The Synthesis of Certain Derivatives and Analogues of (-)- and (+)-Galanthamine and an Assessment of their Capacities to Inhibit Acetylcholine Esterase

Article abstract of DOI:10.1021/acs.joc.7b01062

Syntheses of certain di- and mono-oxygenated derivatives (e.g., 2 and 3, respectively) and analogues (e.g., 4, a D-ring monoseco-analogue of 2) of both the (-)- and (+)-enantiomeric forms of the alkaloid galanthamine [(-)-1] are reported. All have been assessed for their capacities to inhibit acetylcholine esterase but, in contrast to the predictions from docking studies, none bind strongly to this enzyme.

Full text of DOI:10.1021/acs.joc.7b01062

Subscriber access provided by NEW YORK UNIV
Article
The Synthesis of Certain Derivatives and Analogues
of (–)- and (+)-Galanthamine and an Assessment of
their Capacities to Inhibit Acetylcholine Esterase
Joshua N Buckler, Ehab S. Taher, Nicolas J Fraser, Anthony
C Willis, Paul D Carr, Colin J. Jackson, and Martin G Banwell
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01062 • Publication Date (Web): 03 Jul 2017
Downloaded from http://pubs.acs.org on July 3, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Organic Chemistry is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
The Synthesis of Certain Derivatives and Analogues of (–)- and (+)-Galanthamine and
an Assessment of their Capacities to Inhibit Acetylcholine Esterase
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Joshua N. Buckler, Ehab S. Taher, Nicolas J. Fraser, Anthony C. Willis, Paul D. Carr,
Colin J. Jackson and Martin G. Banwell*
Research School of Chemistry, Institute of Advanced Studies
The Australian National University, Canberra, ACT 2601, Australia
OH
OH
A
OH
OH
HO
HO
HO
O
O
O
O
O
CONMe
2
B
(
N
Me
D
N
N
Me
Me
MeO
C
MeO
MeO
MeO
–)ꢀ1
2
3
4
ABSTRACT: Syntheses of certain diꢀ and monoꢀoxygenated derivatives (e.g. 2 and 3,
respectively) and analogues (e.g. 4 – a Dꢀring monosecoꢀanalogue of 2) of both the (–)ꢀ and
(
+)ꢀenantiomeric forms of the alkaloid galanthamine [(–)ꢀ1] are reported. All have been
assessed for their capacities to inhibit acetylcholine esterase but, in contrast to the predictions
from docking studies, none binds strongly to this enzyme.
–
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
INTRODUCTION
The alkaloid (–)ꢀgalanthamine [(–)ꢀ1] (Figure 1) has been isolated from a range of plant
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
sources and is currently used in the clinic for the symptomatic treatment of mild to moderate
1
forms of Alzheimer’s disease. It exerts its beneficial effects by crossing the bloodꢀbrain
barrier and then, in part at least, inhibiting acetylcholine esterase (AChE). It also acts as an
1,2
allosteric modulator of the nicotinic acetylcholine receptor. The nonꢀnatural enantiomer of
compound 1, viz. entꢀ1 or (+)ꢀgalanthamine, has also been shown to accumulate in brain
3
tissue but does so through nonꢀspecific binding.
OH
A
O
B
D
N
Me
C
MeO
(
–)ꢀ1
Figure 1: The Structure of the Alkaloid (–)-Galanthamine
Currently (–)ꢀgalanthamine is produced industrially by extraction from various plants
sources, most notably the red spider lily (Lycoris radia), the wild daffodil (Narcissus
pseudonarcissus), the summer snowflake (Leucojum aestivum) and the Caucasian snowdrop
1
(Galanthus woronowii). However, both the increasing demands for the natural product and
the erosion of habitat of at least some of the producing plants has prompted investigations
into other methods for obtaining it or for identifying analogues with improved efficacy. As
ACS Paragon Plus Environment

Page 3 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
part of such efforts, galanthamine has been the subject of a significant number of total
4
synthesis studies with the first of these being reported by Barton and Kirby and involving
mimicking of the proposed biogenesis. Substantial refinements of this process have been
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
reported in the interim and one of these has formed the basis of a pilotꢀplant scale synthesis
5
a
of the alkaloid although it is not clear if this is contributing significantly to the commercial
6
production of the alkaloid. Magnus and coꢀworkers have described a related and highly
effective approach. Intramolecular Heck reactions have provided another means for
7
assembling the tetracyclic framework of galanthamine, including those accomplished in an
enantioselective manner, while various chirons (corresponding to the Aꢀring) have been
8
,9a
employed for its assembly in either enantiomeric form.
New routes to (–)ꢀgalanthamine
1
e
9
continue to be reported, including approaches from our group.
The identification and biological evaluation of analogues of galanthamine has been another
10
focus of significant activity that is now greatly assisted by data derived from the highꢀ
1
1
resolution Xꢀray analysis of the alkaloid bound to the active site of acetylcholine esterase.
Biomimetic diversityꢀoriented synthesis, sophisticated QSAR analyses, multicomponentꢀ
coupling chemistries as well as more conventional studies have all revealed active
1
2
compounds. The screening of natural products for relevant activities remains an ongoing
area of investigation and has also resulted in the isolation of new inhibitors of the title
1
3
enzyme.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
As part of an ongoing program to establish concise routes to the tetracyclic framework of
1
e,9
galanthamine, we now report chemoenzymatic syntheses of two oxygenated derivatives, 2
1
4
and 3, as well analogues 4ꢀ9 of the natural or (–)ꢀform of the alkaloid (Figure 2).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
OH
OH
OH
HO
HO
HO
O
O
O
O
CONMe2
N
Me
N
Me
MeO
MeO
MeO
3
4
2
OMe
O
OH
MeO
MeO
HO
O
CONHMe
O
CONHMe
O
MeO
5
6
OH
HO
OMe
OMe
OMOM
HO
HO
HO
HO
CONMe2
CONMe2
O
MeO
OMe
OMe
OMe
OMe
CHO
OHC
OHC
8
9
7
Figure 2: The Galanthamine Derivatives/Analogues 2-9 Targeted for Synthesis
Syntheses of the enantiomers of the first five of these derivatives/analogues, viz. compounds
entꢀ2 to entꢀ6, are also described. As detailed below, and contrary to the predictions arising
ACS Paragon Plus Environment

Page 5 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
from molecular docking studies, none of these is an effective inhibitor of acetylcholine
esterase.
Our motivation for undertaking the studies described herein was that the introduction of
additional functionalities within the galanthamine framework should increase water solubility
and/or provide the means for conjugating the drug with various groups including peptide
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
0f
10d
fragments or other entities (e.g. memantine) and thus allowing for the development of a
1
0d
multiꢀtargeted therapeutic approach. In principle, then, the attachment of such moieties
1
0e
could provide new inhibitors (or even just prodrug forms
of galanthamine) that are
superior to existing therapies. As revealed below, the nature of our synthetic strategy is such
that additionally oxygenated forms of the galanthamine Aꢀring were likely to be the most
readily accessible. Accordingly, and since this region of alkaloid has not been “explored”
previously, such derivatives became a major focus of the work detailed below.
RESULTS AND DISCUSSION
Chemical Synthesis Studies
The reaction sequence used to assemble ABCꢀring substructures of (–)ꢀgalanthamine is
shown in Scheme 1 and employs the readily available Lꢀtartaric acid (10) as starting material.
1
5
Thus, as specified in a recent publication, compound 10 can be converted over eleven steps
involving reduction, Grignard addition and ringꢀclosing metatheses as key transformations,
into the 1,2ꢀdiacetal annulated bromocyclohexene 11. Coupling of compound 11 with the
1
6
readily available aryl boronic acid ester 12 proceeded under conventional conditions to give
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the anticipated arylated cyclohexene 13 (68%). Following protocols established during the
17
course of our syntheses of the ribisins, this last compound was engaged in an
intramolecular Mitsunobu reaction using triphenylphosphine in conjunction with diꢀisoꢀ
propyl azodicarboxylate (DIAD) wherein the phenolic hydroxyl group served as the internal
nucleophile and so affording the acidꢀsensitive isobenzofuran 14 (33%). As demonstrated
through work in the enantiomeric series (see below), if appropriate account is taken of this
acidꢀsensitivity then the Mitsunobu reaction can be a high yielding one. Despite concerns
about the potential for competing isomerization of compound 14 to its more conjugated (fully
aromatic) counterpart, on subjecting it to conditions previously employed for effecting the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
18
EschenmoserꢀClaisen rearrangement of allylic alcohols the amide 15 was produced in 84%
yield. The structure of this compound followed not only from the derived NMR, IR and mass
spectral data but also from a singleꢀcrystal Xꢀray analysis of its enantiomer (see below).
Compound 15 embodies both the targeted ABCꢀring substructure of (–)ꢀgalanthamine and
the associated quaternary carbon. In anticipation of installing the final (D) ring of the title
alkaloid the amide residue within compound 15 was reduced using LiBHEt and the 1°ꢀ
3
alcohol 16 thereby obtained in 92% yield.
ACS Paragon Plus Environment

Page 7 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Scheme 1: Synthesis of Key Acid 17 Embodying the ABC-ring Substructure and
Associated Quaternary Carbon of (–)-Galanthamine
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OMe
OH
HO
see ref. 15
MeO
O
CO H
2
O
OH
CO H
2
(eleven steps)
10
HO
OMe
O
Br
MeO
PdCl (dppf)•CH Cl
2
2
2
O
OH
11
Et N
3
+
HO
HO
9:1 v/v THF/water
ca. 66 °C, 2 h
68%
O
O
B
MeO
HO
13
MeO
Ph P, DIAD
3
12
THF, –5 to 22 °C
5
h, 33%
OMe
O
OMe
O
OMe
MeO
MeO
MeO
MeO
MeO
O
O
O
O
OH
O
LiBHEt3
H CC(OMe) NMe
3
2
2
CONMe2
O
CH OH
PhMe, 111 °C
18 h, 84%
O
2
THF
0 to 22 °C
2 h, 92%
MeO
15
14
16
+
–
AcNHꢀTEMPO BF
4
9
:1 v/v MeCN/water
2 °C, 48 h, 76%
2
OMe
MeO
MeO
O
O
O
CO H
2
17
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
19
Oxidation of compound 16 under conditions defined by Bobbitt and Bailey then gave the
acid 17 (76%) that represents the key precursor to the targeted galanthamine
derivatives/analogues 2ꢀ6.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The straightforward manipulations of acid 17 leading to the targeted (–)ꢀgalanthamine
derivatives/analogues 2, 3, 5 and 6 are shown in Scheme 2. Thus, coupling of compound 17
and methylamine, using 1,1’ꢀcarbonyldiimidazole (CDI) for activation of the acid, afforded
7
c,20
amide 5 (79%) and on exposure of the latter to modified PictetꢀSpengler conditions,
involving paraformaldehyde in the presence of trifluoroacetic acid (TFA), the tetracyclic
lactam 2 (47%) was formed as a result of concomitant cleavage of the Ley acetal moiety.
Scheme 2: Conversion of Acid 17 into the (–)-Galanthamine Derivatives/Analogues 2, 3,
5 and 6
OMe
O
OMe
O
OH
MeO
MeO
MeO
MeO
HO
O
(i) CDI
O
(
CH O) , TFA
O
THF, 66 °C, 1 h
2
n
O
O
CO H
O
CONHMe
2
(ii) MeNH2
MeCN, 22 °C
8 h, 47%
N
Me
THF, 0 to 22 °C
MeO
11 h, 79%
5
2
17
NaAlH (OCH CH OCH )
3 2
1:1 v/v TFA/water
2
2
2
THF, 66 ° C, 24 h, 44%
22 °C, 0.5 h, 66%
OH
OH
HO
HO
CONHMe
O
O
N
Me
MeO
MeO
6
3
ACS Paragon Plus Environment

Page 9 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
On treatment with sodium bis(2ꢀmethoxyethoxy)aluminum dihydride, compound 2 was
reduced to the azepine 3 (44%) while hydrolysis of the 1,2ꢀdiacetal residue within compound
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
using aqueous trifluoroacetic acid (TFA) afforded the diol 6 (66%). Compound 6 can be
considered as a hybrid of the title alkaloid and the neurologically active natural product
17,21
ribisin D.
The synthesis of compound 4, a monoꢀseco derivative of 5Rꢀhydroxyꢀ(–)ꢀgalanthamine (3),
simply involved (Scheme 3) acidꢀcatalyzed hydrolysis of precursor 15 under conventional
conditions. This reaction proceeded in 88% yield.
Scheme 3: Hydrolysis of Biacetal 15 Leading to Compound 4
OMe
OH
MeO
MeO
O
HO
O
1:1 TFA/water
2 °C, 0.5 h, 88%
2
O
CONMe2
O
CONMe2
MeO
15
4
All of the spectral data obtained on the targeted compounds 2ꢀ6 were in complete accord
with the assigned structures but final confirmation of that of the first (i.e. 2) followed from a
single crystal Xꢀray analysis of its enantiomer (see below).
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
In an effort to establish a more meaningful SAR profile for the abovementioned analogues,
the enantiomericallyꢀrelated derivatives/analogues entꢀ2 to entꢀ6 (Figure 3) were sought.
While the routes defined above could simply be adapted for this purpose by starting with Dꢀ
rather than Lꢀtaratric acid, we were able to establish a shorter pathway by starting with an
enzymaticallyꢀderived chiron that is readily available in the required enantiomeric form (but
less so in the opposite one required to prepare the derivatives/analogues just described).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
OH
OH
OH
HO
HO
HO
O
O
O
O
CONMe2
N
Me
N
Me
MeO
MeO
MeO
entꢀ2
entꢀ3
entꢀ4
OMe
O
OH
MeO
MeO
HO
O
O
CONHMe
CONHMe
O
MeO
entꢀ5
entꢀ6
Figure 3: The Enantiomeric Series of Derivatives/Analogues
The synthesis of the enantiomeric series of compounds started, as shown in Scheme 4, with
2
2
the enzymaticallyꢀderived and enantiomerically pure cisꢀ1,2ꢀdihydrocatechol 18 that was
2
3
converted into the corresponding and wellꢀknown acetonide 19 under standard conditions.
Immediate treatment of the last compound with mꢀchloroperbenzoic acid (mꢀCPBA) then
ACS Paragon Plus Environment

Page 11 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
23
afforded, in a completely regioꢀ and stereoꢀselective fashion, epoxide 20 (93% from 18).
Reaction of compound 20 with a large excess of pꢀmethoxybenzylalcohol (pꢀMBnOH) in the
presence of boron trifluoride etherate (BF •Et O) resulted in selective nucleophilic opening
3
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
24
of the epoxide ring at the allylic carbon but the protected bromoconduritol thus formed was
not isolated. Rather, it was simply allowed to react with methanol in the presence of
pyridinium tosylate (PPTS) and thereby affording triol 21 (71%). Treatment of a methanolic
solution of this last compound with 2,2,3,3ꢀtetramethoxybutane (2,2,3,3ꢀTMB) in the
presence of pꢀtoluenesulfonic acid monohydrate (pꢀTsOH•H O) then resulted in the selective
2
25
formation of the “Ley”ꢀacetal entꢀ11 (85%) and so establishing an “enantiomeric overlap”
with the synthetic sequence leading to the original sets of analogues.
Scheme 4: Synthesis of Bromoconduritol ent-11 from the Enzymatically-derived and
Homochiral cis-1,2-Dihydrocatechol.
2
,2ꢀDMP
O
HO
HO
O
O
O
O
pꢀTsOH•H O
mꢀCPBA
CH Cl
2
2
Et
2 °C, 1 h
O
2
2
2
0 °C, 48 h
Br
Br
Br
9
3% (from 18)
1
8
19
20
(i) pꢀMBnOH
BF •Et O, CH Cl
(
ii) PPTS
MeOH, 50 °C, 48 h
1% over two steps
3
2
2
2
–
20 to 22 °C,
7
ca. 15 h
OMe
O
2
,2,3,3ꢀTMB
OH
MeO
HC(OMe) , MeOH HO
OPMB
3
O
OH
pꢀTsOH•H O
2
HO
HO
65 °C, 24 h
Br
Br
85%
entꢀ11
21
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
During the conversion 21 → entꢀ11 acetal formation is presumed to take place prior to
cleavage of the PMBꢀether unit since the reverse order of events would lead to an
intermediate conduritol embodying two adjacent transꢀdiol moieties and the formation of two
isomeric Leyꢀtype acetals would therefore be expected. Indeed, when the tetraꢀol derived
from hydrolysis of compound 21 was subjected to reaction with 2,2,3,3ꢀTMB in the presence
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
of pꢀTsOH•H O then a ca. 1:1 mixture of the two possible bisꢀacetals is formed.
2
As shown in Scheme 5, and as was the case in the enantiomeric series, compound entꢀ11
2
6
could be engaged in a SuzukiꢀMiyaura crossꢀcoupling reaction with arylboronic acid ester
1
2 and so affording the anticipated product entꢀ13 (71%) that participated in an
intramolecular Mitsunobu reaction to give compound entꢀ14 (96%). Allylic alcohol entꢀ14
underwent an EschenmoserꢀClaisen rearrangement reaction on thermolysis with the dimethyl
acetal of N,Nꢀdimethylacetamide in refluxing toluene and thereby affording N,Nꢀ
dimethylamide entꢀ15 (86%). The structure of this last compound was confirmed by a singleꢀ
crystal Xꢀray analysis, details of which are provided in the Experimental Section and the
Supporting Information (SI). Reduction of compound entꢀ15 with lithium triethylborohydride
1
9
and oxidation of the resulting 1°ꢀalcohol, entꢀ16 (quant.), using the Bobbitt/Bailey protocol,
then gave acid entꢀ17 (80%).
ACS Paragon Plus Environment

Page 13 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 5: Synthesis of Key Acid ent-17 from Bromoconduritol ent-11
OMe
MeO
O
O
OH
OMe
O
OMe
O
HO
MeO
Br
entꢀ11
+
MeO
MeO
O
OH
PdCl (dppf)•CH Cl
O
O
OH
2
2
2
Et N
Ph P, DIAD
3
3
HO
HO
9:1 v/v THF/water
ca. 66 °C, 2 h
THF, –5 to 22 °C
5 h, 96%
71%
O
O
MeO
B
HO
entꢀ13
entꢀ14
MeO
H CC(OMe) NMe
3
2
2
12
PhMe, 111 °C
18 h, 86%
OMe
OMe
O
OMe
O
MeO
O
MeO
MeO
O
O
O
+
–
LiBHEt3
THF
AcNHꢀTEMPO BF
4
CH OH
CONMe2
CO H 9:1 v/v MeCN/water
O
2
O
O
2
2
2 °C, 48 h, 80%
0 to 22 °C
2 h, quant.
MeO
MeO
MeO
entꢀ17
entꢀ16
entꢀ15
Following precisely the same reaction sequences as detailed above in Schemes 2 and 3,
compound entꢀ17 was converted into the target derivatives entꢀ2 to entꢀ6 (Figure 3) with the
structure of the first of these being confirmed by singleꢀcrystal Xꢀray analysis. The monoꢀ
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
seco analogue of compound entꢀ2, namely amide entꢀ4, was readily obtained in 77% yield by
simply treating bisꢀacetal entꢀ15 with aqueous trifluoroacetic acid (cf Scheme 3).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
As part of an effort to develop galanthamine derivatives containing additional functionality in
the aromatic Cꢀring, especially ones capable of conjugation with motifs that might bind to the
27
soꢀcalled peripheral and anionic binding site of acetylcholine esterase, we sought to exploit
the synthetic chemistry detailed above for this purpose. As such, the relevant arylboronate
acid ester (representing a synthon for the Cꢀring) was required and the route used to obtain
this is shown in Scheme 6. Thus, vanillin (22) was converted, under established conditions,
2
8
into its bromoꢀderivative 23 (90%) and the aldehydic residue within the latter then
protected as the corresponding ethylene acetal using standard conditions and so affording
compound 24 in 55% yield. The readily derived MOMꢀether, 25 (70%), of the last compound
was subjected to a metallation/borylation protocol and the intermediate boronic acid 26
obtained on hydrolytic work up was reacted with sodium iodide/trimethylsilyl chloride (to
cleave the MOM ether) and then pinacol and thus affording the hitherto unreported ester 27
(45% from 25). The structure of this last compound was confirmed by singleꢀcrystal Xꢀray
analysis (see Experimental Section and SI for details).
Scheme 6: Synthesis of Aryl Boronate Ester 27
ACS Paragon Plus Environment

Page 15 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Br
Br
OH
OH
OH
(
2
CH OH)2
Br2
pꢀTsOH•H O
O
2
OMe
OHC
OMe
AcOH, 22 °C
3 h, 90%
OHC
OMe
toluene
O
1
11 °C, 5 h, 55%
22
23
24
MOMꢀCl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
NaH, THF
0
to 22 °C, 18 h
0%
7
O
O
(
i) NaI, TMSCl
MeCN
0 to 22 °C, 4 h
Br
B(OH)2
B
nꢀBuLi
OMOM
OMOM
OMe
OH
B(OiꢀPr)3
O
OHC
OMe
THF
–78 to 22 °C
15 h
OHC
OMe (ii) pinacol, C
0 °C, 2 h
45% (from 25)
6 6
H
O
8
27
26
25
In the opening stages of attempts to exploit ester 27 in the production of galanthamine
analogues related to those described above, the epoxide 19 (Scheme 7) was subject to
2
9
hydrolytic cleavage and thus affording the previously reported transꢀdiol 28 (68%). The
allylic hydroxyl group of diol 28 was selectively protected through its reaction with triꢀisoꢀ
propylsilyl trifluoromethanesulfonate (TIPSꢀOTf) in the presence of 2,6ꢀlutidine and so
3
0
producing the allylic ether 29 (88%). The location of the TIPS group within this product
was established using COSY experiments. On treating compound 29 with methyl iodide in
the presence of sodium hydride the bisꢀether 30 (49%), rather than the anticipated regioꢀ
isomer, was obtained. Since the illustrated locations of the ether residues within compound
3
0 (that thwart the application of the anticipated EschenmoserꢀClaisen rearrangement) were
not appreciated until an Xꢀray analysis was carried out on a derivative, this was carried
forward by first treating it with TBAF and thus affording the homoallylic alcohol 31 (98%).
Compound 31 was engaged in a SuzukiꢀMiyaura crossꢀcoupling reaction with ester 27 to
give the arylated and crystalline cyclohexene 32 (45%), the illustrated structure of which was
established by singleꢀcrystal Xꢀray analysis. Acidꢀcatalyzed hydrolysis of the acetonide
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
residue within the last compound gave triol 33 (81%). Even though the position of the
methoxy group within compound 33 precluded the application of the type of ECꢀ
rearrangement used earlier, it was subjected to an intramolecular Mitsunobu reaction so as to
produce a system, namely compound 7 (85%), that embodies the ABCꢀring ensemble
associated with (–)ꢀgalanthamine.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 7: Synthesis of Analogue 7
ACS Paragon Plus Environment

Page 17 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
KOH
OH
OH
Br
O
9:1 v/v
TIPSꢀOTf
O
O
O
O
OH
O
O
OTIPS
THF/water
2,6ꢀlutidine
66 °C, 48 h
68%
CH
–78 to 22 °C
h, ca. 75%
2 2
Cl
Br
Br
3
2
0
28
29
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
MeI, NaH
THF, 0 to 22 °C
2
h, ca. 38%
OH
OTIPS
O
O
OMe
OMe
O
O
O
O
B
TBAF
THF
+
OH
Br
22 °C, 2 h
Br
30
OHC
OMe
9
8%
2
7
31
PdCl (dppf)•CH Cl
2
2
2
Et N
3
9:1 v/v THF/water
6
6 °C, 2 h, 45%
OH
OH
OH
HO
HO
OMe
OMe
HO
OMe
O
2:1 v/v
AcOH/water
Ph3P
DIAD
O
O
2
2 °C, 18 h
OH
THF, 0 °C
2 h, 85%
OH
81%
MeO
OHC
OMe
OHC
OMe
CHO
32
33
7
The synthesis of analogue 8 is shown in Scheme 8 and involved, as the initial step, the regioꢀ
and diastereoꢀselective cisꢀdihydroxylation of the diene 19. Selective silylation of the allylic
17b,31
hydroxyl group within the resulting and previously reported
diol 34 (62%) was readily
achieved using TIPSꢀOTf in the presence of 2,6ꢀlutidine and the product ether 35 (88%)
subjected to Oꢀmethylation using methyl iodide/base.
Scheme 8: Synthesis of Analogue 8
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
OH
OH
Br
TIPSꢀOTf
2,6ꢀlutidine
O
O
O
O
OH
O
O
OTIPS
OsO , NMO
4
83:17 v/v
CH Cl2
2
–78 to 22 °C
Br
19
acetone/water
to 22 °C, 18 h
62%
Br
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3 h, ca. 75%
0
34
35
NaH, MeI
ca. 77%
OMe
THF
0
to 22 °C, 2 h
OMe
O
O
OH
OTIPS
O
O
O
O
B
TBAF
+
OH
THF, 22 °C
2 h, 88%
Br
Br
36
OHC
OMe
2
7
37
PdCl (dppf)•CH Cl
2
2
2
Et N
3
9
:1 v/v THF/water
66 °C, 3 h, 55%
OMe
5
OMe
OMe
OH
O
HO
HO
O
2:1 v/v
AcOH/water
CONMe2
CONMe2
H CC(OMe) NMe
3
2
2
O
O
50 °C, 22 h
toluene, 111 °C
OMe
OMe
OMe
OMe
OH
ca. 59%
18 h, ca. 33%
OHC
OHC
OHC
OMe
38
39
8
Treatment of the product bisꢀether 36 (92%) with TBAF resulted in cleavage of the OꢀTIPS
bond and formation of the alcohol 37 (88%) that could be crossꢀcoupled with boronate ester
2
7 under SuzukiꢀMiyaura conditions and so affording the anticipated product 38 (55%).
Contrary to expectations, however, the allylic alcohol residue within compound 38 failed to
engage in an EschenmoserꢀClaisen rearrangement on treatment with N,N,ꢀdimethylacetamide
dimethyl acetal. Rather, the replacement of the associated allylic hydroxyl group with a N,Nꢀ
dimethylacetamide residue took place to give, in a stereoselective manner and after
ACS Paragon Plus Environment

Page 19 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
accompanying Oꢀmethylation of the phenolic hydroxyl group, amide 39 (40%). This was
contaminated by small amounts (7%) of its chromatographically inseparable C5ꢀepimer.
3
2
“
Thwarted EꢀC rearrangements” of this type have been reported previously and in this
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
instance the process may be driven by electronꢀrich arene residue facilitating ionization of
the intermediate mixed acetal with the ensuing and extensively stabilized cation then
undergoing nucleophilic capture by 1ꢀmethoxyꢀN,Nꢀdimethylethenꢀ1ꢀamine that is itself
generated through thermal cracking of the starting dimethyl acetal. The basic structure of
product 39 follows from the observation that the diastereotopic methylene hydrogens of the
acetamide sideꢀchain both show vicinal couplings to the adjacent allylic hydrogen. The
illustrated configuration of the C5 acetamide residue is assigned on the basis that this would
be introduced preferentially, during the course of the abovementioned nucleophilic capture
process, from that face of the intermediate cation opposite to the sterically demanding and
nearby acetonide residue. Hydrolysis of this acetonide residue within compound 33 under
standard conditions then gave analogue 8 in 69% yield.
The synthesis of the final galanthamine analogue 9 followed a very similar route to that used
in preparing congener 8. The reaction sequence involved is shown in Scheme 9. Thus,
reaction of epoxide 19 with acetic acid in the presence of a mineral acid gave the previously
9
a
reported transꢀdiol monoꢀacetate 40 (81%), the free hydroxyl of which was protected as the
9a
corresponding MOMꢀether using standard protocols and thus affording compound 41 in
9
1% yield.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Scheme 9: Synthesis of Analogue 9
MOMCl
Hünig's base
DMAP
OH
Br
OMOM
OAc
O
AcOH
H PO
O
O
O
O
OAc
O
O
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
22 °C, 18 h
81%
CH2Cl2
Br
22 °C, 1 h
Br
90%
20
40
41
K CO
2
3
MeOH
95%
22 °C, 1 h
OMOM
OH
O
O
OMOM
PdCl (dppf)•CH Cl
2
2
2
O
O
OH
O
O
Et N
B
3
+
OH
OMe
OH
9:1 v/v THF/water
66 °C, 2 h
Br
OHC
OHC
OMe
49%
43
27
42
H CC(OMe) NMe
2
3
2
toluene, 111 °C
8 h, ca. 26%
1
OMOM
OMOM
5
HO
CONMe2
O
2:1 v/v
AcOH/water
CONMe2
HO
O
5
0 °C, 22 h
OMe
OMe
OMe
OMe
ca. 60%
OHC
OHC
9
44
Cleavage of the acetate unit with the last compound could be effected under conventional
9
a
conditions and the resulting allylic alcohol 42 (95%) then crossꢀcoupled with boronate ester
7 in the usual manner and so giving product 43 (49%). As was the case with congener 32
Scheme 8), on subjecting compound 43 to conditions often used to effect the ECꢀ
2
(
rearrangement this substrate also engaged in both an allylic substitution reaction and Oꢀ
methylation of the phenolic hydroxyl of the precursor. As consequence a
ACS Paragon Plus Environment

Page 21 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
chromatographically inseparable and 3:1 mixture of amide 44 and its βꢀepimer (35%
combined yield) was formed. The salient spectral features of compound 44 resembled those
of congener 33. Hydrolysis of the mixture of acetonide 44 and its C5ꢀepimer under
conventional conditions then gave, after chromatographic purification, diol 9 (69%) that, like
the other galanthamine derivatives/analogues, was subjected to molecular docking studies
and evaluation as a potential inhibitor of acetylcholine esterase. Details are presented in the
following section.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Biological Evaluation and Molecular Docking Studies
The abovementioned derivatives/analogues of (–)ꢀ and (+)ꢀgalanthamine were each evaluated
for their ability to inhibit AChE using a modified method involving addition of DMSO so as
3
3
to ensure dissolution of these otherwise rather insoluble compounds. The inhibitory effect
of DMSO itself on the AChE was taken into account by subtracting a control measurement
for obtaining the IC values of the tested materials. A summary of the inhibition data thus
5
0
obtained is shown in Table 1. These assays reveal that only one of the compounds, namely
derivative 3, showed a measureable IC value (of 420 µM) compared to the positive control
5
0
(
–)ꢀgalanthamine [(–)ꢀ1] which had an IC value of 0.9 µM. Clearly, then, and regardless of
50
the absolute stereochemistries of the systems involved, none of the aboveꢀmentioned
derivatives/analogues is a strong inhibitor of AChE.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1: Outcomes of Evaluating Galanthamine Derivatives and Analogues as Inhibitors of
AChE and the Calculated Docking Binding Energies (BEs) of Them.
Entry
1
Compound
IC50 ( M)
>500
Docking BE (kcal)
2
–8.9
–9.6
–8.4
–9.5
–6.0
–9.1
–
2
entꢀ2
>500
3
4
3
420 ± 57
>500
entꢀ3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
4
>500
6
entꢀ4
>500
7
5
>500
8
entꢀ5
>500
–
9
6
>500
>500
>500
>500
>500
0.9 ± 0.2
–6.3
–9.2
–7.8
–
10
11
12
13
14
entꢀ6
7
8
9
–
1 (+ve control)
–10.2
Analysis of these inhibition data was undertaken through molecular docking simulations and
3
4
using the structure of (–)ꢀgalanthamine bound to human AChE. Crystallographic studies
have revealed that (–)ꢀgalanthamine binds into the active site of AChE with the
tetrahydroazepine or Dꢀring assuming a boatꢀlike conformation and the associated Nꢀmethyl
35
group in a pseudoꢀequatorial orientation spanning the acyl and cholineꢀbinding sites.
Docking simulations, using AUTODOCK, matched the solved structure (see Figure 4A) with
the key interactions between AChE and (–)ꢀgalanthamine being evident and so suggesting
that docking simulations of this type can provide the correct orientation of binding for the
compounds. Surprisingly, with the exception of compounds 5 and entꢀ5, 8 and 9 ꢀ for which
no bound structures could be obtained (in the case of the first two of these compounds this
may be a consequence of their significantly greater size), the derivatives all had significant
docking binding energies, albeit weaker than (–)ꢀgalanthamine (see rightꢀhand column, Table
1
). This highlights some of the known limitations in the prediction of binding affinity by
3
6ꢀ38
docking programs.
The docking studies do, however, reveal the likely structural basis for
ACS Paragon Plus Environment

Page 23 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
the reduced affinity of these analogues. For example, the configuration of compound 3
matches the orientation of (–)ꢀgalanthamine, except that the additional (C5) hydroxyl moiety
is positioned towards tryptophan 84 and so produces a distortion in the shape of the Aꢀring.
This most likely results in destabilization of the πꢀπ stacking interaction between the indole
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
9
ring of Trp84 and this ring (A) with this loss of interaction impairing the compound’s
capacity to inhibit AChE.
Figure 4. Overlap of (–)ꢀGalanthamine (1) (blue) and the Docked Derivatives 3 (A, purple)
and entꢀ3 (B, peach) in the Active Site of AChE
Trp84
Trp84
The (+)ꢀenantiomer, entꢀ3, of compound 3 also has a hydroxyl positioned towards Trp84,
again disrupting the stabilizing cyclohexeneꢀindole interactions, although on this occasion it
is the same hydroxyl moiety that is present in galanthamine, potentially explaining, in part at
least, why (+)ꢀgalanthamine derivatives are not potent inhibitors of AChE.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CONCLUSIONS
The synthetic chemistry studies detailed above have established that the ABCꢀring system of
galanthamine is readily obtained through the SuzukiꢀMiyaura crossꢀcoupling of oꢀ
hydroxyarylboronates with conduritols incorporating a brominated doubleꢀbond and then
engaging the products of such processes in an intramolecular Mitsunobu reaction.
Furthermore, most of the polyhydroxylated tetrahydrodibenzofurans arising from such a
reaction sequence engage in a thermally promoted EschenmoserꢀClaisen type rearrangement
reaction on treatment with N,Nꢀdimethylacetamide dimethyl acetal in refluxing toluene. The
angularlyꢀsubstituted tetrahydrodibenzofurans thus formed, which embody the quaternary
carbon center associated with the title alkaloid and represent monoꢀseco analogues of the
same, can then be elaborated, using PictetꢀSpengler chemistry, to give oxygenated
derivatives of galanthamine, certain variants of which have recently been isolated from
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
40
Chinese medicinal plants. Interestingly, these new natural products were also poor
inhibitors of AChE.
The biological evaluation of the galanthamine derivatives and analogues obtained by the
pathways described above reveals the finely tuned nature of the interactions of the parent
alkaloid with the target enzyme AChE. In particular, structurally “modest” changes to the
galanthamine framework, as embodied in the oxygenated derivatives 2, entꢀ2, 3 and entꢀ3,
can completely disrupt binding and such that the compounds are rendered inactive. These
studies have also revealed that the computational prediction of the likely binding affinity of
galanthamine analogues to AChE is fraught.
ACS Paragon Plus Environment

Page 25 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
EXPERIMENTAL SECTION
1
13
General Protocols. Unless otherwise specified, proton ( H) and carbon ( C) NMR spectra
were recorded at room temperature in baseꢀfiltered CDCl on a spectrometer operating at 400
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
MHz for proton and 100 MHz for carbon nuclei. The signal due to residual CHCl appearing
3
at δ 7.26 and the central resonance of the CDCl “triplet” appearing at δ 77.0 were used to
H
3
C
1
13
1
reference H and C NMR spectra, respectively. H NMR data are recorded as follows:
chemical shift (δ) [multiplicity, coupling constant(s) J (Hz), relative integral] where
multiplicity is defined as: s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet or
combinations of the above. Infrared spectra ( max) were recorded on a FTIR Spectrometer.
Samples were analyzed as thin films or finely divided solids. Lowꢀresolution ESI mass
spectra were recorded on a single quadrupole mass spectrometer interfaced with a liquid
chromatograph, while highꢀresolution measurements were conducted on a timeꢀofꢀflight
instrument. Lowꢀ and highꢀresolution EI mass spectra were recorded on a magneticꢀsector
machine. Melting points were measured on an automated melting point system and are
uncorrected. Analytical thin layer chromatography (TLC) was performed on aluminumꢀ
backed 0.2 mm thick silica gel 60 F254 plates. Eluted plates were visualized using a 254 nm
UV lamp and/or by treatment with a suitable dip followed by heating. These dips included
phosphomolybdic acid:ceric sulfate:sulfuric acid (conc.):water (37.5 g : 7.5 g : 37.5 g : 720
mL), potassium permanganate:potassium carbonate:5% w/v aq. sodium hydroxide
solution:water (3 g : 20 g : 5 mL : 300 mL) ), pꢀanisaldehyde or vanillin:sulfuric acid
(conc.):ethanol (15 g : 2.5 mL : 250 mL). Flash chromatographic separations were carried out
4
1
following protocols defined by Still et al. with silica gel 60 (40–63  m) as the stationary
phase and using the ARꢀ or HPLCꢀgrade solvents indicated. The melting points of solids
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 26 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
purified by such means were recorded directly (ie after they had crystallized from the
concentrated chromatographic fractions). Starting materials, reagents, drying agents and
other inorganic salts were generally commercially available and used as supplied. THF,
methanol and CH Cl were dried using a solvent purification system that is based upon a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
4
2
technology originally described by Grubbs et al.
Specific Chemical Transformations. (2R,3R,4aS,5S,8R,8aS)-6-(2-Hydroxy-3-methoxy-
phenyl)-2,3-dimethoxy-2,3-dimethyl-2,3,4a,5,8,8a-hexahydrobenzo[b][1,4]dioxine-5,8-di
1
5
-ol (13) and Enantiomer ent-13. A magnetically stirred solution of bisꢀacetal 11 (805 mg,
.37 mmol), ester 12 (720 mg, 2.88 mmol) and triethylamine (3 mL) in THF/water (20 mL of
2
a 9:1 v/v mixture) was subject to sonication under an atmosphere of nitrogen for 0.5 h.
PdCl dppf•CH Cl (140 mg, 0.191 mmol) was then added and the ensuing mixture heated
2
2
2
under reflux for 2 h before being cooled and quenched with phosphate buffer (5 mL of a 1 M
aqueous solution at pH 7). The mixture thus obtained was cooled to 0 ºC, treated with
methanol/30% aq. hydrogen peroxide (10 mL of a 1:1 v/v mixture) and then allowed to warm
to room temperature over 1 h. The resulting mixture was diluted with water (50 mL) and
extracted with ethyl acetate (5 × 20 mL). The combined organic phases were washed with
brine (2 × 20 mL) before being dried (Na SO ), filtered and concentrated under reduced
2
4
pressure. The ensuing thick, orange oil was triturated with diethyl ether (5 × 2 mL) and the
resulting yellow solid subject to flash column chromatography (silica, 4:15:1 v/v ethyl
acetate/hexane/methanol → 12:7:1 v/v ethyl acetate/hexane/methanol gradient elution).
Concentration of the appropriate fractions (R = 0.3 in 10:9:1 v/v/v ethyl acetate/hexane/
f
2
0
methanol) afforded phenol 13 (618 mg, 68%) as a white powder, m.p. = 202–210 ºC, [α]
=
D
1
–
93.8 (c = 0.1, methanol). H NMR (400 MHz, CD OD) δ 6.89 (dd, J = 7.5 and 1.8 Hz,
3
ACS Paragon Plus Environment

Page 27 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
1
H), 6.83–6.73 (complex m, 2H), 5.76 (d, J = 2.5 Hz, 1H), 4.60 (d, J = 3.7 Hz, 1H), 4.23 (dd,
J = 8.0 and 2.5 Hz, 1H), 4.09 (dd, J = 11.0 and 8.0 Hz, 1H), 3.85 (s, 3H), 3.69 (dd, J = 11.0
and 3.7 Hz, 1H), 3.34 (s, 3H), 3.28 (s, 3H), 1.34 (s, 3H), 1.33 (s, 3H) (signals due to hydroxyl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13
protons not observed); C NMR (100 MHz, CD OD) δ 149.0, 145.0, 138.9, 132.9, 128.6,
3
123.6, 120.3, 112.2, 100.7, 100.3, 71.4, 70.7, 69.7, 69.0, 56.6, 48.3, 48.2, 18.1(5), 18.1(0);
–1
IR νmax 3456, 3283, 2941, 1589, 1468, 1218, 1129, 1119, 914, 594 cm ; MS (ESI, +ve) m/z
+
+
+
7
88 ([(2M + Na) 80%], 405 [(M + Na) 100]; HRMS (ESI, +ve) m/z (M + Na) calcd for
, ,
C H NaO 405.1525, found 405.1526.
19
26
8
Compound entꢀ13 was prepared in an analogous fashion from entꢀ11 (2.04 g, 5.34
2
0
mmol) to give 1.60 g (71%) of product, [α]
= + 86.4 (c = 0.1, methanol). All the other
D
spectral data acquired on this material were identical with those reported above for
compound 13.
(
2R,3R,4aS,5R,11aR,11bS)-2,3,10-Trimethoxy-2,3-dimethyl-2,3,4a,5,11a,11b-hex-
ahydrobenzo[b][1,4]dioxino[2,3-g]benzofuran-5-ol (14) and Enantiomer ent-14. A
magnetically stirred solution of phenol 13 (605 mg, 1.58 mmol) and PPh (456 mg, 1.74
3
mmol) in THF (70 mL) maintained at –5 ºC was treated, dropwise over 0.5 h, with diꢀisoꢀ
propyl azodicarboxylate (364 µL, 1.74 mmol). The resulting solution was stirred at –5 ºC for
4
h then allowed to warm to room temperature over 1 h before being concentrated under
reduced pressure. The orange oil thus obtained was subjected to two successive flash
chromatographic separations (silica, hexane → 1:1 v/v ethyl acetate/hexane gradient elution)
and concentration of appropriate fractions (R = 0.7, 2:1 v/v ethyl acetate/hexane) afforded
f
20
1
allylic alcohol 14 (193 mg, 33%) as a white foam, [α] = –157 (c = 0.6, CHCl ). H NMR
D
3
(
400 MHz, CDCl ) δ 6.99 (dd, J = 7.5 and 1.2 Hz, 1H), 6.89 (apparent t, J = 7.5 Hz, 1H),
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 28 of 63
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
.82 (dd, J = 8.1 and 1.2 Hz, 1H), 5.81 (m, 1H), 5.14 (dt, J = 9.0 and 3.6 Hz, 1H), 4.67–4.61
(complex m, 1H), 4.04 (dd, J = 10.5 and 9.0 Hz, 1H), 3.93 (dd, J = 10.5 and 7.7 Hz, 1H),
13
3
.88 (s, 3H), 3.34 (s, 6H), 2.34 (d, J = 4.8 Hz, 1H), 1.36(3) (s, 3H), 1.35(9) (s, 3H); C NMR
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(100 MHz, CDCl ) δ 151.6, 145.4, 139.1, 125.7, 122.1, 116.2, 113.6, 113.5, 99.2, 98.9, 84.0,
3
74.1, 71.3, 70.1, 56.0, 48.4, 48.0, 17.7, 17.6; IR νmax 3486, 2959, 2897, 1614, 1597, 1494,
−
1
+
1
445, 1133, 1114, 1098, 1035, 1015, 786 cm ; MS (ESI, +ve) m/z 752 [(2M + Na) 25%],
,
+
+
3
87 [(M + Na) 100%]; HRMS (ESI, +ve) m/z (M + Na) calcd for C H NaO 387.1420,
,
19 24
7
found 387.1423.
Compound entꢀ14 was prepared in an analogous fashion from entꢀ13 (345 mg, 0.90
20
D
mmol) to give 314 mg (96%) of product, [α]
= +102 (c = 1.0, CHCl ). All the other
3
spectral data acquired on this material were identical with those reported above for
compound 14.
N,N-Dimethyl-2-((2R,3R,4aS,6aS,11aR,11bR)-2,3,10-trimethoxy-2,3-dimethyl-
2
,3,11a,11b-tetrahydrobenzo[b][1,4]dioxino[2,3-g]benzofuran-6a(4aH)-yl)acetamide
(15) and Enantiomer ent-15. A magnetically stirred solution of allylic alcohol 14 (180 mg,
0.494 mmol) in toluene (20 mL) maintained at 22 °C was treated with N,Nꢀ
dimethylacetamide dimethyl acetal (900  L, 4.9 mmol) and the resulting solution heated
under reflux for 18 h. The cooled reaction mixture was concentrated under reduced pressure
and the residue thus obtained was triturated with diethyl ether. The resulting waxy solid was
subjected to flash chromatography (silica, 20:79:1 v/v ethyl acetate/hexane/methanol →
6
0:39:1 v/v ethyl acetate/hexane/methanol gradient elution) and concentration of appropriate
fractions (R = 0.2 in 2:1 v/v ethyl acetate/hexane) afforded amide 15 (178 mg 84%) as a
f
20
1
white foam, [α]
= –89.9 (c = 1.0, CHCl ). H NMR (400 MHz, CDCl ) δ 6.83 (m, 1H),
D
3
3
ACS Paragon Plus Environment

Page 29 of 63
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
6
.75 (d, J = 7.8 Hz, 1H), 6.72 (d, J = 7.6 Hz, 1H), 6.02 (dd, J = 10.0 and 2.7 Hz, 1H), 5.74
(dd, J = 10.0 and 2.0 Hz, 1H), 5.04 (d, J = 9.6 Hz, 1H), 4.55 (dt, J = 9.6 and 2.0 Hz, 1H),
3
.85 (s, 3H), 3.76 (t, J = 9.6 Hz, 1H), 3.30 (s, 3H), 3.16 (s, 3H), 2.89 (s, 3H), 2.78 (s, 3H),
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13
2
.71 (d, J = 15.8 Hz, 1H), 2.61 (d, J = 15.8 Hz, 1H), 1.34 (s, 3H), 1.28 (s, 3H); C NMR
(100 MHz, CDCl ) δ 169.0, 145.6(4), 145.5(6), 134.2, 128.9, 126.9, 121.5, 115.3, 112.0,
3
9
9.8, 87.1, 70.5, 65.9, 56.1, 51.0, 48.1, 47.4, 42.5, 37.3, 35.3, 17.7(4), 17.7(0) (one signal
obscured or overlapping); IR νmax 2954, 2889, 1651, 1491, 1459, 1380, 1276, 1115, 1061,
−
1
+
1000, 954, 739 cm ; MS (ESI, +ve) m/z 456 [(M + Na) 100%], 201 (35); HRMS (ESI,
,
+
+
ve) m/z (M + Na) calcd for C H NNaO 456.1998, found 456.2000.
2
3
31
7
Compound entꢀ15 was prepared in an analogous fashion from entꢀ14 (234 mg, 0.64
mmol) to give 238 mg (86%) of product as a white foam. A small sample was crystallised
(
diethyl ether/methanol/hexane) to give a white, crystalline solid, mp = 160–165 ºC
20
(
decomposition), [α] = +87.1 (c = 1.0, CHCl ). All the other spectral data acquired on this
D
3
material were identical with those reported above for compound 15.
-((2R,3R,4aS,6aS,11aR,11bR)-2,3,10-Trimethoxy-2,3-dimethyl-2,3,11a,11b-tetra
2
-hydrobenzo[b][1,4]dioxino[2,3-g]benzofuran-6a(4aH)-yl)ethan-1-ol (16) and Enantio-
mer ent-16. A magnetically stirred solution of amide 15 (146 mg, 0.337 mmol) in THF (17
mL) maintained at 0 ºC was treated, dropwise, with lithium triethylborohydride (1.7 mL of a
1 M solution in THF, 1.7 mmol). The resulting mixture was warmed to room temperature
over 2 h, reꢀcooled to 0 ºC, quenched with methanol (2 mL) and then treated with silica (200
mg of flash chromatographic grade material) before being subjected to flash chromatography
(
silica, 1:4 v/v ethyl acetate/hexane → 1:1 v/v ethyl acetate/hexane gradient elution).
Concentration of relevant fractions (R = 0.6, 2:1 v/v ethyl acetate/hexane) afforded
f
ACS Paragon Plus Environment