Catalytic deacylative alkylations (DaA) of enolcarbonates: Total synthesis of (±)-Crinane

Article abstract of DOI:10.1016/j.tetlet.2020.152129

An efficient Pd(0)-catalyzed deacylative allylation (DaA) of enolcarbonates (as pro-nucleophile) of cycloalkanones sharing acyl functionality at C2-position with readily available allylic alcohols (as pro-electrophiles) is disclosed under mild reaction conditions. A wide variety of cycloalkanones with an aromatic ring and allyl group at C-2 position (all-carbon quaternary center) are obtained in good to excellent yields (36 examples). The usefulness of this methodology has been shown by a total synthesis of Amaryllidaceae alkaloid, (±)-crinane from 2-aryl cyclohexanone in 5 steps.

Full text of DOI:10.1016/j.tetlet.2020.152129

Journal Pre-proofs
Catalytic Deacylative Alkylations (DaA) of Enolcarbonates: Total Synthesis
of (±)-Crinane
Mrinal K. Das, Abhinay Yadav, Satyajit Majumder, Alakesh Bisai
PII:
S0040-4039(20)30584-0
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https://doi.org/10.1016/j.tetlet.2020.152129
TETL 152129
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Tetrahedron Letters
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7 June 2020
Please cite this article as: Das, M.K., Yadav, A., Majumder, S., Bisai, A., Catalytic Deacylative Alkylations
(DaA) of Enolcarbonates: Total Synthesis of (±)-Crinane, Tetrahedron Letters (2020), doi: https://doi.org/
10.1016/j.tetlet.2020.152129
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Catalytic Deacylative Alkylations (DaA) of Enolcarbonates: Total Synthesis of (±)-Crinane
Mrinal K. Das,a Abhinay Yadav,a Satyajit Majumder,a and Alakesh Bisai*a,b

1
Tetrahedron Letters
Catalytic Deacylative Alkylations (DaA) of Enolcarbonates: Total Synthesis of (±)-
Crinane
Mrinal K. Das,a Abhinay Yadav,a Satyajit Majumder,a and Alakesh Bisai*a,b
aDepartment of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhauri, Bhopal - 462 066, Madhya Pradesh, India
bDepartment of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, Nadia - 741 246, West Bengal, India
Contact: +91-755-269-1328, Fax: +91-755-269-2392. Address for correspondence: alakesh@iiserb.ac.in; alakesh@iiserkol.ac.in
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient Pd(0)-catalyzed deacylative allylation (DaA) of enolcarbonates (as pro-
nucleophile) of cycloalkanones sharing acyl functionality at C2-position with readily
available allylic alcohols (as pro-electrophiles) is disclosed under mild reaction conditions.
A wide variety of cycloalkanones with an aromatic ring and allyl group at C-2 position
(all-carbon quaternary center) are obtained in good to excellent yields (36 examples). The
usefulness of this methodology has been shown by a total synthesis of Amaryllidaceae
alkaloid, (±)-crinane from 2-aryl cyclohexanone in 5 steps.
Available online
Keywords:
Deacylaive alkylations
enolcarbonates
2009 Elsevier Ltd. All rights reserved.
Pd(0)-catalyzed
Amaryllidaceae alkaloids
Introduction
We were particularly interested in the synthesis of 2-
Palladium(0)-catalyzed decarboxylative allylation (DcA) [the
Tsuji-Trost allylation]1,2 provides opportunity for the allylation
via elimination of CO2 from allyl -ketoesters or allylenol
carbonates.3 Since this reaction works under mild condition to
produce a number of functionalized intermediates,4 it finds huge
application in the synthesis of valuable pharmaceuticals and
natural products.5-6 However, the incorporation of allyl
electrophile and the nucleophile into the same reactant molecule
via an ester linkage6a-b or carbonate6c-d limits the utilization of
decarboxylative allylation (DcA) reaction. Therefore, there is
urgency in developing similar type C-C bond formation strategy
which can takes place intermolecularly to address synthetically
viable challenging substrates. Towards this direction, Tunge
and Grenning’s elegant palladium(0)-catalyzed three-
component deacylative allylation (DaA) of -nitroketone
arylcyclohexanones bearing an all-carbon quaternary center (see, 1f
in Figure 1), as these type of compounds are common in a wide
variety of pharmaceuticals,12 and natural products such as
Amaryllidaceae alkaloids (Figure 1).13 The members of
Amaryllidaceae alkaloids (1a-e, Figure 1) have long attracted the
attention of synthetic chemists not only due to their fascinating
architecture but also because of significant bioactivities such as
antitumor, antiviral, and anti-acetylcholinesterase activities.13a-c
Structurally, the alkaloids 1a-e are characterized by a 5,10b-
ethanophenathridine skeleton bearing an all-carbon quaternary
center. More than 500 Amaryllidaceae alkaloids have been isolated
from different Amaryllidaceae species till date.13d
7
derivatives (pKa ~ 17) opens up a unique avenue to access
unsymmetrically substituted 1,6-dienes under Pd(0)-catalyzed
9
three-component coupling conditions (Scheme 1).8,
The
reaction goes through in-situ formation of a Pd(II)--allyl
complex from an external allyl alcohol as a pro-electrophile via
a retro-Claisen activation of -nitro cyclohexanone derivative
(Scheme 1). This report also suggests that enolates with pKa ~
18-25,10a and nitrile stabilized anions with pKa ~ 2310b undergo
alkylations to afford
a variety of synthetically useful
compounds.11
Figure 1. Representative Amaryllidaceae alkaloids 1a-e and key
intermediates.
We envisioned that, the 5,10b-ethanophenathridine skeleton 1h
(Figure 1) of Amaryllidaceae alkaloids could be achieved from a
sequential reductive amination of -ketoaldehyde 1g with ammonium
acetate followed by a Pictet-Spengler reaction using formaldehyde.
Scheme 1. Synthesis of 1,6-diene via deacylative allylation.

2
Tetrahedron
Compound -ketoaldehyde of type 1g could be accessed from a
entr
y
Catalyst
base
solvent
temp
time
7aa,b
11aa,
one-pot oxidative cleavage of 2-aryl-2’-allyl cyclohexanone of type
1f (Figure 1). Encouraged by Tunge’s work on deacylative process,8,
b
11a-b
we became interested in developing methods that would allow
1
2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
KOtBu
KOtBu
NaH
NaH
NaH
NaH
NaH
KOtBu
KOtBu
NaH
NaH
NaH
NaH
NaH
NaH
KH
THF
PhMe
THF
THF
PhMe
PhH
55°C
55°C
55°C
25°C
25°C
25°C
55°C
55°C
55°C
55°C
55°C
55°C
55°C
55°C
25°C
25°C
55°C
3 h
4 h
4 h
6 h
7 h
7 h
4 h
4 h
6 h
4 h
4 h
2 h
3 h
3 h
7 h
7 h
5 h
49%
40%
63%
58%
40%
43%
40%
45%
48%
60%
67%
71%
77%
84%
81%
70%
92%
14%
20%
13%
19%
17%
17%
14%
20%
9%
rapid entry to 2-aryl 2’-allylcyclohexanones via an intermolecular
allylation of enolates (Scheme 2).
3
4
Results and Discussions
5

6
-Nitrocyclohexanone derivatives having an electron-withdrawing
group could undergo a retro-Claisen type activation in the presence
7c
THF
THF
PhMe
THF
THF
THF
THF
THF
THF
THF
THF
8c
of in-situ generated allyl alkoxide, thereby breaks cyclohexanone
ring in Tunge’s reports.8,
Therefore, we wanted to utilize
9
9c
enolcarbonates of 2-arylcyclohexanone derivatives for the synthesis
of 2-aryl-2’-allyl cyclohexanone. Our rationale of deacylative
allylations (DaA) of enolcarbonate 2 is shown in Scheme 2. We
argued that an allylic alkoxide may induce a deacylative process of
aryl substituted enolcarbonate 2 to form carbanion 4b (Scheme 2).
The carbanion 4b would then react with Pd(II)--allyl complex
generated in situ by reaction of allylacetate and Pd(0) to furnish
various 2-arylated cyclohexanones 7-9 with a C2-quaternary center.
Herein, we report Pd(0)-catalyzed deacylative allylation of
enolcarbonates via coupling of in situ generated nucleophiles with
allyl electrophiles (Scheme 2).
10d
9e
7%
12%
9%
10f
11g
12h
13
14
15
10%
---
8%
10%
---
NaH
areactions were carried out by using 0.20 mmol of 1a with 0.30 mmol of allyl
alcohol under argon atmosphere. bisolated yield after column purification.
d
e
f
c10 mol% PPh3 was used. 5 mol% L1 was used. 5 mol% L2 was used. 5
mol% L3 was used. g5 mol% L4 was used. h5 mol% L5 was used.
Scheme 2: Our hypothesis of deacylative allylations (DaA).
We selected enolcarbonate 2a and allyl alcohol as the pro-
nucleophile and pro-electrophile, respectively, for optimization
studies. The optimization reaction was performed using a number of
different ligands L1-L5, different bases such as potassium tert-
butoxide, sodium hydride, and potassium hydride, as well as
different solvents (Table 1). 2.5 mol% Pd2(dba)3 as catalyst (entries
1-6) afforded product in 63% yield in tetrahydrofuran (entry 3). A
number of phosphine based ligands such as PPh3 (10 mol%), and L1-
L5 (5 mol%) were used along with 2.5 mol% Pd(OAc)2 (entries 7-
12), which afforded a maximum of 84% yield (entry 12). Later, a
number of conditions were employed by using 2.5 mol% Pd(PPh3)4
as catalyst with different bases and solvents (see, SI for details).
Following exhaustive optimization, as shown in Table 1, it was
found that, 2.5 mol% Pd(PPh3)4 in combination with NaH (2
equivalents) in THF furnished product 7a in 92% yield in 5 h (entry
15, Table 1). Therefore, this condition was considered as standard for
evaluation of substrate scope.
With the standard condition in hand, a number of enolcarbonates of
2-arylcyclohexanone 2a-m were tested as pro-nucleophiles by
reaction with allyl alcohols as pro-electrophiles and the results are
shown in Scheme 3. Gratifyingly, we could synthesize a wide range
of 2-aryl 2’-allyl cyclohexanones 7a-g having electronically different
aromatic rings with a C2-quaternary center in good to excellent
yields (Scheme 3). Further, methallyl alcohol as pro-electrophile
afforded products 2-aryl 2’-methallyl cyclohexanones 7h-k in 85-
87% yields in 4-6 h (Scheme 3). Gratifyingly, phenallyl alcohol
could also be employed as pro-electrophiles to offer products 7l-m in
95-97% yields under the standard condition (Scheme 3).
Table 1: Optimization of deacylative allylation (DaA).a,b

3
areactions were carried out using 0.20 mmol (1 equivalent) of 2a-g with 0.30
mmol (1.5 equivalents) of allylalcohol and 0.40 mmol (2 equivalents) of NaH
in 3.0 mL THF in the presence of 2.5 mol% of Pd(PPh3)4 at 55 °C under
argon condition. bisolated yields after column purification.
areactions were carried out using 0.20 mmol (1 equivalent) of 2 with 0.30
mmol (1.5 equivalents) of allylalcohol 3 and 0.40 mmol (2 equivalents) of
NaH in 3.0 mL THF at 55 °C in the presence of 2.5 mol% of Pd(PPh3)4 under
argon condition. bisolated yields after column purification.
Scheme 3: Substrates scope of Pd(0)-catalyzed DaA using esters.a,b
Scheme 4: Substrates scope of Pd(0)-catalyzed DaA using esters.a,b
Further, highly regioselective nature of DaA reaction was shown by
employing reverse-prenyl alcohol as a proelectrophile (Scheme 5).
This reaction afforded linear product 8f as sole regioisomer in 92%
yield and no trace of branched product was isolated (Scheme 5).
Next, the regioselectivity issues were tested with 3-substituted and
3,3’-disubstituted allyl alcohols (Scheme 4). In these cases, one
would expect of mixture of regioisomeric product formation such as
linear and branch products. In this regard, a number of substituted
allylalcohols particularly 3-substituted allyl alcohols with E-
geometry, 3,3-dimethyl allyl alcohol (prenyl alcohol), and geraniol
were tested as pro-electrophiles under optimized conditions (Scheme
4). Gratifyingly, we found that the DaA reaction of all allyl alcohols
with E-geometry was highly regioselective in nature, where linear
products (such as 8a-h) were obtained as sole regioisomer in
excellent yields (up to 95% yield) and no trace of branched products
were observed (Scheme 4). Along similar line, 3,3-dimethyl allyl
alcohol (prenyl alcohol) and geraniol also afforded linear products 8f
and 8e, respectively, as a sole regioisomer (Scheme 4).
Scheme 5: Substrates scope of Pd(0)-catalyzed DaA using tertiary
alcohol.a,b
Next, a number of enolcarbonates with an aromatic ring containing
electron-withdrawing groups were tested to afford compounds 9a-i
in 80-96% yields in 3-5 hours (Scheme 6). Once again, a number of
substituted allylalcohols, such as crotyl alcohol, and cinnamyl
alcohol with E-geometry, as pro-electrophiles furnished only linear
products (9b-c, 9e-f, 9j, and 9m) under optimized conditions.
Further, 3,3-dimethyl allyl alcohol (prenyl alcohol) as pro-
electrophile afforded only linear products 9i and 9l (Scheme 6).
Interestingly, cyclohexane sharing an ethylester at C-2 position,
which is a challenging substrate due to the possibilities of sequential
allylations,13c also afforded product 9j in 68% yield. Gratifyingly,
our optimized condition well tolerated with 2-arylcyclopentanones
(9k-n) and 2-arylcycloheptanones (9o-p) based structural scaffolds
to afford products in 81-98% yields (Scheme 6).

4
Tetrahedron
Scheme 7: Intermolecular nature of Pd(0)-catalyzed DaA.
Later, with a successful studies of Pd(0)-catalyzed deacylative
reactions with enolcarbonates of 2-arylcyclohexanones, our effort
was thereafter to develop enantioselective version of this
methodology (Scheme 8). Towards this a variety of Pd(0)-species
were utilized in combination with a number of enantioenriched
ligands. Following an exhaustive optimization (See, Supporting
Information for detailed optimization studies), it was revealed that
product (-)-7f could be obtained in 77% ee (92% yield), when the
DaA of (±)-2f was carried out with NaH in MTBE (methyl tert-butyl
ether) at 10 C for 29 h in the presence of 2.5 mol% Pd2(dba)3 and
7.5 mol% of C2-symmetric phosphino-carboxamide ligand (S,S)-L
(Scheme 8).
areactions were carried out using 0.20 mmol (1 equivalent) of 2 with 0.30
mmol (1.5 equivalents) of allylalcohol 8 and 0.40 mmol (2 equivalents) of
NaH in 3.0 mL THF at 55 °C in the presence of 2.5 mol% of Pd(PPh3)4 under
argon condition. bisolated yields after column purification.
areactions were carried out on 0.15 mmol of substrates in 3 mL of MTBE
under an N2 atmosphere. byield of isolated product after purification by
column chromatography. cee’s were determined by chiralpak AS-H column.
Scheme 6: Substrates scope of Pd(0)-catalyzed DaA using esters.
Further, to study the competitive reactions of allylation and
methallylation, we carried out a control experiment with 2f in the
presence of methallylalcohol and allylacetate (Scheme 7). In this
regard, our observation of formation of 7f as major product as
compared to 7h (7f:7h = 1.8:1) might possibly indicate that the
sterics play crucial role in our DaA (Scheme 7). In this reaction
allylation afforded the major product 7f over methallylation product
7h. The methallylation goes through the in-situ preparation of
methallyl carbonate (Scheme 7). This was further confirmed by a
control experiment with 2f in the presence of allylalcohol and
methallylacetate, where 1.7:1 of product distribution was observed
for allylation (7f) over methallylation (7h). In the latter case, the
allylation goes through the in-situ preparation of allyl carbonate
(Scheme 7). These cross-over experiments also suggest that the allyl
acetate and allyl carbonate [or methallyl acetate and methallyl
carbonate] have similar type of reactivities under Pd(0)-catalyzed
DaA methodology.
Scheme 8: Development of enantioselective variant of DaA.a-c
Next, as per our hypothesis shown in Figure 1, we wanted to utilize
our methodology for total synthesis of Amaryllidaceae alkaloid,13c
(±)-crinane (1a) (Figure 1). Therefore, with 2-(3,4-methylenedioxy
phenyl)-2’-allyl cyclohexanone (7f) in hand, a concise total synthesis
(±)-crinane (1a) was undertaken (Scheme 9). Towards this, a one-pot
oxidative cleavage of terminal olefin of 7f with NaIO4 in the
presence of catalytic amount of OsO4 and stoichiometric amount
DABCO afforded -ketoaldehyde 10 in 92% yield. A sequential
reductive amination of -ketoaldehyde 10 with ammonium acetate
afforded C-3a-cis-octahydroindole core 11 as a single diastereomer
in 82% yield. Finally, 5,10b-ethanophenathridine skeleton of (±)-
crinane (1a) was installed by a reaction using Eschenmoser’s salt
(dimethylmethylideneammonium iodide) in tetrahydrofuran at 40 C
(86% yield).

5
Jiménez-Osés, G.; Wang, B.; Houk, K. N.; Stoltz, B. M. Org. Lett. 2015,
17, 1082.
6. For sequential enantioselective allylations in total synthesis of natural
products, see; (a) Enquist Jr., J. A.; Stoltz, B. M. Nature 2008, 453, 1228. (b)
Trost, B. M.; Osipov, M. Angew. Chem., Int. Ed. 2013, 52, 9176. (c) Ghosh,
S.; Bhunia, S.; De, S.; Kakde, B. N.; Bisai, A. Chem. Commun. 2014, 50,
2434.
7. Nitroalkane pKa’s: Bordwell, F. G.; Vanier, N. R.; Matthews, W. S.;
Hendrickson, J. B.; Skipper, P. L. J. Am. Chem. Soc. 1975, 97, 7160.
8. Grenning, A. J.; Tunge, J. A. Angew. Chem., Int. Ed. 2011, 50, 1688.
9. References pertaining to C-C bond cleavage by retro-Claisen reaction, see:
(a) Han, C.; Kim, E. H.; Colby, D. A. J. Am. Chem. Soc. 2011, 133, 5802. (b)
He, C.; Guo, S.; Huang, L.; Lei, A. J. Am. Chem. Soc. 2010, 132, 8273 and
references therein.
Scheme 9: Total synthesis of (±)-crinane (1a).
Conclusions
10. (a) Ketone pKa’s: Bordwell, F. G.; Harrelson, Jr. J. A. Can. J. Chem.
1990, 68, 1714. (b) Nitrile pKa’s: Bordwell, F. G.; Cheng, J.-P.; Bausch, M.
J.; Bares, J. E. J. Phys. Org. Chem. 1988, 1, 209.
In summary, we have developed an effective protocol involving
Pd(0)-catalyzed deacylative allylation (DaA) of enolcarbonates (pro-
nucleophile) from cyclohexanones with readily available allyl
11. (a) Grenning, A. J.; Tunge, J. A. J. Am. Chem. Soc. 2011, 133, 14785. (b)
Grenning, A. J.; Allen, C. K. V.; Maji, T.; Lang, S. B.; Tunge, J. A. J. Org.
Chem. 2013, 78, 7281. (c) For a deacylative process on 2-oxindoles, see;
Kumar, N.; Das, M. K.; Ghosh, S.; Bisai, A. Chem. Commun. 2017, 53, 2170.
alcohol as pro-electrophile.
A wide range of 2-aryl-2-allyl
cycloalkanones, with electronically different aromatic rings, were
synthesized in good to excellent yield (36 examples). A trial on the
development of catalytic asymmetric version of DaA methodology
afforded product ()-7f in 77% ee (92% yield). The present method
represents the first utilization of 2-arylcyclohexanones as
nucleophiles in deacylative alkylation process and their eventual
application in the total synthesis of complex Amaryllidaceae
alkaloid, (±)-crinane (1a) in few steps sharing 5,10b-
ethanophenathridine core.14 Further studies towards the utilization of
DaA is under active investigation in our laboratory.
12. Jin, Z. and Xu, X.-H. in Natural Products: Phytochemistry, Botany and
Metabolism of Alkaloids, Phenolics and Terpenes, ed. K. G. Ramawat and J.
M. M´erillon, Springer-Verlag, Berlin Heidelberg, 2013, p. 479.
13. (a) Das, M. K.; De, S.; Shubhashish, S.; Bisai, A. Org. Biomol. Chem.
2015, 13, 3585. (b) Das, M. K.; De, S.; Shubhashish, S.; Bisai, A. Synthesis
2016, 48, 2093. (c) Chandra, A.; Verma, P.; Negel, A.; Pandey, G. Eur. J.
Org. Chem. 2017, 6788. (d) Review: Jin, Z. Nat. Prod. Rep. 2009, 26, 363.
14. For asymmetric synthesis of crinane, see; (a) Kano, T.; Hayashi, Y.;
Maruoka, K. J. Am. Chem. Soc., 2013, 135, 7134. (b) Gao, Y.-R.; Wnag, D.-
Y.; Wang, Y-Q. Org. Lett. 2017, 19, 3516. (c) Das, M. K.; Kumar, N.; Bisai,
A. Org. Lett. 2018, 20, 4421. (d) Verma, P.; Chandra, A.; Pandey, G. J. Org.
Chem. 2018, 83, 9968.
Acknowledgments
Financial supports from the Science Engineering Research Board
(SERB), DST [CRG/2019/000113], Ministry of Earth Sciences
[MoES (09-DS/11/2018-PC-IV)], and Council of Scientific and
Industrial Research [CSIR (02(0295)/17/EMR-II)], Govt. of India,
are gratefully acknowledged. S.M. thanks the CSIR for Junior
Research Fellowships (JRF). We sincerely thank the Department of
Chemistry and Central Instrumental Facility (CIF), IISER Bhopal for
research facilities.
Supplementary Material
Supplementary material for this work is available via online at:
References and notes
1. (a) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461. (b)
Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (c) Denmark, S. E.;
Regens, C. S. Acc. Chem. Res. 2008, 41, 1486.
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Tsuji, J.; Minami, I.; Shimizu, I. Tetrahedron Lett. 1983, 24, 1793.
3. For numerous examples, see; (a) Trost, B. M.; Crawley, M. L. Chem. Rev.
2003, 103, 2921. (b) Weaver, J. D.; Recio III, A.; Grenning, A. J.; Tunge, J.
A. Chem. Rev. 2011, 111, 1846.
4. (a) Mohr, J. T.; Behenna, D. C.; Harned, A. W.; Stoltz, B. M. Angew.
Chem., Int. Ed. 2005, 44, 6924. (b) Burger, E. C.; Tunge, J. A. J. Am. Chem.
Soc. 2006, 128, 10002. (c) Recio III, A.; Tunge, J. A. Org. Lett. 2009, 11,
5630. (d) Grenning, A. J.; Tunge, J. A. Org. Lett. 2010, 12, 740.
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Lotz, M. D.; Martin, S. F. Org. Lett. 2010, 12, 1576. (c) Numajiri, Y.;

6
Tetrahedron
Highlights
1. Pd(0)-catalyzed DaA of enolcarbonates (pro-
nucleophile) has been developed.
2. A variety of allyl alcohols (pro-electrophile)
afforded products under mild condition.
3. The allylation is highly regioselective, preferring
a linear product over branched product.
4. Total synthesis of Amaryllidaceae alkaloid, (±)-
crinane has been accomplished.