
`
`indicated as s, singlet; d, doublet; t, triplet; q, quartet; m,
`multiplet; br, broad peak. Measurements of mass spectra
`were performed with a JMS-SX102A mass spectrometer.
`Data for elemental analyses are within ^0.3% of the
`theoretical values, and were determined by a Yanaco
`CHN-corder MT-5. Unless otherwise noted, all
`the
`experiments were carried out using anhydrous solvents
`under an atmosphere of dry argon. Throughout
`this
`study, Merck precoated TLC plates (Silica gel 60
`F254, 0.25 mm; Art. 5715) were used for thin layer
`chromatographic (TLC) analysis, and all the spots were
`visualized using UV light followed by coloring with
`phosphomolybdic acid. Wako Gel C-200, Wako Gel
`C-300, Silica gel 60 (0.040 – 0.063 mm, F254; Art. 9385,
`Merck Co., Ltd.), or Chromatorexw NH-DM 1020
`(100 – 200 mesh, Fuji Silysia Chemical, Ltd.) was used as
`an adsorbent for the flash column chromatography.
`
`4.1.1. 3,4,5,6-Tetrahydrobenzo[c]furan (8). 4,5,6,7-Tetra-
`hydroisobenzofuran-5-ol was prepared as a colorless oil
`starting with furfuryl alcohol according to the reported
`103 – 1058C/3 mmHg.
`procedure.14a
`1H NMR
`Bp
`(400 MHz, CDCl3): d 1.56 (d, J¼4.9 Hz, 1H), 1.77 – 1.84
`J¼16.6, 6.0 Hz, 1H), 2.91 (dd, J¼15.7, 4.9 Hz, 1H), 4.10 –
`4.16 (m, 1H), 7.16 – 7.18 (m, 2H). IR (neat): 3360, 2930,
`þ
`1640, 1440 cm21. MS (EI) m/z: 138 (M
`), 120, 94 (100).
`þ
`HRMS (EI) (m/z): calcd for C8H10O2 (M
`): 138.0681.
`Found, 138.0686.
`
`(m, 1H), 1.90 – 1.97 (m, 1H), 2.49 – 2.63 (m, 2H), 2.77 (dt,
`
`To a solution of 4,5,6,7-tetrahydroisobenzofuran-5-ol
`(34.1 g, 0.25 mol) and triethylamine (68.8 mL, 0.49 mol)
`in CH2Cl2 (400 mL), methanesulfonyl chloride (28.7 mL,
`0.37 mol) was added dropwise at 08C, and the resulting
`mixture was stirred at the same temperature for 1.5 h, then
`gradually warmed to rt. After concentration in vacuo, the
`residue was poured into water (500 mL), and the aqueous
`
`mixture was extracted with diethyl ether (100 mL£3). The
`
`combined organic extracts were washed with brine
`(100 mL), dried over anhydrous Na2SO4, filtered,
`then
`concentrated in vacuo, to give 4,5,6,7-tetrahydroisobenzo-
`furan-5-yl methanesulfonate (53.3 g, 100%) as an oil. This
`material was immediately used for the next reaction without
`further purification. 1H NMR (400 MHz, CDCl3): d 1.99 –
`2.07 (m, 1H), 2.10 – 2.19 (m, 1H), 2.64 – 2.71 (m, 1H), 2.76 –
`2.85 (m, 1H), 2.90 (dd, J¼16.6, 6.4 Hz, 1H), 2.99 – 3.04 (m,
`1H), 3.04 (s, 3H), 5.11 – 5.16 (m, 1H), 7.14 – 7.22 (m, 2H).
`IR (neat): 3650, 2940, 1350, 1170 cm21. MS (EI) m/z: 216
`þ
`), 120 (100). HRMS (EI) (m/z): calcd for C8H12O2
`(M
`þ
`(M
`): 216.0