DOI: 10.1002/chem.201404229
Full Paper
& Gold-Catalyzed Reactions
Synthesis of Highly Substituted N-(Furan-3ylmethylene)benzenesulfonamides by a Gold(I)-Catalyzed Oxidation/1,2-Alkynyl Migration/Cyclization Cascade Tao Wang,[a] Long Huang,[a] Shuai Shi,[a] Matthias Rudolph,[a] and A. Stephen K. Hashmi*[a, b]
Abstract: A variety of N-(furan-3-ylmethylene)benzenesulfonamides were obtained by a gold(I)-catalyzed cascade reaction from easily accessible starting materials. The reaction
pathway involves a rarely observed 1,2-alkynyl migration onto a gold carbenoid. This observation further enriches gold carbenoid chemistry with regard to group migration.
a-Oxo gold carbenoids have been postulated as key intermediates in a number of gold-catalyzed oxygen transfer reactions.[1] This intermediate, which is usually generated by the gold-catalyzed conversion of sulfoxides or nitrogen oxides with alkynes, can undergo a variety of valuable transformations, for example C H insertions,[2] cyclopropanations of olefins,[3] ring-expansions,[4] and reactions with various nucleophilies.[5] As an interesting feature of metal carbenoid chemistry, group migration has attracted significant interest among chemists[6] and is an active research area of a-oxo gold carbenoid chemistry, too. Among the group migrations to a-oxo gold carbenoids, the 1,2- Scheme 1. Previous work and our hypothesis. migration of hydride,[4, 7] alkyl,[4] aryl,[4] or alkenyl groups[4c] are dominantly observed, whereas a 1,2-alkynyl migration[4d] is rarely reported. Our ongoing efforts to erate intermediate B, which upon cyclization should deliver Nexpand the scope of gold chemistry[8] and our interest in gold (furan-3-ylmethylene)benzenesulfonamide 3 as a product carbenoids[9] encouraged us to investigate the rarely observed (Scheme 1b). The commonly used methods for the synthesis of 1,2-alkynyl migration. these types of useful intermediates and building blocks in orVery recently, we developed a highly efficient approach for ganic synthesis usually require pre-synthesized 3-formyl furans the synthesis of 3-formyl furans from easily available 1,4-diynand harsh reaction conditions such as very high tempera3-ols.[10] The mechanistic investigation indicated that a 1,2-alture.[11] Herein we report an efficient strategy for the synthesis kynyl migration onto a gold carbenoid was involved in the reof N-(furan-3-ylmethylene)benzenesulfonamides from easily acaction pathway (Scheme 1a). We envisioned that by replacing cessible starting materials by a gold(I)-catalyzed oxidation/1,2the hydroxyl group with suitable amine groups, for example palkynyl migration/cyclization cascade. toluenesulfonamide, in the presence of pyridine-N-oxides, inN-(1,5-Diphenylpenta-1,4-diyn-3-yl)-4-methylbenzenesulfonatermediate A should be formed under gold catalysis. This intermide (1 a), which can be easily prepared in one pot by treating mediate is expected to undergo a 1,2-alkynyl migration to genphenylacetylene with n-butyllithium followed by addition of ethyl N-tosylformimidate and BF3 .OEt2, was chosen as test sub[a] M. Sc. T. Wang, M. Sc. L. Huang, M. Sc. S. Shi, Dr. M. Rudolph, strate. As shown in Table 1, treating 1 a with 3,5-dichloropyriProf. Dr. A. S. K. Hashmi dine-N-oxide (2 a) in the presence of IPrAuCl/AgSbF6 in toluene Organisch-Chemisches Institut delivered the desired product 3 a in good yield (entry 1). EnRuprecht-Karls-Universitt Heidelberg couraged by this result, we tested a variety of pyridine-NIm Neuenheimer Feld 270, 69120 Heidelberg (Germany) Fax: (+ 49) 6221-54-4205 oxides but no improvement in yield was achieved (entries 2– E-mail: [email protected] 4). 4-Picoline-N-oxide (2 c) and 3-bromopyridine-N-oxide (2 d) [b] Prof. Dr. A. S. K. Hashmi afforded 3 a in a slightly lower yield (entries 3 and 4), whereas Chemistry Department, Faculty of Science, King Abdulaziz University unsubstituted pyridine-N-oxides (2 b) only delivered an incomJeddah 21589 (Saudi Arabia) plete conversion of 1 a (entry 2). We next optimized the reacSupporting information for this article is available on the WWW under tion by examining a variety of different catalysts (entries 5–9). http://dx.doi.org/10.1002/chem.201404229. Chem. Eur. J. 2014, 20, 14868 – 14871
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Full Paper Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst (5 mol %)
N-oxide (1.2 equiv)
Solvent/Temperature/ Time
Yield [%][b] 3a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2a 2b 2c 2d 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a
toluene/rt/10 min toluene/rt/24 h toluene/rt/24 h toluene/rt/2 h Toluene/rt/7 h toluene/rt/24 h toluene/rt/24 h toluene/rt/30 min toluene/rt/5 h toluene/rt/10 min toluene/rt/10 min toluene/rt/10 min toluene/rt/24 h DCM/rt/30 min DCE/rt/10 min THF/rt/10 h DCE/rt/1 h
88 51[c] (30) 83[c] 85 70 NR[d] 44[c] (50) 83 81 83 86 85 NR[d] 85 91 87[c] 85
IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 Ph3PAuCl/AgSbF6 AgSbF6 IMesAuCl/AgSbF6 L1 AuCl/AgSbF6 L2 AuCl/AgSbF6 IPrAuCl/AgOTf IPrAuCl/AgBF4 IPrAuCl/AgNTf2 IPrAuCl/AgOTs IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 (3 mol %)
[a] All reactions were carried out on a 0.2 mmol scale in 2 mL of solvent. [b] Isolated yield, the number in parentheses is recovered 1 a after the column. [c] Hydrolysis of 3 a was observed leading to 4 a. [d] No reaction was observed.
Ph3PAuCl combined with AgSbF6 afforded 3 a in moderate yield (entry 5). No transformation was observed when AgSbF6 was used alone as catalyst (entry 6). Three gold complexes bearing N-heterocyclic harbene (NHC) ligands were also investigated, among which L1[8a] gave a similar result with IPrAuCl (entry 8). Combined with IPrAuCl different silver salts were also tested but no significant effect of the counter ion was observed (entries 10–12), the only exception being AgOTs which did not catalyze this reaction at all (entry 13). A solvent screening revealed that 1,2-dichloroethene (DCE) was the best solvent for this transformation (entry 15). A reduction of the catalyst loading from 5 mol % to 3 mol % led to a decrease in yields from 91 % to 85 % (entry 17). After prolonged reaction times, hydrolysis of 3 a was observed in some cases (entries 2, 3, 7, 16). Chem. Eur. J. 2014, 20, 14868 – 14871
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After completion of the optimization process the scope of the reaction was investigated (Table 2). Symmetrical starting materials were examined first. In analogy to the synthesis of 3formylfurans,[10] substrates bearing substituents in para- or meta-position of the phenyl group smoothly delivered the corresponding products 3 in good to excellent yields (3 a–3 h). The partial hydrolysis of 3 h during the column chromatography on silica gel is responsible for the relatively low yield of this product. A heteroaromatic substrate and a vinyldiyne starting material also smoothly afforded the corresponding N(furan-3-ylmethylene)benzenesulfonamides in excellent and good yields, respectively (3 i and 3 j). An aliphatic diyne starting material was also investigated, which gave 3 k in good yield, but longer reaction times were necessary. Replacing the p-toluenesulfonyl by a 4-nitrobenzenesulfonyl group in the starting material gave the corresponding product in good yield as well (3 l). Diyne 1 m showed no reaction even after a long time. The steric bulk of the TMS group is considered to account for the inactivity of 1 m. Unsymmetrical starting materials were also investigated (Table 3). Substrate 1 n bearing a 4-trifluoromethylphenyl group and a 4-methoxy phenyl group showed good regioselectivity, affording 3 n as a single isomer in good yield. In accordance with the synthesis of 3-formylfurans,[10] the electronrich 4-methoxy phenyl group is placed in the 2-position of the furan. Starting materials with sterically demanding substituents were also investigated. Compounds 3 p and 3 q were obtained as single products, whereas 3 o and 3 o’ were obtained as a mixture in a 3:1 ratio. This might be explained by the enormous steric demand of the 2,4,6-trimethylphenyl group, which pushes the Ts group towards the phenyl group on the other side, leading to two equivalently shielded alkynes on both sides of the starting material. Interestingly, 1 r, bearing one TMS group, afforded TMS-deprotected 3 r in moderate yield. Notably, the corresponding alcohol starting materials showed no deprotection, but even under the conditions of our previous work deprotection took place for the amine starting materials.[10] Attempts at the preparation of starting materials bearing terminal alkynes via TMS removal from 1 m and 1 r failed because of their instability under basic conditions. An alternative synthesis based on the nucleophilic addition of ethynylmagnesium bromide to ethyl N-tosylformimidate also failed. When N-iodosuccinimide (NIS) was added under the standard conditions, 1 a afforded the iodination product 4 a in moderate yield (Scheme 2), which further enhances the utility of N-(furan-3-ylmethylene)benzenesulfonamides. One might wonder whether the synthesis of 3-formylfurans from the analogous bispropargylic alcohols (Scheme 1a) and
Scheme 2. Conversion in the presence of NIS.
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Full Paper group (enol ether substructure in conjugation with the aldehyde) the electrophilicity of the aldehyde is reduced and for the condensation harsh reaction conditions are needed (120 8C, 16 h) and lower overall yields are obtained. This is in accord with previous reports,[11] and for an efficient condensation of the related 2-furylfurans with tosylamide even TiCl4 is required.[12] Thus for an application in the synthesis of functionalized substrates, our new and mild conditions are superior to a subsequent condensation with tosylamide. As substrates 1 are prepared in one step from ethyl N-tosylformimidate, the number of steps for the substrate preparation is the same for both routes discussed. In conclusion, an efficient and mild gold(I)-catalyzed cascade reaction towards N-(furan-3-ylmethylene)benzenesulfonamides from easily accessible starting materials has been developed. As an expansion of the synthesis of 3-formylfurans, 1,2-alkynyl migration to gold carbenoids was also involved in this transformation. This work shows that these reactions proceeding to carbenoid intermediates can be extended to nitrogen-containing substrates, widening the substrate scope to a very important class of functional groups for organic synthesis.
Table 2. Scope of the reaction for symmetrical starting materials.[a]
[a] All reactions were carried out on a 0.2 mmol scale in 2 mL of DCE; all yields given in this table refer to isolated products. [b] The hydrolysis of 3 h was observed during column chromatography. [c] Ns = 4-nitrobenzenesulfonyl.
Table 3. Scope of the reaction for unsymmetrical starting materials.[a]
Experimental Section General procedure: N-(Penta-1,4-diyn-3-yl)-4-methylbenzenesulfonamide (1, 0.2 mmol) and 3,5-dichloropyridineN-oxide (2 a, 39 mg, 0.24 mmol) were added to a suspension of IPrAuCl (6.2 mg, 0.01 mmol) and AgSbF6 (3.4 mg, 0.01 mmol) in 1,2-dichloroethane (2 mL). The resulting mixture was allowed to stir at room temperature and was monitored by thin layer chromatography (TLC). After the complete conversion of 1, the solvent was removed and the product was purified by silica gel column chromatography using petroleum ether–ethyl acetate mixture as eluent.
Acknowledgements The authors thank Umicore AG & Co. KG for the generous donation of gold salts. T.W. and S.S. are grateful for the fellowships from the China Scholarship Council (CSC). L.H. appreciates HGMF scholarships from the Heinz Gçtze Memorial Fellowship program of the Athenaeum Foundation fellowship. [a] All reactions were carried out on a 0.2 mmol scale in 2 mL of DCE; all the yields given in this table refer to isolated products. [b] The hydrolysis of 3 n was observed during column chromatography.
a subsequent condensation with tosylamide might provide an easier access to the products 3. We tested this alternative; however, due to the electron-donating properties of the furyl Chem. Eur. J. 2014, 20, 14868 – 14871
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14870
Keywords: alkynes · furans · gold catalysis · imines · sulfonamides
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Received: July 3, 2014 Published online on September 18, 2014
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Full Paper
& Gold-Catalyzed Reactions
Synthesis of Highly Substituted N-(Furan-3ylmethylene)benzenesulfonamides by a Gold(I)-Catalyzed Oxidation/1,2-Alkynyl Migration/Cyclization Cascade Tao Wang,[a] Long Huang,[a] Shuai Shi,[a] Matthias Rudolph,[a] and A. Stephen K. Hashmi*[a, b]
Abstract: A variety of N-(furan-3-ylmethylene)benzenesulfonamides were obtained by a gold(I)-catalyzed cascade reaction from easily accessible starting materials. The reaction
pathway involves a rarely observed 1,2-alkynyl migration onto a gold carbenoid. This observation further enriches gold carbenoid chemistry with regard to group migration.
a-Oxo gold carbenoids have been postulated as key intermediates in a number of gold-catalyzed oxygen transfer reactions.[1] This intermediate, which is usually generated by the gold-catalyzed conversion of sulfoxides or nitrogen oxides with alkynes, can undergo a variety of valuable transformations, for example C H insertions,[2] cyclopropanations of olefins,[3] ring-expansions,[4] and reactions with various nucleophilies.[5] As an interesting feature of metal carbenoid chemistry, group migration has attracted significant interest among chemists[6] and is an active research area of a-oxo gold carbenoid chemistry, too. Among the group migrations to a-oxo gold carbenoids, the 1,2- Scheme 1. Previous work and our hypothesis. migration of hydride,[4, 7] alkyl,[4] aryl,[4] or alkenyl groups[4c] are dominantly observed, whereas a 1,2-alkynyl migration[4d] is rarely reported. Our ongoing efforts to erate intermediate B, which upon cyclization should deliver Nexpand the scope of gold chemistry[8] and our interest in gold (furan-3-ylmethylene)benzenesulfonamide 3 as a product carbenoids[9] encouraged us to investigate the rarely observed (Scheme 1b). The commonly used methods for the synthesis of 1,2-alkynyl migration. these types of useful intermediates and building blocks in orVery recently, we developed a highly efficient approach for ganic synthesis usually require pre-synthesized 3-formyl furans the synthesis of 3-formyl furans from easily available 1,4-diynand harsh reaction conditions such as very high tempera3-ols.[10] The mechanistic investigation indicated that a 1,2-alture.[11] Herein we report an efficient strategy for the synthesis kynyl migration onto a gold carbenoid was involved in the reof N-(furan-3-ylmethylene)benzenesulfonamides from easily acaction pathway (Scheme 1a). We envisioned that by replacing cessible starting materials by a gold(I)-catalyzed oxidation/1,2the hydroxyl group with suitable amine groups, for example palkynyl migration/cyclization cascade. toluenesulfonamide, in the presence of pyridine-N-oxides, inN-(1,5-Diphenylpenta-1,4-diyn-3-yl)-4-methylbenzenesulfonatermediate A should be formed under gold catalysis. This intermide (1 a), which can be easily prepared in one pot by treating mediate is expected to undergo a 1,2-alkynyl migration to genphenylacetylene with n-butyllithium followed by addition of ethyl N-tosylformimidate and BF3 .OEt2, was chosen as test sub[a] M. Sc. T. Wang, M. Sc. L. Huang, M. Sc. S. Shi, Dr. M. Rudolph, strate. As shown in Table 1, treating 1 a with 3,5-dichloropyriProf. Dr. A. S. K. Hashmi dine-N-oxide (2 a) in the presence of IPrAuCl/AgSbF6 in toluene Organisch-Chemisches Institut delivered the desired product 3 a in good yield (entry 1). EnRuprecht-Karls-Universitt Heidelberg couraged by this result, we tested a variety of pyridine-NIm Neuenheimer Feld 270, 69120 Heidelberg (Germany) Fax: (+ 49) 6221-54-4205 oxides but no improvement in yield was achieved (entries 2– E-mail: [email protected] 4). 4-Picoline-N-oxide (2 c) and 3-bromopyridine-N-oxide (2 d) [b] Prof. Dr. A. S. K. Hashmi afforded 3 a in a slightly lower yield (entries 3 and 4), whereas Chemistry Department, Faculty of Science, King Abdulaziz University unsubstituted pyridine-N-oxides (2 b) only delivered an incomJeddah 21589 (Saudi Arabia) plete conversion of 1 a (entry 2). We next optimized the reacSupporting information for this article is available on the WWW under tion by examining a variety of different catalysts (entries 5–9). http://dx.doi.org/10.1002/chem.201404229. Chem. Eur. J. 2014, 20, 14868 – 14871
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Full Paper Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst (5 mol %)
N-oxide (1.2 equiv)
Solvent/Temperature/ Time
Yield [%][b] 3a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2a 2b 2c 2d 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a
toluene/rt/10 min toluene/rt/24 h toluene/rt/24 h toluene/rt/2 h Toluene/rt/7 h toluene/rt/24 h toluene/rt/24 h toluene/rt/30 min toluene/rt/5 h toluene/rt/10 min toluene/rt/10 min toluene/rt/10 min toluene/rt/24 h DCM/rt/30 min DCE/rt/10 min THF/rt/10 h DCE/rt/1 h
88 51[c] (30) 83[c] 85 70 NR[d] 44[c] (50) 83 81 83 86 85 NR[d] 85 91 87[c] 85
IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 Ph3PAuCl/AgSbF6 AgSbF6 IMesAuCl/AgSbF6 L1 AuCl/AgSbF6 L2 AuCl/AgSbF6 IPrAuCl/AgOTf IPrAuCl/AgBF4 IPrAuCl/AgNTf2 IPrAuCl/AgOTs IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 IPrAuCl/AgSbF6 (3 mol %)
[a] All reactions were carried out on a 0.2 mmol scale in 2 mL of solvent. [b] Isolated yield, the number in parentheses is recovered 1 a after the column. [c] Hydrolysis of 3 a was observed leading to 4 a. [d] No reaction was observed.
Ph3PAuCl combined with AgSbF6 afforded 3 a in moderate yield (entry 5). No transformation was observed when AgSbF6 was used alone as catalyst (entry 6). Three gold complexes bearing N-heterocyclic harbene (NHC) ligands were also investigated, among which L1[8a] gave a similar result with IPrAuCl (entry 8). Combined with IPrAuCl different silver salts were also tested but no significant effect of the counter ion was observed (entries 10–12), the only exception being AgOTs which did not catalyze this reaction at all (entry 13). A solvent screening revealed that 1,2-dichloroethene (DCE) was the best solvent for this transformation (entry 15). A reduction of the catalyst loading from 5 mol % to 3 mol % led to a decrease in yields from 91 % to 85 % (entry 17). After prolonged reaction times, hydrolysis of 3 a was observed in some cases (entries 2, 3, 7, 16). Chem. Eur. J. 2014, 20, 14868 – 14871
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After completion of the optimization process the scope of the reaction was investigated (Table 2). Symmetrical starting materials were examined first. In analogy to the synthesis of 3formylfurans,[10] substrates bearing substituents in para- or meta-position of the phenyl group smoothly delivered the corresponding products 3 in good to excellent yields (3 a–3 h). The partial hydrolysis of 3 h during the column chromatography on silica gel is responsible for the relatively low yield of this product. A heteroaromatic substrate and a vinyldiyne starting material also smoothly afforded the corresponding N(furan-3-ylmethylene)benzenesulfonamides in excellent and good yields, respectively (3 i and 3 j). An aliphatic diyne starting material was also investigated, which gave 3 k in good yield, but longer reaction times were necessary. Replacing the p-toluenesulfonyl by a 4-nitrobenzenesulfonyl group in the starting material gave the corresponding product in good yield as well (3 l). Diyne 1 m showed no reaction even after a long time. The steric bulk of the TMS group is considered to account for the inactivity of 1 m. Unsymmetrical starting materials were also investigated (Table 3). Substrate 1 n bearing a 4-trifluoromethylphenyl group and a 4-methoxy phenyl group showed good regioselectivity, affording 3 n as a single isomer in good yield. In accordance with the synthesis of 3-formylfurans,[10] the electronrich 4-methoxy phenyl group is placed in the 2-position of the furan. Starting materials with sterically demanding substituents were also investigated. Compounds 3 p and 3 q were obtained as single products, whereas 3 o and 3 o’ were obtained as a mixture in a 3:1 ratio. This might be explained by the enormous steric demand of the 2,4,6-trimethylphenyl group, which pushes the Ts group towards the phenyl group on the other side, leading to two equivalently shielded alkynes on both sides of the starting material. Interestingly, 1 r, bearing one TMS group, afforded TMS-deprotected 3 r in moderate yield. Notably, the corresponding alcohol starting materials showed no deprotection, but even under the conditions of our previous work deprotection took place for the amine starting materials.[10] Attempts at the preparation of starting materials bearing terminal alkynes via TMS removal from 1 m and 1 r failed because of their instability under basic conditions. An alternative synthesis based on the nucleophilic addition of ethynylmagnesium bromide to ethyl N-tosylformimidate also failed. When N-iodosuccinimide (NIS) was added under the standard conditions, 1 a afforded the iodination product 4 a in moderate yield (Scheme 2), which further enhances the utility of N-(furan-3-ylmethylene)benzenesulfonamides. One might wonder whether the synthesis of 3-formylfurans from the analogous bispropargylic alcohols (Scheme 1a) and
Scheme 2. Conversion in the presence of NIS.
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Full Paper group (enol ether substructure in conjugation with the aldehyde) the electrophilicity of the aldehyde is reduced and for the condensation harsh reaction conditions are needed (120 8C, 16 h) and lower overall yields are obtained. This is in accord with previous reports,[11] and for an efficient condensation of the related 2-furylfurans with tosylamide even TiCl4 is required.[12] Thus for an application in the synthesis of functionalized substrates, our new and mild conditions are superior to a subsequent condensation with tosylamide. As substrates 1 are prepared in one step from ethyl N-tosylformimidate, the number of steps for the substrate preparation is the same for both routes discussed. In conclusion, an efficient and mild gold(I)-catalyzed cascade reaction towards N-(furan-3-ylmethylene)benzenesulfonamides from easily accessible starting materials has been developed. As an expansion of the synthesis of 3-formylfurans, 1,2-alkynyl migration to gold carbenoids was also involved in this transformation. This work shows that these reactions proceeding to carbenoid intermediates can be extended to nitrogen-containing substrates, widening the substrate scope to a very important class of functional groups for organic synthesis.
Table 2. Scope of the reaction for symmetrical starting materials.[a]
[a] All reactions were carried out on a 0.2 mmol scale in 2 mL of DCE; all yields given in this table refer to isolated products. [b] The hydrolysis of 3 h was observed during column chromatography. [c] Ns = 4-nitrobenzenesulfonyl.
Table 3. Scope of the reaction for unsymmetrical starting materials.[a]
Experimental Section General procedure: N-(Penta-1,4-diyn-3-yl)-4-methylbenzenesulfonamide (1, 0.2 mmol) and 3,5-dichloropyridineN-oxide (2 a, 39 mg, 0.24 mmol) were added to a suspension of IPrAuCl (6.2 mg, 0.01 mmol) and AgSbF6 (3.4 mg, 0.01 mmol) in 1,2-dichloroethane (2 mL). The resulting mixture was allowed to stir at room temperature and was monitored by thin layer chromatography (TLC). After the complete conversion of 1, the solvent was removed and the product was purified by silica gel column chromatography using petroleum ether–ethyl acetate mixture as eluent.
Acknowledgements The authors thank Umicore AG & Co. KG for the generous donation of gold salts. T.W. and S.S. are grateful for the fellowships from the China Scholarship Council (CSC). L.H. appreciates HGMF scholarships from the Heinz Gçtze Memorial Fellowship program of the Athenaeum Foundation fellowship. [a] All reactions were carried out on a 0.2 mmol scale in 2 mL of DCE; all the yields given in this table refer to isolated products. [b] The hydrolysis of 3 n was observed during column chromatography.
a subsequent condensation with tosylamide might provide an easier access to the products 3. We tested this alternative; however, due to the electron-donating properties of the furyl Chem. Eur. J. 2014, 20, 14868 – 14871
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Keywords: alkynes · furans · gold catalysis · imines · sulfonamides
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Received: July 3, 2014 Published online on September 18, 2014
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