An efficient route for the synthesis of chiral conduritol-derivative carboxamides via palladium-catalyzed aminocarbonylation of bromocyclohexenetetraols Rui M.B. Carrilho a , Viviana Heguaburu b , Valeria Schapiro b , Enrique Pandolfi b ,L aszl o Koll ar c , Mariette M. Pereira a, * a Departamento de Química, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal b Departamento de Química Organica, Facultad de Química, UdelaR, C. P. 11800, Montevideo, Uruguay c Department of Inorganic Chemistry, University of Pecs, Ifj usag u. 6, H-7624 Pecs, Hungary article info Article history: Received 12 March 2012 Received in revised form 28 May 2012 Accepted 30 May 2012 Available online 8 June 2012 Keywords: Glycomimic Vinyl-bromides Conduritol Carboxamide Palladium Carbon monoxide Aminocarbonylation abstract A family of chiral conduritol-derivative carboxamides was synthesized through palladium-catalyzed aminocarbonylation of diastereoisomeric bromocyclohexenetetraols, previously prepared through bio- transformation of bromobenzene by mutant strains of Pseudomonas putida F39/D. The coupling reactions of bromocyclohexenetetraols with CO and different amines, such as tert-butyl- amine, aniline, and piperidine, were performed in the presence of in situ generated Pd(0)/PPh 3 catalyst. The methodology was applied to the corresponding iodo-cyclohexenetetraol derivative, using (L)-alanine and (L)-valine methyl ethers as N-nucleophiles. The resulting carboxamides were obtained in highly chemoselective reactions, isolated, and fully characterized. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The biotransformation of monosubstituted arenes by mutant strains of Pseudomonas putida, 1 producing cis-dihydrodiols, is an ef- ficient tool for the asymmetric synthesis of natural products. 2 Further functionalization of these compounds leads to cyclohexenetetraol derivatives that resemble the structure of sugars (i.e., conduritols). 3 The design and synthesis of small molecules, which can mimic complex carbohydrates involved in diverse cellular proc- essesdglycomimics, 4 may lead to a better understanding of carbo- hydrates functions and can eventually culminate in potential drug candidates. 5 These glycomimics include carbasugars and amino- carbasugars, which usually display various biological properties as enzyme inhibitors, 6 and may be involved in relevant biological processes or biotechnological applications. 7 For example, amino- cyclitols 8 as valienamine have demonstrated strong glucosidase inhibitory activity, thus showing particular importance as chemo- therapeutic agents (Fig. 1). 9 Since the seminal reports by Heck in 1974, which described the use of the first palladium-catalyzed three-component coupling reaction of an aryl halide, CO and nucleophiles such as alcohols or amines, as effective carbonylation catalytic systems, 10 many aspects of carbon- ylation reactions have been recently reviewed. 11 Particularly, palladium-catalyzed carbonylation of iodoaromatics and iodoalkenes has been extensively used in synthetic chemistry for the preparation of a wide range of carboxylic acid derivatives. 12 Boyd 13 and Fang 14 have applied the alkoxycarbonylation of iodine derivatives as one of the steps for the total synthesis of, respectively, carbasugars and the antiviral agent oseltamivir (Tamiflu Ò )(Scheme 1). Nevertheless, car- bonylation reactions of bromoalkenes are less explored and they still represent a great challenge, due to their lower reactivity, relatively to corresponding iodine counterparts. Moreover, their application in homogeneous catalytic transformations 15 is highly supported by both lower cost and easier synthesis. Encouraged by the growing interest in glycomimics with potential biological activity, herein we report the synthesis of a family of chiral conduritol-derivative carboxamides, through Pd/PPh 3 -catalyzed Fig. 1. Structure of valienamine. * Corresponding author. Tel.: þ351 239854474; fax: þ351 239827703; e-mail address: [email protected](M.M. Pereira). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.05.128 Tetrahedron 68 (2012) 6935e6940
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Tetrahedron 68 (2012) 6935e6940
Contents lists available
Tetrahedron
journal homepage: www.elsevier .com/locate/ tet
An efficient route for the synthesis of chiral conduritol-derivative carboxamidesvia palladium-catalyzed aminocarbonylation of bromocyclohexenetetraols
Rui M.B. Carrilho a, Viviana Heguaburu b, Valeria Schapiro b, Enrique Pandolfi b, L�aszl�o Koll�ar c,Mariette M. Pereira a,*
aDepartamento de Química, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, PortugalbDepartamento de Química Org�anica, Facultad de Química, UdelaR, C. P. 11800, Montevideo, UruguaycDepartment of Inorganic Chemistry, University of P�ecs, Ifj�us�ag u. 6, H-7624 P�ecs, Hungary
a r t i c l e i n f o
Article history:Received 12 March 2012Received in revised form 28 May 2012Accepted 30 May 2012Available online 8 June 2012
0040-4020/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.tet.2012.05.128
a b s t r a c t
A family of chiral conduritol-derivative carboxamides was synthesized through palladium-catalyzedaminocarbonylation of diastereoisomeric bromocyclohexenetetraols, previously prepared through bio-transformation of bromobenzene by mutant strains of Pseudomonas putida F39/D.The coupling reactions of bromocyclohexenetetraols with CO and different amines, such as tert-butyl-amine, aniline, and piperidine, were performed in the presence of in situ generated Pd(0)/PPh3 catalyst.The methodology was applied to the corresponding iodo-cyclohexenetetraol derivative, using (L)-alanineand (L)-valine methyl ethers as N-nucleophiles. The resulting carboxamides were obtained in highlychemoselective reactions, isolated, and fully characterized.
� 2012 Elsevier Ltd. All rights reserved.
Fig. 1. Structure of valienamine.
1. Introduction
The biotransformation of monosubstituted arenes by mutantstrains of Pseudomonas putida,1 producing cis-dihydrodiols, is an ef-ficient tool for the asymmetric synthesis ofnatural products.2 Furtherfunctionalization of these compounds leads to cyclohexenetetraolderivatives that resemble the structure of sugars (i.e., conduritols).3
The design and synthesis of small molecules, which can mimiccomplex carbohydrates involved in diverse cellular proc-essesdglycomimics,4 may lead to a better understanding of carbo-hydrates functions and can eventually culminate in potential drugcandidates.5 These glycomimics include carbasugars and amino-carbasugars, which usually display various biological properties asenzyme inhibitors,6 and may be involved in relevant biologicalprocesses or biotechnological applications.7 For example, amino-cyclitols8 as valienamine have demonstrated strong glucosidaseinhibitory activity, thus showing particular importance as chemo-therapeutic agents (Fig. 1).9
Since the seminal reportsbyHeck in1974,whichdescribed theuseof the first palladium-catalyzed three-component coupling reaction
x: þ351 239827703; e-mail
All rights reserved.
of an aryl halide, CO and nucleophiles such as alcohols or amines, aseffective carbonylation catalytic systems,10 many aspects of carbon-ylation reactions have been recently reviewed.11 Particularly,palladium-catalyzedcarbonylationof iodoaromatics and iodoalkeneshas been extensively used in synthetic chemistry for the preparationof a wide range of carboxylic acid derivatives.12 Boyd13 and Fang14
have applied the alkoxycarbonylation of iodine derivatives as one ofthe steps for the total synthesis of, respectively, carbasugars and theantiviral agent oseltamivir (Tamiflu�) (Scheme 1). Nevertheless, car-bonylation reactions of bromoalkenes are less explored and they stillrepresent a great challenge, due to their lower reactivity, relatively tocorresponding iodine counterparts. Moreover, their application inhomogeneouscatalytic transformations15 ishighly supportedbybothlower cost and easier synthesis.
Encouraged by the growing interest in glycomimicswith potentialbiological activity, hereinwe report the synthesis of a family of chiralconduritol-derivative carboxamides, through Pd/PPh3-catalyzed
Scheme 1. Alkoxycarbonylation step in the synthesis of Tamiflu�.
Scheme 3. Aminocarbonylation reaction of bromocyclohexenetetraol derivatives.
R.M.B. Carrilho et al. / Tetrahedron 68 (2012) 6935e69406936
aminocarbonylation of bromocyclohexenetetraols, using primaryand secondary amines as nucleophiles. Aiming to expand the struc-tural motifs of these sugar mimics, retaining the resemblance withnatural products, two amino acid methyl esters were used as N-nu-cleophiles in the aminocarbonylation of a similar substrate withiodoalkene functionality.
2. Results and discussion
2.1. Synthesis of bromocyclohexenetetraols
The substrates 1 and 2 have been synthesized, following the se-quential pathway, represented inScheme2. Thebiotransformationofbromobenzenewith P. putida F39/D, in 2 g/L culture mediumyield,1b
followed by the reaction with 2,2-dimethoxypropane16 yielded theprotected cis-cyclohexadienediol derivative (88% yield), whose fur-ther oxygenation was followed by two different synthetic ap-proaches. In the first one, the hydroxylation was performed usingOsO4 and NMO (N-methylmorpholine N-oxide) attaining the syndihydroxylated precursor as only product, with 80% yield.17 In a sec-ond process, the anti dihydroxylated compound was exclusivelyobtained through the reaction of the protected cis-cyclo-hexadienediol with MCPBA (meta-chloroperoxybenzoic acid) fol-lowed by epoxide ring opening with KOH.16 The subsequentprotections of the hydroxyl groups were carried out with acetic an-hydride and triethylamine, reaching (1R,2R,3S,4S)-1,2-diacetyl-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol (1) and(1S,2R,3S,4S)-1,2-diacetyl-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol (2), both in 99% yields.
Scheme 2. Synthesis of bromocyclohexenetetraol precursors.
2.2. Aminocarbonylation of halocyclohexenetetraols
The bromocyclohexenetetraol derivatives 1 or 2 were subjectedto reaction with the desired amine (tert-butylamine (a), aniline (b),
and piperidine (c)), in the presence of carbon monoxide and in situgenerated Pd(0) catalysts, in a stainless steel autoclave (Scheme 3).
The reactions’ progress was followed by TLC and GC analysis ofaliquots taken from the reactor via cannula, and the final conver-sions were determined by GC and confirmed by 1H NMR spec-troscopy of the crude mixture, through integration of the olefinicprotons of both substrate and reaction products.
The optimization for the aminocarbonylation reaction condi-tions was performed using substrate 1 and tert-butylamine (a) asnucleophile as models (Table 1).
In the first assessment, using the Pd/PPh3 catalyst, at 50 �Ctemperature and CO pressure of 30 bar, the reaction did not pro-ceed after 24 h (Table 1, entry 1). To overcome the issues associatedwith the low reactivity of this type of bromoalkenes, a temperatureof 100 �C was employed, keeping the CO pressure in 30 bar. Underthese conditions, a conversion of 60% was achieved (Table 1, entry2) with the same catalyst. However, at longer reaction time (48 h),a complex mixture of products, containing mono- and doublecarbonylated and hydrolyzed products, was evidenced by CG, 1HNMR, and 13C NMR (Table 1, entry 3). With a 20 bar CO pressure,a conversion of only 30% was obtained in 24 h (Table 1, entry 4).Using methyl ethyl ketone as solvent, instead of DMF, higher con-version was observed (90%), however with lower selectivity for thetarget monocarboxamide (a 1:1 mixture of mono- and doublecarbonylated products was obtained) (Table 1, entry 5). Further-
more, another ligand was evaluated in the reaction, namely the tris[(S)-20-(benzyloxy)-1,10-binaphthyl-2-yl]phosphite, recently syn-thesized by our group.18 However, no conversion was observedwith this Pd/monophosphite catalyst, in 24 h (Table 1, entry 6).
Table 1Optimization of reaction parameters for aminocarbonylation of 1 with tert-butyl-amine (a) as nucleophile
Reaction conditions: 0.3 mmol of substrate 1; 0.015 mmol of Pd(OAc)2; 0.03 mmolof ligand; 0.9 mmol tBuNH2; 0.25 mL Et3N, 7 mL solvent.
a % of substrate converted in carboxamides at the indicated time, determined byGC and 1H NMR.
b 1:1 mixture of carboxamide/ketocarboxamide.
R.M.B. Carrilho et al. / Tetrahedron 68 (2012) 6935e6940 6937
Therefore, in order to convert this reaction into a real synthetictool for the preparation of conduritol-derivative carboxamides, thereactions were performed using the following parameters asstandard: T¼100 �C; P(CO)¼30 bar; Pd(OAc)2/PPh3 as catalyst; DMFas solvent; reaction time¼24 h.
The isolated yields of the aminocarbonylation reactions, obtainedafter the work-up and products purification using silica gel columnchromatography, are summarized in Table 2. While the amino-
Table 2Aminocarbonylation of bromocyclohexenetetraol derivatives 1 and 2
Reaction conditions: 0.3 mmol of substrate; 0.015 mmol of Pd(OAc)2; 0.03 mmol ofPPh3; 0.25 mL of triethylamine; solvent: 7 mL of DMF; T¼100 �C; P(CO)¼30 bar;amine/substrate molar ratios: a/substrate¼3; b/substrate¼2; c/substrate¼1.5.
a Determined by GC and 1H NMR.
Scheme 4. Aminocarbonylation of iodo-cyclohexenetetraol derivative, using (L)-ala-nine (d) and (L)-valine (e) methyl esters as N-nucleophiles.
carbonylation of 1, using tert-butylamine (a) as nucleophile, attainedproduct 3a with isolated yield of 50%, when substrate 2 wasemployed, using the same amine, a 52% isolated yield of product 4awasreached (Table2, entries1and4).Usinganiline (b) asnucleophilewith the same reaction conditions, products 3b and 4b wereobtained, with 45% and 27% isolated yields, respectively (Table 2,entries 2 and 5). The experiments with piperidine (c) gave 56% and53% yields of products 3c and 4c, respectively (Table 2, entries 3 and6). The overall results show that, even under moderate pressures(30 bar CO) and temperature of 100 �C, a total selectivity for the-monocarboxamide product was achieved since no double carbonyl-ation products (2-ketocarboxamides) were obtained.19 The reaction
yields did not dependon the stereoconfigurationof the substrate, butwere influencedby theaminenucleophilicity, beinghigherwithalkylamines a and c than those achieved with the aromatic amine b. Thesensitivenessof theacetateprotectinggrouptohydrolysis, duringtheisolation/purification process resulted in a slight decrease of isolatedyields, comparatively to reaction conversions (Table 2). It should benoticed that the employed amine/substrate ratios were based onseveral previous optimization studies, using a wide range of haloar-ene and haloalkene substrates, and have mainly into account thevolatility properties of each amine.11,12 For instance, themost volatileamine, tert-butylamine (a) has tobeused in (at least) threefold excessto achieve good yields in reasonable reaction time, while in the caseof less volatile piperidine (c), a ratio of 1.5 to substrate is sufficient.
To extend the structural patterns of the conduritol-derivativecarboxamides, the aminocarbonylation of the correspondingiodo-conduritol was performed, with (L)-alanine and (L)-valinemethyl esters as N-nucleophiles (Scheme 4), using the same stan-dard conditions employed for bromocyclohexenetetraols. Theseamino acid methyl esters were used in hydrochloride form (solids),and the ‘free’ nucleophiles were made available ‘in situ’ upon theeffect of triethylamine as HCl acceptor. Therefore only a slight ex-cess of N-nucleophile to the substrate was necessary.
As expected, the aminocarbonylation of the iodo-cyclo-hexenetetraol derivative occurred more efficiently than with thevinylbromide counterpart, achieving complete conversions (99% inthe case of (L)-alanine methyl ester and 93% with (L)-valine). Afterwork-up and purification, the carboxamide compounds 3d and 3ewere obtained with 85 and 66% isolated yields, respectively.
3. Conclusion
An effective route for a family of chiral conduritol-derivativecarboxamides was developed through the synthesis of new di-astereoisomeric bromocyclohexenetetraol derivatives, obtainedfrom bromobenzene biotransformation with mutant strains of P.putida F39/D, followed by palladium-catalyzed coupling reaction ofthe bromoalkenes with CO and different amines. Remarkable se-lectivity for monocarboxamide compounds was achieved under30 bar CO pressure and 100 �C. The syn or anti stereochemistry ofthe acetate groups in positions 1 and 2 of the ring did not affect thereactivity, whereas significant differences in product yields wereapparently due to the nucleophilicity of the amine.
In spite of the widely known low reactivity of bromoalkenes inhomogeneous catalytic reactions, the carboxamides obtainedthrough aminocarbonylation of the corresponding tetraol de-rivatives possessing bromoalkene functionality were isolated inmoderate isolated yields. The versatility of thismethodology for thepreparation of conduritol-derivative carboxamides is demonstratedby the diverse range of N-nucleophiles, as well as the possibility ofusing either vinyl iodides or vinyl-bromides in such reactions.
R.M.B. Carrilho et al. / Tetrahedron 68 (2012) 6935e69406938
4. Experimental section
4.1. General
1H and 13C NMR spectra were recorded in CDCl3 solutions ona Bruker Avance 400 spectrometer, at 400.13 MHz and 100.61 MHz,respectively. Chemical shifts are expressed in parts per million, rela-tively to an internal TMS standard. The absolute assignments of 1Hand 13C signals were based on two-dimensional experiments (COSY,HSQC, and HMBC). High-resolution mass spectrometry analysis wascarried out on a Bruker Microtof apparatus, equipped with selectiveESI detector. Optical rotation was measured with an Optical ActivityLda.polarimeter, using a2 mLcell. All reagentswere fromcommercialorigin (SigmaeAldrich and Fluka), except the chiral cyclo-hexadienediols, which were synthesized according to the published procedure for the use of P. putida F39/D.20 Compounds(1S,2R,3S,4S)-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol,16 (1R,2R,3S,4S)-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol,17 and (1R,2R,3S,4S)-5-iodo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol21 were prepared according to pre-viously described procedures. Spectroscopic data are in good agree-ment with those reported.
Note: OsO4 is highly poisonous, so the appropriate safetyprocedures were taken for its manipulation. R: 26/27/28-34;S: 1/2-7/9-26-45.
4.2. Synthesis of substrates
A solution of (1S,2R,3S,4S)-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol or (1R,2R,3S,4S)-5-bromo-3,4-O-iso-propylidenecyclohex-5-en-1,2,3,4-tetraol (1 g, 3.8 mmol) in 20 mLCH2Cl2, was cooled in an ice bath. Under N2 atmosphere, acetic an-hydride (1.4 mL, 15.9 mmol), NEt3 (4.2 mL, 30.2 mmol), and a smallspatula tip of DMAP were added. After stirring for 2 h, a cooled sat-urated solution of Na2CO3 was added. Upon extraction with diethylether, the combined organic phase was successively washed withwater, a saturated solution of CuSO4, and brine, dried over Na2SO4,and the solvent was evaporated under vacuum. The products werepurified by silica gel column chromatography, using a mixture of n-hexane/EtOAc (7:3) as eluent.
The substrate (1R,2R,3S,4S)-1,2-diacetyl-5-iodo-3,4-O-iso-propylidenecyclohex-5-en-1,2,3,4-tetraolwasprepared through thesame synthetic methodology used for (1R,2R,3S,4S)-1,2-diacetyl-5-bromo-3,4-O-isopropylidenecyclohex-5-en-1,2,3,4-tetraol.
4.3. General procedure for aminocarbonylation reactions
In a typical experiment Pd(OAc)2 (3.4 mg, 0.015 mmol), PPh3(7.9 mg, 0.03 mmol), the halocyclohexenetetraol derivative 1, 2, or 5(0.30 mmol), and the desired amine (0.33e0.90 mmol) (see Table 1)were dissolved in 7 mL of DMF under N2 atmosphere. The homo-geneous yellow solution was transferred to a stainless steel auto-clave and triethylamine (0.25 mL) was added. The reaction vesselwas then pressurized with 30 bar of carbon monoxide and themixture was magnetically stirred for 24 h at 100 �C. At the end ofthe reaction, somemetallic palladiumwas formed and precipitated,which was filtered off. The mixture was then concentrated andevaporated to dryness. The residue was dissolved in chloroform(20 mL), then washed with water (3�20 mL), dried over Na2SO4,and concentrated to a red/orange waxy oil. The chemically pureproducts 3aee and 4aec were isolated from the reaction mixturesby column chromatography (Silicagel, chloroform/ethyl acetate orn-hexane/ethyl acetate; exact ratios are specified in the character-ization for each compound).
Authors are thankful to Portuguese FCT (Fundac~ao para a Cie
ˇ
nciae a Tecnologia), QREN/FEDER/COMPETE-Programa Operacional
R.M.B. Carrilho et al. / Tetrahedron 68 (2012) 6935e69406940
Factores de Competitividade, PTDC/QUI-QUI/112913/2009, ANII(Agencia Nacional de Investigaci�on e Innovaci�on) from Uruguayangovernment and PEDECIBA (Programa de Desarrollo de CienciasB�asicas, PNUD/URU/06/004) and Hungarian Scientific ResearchFund (OTKA CK78553). R.M.B.C. thanks FCT for PhD grant SFRH/BD/60499/2009. V.H. thanks AUGM (Asociaci�on de Universidades delGrupo Montevideo) and the Coimbra Catalysis & Fine Chemistrygroup, for a fellowship.
Supplementary data
Supplementary data related to this article can be found online athttp://dx.doi.org/10.1016/j.tet.2012.05.128.
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