One-step synthesis of strained bicyclic carboxylic and boronic amino esters via ruthenium-catalysed tandem carbene addition/cyclopropanation of enynes

Article abstract of DOI:10.1002/adsc.201200117

The reaction of 1,6- and 1,7-enynes, derived from carboxylic and boronic amino acids, with diazo compounds in the presence of the (cyclooctadiene) (pentamethylcyclopentadiene)ruthenium chloride complex [RuCl(cod)(C ₅Me₅)] catalyst le

Full text of DOI:10.1002/adsc.201200117

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DOI: 10.1002/adsc.201200117
One-Step Synthesis of Strained Bicyclic Carboxylic and Boronic
Amino Esters via Ruthenium-Catalysed Tandem Carbene
Addition/Cyclopropanation of Enynes
Chloꢀ Vovard-Le Bray,a Hubert Klein,a Pierre H. Dixneuf,a Aurꢀlie Macꢀ,a
Fabienne Berrꢀe,a Bertrand Carboni,a and Sylvie Dꢀriena,*
a
UMR 6226 – Institut des Sciences Chimiques de Rennes, CNRS – Universitꢀ de Rennes 1, Campus de Beaulieu, 35042
Rennes, France
Fax : (+33)-2-2323-6939; e-mail: sylvie.derien@univ-rennes1.fr
Received: February 13, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200117.
Abstract: The reaction of 1,6- and 1,7-enynes, de-
rived from carboxylic and boronic amino acids,
with diazo compounds in the presence of the (cyclo-
based on the variation of the ring size and on the
scaffold decoration.[3] In parallel, a boronic acid can
also profitably replace the carboxylic function and
boron-containing compounds were found to be attrac-
tive drug candidates against various categories of dis-
eases,[4] especially boropeptides that constitute prom-
ising targets for medicinal chemists.[5]
Recently, non-conjugated enynes have been shown
to be key starting substrates for the metal-catalysed
creation of cyclic complex architectures.[6] The catalyt-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
to the formation of strained bicyclic proline or ho-
moproline derivatives in good yields. This catalytic
transformation proceeds under mild conditions, in
one step from easily accessible enynes and was ap-
plied to various protecting groups. High stereoselec-
tivities for the created alkenyl chain and excellent
diastereoselectivities for proline derivatives were
obtained.
ic activity of the complex RuClACTHNUTRGENN(UG C5Me5)ACHTUNGTRENN(UGN cod) has been
exploited to produce ruthenium-carbene species, in
situ generated from diazo compounds.[7] Their addi-
tion to enynes led to bicyclic derivatives after a cata-
lytic cascade rearrangement.[8]
Keywords: bicyclic amino acids; boronic amino
acids; diazoalkanes; enynes; ruthenium
The mild conditions of this ruthenium-catalysed re-
action, a priori compatible with classical protecting
groups of amino acid functionalities, prompted us to
explore the access to novel bicyclic constrained car-
boxylic and boronic amino esters. Following our first
The incorporation of non-proteinogenic cyclic a- positive results obtained in the synthesis of fluorinat-
amino acids into key positions of peptide chains plays ed derivatives,[9] we explored the reactivity of differ-
a major role in drug discovery processes. It constitutes ent 1,6- and 1,7-enynes containing a-amino carboxylic
one of the most effective ways to confer higher bio- ester and a-aminoboronate groups. We report here
logical activity, as well as increasing biostability. Fur- the synthesis of new strained bicyclic pipecolic acid
thermore, the introduction of conformational con- and proline derivatives under mild conditions
straints that affect secondary and tertiary peptide via a tandem carbene addition/cyclopropanation in
structures may also provide useful information on the presence of trimethylsilyldiazomethane and
structural requirements for bioactivity.[1] For example, RuCl
tyrosine has been successfully replaced by a 1,2,3,4-
tetrahydroisoquinoline-3-carboxylic acid or
4(S)-amino-1,3,4,5-tetrahydro-3-oxo-2H-benzazepine-
ACHTUGTNRENNUG(C5Me5)ACHTUNGTERN(NUGN cod) catalyst (Scheme 1).
The synthesis of 1,7-enynes 1a–d was first carried
out according to reported procedures from amino
esters, allyl and propargyl halides.[10] These enynes re-
a
2-acetic acid to develop highly selective substrates acted with 1.2 equiv. of trimethylsilyldiazomethane in
suitable for the specific monitoring of spleen tyrosine diethyl ether in the presence of 5 mol% of the preca-
kinase (Syk) activity.[2] In recent years, many synthetic talyst RuCl
N
ACHTUNGTNER(NUNG cod) affording after 5 h at room
AHCTUNGTRENNUNG
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amino esters 2a, b can be separated using silica gel
chromatography. NOESY NMR experiments showed
a relative cis configuration of the vinyl and ester
groups for the major isomer of 2a, b, while the re-
verse stereochemistry was observed for the major
isomer of 2c.
We then examined the synthesis of amino acid de-
rivatives containing a strained bicycloACTHUNRTGNEUNG[3.1.0]hexane
structure. The preparation of the starting 1,6-enynes
3a–d and 3f was first achieved via a Petasis three-
component coupling reaction of an alkenylboronic
acid, glyoxylic acid and propargylic amine
Scheme 1. Synthesis of bicyclic amino ester derivatives.
were isolated in good yields when R3 =H, whether (Scheme 2).[11] Enyne 3e was synthesized by treatment
with a Ts or a Cbz protecting group (entries 1-3), of 3a with benzyl chloroformate in the presence of
whereas the presence of a phenyl group at the ex- sodium bicarbonate. The enantiomerically pure deriv-
tremity of the double bond had a detrimental effect ative 3f was isolated by recrystallization from a 4/
on the cyclization process (entry 4). For compounds 1 diastereomer mixture when (R)-2-phenyl-N-propar-
derived from glycine (R2 =H, 2a, b), the newly creat- gylglycinol was used.[12,13] The cis configuration of the
ed double bond was formed in a Z/E mixture with an substituents on the morpholinone core has been es-
80/20 ratio when R1 =Ts (entry 1), but with a higher tablished by X-ray crystallographic analysis.[14]
ratio of 95/5 in the case of a Cbz group (entry 2). The
The conditions of Table 1 were used to perform the
introduction of a methyl substituent at the a-position cyclization of 1,6-enynes 3a–e (Table 2). Enyne 3a
of the ester group led to a total Z stereoselectivity with a non-protected nitrogen atom was first evaluat-
(entry 3). Thus, the reaction of alanine-derived enynes ed to determine if the protection of the amino group
led specifically to bicyclic products with only a Z con- was essential. No reaction occurred and the starting
figuration for the double bond.
material was recovered (entry 1). No improvement
These reactions proceeded with low diastereoselec- was observed with a spirocyclohexyl substituent a to
tivity (10 to 28%). Each diastereomer of bicyclic the triple bond (entry 2). On the other hand, N-pro-
Table 1. Reaction of 1,7-enynes 1a–d with trimethylsilydiazomethane.
Entry
1
Enyne 1
Product 2
Yield [%][a]
68
Z/E
cis/trans[b]
80/20
60/40
2
3
60
63
95/5
55/45
36/64
100/0
4
25
nd[c,d]
nd
[a]
1.2 equiv. of N2CHSiMe3 (2M in diethyl ether), in diethyl ether, room temperature, 5 mol% catalyst. Isolated product
yields obtained after purification by silica gel chromatography.
The cis/trans ratios were determined by H NMR. The cis isomer corresponds to the relative cis configuration of the vinyl
[b]
1
and ester groups.
nd: not determined.
[c]
[d]
1
Four diastereomers (four trimethylsilyl signals were detected by H NMR), with a Z configuration of the double bond for
the major isomer.
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One-Step Synthesis of Strained Bicyclic Carboxylic and Boronic Amino Esters
Scheme 2. Synthesis of 1,6-enynes 3a–f.
Table 2. Reaction of 1,6-enynes 3c–e with trimethylsilydiazomethane.
Entry
1
Enyne 3
Product 4
Yield [%][a]
–
dr[b]
nr[c]
–
2
nr[c]
–
–
3
4
60
95/5
52
77
100/0
100/0
5
[a]
1.2 equiv. of N2CHSiMe3 (2M in diethyl ether), in diethyl ether, room temperature, 5 mol% catalyst. Isolated product
yields obtained after purification by silica gel chromatography.
The cis/trans ratios were determined by H NMR. The cis isomer corresponds to the relative cis configuration of the vinyl
and ester groups.
No reaction.
[b]
[c]
1
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Scheme 3. Synthesis of tricyclic compound 4f.
tected enynes 3c–e led to constrained bicyclic proline
derivatives 4c–e in reasonable to good yields (en-
tries 3–5). Even though the presence of a Cbz protect-
ing group gave the best yield, it is noteworthy that
the use of enynes containing a trisubstituted basic
amino group allowed the reaction to take place.
Scheme 5. Reaction of 1,6-enynes 6 and 7 with trimethylsily-
diazomethane.
thesised from commercially available l-phenylalanine,
Apparently the presence of a substituent on the E following classical literature procedures.[17]
double bond of the 1,6-enyne moiety induced the se-
The Ru-catalysed reaction of 6 with trimethylsilyl-
lective formation of the Z configuration for the diazomethane led to the bicyclo
AHCTUNGTRENNUNG
carbon-carbon double bond of the vinylsilane moiety. tive 8 with an exocyclic amino ester function in 78%
Furthermore, in the case of the formation of com- yield (Scheme 5). We can notice that, if the Z stereo-
pounds 4c–e, one diastereomer was predominantly isomer was predominantly formed, there was no con-
obtained (ratio: 95/5 to 100/0). NOESY NMR experi- trol of the diastereomeric ratio. Under the same ex-
ments for 4c showed cis relationship between the perimental conditions, the yield of 9 was notably
ester, the vinyl and the phenyl groups for the major lower (25%) with a close stereochemical course. The
isomer. The same assignments were proposed for 4d addition of 20 mol% of Ti
and 4e according to NMR data.
effect,[17] while the yield of 9 can be increased to 53%
As a first approach to the enantiomerically en- with a ruthenium catalyst loading up to 10 mol%.
riched version of this tandem cyclization process, we It was previously shown that boronated enynes can
ACHTUNGTERN(NUNG O-i-Pr)4 has no significant
then used the derivative 3f as starting 1,6-enyne. be engaged successfully in various metal-catalysed cy-
NMR data showed that the same major diastereomer cloisomerisation reactions allowing the construction
for 4f was synthesized, but with a lower ratio (70/30) of a range of stereodefined skeletally diverse mole-
(Scheme 3).
cules.[12,18–20] The introduction of a pinacol boronic
To improve the panel of bicyclic amino acids avail- ester group in place of the ester function of enyne 1a
able via this ruthenium-catalysed tandem sequence, was obtained via the synthesis of enyne 10.[18a] In the
the preparation of bicyclic compounds with an exocy- presence of diazoalkane the Ru-catalysed reaction of
clic amino acid function was attempted. This kind of 10 successfully led to boron analogue 11 with a good
amino acid is more amenable for further functionali- yield (Scheme 6). The Z/E ratio and the cis/trans ratio
sation to obtain new functionalised bicyclic com- of the bicyclic compound are identical to those ob-
pounds. Two enynes 6 and 7 were synthesised accord- tained with the corresponding carboxylic amino ester
ing to Scheme 4. The diester 5[15] was converted into under the same catalytic conditions. Similarly, bicyclic
its acyl azide by reaction with thionyl chloride and boroproline derivative 13 was synthesised from borat-
sodium azide. The last step was a Curtius rearrange- ed 1,6-enyne 12,[19] with a good yield as a major dia-
ment trapping the isocyanate intermediate with stereomer (95/5). Besides a potential interest in me-
benzyl alcohol to give enyne 6.[16] Enyne 7 was syn- dicinal chemistry, these compounds should be also of
Scheme 4. Synthesis of 1,6-enynes 6 and 7.
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One-Step Synthesis of Strained Bicyclic Carboxylic and Boronic Amino Esters
(Scheme 7).[8] Classical reaction steps[7–9] lead to the
metallacyclobutane intermediate C and DFT calcula-
tions have shown that the (C5R5RuCl) moiety favours
the formation of a cyclopropanation product.[8] As ex-
pected, the stereochemistry of the newly created
double bond resulted from the opening of the metal-
lacyclobutene A. A Z configuration was mainly ob-
served due to a strong interaction between the SiMe3
and Cl groups. However, this stereoselectivity also de-
pends on the allylic or homoallylic arm substitution in
the starting enyne. A possible coordination of this
double bond on the ruthenium centre in intermediate
A can disturb the Cl····SiMe3 interaction and partially
leads to E stereochemistry (compounds 2a, 2b, 8, 9,
11). The presence of a substituent on the double bond
or at the a-position of the ester group (when R=
CO2Me) avoided this coordination and led to a total
Z stereoselectivity. Two diastereomers are generally
Scheme 6. Reaction of 1,6-enynes 10 and 12 with trimethyl-
silydiazomethane.
great significance for the synthesis of various 2-substi- formed. A cis configuration of the vinyl and ester
tuted constrained bicyclic proline and pipecolic acid groups for the major isomer is observed when R2 =H
analogues via Suzuki–Miyaura couplings.[21]
but the best ratios are obtained for the synthesis of
These results are in agreement with the mechanism proline derivatives. This diastereoselectivity results
already suggested for the cyclisation of simple enynes from the cyclization of the intermediate B and strong-
Scheme 7. Proposed catalytic cycle.
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ly depends on the size of the created ring. More im-
portant geometrical constraints for the 5-membered
ring could explain this best diastereoselectivity.
In conclusion, we have opened a route for the syn-
thesis of novel highly strained bicyclic proline and pi-
pecolic acid derivatives via a stereoselective RuCl-
ACHTUNGTRENNUNG(C5Me5)ACHTUNGTRENNUNG(cod)-catalysed tandem carbene addition/cy-
clopropanation sequence. This one-pot catalytic pro-
cess leads to carboxylic and boronic amino acid deriv-
atives from easily accessible enynes in good yields
and under mild conditions. The cyclization reaction
was found to be tolerant with various N-protecting
groups, while the presence of hydrogen on the nitro-
gen atom prevents any cyclization. High Z stereose-
lectivities for the newly created alkenyl chain and ex-
cellent diastereoselectivities for proline derivatives
are observed. Further studies aimed at the synthesis
of enantiomerically enriched constrained a-amino
acids using this strategy are in progress, as is their in-
corporation into peptidomimetics.
Experimental Section
Typical Procedure for Catalytic Carbene Addition/
Cyclopropanation of Enynes
In a Schlenk tube under an inert atmosphere, to a solution
of the enyne (1 mmol) in degassed diethyl ether were added
1.2 mmol of a 2.0M (trimethylsilyl)diazomethane solution in
diethyl ether. 5 mol% of the precatalyst RuClACTHNUTRGENN(UG C5Me5)ACHTUNTGREN(NUGN cod)
were then introduced. The mixture was stirred at room tem-
perature for 3–5 h. Reaction completion was monitored
using GC or TLC techniques. The solvent was removed
under vacuum and diastereomers were separated as pure
compounds using standard chromatography over silica gel
with a diethyl ether/pentane eluting mixture.
All procedures and characterisation data are presented in
the Supporting Information.
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Acknowledgements
The authors are grateful to the CNRS and the Ministꢀre de la
recherche for support, the latter for a PhD grant to CV-LB
and HK. They thank Dr T. Roisnel, Centre de Diffractomꢁ-
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CNRS-Universitꢁ de Rennes 1, for performing the crystallo-
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One-Step Synthesis of Strained Bicyclic Carboxylic and Boronic Amino Esters
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8 One-Step Synthesis of Strained Bicyclic Carboxylic and
Boronic Amino Esters via Ruthenium-Catalysed Tandem
Carbene Addition/Cyclopropanation of Enynes
Adv. Synth. Catal. 2012, 354, 1 – 8
Chloꢀ Vovard-Le Bray, Hubert Klein, Pierre H. Dixneuf,
Aurꢀlie Macꢀ, Fabienne Berrꢀe, Bertrand Carboni,
Sylvie Dꢀrien*
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