Rhodium-Catalyzed Asymmetric Cycloisomerization of 1,3-Diketones with Keto-Vinylidenecyclopropanes: Synthesis of Enantiomerically Enriched Cyclic β-Amino Alcohols

Article abstract of DOI:10.1002/adsc.202001167

We report here an effective and atom-economical method to synthesize enantiomerically enriched cyclic β-amino alcohols via a rhodium-catalyzed asymmetric cycloisomerization of 1,3-diketones with keto-vinylidenecyclopropanes. The reactions proceed through a Rh-catalyzed transformation of keto-vinylidenecyclopropanes via cleavage of the distal C?C bond of the three-membered ring as a three-carbon synthon, allowing the generation of a range of enantiomerically enriched cyclic β-amino alcohols tethered to an alkene and an 1,3-dione moiety in good yields with high ee values under mild conditions. Derivatizations including allylation of the functionalized β-amino alcohols and subsequent RCM reaction as well as the preparation of a pyrazole derivative were carried out as well. (Figure presented.).

Full text of DOI:10.1002/adsc.202001167

UPDATES
DOI: 10.1002/adsc.202001167
Rhodium-Catalyzed Asymmetric Cycloisomerization of 1,3-
Diketones with Keto-Vinylidenecyclopropanes: Synthesis of
Enantiomerically Enriched Cyclic β-Amino Alcohols
Chen Xu,+a Chao Ning,+a Song Yang,a Yin Wei,b and Min Shia, b,*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East
China University of Science and Technology, 130 Mei Long Road, Shanghai 200237, People’s Republic of China
Fax: (+86)-21-6416-6128;
E-mail: mshi@mail.sioc.ac.cn
b
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, People’s Republic of China
+
These authors contributed equally to this work
Manuscript received: September 23, 2020; Revised manuscript received: January 14, 2021;
Version of record online: Februar 9, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001167
pyrrolidinoindoline-type alkaloid.[4] In addition, Mar-
Abstract: We report here an effective and atom-
emycin B, with high probability to be introduced as a
fragment of the cysteine structure used in biosynthesis,
economical method to synthesize enantiomerically
enriched cyclic β-amino alcohols via a rhodium-
catalyzed asymmetric cycloisomerization of 1,3-
diketones with keto-vinylidenecyclopropanes. The
was first isolated from a marine streptomyces
species.[5] Besides, hyacinthacine alkaloids are a
relatively recent addition to the group of polyhydroxy-
reactions proceed through a Rh-catalyzed trans-
lated 3-(hydroxymethyl)pyrrolizidine natural products,
formation of keto-vinylidenecyclopropanes via
in which over nineteen hyacinthacine alkaloids have
cleavage of the distal CÀ C bond of the three-
membered ring as a three-carbon synthon, allowing
the generation of a range of enantiomerically
enriched cyclic β-amino alcohols tethered to an
alkene and an 1,3-dione moiety in good yields with
high ee values under mild conditions. Derivatiza-
tions including allylation of the functionalized β-
amino alcohols and subsequent RCM reaction as
well as the preparation of a pyrazole derivative were
carried out as well.
been isolated thus far.[6] The class of enantiomerically
pure compounds are often prepared by direct reduction
of β-amino acids. Thus, the traditional methods for the
preparation of β-amino alcohols mostly required
enantiopure substrates or reagents as chiral pool or
involved with stepwise catalytic asymmetric synthetic
processes.[7] However, these synthetic strategies often
suffered from either structurally limited substrates/
products
or
relatively
low
regio-
and
stereoselectivities.[8] Therefore, nowadays, the develop-
ment of direct asymmetric synthesis of cyclic β-amino
alcohols through novel domino or cascade reaction
process in good yields with high stereoselectivities is
highly desirable. On the basis of such circumstance,
we wish to explore a new synthetic protocol for the
rapid construction of enantiomerically enriched cyclic
β-amino alcohols from simple starting materials by an
Keywords: β-amino alcohols; trimethylenemethane-
rhodium (TMM-Rh) complex; vinylidenecyclopro-
panes (VDCPs); diketone; CÀ H alkylation;
In bioactive natural products, β-amino alcohols and atom-economical process.
cyclic β-amino alcohols as well as their derivatives are
Asymmetric allylic substitution is a primary strat-
vital structural components,[1] and have widespread egy for stereoselective construction of carbon-heter-
applications in the medicinal chemistry and pharma- oatom and carbon-carbon bond. In this context, allenes
ceutical industry.[2] Figure 1 presents several bioactive could be transformed into electrophilic metal-π-allyl
molecules and medical substances containing cyclic β- intermediates by iridium, rhodium, ruthenium and
amino alcohol structures. For example, Febrifugine is other transition metal catalysts to undergo nucleophilic
well-known for its antimalarial effect.[3] CPC-1 is a attack.[9] For example, chiral phosphine-coordinated
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the three-membered ring and undergo a structural
rearrangement to give a new type of trimeth-
ylenemethane-rhodium (TMM-Rh) complex (Sche-
me 1B-a).[12a] In our previous work, we have demon-
strated that terminal alkyne could react with cationic
Ag species to provide alkynyl-Ag intermediate, which
rapidly underwent trans-metalation to TMM-Rh com-
plex. After reductive elimination, the reaction even-
tually delivered the functionalized cyclic β-amino
alcohols in high ee values (when X=N, Scheme 1B-
b).[12a] Furthermore, we also succeeded in using a
variety of secondary amines as “soft” nucleophiles
(those from conjugate acids with a pKa less than 25) to
achieve direct allylic substitution, affording the corre-
sponding functionalized cyclic β-amino alcohols in
good yields along with high ee values (Scheme 1B-
c).[12b] Encouraged by these findings, we envisaged that
if using 1,3-diketones as nucleophiles in this
Figure 1. Representative bioactive molecules containing cyclic
β-amino alcohol moiety.
ruthenium(II) complex bearing carbonyl ligands could transformation,[13] a new type of functionalized cyclic
be used to catalyze the reactions of allyl amides with β-amino alcohols bearing a diketone moiety could be
alcohols or aldehydes, giving β-amino alcohols in high generated (Scheme 1C, this work).
ee values (Scheme 1A).[10]
To test the feasibility of our hypothesis, we
Vinylidenecyclopropanes (VDCPs), bearing an al- surveyed the reaction of keto-VDCP 1a as our initial
lene group linked to a highly strained cyclopropane substrate with dibenzoyl methane 2a using in situ
ring, act as an essential element in organic synthesis generated cationic Rh(I) complex as catalyst.[12] After
and have continuously fascinated organic chemists for preliminary screening, the combination of [Rh(cod)
decades.[11] Our processing study on metal-catalyzed Cl]2 and NaBArF was found to be an outstanding
4
conversions of VDCPs, indicates that cationic Rh(I) catalyst system, affording the desired product rac-3a
°
complexes could insert into the weaker distal bond of in 96% yield at 90 C in toluene (Table 1, entries 1–4).
When commercially available [Rh(cod)2]BF4 (pur-
chased from Adamas) was used as the catalyst, product
3a was furnished in 74% total yield after stirring the
°
°
reaction mixture at 90 C for 24 hours and at 100 C for
another 24 hours (Table 1, entry 5). Changing the
solvent from toluene to chlorobenzene, 1,2-dichloro-
ethane, 1,4-dioxane or acetonitrile, we found that the
reaction proceeded very well in chlorobenzene to give
3a in 93% yield; in 1,2-dichloroethane or 1,4-dioxane,
3a was given in 40% or 50% yield, respectively and in
acetonitrile, only trace of 3a was afforded, indicating a
drastic solvent effect in this reaction (Table 1, en-
tries 6–9). Next, we inspected asymmetric variant of
this reaction using a bunch of chiral binaphthyl bis-
diarylphosphine ligands L2–L5 under the temporally
optimized conditions. To our delight, when (R)-
XylBINAP (L5) was employed as a chiral ligand, the
desired product 3a was furnished in 97% yield with
91% ee value (Table 1, entries 10–13). To further
improve the enantiomeric excess of 3a, we decided to
reexamine the reaction conditions with catalyst combi-
nation of [Rh(cod)Cl]2 and AgNTf2 and the results are
shown in entries 14–18 of Table 1 in a variety of
solvents such as 1,2-dichloroethane, 1,4-dioxane,
acetonitrile or chlorobenzene. We found that perform-
°
ing the reaction in chlorobenzene at 90 C afforded 3a
in 60% yield; the yield of 3a could be improved to
Scheme 1. Previous work and this work.
°
91% when the reaction was carried out at 100 C
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Table 1. Optimization of Reaction Conditions.
Table 2. Substrate Scope of Keto-VDCPs 1.
(Table 1, entries 17–18). Gratifyingly, using (R)-TolBI-
NAP (L3) as a chiral ligand with this catalytic system,
3a could be obtained in 86% yield along with 96% ee Moreover, regardless of whether methyl group or a
value (Table 1, entry 19). The use of L2, L4 and L5 as fluorine atom was introduced into the benzene ring of
the chiral ligands in this reaction provided 3a in 70– R2 group in keto-VDCP 1, the reactions delivered the
93% yields but along with 92%–93% ee values desired products 3j and 3k in 95% yield along with
(Table 1, entries 20–22). The catalyst combination of 98% ee and 74% along with 86% ee. The further
[Rh(cod)Cl]2 and NaBArF afforded 3a in 86% yield screening of R1 group revealed that methyl group,
along with 92% ee value under this condition (Table 1, substituted aromatic group and heteroaromatic group
entry 23).
were all compatible, affording the desired products 3l–
With the optimal conditions in hand, the generality 3o in 89–95% yields along with 95%–>99% ee
of this catalytic asymmetric synthesis of six-membered values. However, changing the linker as an oxygen
β-amino alcohol was examined. First, various linear atom or a carbon atom such as keto-VDCPs 1p and
alkyl substituents including long-chain alkane or long- 1q, none of corresponding products 3p and 3q could
chain alkane with a terminal olefin, an aldehyde be obtained under the standard conditions, perhaps due
protected group and benzyl groups were introduced as to steric or electronic effects.
R2 into keto-VDCP 1, affording the desired products
The scope of 1,3-bisaryl-1,3-propanedione was also
3a–3f in good yields ranging from 75% to 95% along examined with a variety of para or meta-substituted
with 98% to >99% ee values (Table 2). The branched aromatic rings, affording the desired products 3r–3w
alkyl substituents such as isopropyl group and in good yields ranging from 80% to 95% along with
cyclohexyl group were tolerated, giving the corre- 94% to >99% ee values (Table 3). Unexpectedly,
sponding products 3g and 3h in 96% yield with 87% when acetylacetone was used as the nucleophile, the
ee and 91% yield with 90% ee, respectively. R2 group desired product 3v was obtained as enol and diketone
could be also a phenyl group, furnishing the desired isomeric mixture in 86% total yield, in which the enol
product 3i in 81% yield along with 91% ee value. product was the major one on the basis of NMR
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Table 3. Substrate Scope of 1,3-Diketones.
Table 4. Non-reactive Nucleophiles 2.
spectroscopic data along with 96% ee value. Next, we
further examined the reaction of 1a with asymmetrical
1-phenylbutane-1,3-dione and found that the corre-
sponding product mixture of 3w was afforded in 85%
total yield along with 4:1 ratio of diketone and enol, in
which the diketone was produced as a 1.5:1 diastereo-
meric mixture along with 95% and 88% ee values,
respectively. However, when cyclic diketones or
substituted dibenzoyl methanes as well as ethyl 3-
oxobutanoate or dimethyl malonate were used as the
nucleophiles, none of the desired products could be
formed under the standard conditions presumably due
to the steric effects (Table 4). For the detailed
examinations, see Table S1 in the Supporting Informa-
tion.
Scheme 2. Gram Scale-Up Experiment.
Furthermore, this catalytic asymmetric synthesis of
functionalized six-membered β-amino alcohol could be
conducted at 2.0 mmol scale, affording the desired
product 3a in 86% yield along with 90% ee value
(Scheme 2).
Considering that product 3 has two acidic hydrogen
atoms, including the remaining α-H of diketone and
the OÀ H of hydroxy group of β-amino alcohol, we
performed the further allylation reactions under two
different conditions (Scheme 3). Using 3.0 equivalents
of sodium hydride as a base, single allylated product
°
4a was obtained in 60% yield with 71% ee at 0 C in
DMF based on NMR spectroscopic data. On the other
hand, when using K2CO3 (3.0 eq) as a base, the
carbonyl α-H of diketone was allylated, giving 4b in
82% yield with 98% ee. This may be because the
hydroxy group in 3a can be more easily deprotonated
by a stronger base such as NaH in DMF at lower
temperature and the acidic proton between two
Scheme 3. Derivatizations of Product 3a.
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carbonyl groups can be selectively deprotonated by a pane in VDCP to give a TMM-Rh complex, an
weaker inorganic base K2CO3 at higher temperature. intramolecular ketone carbometallation and the nucleo-
The obtained product 4a could be further used for the philic attack of 1,3-diketone, yielding the correspond-
RCM reaction using Zhan-1B as the catalyst, affording ing functionalized enantiomerically enriched cyclic β-
the corresponding intramolecular RCM product 5a in amino alcohols. Further investigations to examine the
42% yield with 99% ee (Scheme 3). In addition, we mechanistic details more extensively and exploration
also performed the reaction of 3a with phenylhydra- of new methodology based on this novel TMM-metal
zine upon heating in mixing solvent of methanol and complex generated from functionalized VDCPs are
acetic acid, delivering the product 6a with pyrazole currently underway in our laboratory.
structure in 87% yield with 94% ee value
(Scheme 3).[14]
Experimental Section
On the basis of above results and the previous
reports,[10] a plausible mechanism was proposed for General Procedure for Synthesis of 3
this domino reaction procedure using 1a and 2a as
model substrates. As shown in Scheme 4, the catalytic
cycle started from the insertion of Rh(I) complex into
To a 20 mL flame-dried tube was charged with keto-VDCPs 1
(0.2 mmol, 1.0 equiv.), 1,3-diones 2 (0.24 mmol, 1.2 equiv.),
[Rh(cod)Cl]2 (5 mol%, 0.05 equiv.), AgNTf2 (10 mol%,
0.10 equiv.), and (R)-TolBINAP (10 mol%, 0.10 equiv.). The
the weaker distal CÀ C bond of VDCP 1a via an
oxidative addition and the subsequent isomerization to
reaction tube was evacuated and backfilled with argon (repeated
give
a TMM-Rh complex A (TMM: trimeth-
three times). Then, super dry chlorobenzene (2.0 mL) was
°
ylenemethane), which underwent an intramolecular added into the tube. The reaction mixture was stirred at 100 C
for 10 h. After the reaction was complete, the mixture was
filtered through absorbent cotton, and solvent was removed
under reduced pressure. The residue was purified by a flash
column chromatography (SiO2) to give the corresponding
product 3.
ketone carbometallation to generate an electrophilic
Rh-π-allyl intermediate B. On the other hand, an
enolate could be formed from 2a via the hydrogen
À
abstraction with NTf2 , which nucleophilically at-
tacked intermediate B to give intermediate C. The
subsequent protonolysis would give the desired prod-
uct 3a and regenerated the Rh(I) cationic catalyst.
In summary, we have developed a novel domino
Supporting Information Available
reaction process of rhodium-catalyzed asymmetric Detailed descriptions of experimental procedures and
keto-VDCPs’ cycloisomerization with 1,3-ketones as their spectroscopic data are presented in the Supporting
nucleophiles, affording cyclic β-amino alcohols in Information.
good yields with good to excellent ee values in an
atom economic way. The reaction proceeded through a
rhodium(I) catalyzed cycloisomerization of cyclopro-
Acknowledgements
We are grateful for the financial support from the Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant No. XDB20000000) and sioczz201808, the National
Natural Science Foundation of China (21372250, 21121062,
21302203, 20732008, 21772037, 21772226, 21861132014 and
91956115).
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