Programmed serial stereochemical relay and its application in the synthesis of morphinans

Article abstract of DOI:10.1039/c7sc03189k

Herein we report a rationally designed, serial point-to-axial and axial-to-point stereoinduction and its integration into multi-step and target-oriented organic synthesis. In this proof-of-concept study, the configurational stability of several carefully designed atropisomeric intermediates and the fidelity of their unconventional stereoinductions were systematically investigated. The highly functionalized prepared synthetic intermediate was further applied in a novel chemical method to access the morphinans and it is potentially applicable to other structurally related alkaloids.

Full text of DOI:10.1039/c7sc03189k

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Programmed serial stereochemical relay and its
application in the synthesis of morphinans†
Cite this: DOI: 10.1039/c7sc03189k
*
Kun Ho (Kenny) Park, Rui Chen and David Y.-K. Chen
Herein we report a rationally designed, serial point-to-axial and axial-to-point stereoinduction and its
integration into multi-step and target-oriented organic synthesis. In this proof-of-concept study, the
configurational stability of several carefully designed atropisomeric intermediates and the fidelity of their
unconventional stereoinductions were systematically investigated. The highly functionalized prepared
synthetic intermediate was further applied in a novel chemical method to access the morphinans and it
is potentially applicable to other structurally related alkaloids.
Received 21st July 2017
Accepted 15th August 2017
DOI: 10.1039/c7sc03189k
rsc.li/chemical-science
which allow for the subsequent programmed-stereoinduction
events.
Introduction
Thoughtful orchestration of substrate and reagent controlled
stereochemical induction is essential in any stereoselective
synthesis.1 To date, while the fundamental principles behind
stereochemical induction are well-grounded, some forms of
stereoinduction are much less conventional than others. For
example, if we consider “point” and “axial” as two of the most
common forms of stereochemical elements, one can appreciate
that while the intermolecular point-to-point (for example, oxa-
zaborolidine-mediated reduction)2 and axial-to-point (for
example, Noyori BINAP hydrogenation)3 stereoinductions are
routinely practiced, intramolecular point-to-axial, axial-to-
point, and axial-to-axial stereoinductions are considerably
more rare.4 In the asymmetric synthesis of longithorone A5a and
rhazinilam,5b unconventional yet highly effective stereo-
inductions were elegantly illustrated by the Shair and Zakarian
groups, respectively (Scheme 1a). Furthermore, we pondered if
a series of these unconventional forms of stereoinduction could
be logically sequenced as part of a multi-step synthetic scheme,
and in doing so demonstrate their utility in the context of target-
oriented synthesis (Scheme 1b).
Results and discussion
We rst investigated if a “point” stereochemistry that resides in
close proximity to the biaryl axis can render, and hence induce,
an axial stereochemical property i.e. a point-to-axial stereo-
induction. In this context, the synthesis of biaryl aldehyde 4 was
In preparation for the proposed studies, it is important to
recognize that while a “point” stereochemical element can be
readily generated and/or identied, an “axial” stereochemical
element is only observable if the system can attain a sufficiently
high rotational energy barrier. In line with this requirement, we
began our proof-of-concept studies by leveraging the well-
established biaryl system as the source of the “axial” stereo-
chemical element (Scheme 1b). Furthermore, the proposed
biaryl system harbors additional functional group (FG) handles
Department of Chemistry, Seoul National University, Gwanak-1 Gwanak-ro,
Gwanak-gu, Seoul 151-742, South Korea. E-mail: davidchen@snu.ac.kr
Scheme 1 (a) Selected examples of unconventional stereochemical
induction in target-oriented organic synthesis; (b) proposed point-to-
axial-to-point serial stereochemical relay and its application in the
synthesis of morphinans. TBS ¼ tert-butyldimethylsilyl.
† Electronic supplementary information (ESI) available. CCDC 1526432. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7sc03189k
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reagents, and gratifyingly, each organometallic addition reac-
tion afforded a separable mixture of two stereoisomers in high
yield (5a/5a0–5d/5d0, 85–97%). Recognizing that the biaryl
aldehyde 4 does not have atropisomeric properties at ambient
temperature,8 this nding indicated that the newly formed
biaryls 5a/5a0–5d/5d0 exhibit restricted rotation and constitute
an illustration of point-induced atropisomerism.
Before pressing onto our next objective, namely the “axial-to-
point” stereochemical induction, two crucial criteria had to be
considered. First, the substrate must exhibit sufficient atropi-
someric congurational stability during the axial-to-point ster-
eoinduction event. Second, any preparatory transformations
leading to the axial-to-point stereoinduction event must
preserve the atropisomeric property within the substrate. In
order to assess the congurational stability of the organome-
tallic addition products (5a/5a0–5d/5d0), chromatographically
separated and atropisomerically pure biaryl benzylic alcohols
were subjected to thermal conditions and their atropisomeri-
Scheme 2 Synthesis of biaryl benzylic alcohols 5a/5a0 to 5d/5d0
exhibiting atropisomeric properties. Reagents and conditions: (a)
Pd(OAc)2 (0.04 equiv.), K2CO3 (4.0 equiv.), TBAB (1.0 equiv.), DMF,
120 ꢀC, 16 h, 86%; (b) DDQ (1.0 equiv.), CH2Cl2/pH 9.2 buffer (10 : 1),
1
zation was monitored by H NMR analysis (Table 1). Our qual-
0
ꢀC, 1.5 h, 76%; (c) for 5a/5a0: methylmagnesium bromide (3.0 M in
itative analysis revealed that all of the substrates exhibited good
congurational stability from low to moderately elevated
temperatures (up to 70 ꢀC), with the t-butyl substrates 5c and 5c0
Et2O, 3.3 equiv.), THF, ꢂ78 to 23 ꢀC, 16 h, 92% (5a : 5a0 ꢁ 1 : 1); for 5b/
5b0: phenylmagnesium bromide (1.0 M in THF, 4.7 equiv.), THF, ꢂ78 to
23 ꢀC, 16 h, 85% (5b : 5b0 ꢁ 6 : 1); for 5c/5c0: t-butyllithium (1.7 M in
pentane, 3.5 equiv.), THF, ꢂ78 to 23 ꢀC, 16 h, 92% (5c : 5c0 ꢁ 4 : 1); for
5d/5d0: allylmagnesium bromide (1.0 M in THF, 2.6 equiv.), THF, ꢂ78 to
23 ꢀC, 16 h, 97% (5d : 5d0 ꢁ 3 : 1). DDQ ¼ 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone; DMF ¼ N,N0-dimethylformamide; OAc ¼ acetate;
TBAB ¼ tetra-n-butylammonium bromide.
ꢀ
demonstrating extended stability up to 100 C (for details, see
the ESI†).
Next, we envisaged that the proposed axial-to-point stereo-
induction would take place via a dearomatization process, in
which the dearomatized product or its chemically transformed
derivative would possess new “point” stereochemical
element(s).9 Aer contemplating a variety of well-documented
dearomatization protocols, hypervalent iodine mediated
oxidative dearomatization appeared the most appealing owing
to its operational ease, substrate scope and product versatility.10
Accordingly, benzylic alcohols 5a/5a0–5d/5d0 (except 5c/5c0,
which remained as benzylic alcohols) were protected as their
corresponding TBS ethers to prevent any unwanted side reac-
tions and to enhance their atropisomeric stabilities, followed by
treatment with PIFA in the presence of methanol (5d/5d0 in
Scheme 3a, for 5a/5a0–5d/5d0 see the ESI†). Much to our
surprise, each of the isomerically pure phenols 6d and 6d0
underwent oxidative dearomatization to afford an identical
mixture of two dienone products 7d and 7d0 (7d : 7d0 ꢁ 1 : 1).
The fact that 7d and 7d0 were observed as two distinct diaste-
Table 1 Configurational stability study of biaryl benzylic alcohols 5a/
5a0 to 5d/5d0. Diastereoisomeric pairs: 5a/5a0: R ¼ Me; 5b/5b0: R ¼ Ph;
5c/5c0: R ¼ t-Bu; 5d/5d0: R ¼ allyla
Temp. ꢀ
C
40
70
90
100
110
120
Compd.
(1H NMR analysis aer 1 h at each temperature up to 110 ꢀC)
5a
1 : 0
0 : 1
1 : 0
0 : 1
1 : 0
0 : 1
1 : 0
0 : 1
1 : 0.10
0.07 : 1
1 : 0.13
0.06 : 1
1 : 0
0 : 1
1 : 0
0 : 1
1 : 0.43
0.16 : 1
1 : 0.43
0.23 : 1
1 : 0
0 : 1
1 : 0.36
0.53 : 1
1 : 0.88
0.39 : 1
1 : 0.98
0.44 : 1
1 : 0.09
0.06 : 1
0.93 : 1
0.70 : 1
0.79 : 1
0.60 : 1
0.68 : 1
0.58 : 1
1 : 0.35
0.18 : 1
0.71 : 1
0.70 : 1
0.69 : 1
0.70 : 1
0.63 : 1
0.65 : 1
0.53 : 1
0.52 : 1
0.71 : 1
0.70 : 1
5a0
5b
5b0
5c
5c0
5d
5d0
1
reoisomers based on our H NMR analysis suggested that the
epimerization of the biaryl axis must take place during the
course of the reaction. However, a postulated transition state (6-
PIFA-TS, Scheme 4a), formed through an associative mecha-
nism by invoking a coordinated phenol–aryliodide complex,
should increase the rotational energy barrier and thus render
higher congurational stability. While seeking a more concrete
explanation, we also became aware of a recent report from the
Pappo laboratory describing that several optically pure BINOL
substrates underwent metal-catalyzed racemization under
single-electron-transfer (SET) conditions (Scheme 4b), although
the precise mechanistic origin behind this racemization
process remained unclear.11 Very recently, NMR and DFT
studies reported by the Koltunov group also suggested an acid-
mediated atropisomerization of optically pure BINOL via
a
1
Ratio determined by H NMR integration. For details, see the ESI.†
originally envisioned based on the desymmetrization of the
readily accessible (and potentially optically active) dialdehyde 1
(Scheme 2).6 However, a more direct approach was later realized
through one of the earliest illustrations of CH-activation
chemistry pioneered by Dyker,7 and the as-obtained biaryl
system 3 underwent smooth benzylic oxidation (DDQ) to afford
the targeted aldehyde 4 together with its equilibrating hemi-
acetal 40 in 65% yield over the two steps. The phenolic aldehyde
4 (and 40) was next subjected to a selection of organometallic
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Scheme 3 (a) Oxidative dearomatization of atropisomerically pure biaryl phenols 6d and 6d0; (b) oxidative dearomatization and configurational
stability studies of biaryl phenols 6e and 6e0; and (c) oxidative dearomatization and configurational stability studies of biaryl phenols 6f and 6f0.
PCC ¼ pyridinium chlorochromate; PIDA ¼ [bis(acetoxy)iodo]benzene; PIFA ¼ [bis(trifluoroacetoxy)iodo]benzene; and TBSCl ¼ tert-butyldi-
methylsilyl chloride.
Continuing our studies, several related phenolic biaryl
substrates were conceived in order to understand and to
preserve the atropisomeric information (Scheme 3b and c). In
this context, the phenolic substrates 6e and 6e0 with the phenol
and methoxy positions switched compared to those in 6d and
6d0 demonstrated much improved thermal stability, however,
when separately treated with PIDA in the presence of methanol,
also afforded an identical mixture of dienones 7e and 7e0
(ꢁ1 : 1). Gratifyingly, phenols 6f and 6f0 with an additional
methyl substituent were found to retain their atropisomeric
purity upon PIDA-mediated oxidative dearomatization, and the
atropisomerically pure dienones 7f and 7f0 faithfully underwent
intramolecular Diels–Alder reactions to afford tetracycles 8f and
8f0 in 70% and 61% yield, respectively, with their stereochemical
relationship conrmed upon oxidation with PCC. An analogous
intramolecular Diels–Alder reaction could also be realized with
a 1 : 1 mixture of dienones 7d and 7d0 to afford a near 1 : 1
mixture of tetracycles 8 and 80 (Scheme 3a). Considering that
dienones 7d and 7d0 are likely to be congurationally labile at
elevated temperatures (see Table 1), this latter result implied
that the benzylic OTBS stereocenter offered essentially no ster-
eoinduction during the intramolecular Diels–Alder process.
Furthermore, the selective formation of the Diels–Alder product
8f from 7f (and 8f0 from 7f0) strongly suggested that the ster-
eoinduction arose exclusively from the atropisomeric property
Scheme
4 (a) Oxidative dearomatization of 6 via an associative
mechanism; (b) racemization of (R)-BINOL under SET conditions; and
(c) acid promoted racemization of (R)-BINOL. BINOL ¼ 1,10-bi-2-
naphthol;
DCE
¼
1,2-dichloroethane;
and
HFIP
¼
hexafluoroisopropanol.
several enone intermediates (e.g. BINOL-H+) that structurally
closely resemble those generated during the hypervalent iodine
and metal–salt mediated phenol oxidations (Scheme 4c).12
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that the highly functionalized phenanthrene-type tetracycle 8/80
bearing a quaternary stereocenter shared some similarities with
the core structure of the morphinan family of natural prod-
ucts.14 Further structural analysis implied that the [2.2.2]-
bicyclic domain within 8 (or 80) had to be ruptured while
retaining its quaternary stereocenter. As shown in Scheme 7,
Diels–Alder product 8 (and 80) was readily converted to diketone
9 via a regioselective hydroboration and an oxidative work-up
with PCC,15 followed by an acid-induced elimination. It is
worth noting that, while tetracycles 8 and 80 could be used in
combination in this racemic illustration, enantio- and
diastereo-isomerically pure 8 (or 80) will be required to access
optically pure intermediates. The more sterically accessible
ketone in 9 was converted to its corresponding oxime and
thermally equilibrated to a single geometric isomer 10, which
was subsequently tosylated (TsCl, 87% yield from 9) to set the
stage for the Beckmann rearrangement.16 Under optimized
conditions, tosyloxime 11 underwent regioselective ring
expansion under the inuence of ZnCl2 to afford lactam 12 in
73% yield. Further cleavage of the bridgehead C–N bond in 12
was realized through a Hoffmann elimination17 of its quater-
nary ammonium salt derivative 13 to furnish dimethylamine 14.
Unmasking the dimethoxy acetal in 14 under acidic conditions,
very surprisingly, afforded a 1,2-migratory product 15 (aer
treatment with ethyl chloroformate).18 This undesired bond
migration was easily rectied upon partial reduction of diene
14,19 followed by acidic deketalization to yield hydroxy ketone 16
in 66% overall yield. a-Deoxygenation and N-demethylation
were sequentially realized through the action of SmI2 and 1-
chloroethyl chloroformate and the as-obtained ketone
18 represents a valuable common intermediate that is also
amenable for a synthetic method to access the hasubananes
(bridgehead cyclization “a”).20 Advancing tricyclic ketone 18
next called for the formation of a ve-membered oxacycle that
resides in several agship morphinans. To this end, phenolic
ketone 18 was converted to its dioxolane derivative followed by
Scheme 5 Atropisomerism dictated stereoinduction leading to the
stereocontrolled formation of Diels–Alder products 8f and 8f0.
Scheme 6 Oxidative dearomatization and intramolecular Diels–Alder
reaction of biaryl phenols 6g and 6h.
a
dioxolane-directed phenolic demethylation.22 Dioxolane
of 7f (and 7f0) (Scheme 5). In retrospect, the benzylic “point”
stereochemical directing element was positioned in proximity
to the biaryl-axis for the rst “point-to-axial” stereoinduction,
while it was distant from the second “axial-to-point” stereo-
induction event to suppress its stereo-directing ability. From
a design perspective, a single stereo-directing element at each
stereochemistry inducing step is synthetically more attractive to
avoid any complicated synergistic stereo-directing phenomena.
To conclude our studies in remote stereoinductions, we
found that substrates 6g and 6h (racemic or optically active)
with their OH/OTBS stereocenter relocated in proximity to the
intramolecular Diels–Alder transition-state could render
a signicantly higher level of point-to-point stereoinduction
than 6d/6d0 (Scheme 6), an observation that was consistent with
our previous synthetic studies towards platencin.13
removal followed by treatment of the resulting phenolic ketone
(19) with pyridinium tribromide23 and subsequent heating
smoothly delivered an inconsequential mixture of oxa-tetracycle
20 and arylbromide 20a. Next, relocation of the bridgehead
olen24 in 20/20a rst converged both compounds through
hydrogenation and dioxolane formation, and on further expo-
sure to NBS under radical conditions afforded styrene 21.
Reductive detosylation of 21 under Birch-type conditions took
place with concomitant piperidine formation21s and furnished
synthetic dihydrocodeinone upon dioxolane removal (HCl,
MeOH).25 Dihydrocodeinone serves as a valuable intermediate
to readily access a diverse array of naturally occurring and
designed morphinans, including but not limited to dihy-
drocodeine, codeine, morphine, thebaine and oxycodone.26
Conclusions
Synthetic applications
In summary, we have demonstrated proof-of-concept, rationally
Finally, we turned our attention to the synthetic utility of the designed serial point-to-axial-to-point stereoinductions and
synthesized advanced intermediates. To this end, we recognized examined the stereochemical delity of these processes. During
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Scheme 7 Synthesis of morphinans. Reagents and conditions: (a) BH3$THF (5.0 equiv.), THF, ꢂ78 to 70 ꢀC, 4 h; then PCC (5.0 equiv.), 23 ꢀC, 5 h,
41%; (b) p-TsOH$H2O (1.0 equiv.), toluene, 110 ꢀC, 20 min, 95%; (c) H2NOH$HCl (3.0 equiv.), NaOAc (3.0 equiv.), MeOH, 23 to 70 ꢀC, 16 h (heat to
neat), 90%; (d) TsCl (1.5 equiv.), NaOH (2.5 equiv.), acetone/H2O (1 : 1), 23 ꢀC, 2 h, 97%; (e) ZnCl2 (2.0 equiv.), MeCN, 100 ꢀC, 4 h; then ZnCl2 (2.0
equiv.), 100 ꢀC, 4 h, 73%; (f) LiAlH4 (10 equiv.), THF, 0 to 70 ꢀC, 8 h; (g) MeI (14 equiv.), CH2Cl2, 23 ꢀC, 10 h, 42% for two steps; (h) KOH (20% in
MeOH), 70 ꢀC, 10 h, 80%; (i) HCl (10% aq.)/MeOH (4 : 1), 23 ꢀC, 1 h, 67%; (j) EtOCOCl (5.0 equiv.), NaHCO3 (20 equiv.), 1,2-dichloroethane, 100 ꢀC,
2 h, 93%; (k) Pd/CaCO3 (1.0 equiv.), MeOH, H2 (1 atm), 23 ꢀC, 16 h, (1,4 : 1,2-hydrogenated compound ꢁ 5 : 1); then HCl (10% aq.)/MeOH (2 : 1),
23 ꢀC, 1 h, 66% for two steps; (l) Ac2O (10 equiv.), Et3N (10 equiv.), DMAP (0.1 equiv.), CH2Cl2, 23 ꢀC, 4 h; (m) SmI2 (0.1 M in THF), THF/MeOH (1 : 1),
ꢂ78 ꢀC, 0.5 h, 78% for two steps; (n) 1-chloroethyl chloroformate (20 equiv.), NaHCO3 (20 equiv.), 1,2-dichloroethane, 100 ꢀC, 2 h; then MeOH,
60 ꢀC, 1.5 h; then TsCl (2.0 equiv.), DMAP (0.4 equiv.), Et3N (2.3 equiv.), CH2Cl2, 23 ꢀC, 1.5 h, 64% for two steps; (o) TMSCl (2.0 equiv.), ethylene
glycol/CH2Cl2 (1 : 1), 23 to 50 ꢀC, 5 h, 92%; (p) L-selectride (1.0 M in THF, 5.0 equiv.), THF, 80 ꢀC, 24 h; then HCl (4.0 N aq.)/MeOH (1 : 15), 23 to
45 ꢀC, 3 h, 77% for two steps; (q) PyH$Br3 (2.0 equiv.), CH2Cl2/AcOH (2 : 5), 23 ꢀC, 30 min; then LiBr (5.0 equiv.), Et3N (10.0 equiv.), MeCN, 60 ꢀC,
20 min, 53%; (r) Pd/C (10% wt/wt, 2.0 equiv.), EtOAc/MeOH (1 : 3), H2, 23 ꢀC, 30 min, 90%; then TMSCl (excess), ethylene glycol/CH2Cl2 (1 : 1), 23
to 50 ꢀC, 5 h, 80%; (s) NBS (1.05 equiv.), benzoyl peroxide (0.5 equiv.), CCl4, 80 ꢀC, 1 h; then Et3N (7.8 equiv.), 80 ꢀC, 15 min, 60%; (t) Li (excess),
NH3, ꢂ78 ꢀC, 10 min, 69%; then HCl (4.0 N aq.)/MeOH (1 : 15), 70 ꢀC, 6 h, 75%; and (u) LiAlH4 (excess), THF, 0 ꢀC, 45 min, 74%. DMAP ¼ N,N0-
dimethylaminopyridine; EtOAc ¼ ethyl acetate; L-selectride ¼ lithium tri-sec-butylborohydride; NBS ¼ N-bromosuccinimide; PyH$Br3 ¼ pyr-
idinium tribromide; TMSCl ¼ trimethylsilyl chloride; p-TsOH$H2O ¼ para-toluenesulfonic acid monohydrate; TsCl ¼ para-toluenesulfonyl
chloride; and Ts ¼ para-toluenesulfonyl.
this investigation, an unexpected atropisomeric epimerization method to access the morphinan family of natural products and
upon hypervalent iodine-mediated oxidative dearomatization of potential access to other related alkaloid structures. Concep-
isomerically pure biaryl phenols was discovered and systemat- tually related programmed serial-stereoinductions, particularly
ically investigated. This nding bears important ramications those involving unconventional intramolecular axial-to-point
in related oxidative generation11 and transformations of and point-to-axial processes, are currently being designed and
designed and naturally occurring phenolic biaryl systems and implemented in the context of target-oriented synthesis in our
their atropisomeric integrity. While the undesired atropiso- laboratory.
meric epimerization could be suppressed by increasing the
steric pressure about the biaryl axis of the substrate (e.g. 6f/6f0),
more in depth mechanistic studies are in progress to render
a substrate-independent solution.27 Further application of the
developed synthetic strategy also enabled a novel synthetic
Conflicts of interest
There are no conicts to declare.
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12 A. M. Genaev, G. E. Salnikov, A. V. Shernyukov, Z. Zhu and
K. Y. Koltunov, Org. Lett., 2017, 19, 532.
Acknowledgements
13 G. Y. C. Leung, H. Li, Q.-Y. Toh, A. M.-Y. Ng, R. J. Sum,
J. E. Bandow and D. Y.-K. Chen, Eur. J. Org. Chem., 2011, 183.
14 For recent books and review chapters, see: (a) A. Brossi, in
The Chemistry and Biology of Isoquinoline Alkaloids, ed. J. D.
Phillipson, M. R. Roberts and M. H. Zenk, Springer-Verlag,
Berlin, 1985, pp. 171–189; (b) H. Nagase, in Chemistry of
Opioids; Top. Curr. Chem., Springer-Verlag, Berlin
Heidelberg, 2011, vol. 299, pp. 1–312.
This paper is dedicated to Professor William S. Knowles on the
occasion of his centennial. This work was supported by the
Seoul National University Foreign Faculty Fund, the New Faculty
Resettlement Fund, the National Research Foundation of Korea
(NRF) grant funded by the Korean government (MSIP) (No.
2012R1A2A2A01002895, 2013R1A1A2057837, and 2014-011165,
Center for New Directions in Organic Synthesis), and Novartis.
Rui Chen and Kenny Park were supported by the BK21Plus
Program, Ministry of Education. We thank Youngwook Park
and Sungmin Song for the preliminary studies. We thank
Professor Doron Pappo, Ben-Gurion University of the Negev,
Israel, for helpful discussions regarding the racemization/
epimerization of phenolic biaryl/binaphthyl systems under
oxidative conditions.
15 For pioneering discovery and examples of
a one-pot
hydroboration-PCC oxidation, see: (a) H. C. Brown,
S. U. Kulkarni and C. G. Rao, Synthesis, 1980, 151; (b)
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1990, 20, 3265.
16 T. Tatsumi, Beckmann rearrangement, ed. R. A. Sheldon and
H. Bekkum, Wiley-VCH, Weinheim, New York, 2001, p.
185–204.
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epimerization process, judicious design and choice of
reagent(s) and reaction conditions could provide
a substrate-independent solution. Theoretical investigation
of the racemization/epimerization of phenolic biaryl/
binaphthyl systems under oxidative conditions is currently
in progress.
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Chem. Sci.