Review pubs.acs.org/CR
Recent Developments in the Synthesis and Application of Sultones Shovan Mondal* Institut de Chimie Radicalaire, CNRS UMR 7273, Equipe CMO, Aix-Marseille Université, 13397 Cedex 20, Marseille, France For example, the aliphatic sultone 2 can be named as 3hydroxy-1-propanesulfonic acid sultone, propane-1,3-sultone, or more simply as propane γ-sultone. Sultones are important heterocyclic compounds that can introduce alkyl chains carrying the sulfonic acid functionalities and can therefore be used as sulfoalkylating agents. Some review articles have also been published on sultone chemistry in earlier literature,4−8 but there is no literature survey covering the recent developments in sultone synthesis. This review compiles the recent developments (focusing on the period 2000−2011) in the synthesis of sultones.
2. SYNTHESIS OF SULTONES Because sultones are synthetically very useful heterocycles in organic synthesis, many groups are currently working on sultone chemistry.9−26 Most of the earlier procedures for sultone synthesis involved either carbanion-mediated sulfonate intermolecular or intramolecular coupling reactions (CSIC reactions)27 or sulfonation of olefins with dioxane−sulfur trioxide. Recently, many powerful methodologies have been developed for the synthesis of the sultones, such as intramolecular Diels−Alder reactions, ring-closing metathesis, Pdcatalyzed intramolecular coupling reactions, Rh-catalyzed C−H insertion, Rh-catalyzed carbene cyclization cycloaddition cascade reactions, etc. For clear presentation, the syntheses of sultones have been divided into two categories: (i) asymmetric synthesis and (ii) nonasymmetric synthesis, which were then subdivided according to their reaction types.
CONTENTS 1. Introduction 2. Synthesis of Sultones 2.1. Asymmetric Synthesis of Sultones 2.1.1. C−H Insertion Reactions 2.1.2. Cycloaddition Reactions 2.1.3. Olefin Sulfonation Reactions 2.1.4. Miscellaneous Reactions 2.2. Non-Asymmetric Synthesis of Sultones 2.2.1. Ring-Closing Metathesis Reactions 2.2.2. Cycloaddition Reactions 2.2.3. Nucleophilic Addition Reactions 2.2.4. Pd-Catalyzed Intramolecular Cyclization Reactions 2.2.5. Miscellaneous Reactions 3. Synthetic Applications of Sultones 3.1. Sultones in General Organic Synthesis 3.2. Sultones in Natural Product Synthesis 4. Biological Activities of Sultones 5. Conclusion Author Information Corresponding Author Notes Biography Acknowledgments References
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2.1. Asymmetric Synthesis of Sultones
Asymmetric syntheses of sultones became an attractive goal for the researchers because chiral sultones can offer novel possibilities for stereoselective transformations. There have been several new developments in the asymmetric synthesis of sultones in the recent literature, which are discussed in the following sections. 2.1.1. C−H Insertion Reactions. Du Bois and co-workers exploited the chemoselective synthesis of δ-sultones by Rhcatalyzed C−H 1,6-insertion by attempting two parallel procedures.28 They utilized diazo intermediates or in situgenerated aryl iodonium ylides for the formation of Rh-carbene species. For example, when diazosulfonate (1.1) was refluxed with 2 mol % bis[rhodium(α,α,α′,α′-tetramethyl-1,3-benzenedipropionic acid)] [Rh2(esp)2], it led to the corresponding sultone (1.2) in 84% yield. On the other hand, when sulfonate ester (1.3) was treated with PhIO, Cs2CO3 and 3 Å molecular sieves in the presence of a catalytic amount of Rh2(OAc)4, the sultone 1.2 was obtained in 52% yield (Scheme
1. INTRODUCTION Sultones are the internal esters of hydroxy sulfonic acids and are, as such, sulfur analogues of lactones. Most known sultones are 4- to 6-membered rings, and fewer are 7-membered or larger. Some representative structures of sultones are depicted in Figure 1. The term “sultone” was first introduced by Erdmann1 in 1888 to describe one of the simpler aromatic sultone, 1,8-naphthosultone (1). Various systems of nomenclature are in use in Chemical Abstracts and in the literature.2,3 © 2012 American Chemical Society
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Scheme 4. Enantio- and Diastereoselective Syntheses of βSultones
Figure 1. Representative structures of sultones.
then 10 h stirring at r.t., a diastereomeric mixture of bicyclic sultones (2.2a,b) was obtained (Scheme 2). However, the main problem was that the diastereomeric ratio (2.2a/2.2b) erratically changed from experiment to experiment, ranging from complete selectivity in favor of 2.2a to 2:1 in favor of the same diastereomer. 2.1.2. Cycloaddition Reactions. In their continuous efforts on the synthesis of sultones by Diels−Alder reactions,30−34 Metz et al. reported a stereoselective preparation of bicyclic sultones by the thermal and high-pressure intramolecular Diels−Alder reaction of vinylsulfonates.35 When the intramolecular Diels−Alder reactions of vinylsulfonates (3.1) were performed in refluxing toluene, the bicyclic sultones 3.2 and 3.3 were formed along with undesired byproduct. To suppress side-reactions at this elevated temperature, addition of a small amount of the radical scavenger, butylated hydroxytoluene (BHT), was required. However, the vinylsulfonates (3.1) cyclized smoothly at room temperature when a pressure of 13 kbar was applied (Scheme 3, Table 1). Moreover, Koch and Peters developed a new methodology for the enantio- and diastereoselective formation of βsultones.36,37 β-Sultones with α-CH2 groups readily rearranged to yield mixtures of isomeric sulfonic acids as well as γ- and δsultones within several hours at room temperature.38 But when an electron-withdrawing and bulky group like CCl3 is present at the α-position, the β-sultones are exceptionally stable. Therefore, when chloral (4.2) was treated with alkylsulfonyl chlorides (4.1) in the presence of (DHQ)2PYR (dihydroquinine-2,5diphenyl-4,6-pyrimidinediyl diether) as catalyst, metal triflate salts as cocatalysts and PMP (1,2,2,6,6-pentamethylpiperidine) as stoichiometric base in CH2Cl2 at −15 °C, the β-sultones (4.3a−f) were readily formed in moderate to good yields (Scheme 4). From screening of the various metal triflate salts, In(OTf)3 (best enantiomeric ratio (e.r.) values) and Bi(OTf)3 (best yields) emerged as the most promising cocatalysts (Table 2). Recently, Zhang et al. reported preparation of a series of bicyclic sultones by the asymmetric 1,3-dipolar cycloadditions of chiral nitrones with α,β-unsaturated γ-sultone.39 When chiral nitrones 5.2a−e were subjected to 1,3-dipolar cycloaddition with α,β-unsaturated γ-sultones 5.1 in toluene at 90 °C for 24− 36 h, four diastereomeric mixtures of bicyclic sultones (5.3 to 5.6) (isoxazolidine derivatives) were obtained (Scheme 5). The major products 5.3 and 5.4 could be isolated by flash chromatography. Alhough the reactions showed excellent endo selectivities (endo/exo > 96:100:1 >100:1 >100:1 >50:1 >50:1 >50:1 15:1 22:1 12:1 >100:1 >100:1
83.5:16.5 89:11 96:4 97:3 91.5:8.5 94.5:5.5 97.5:2.5 90.5:9.5 95.5:4.5 92:8 99.7:0.3 99.6:0.4
a
Yield of isolated product. bDiastereomeric ratio (d.r.) values were determined by 1H NMR spectroscopy. cThe e.r. values were determined by chiral column high-performance liquid chromatography (HPLC). d18 mol % of M(OTf)3 was used. ei-Pr2NEtwas used instead of PMP.
Scheme 5. Asymmetric Cycloaddition of Nitrones with α,βUnsaturated γ-Sultones
reactions affording the tricyclic sultones 6.10a−d in good yields as well as good diastereoselectivities (Scheme 6). 2.1.3. Olefin Sulfonation Reactions. Enders et al. described the auxiliary-controlled synthesis of enantiopure α,γ-substituted γ-sultones via α-allylated chiral sulfonates (Scheme 7).42,43 The enantiopure sulfonates 7.1a−e were lithiated with 1 equiv of n-butyllithium in tetrahydrofuran (THF) at −90 to −95 °C. The lithiated sulfonic esters were then allowed to react with different allylic halides at −90 to −95 °C for 2 h and then at −78 °C for 15 h to give the α-allylated sulfonates (R)-7.2a−e in good-to-excellent yields and high diastereomeric excesses (Table 4). The racemization-free removal of the chiral auxiliary to form the corresponding sulfonic acids 7.3a−e proceeded by refluxing 7.2a−e in an EtOH solution containing 2% trifluoroacetic acid (TFA). The crude sulfonic acids 7.3a−e were again refluxed in a TFA/ CH2Cl2 solution to yield the enantiopure α,γ-substituted γsultones in moderate-to-excellent yields with high diastereoselectivities and enantiomeric excesses (Table 5). In a subsequent report, the same group also investigated the asymmetric synthesis of γ-sultones.44 The key step of the synthesis is the zinc bromide-catalyzed aza-Michael addition of the enantiopure hydrazine (S)-1-amino-2-methoxymethylpyrrolidine (S-SAMP) or (R,R,R)-2-amino-3-methoxymethyl-2azabicyclo[3.3.0]octane (R,R,R-RAMBO) (8.2) to alkenylcyclohexyl sulfonates 8.1, which leads to β-hydrazino sulfonates 8.3a−k. Subsequent reductive N−N bond cleavage with BH3·THF and protection of the resulting amines with benzyl chloroformate (CbzCl) gave the sulfonates 8.5a−k in moderate-to-good yields. α-Alkylation of 8.5 with allyl iodide afforded the sulfonates 8.6a,b, which were converted to the corresponding sultones 8.7a,b by diastereoselective iodosultonization (Scheme 8). 2.1.4. Miscellaneous Reactions. In 2000, Morimoto et al. described45 the formation of γ-sultones (9.3a−d) by the treatment of 2,4,6-trimethylpyridine N-oxide (9.1) with BnSO2Cl and Et3N in the presence of various olefins (9.2) (Scheme 9). It was suspected that the α-sultone (9.5) acted as the reactive intermediate for the transformation of γ-sultones (9.3a−d). The stereoisomeric mixtures and poor yields were the main drawbacks of the reactions.
Table 3. Summarized Results of Cycloaddition Reactions
a Entries 2 and 3 were determined by 1H NMR, and others were by isolated yield. b5.3 and 5.4, isolated yields.
protected and converted to the Weinreb amides 6.3a−d, which were homologated using Normant’s Grignard reagent to give the hydroxy ketones 6.4a−d. Oxidation to aldehydes 6.5a−d and subsequent conversion to keto esters 6.6a−d provide the substrates for diazo transfer, which afforded the diazoketones 6.7a−d. Cleavage of the silyl protecting group in the presence of the diazoketone was successfully achieved using HF/MeCN, giving 6.8a−d in good yields. Sulfonylation finally furnished the required substrates 6.9a−d for the CCCC reactions. When the vinylsulfonates 6.9a−d were treated with rhodium(II) octanoate [Rh2(oct)4] in CH2Cl2, they underwent CCCC
2.2. Non-Asymmetric Synthesis of Sultones
2.2.1. Ring-Closing Metathesis Reactions. Metz and coworkers reported preparation of a series of normal (5-, 6-, and 7-membered), medium (8- and 9-membered), and large (155341
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Scheme 6. CCCC Reactions of Vinylsulfonates for the Synthesis of Tricyclic Sultones
Scheme 7. Asymmetric Synthesis of α,γ-Substituted γ-Sultones
oxides with α,β-unsaturated γ-sultones.49 When substituted chlorobenzaldoximes (11.1) were treated with triethylamine, the nitrile oxides 11.2 were generated in situ and underwent [3 + 2]-cycloaddition reactions with γ-sultone 5.1 to afford the bicyclic sultones 11.3a−l in moderate-to-good yields. The main byproduct of the cycloaddition reactions were the dimers (11.4) of the 1,3-dipolar reagents 11.2 (Scheme 11). 1,3Dipolar cycloaddition reactions of α,β-unsaturated γ-sultone 5.1 with a variety of nitrones were also described by the same group.50
membered) ring size sultones by ring-closing metathesis (RCM).46,47 Vinylsulfonates 10.3 were prepared by esterification of the corresponding alcohols 10.2 with vinylsulfonyl chlorides 10.1. RCM of sulfonates 10.3 using the secondgeneration Grubbs’ catalyst A in refluxing CH2Cl2 gave the corresponding sultones 10.4 in moderate-to-excellent yields (Scheme 10). In a subsequent report Cossy and co-workers48 disclosed similar results in benzene at 70 °C. 2.2.2. Cycloaddition Reactions. Liu and co-workers investigated the 1,3-dipolar cycloaddition reactions of nitrile 5342
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Table 4. Asymmetric α-Allylation of Sulfonates 7.1 to Afford the Products (R)-7.2a−e
Scheme 9. Synthesis of γ-Sultones in the Presence of Several Olefins
a
Determined by 13C NMR spectroscopy. bThe value in parentheses denotes diastereomeric excess (d.e.) after recrystallization from 2propanol.
12). Hydrogenation of the α,β-unsaturated γ-sultones (12.2a− e) led to the corresponding β-alkylpropane γ-sultones (12.3a− e) in good yields (Table 6). In the same year, Arava et al. reported the preparation of bicyclic sultone oximes, which are important intermediates for the synthesis of zonisamide, an anticonvulsant drug.52 The methanesulfonate derivatives (13.1a−i) were treated with a strong base such as NaH in dimethylsulfoxide (DMSO), giving the sultones 13.2a−i, which were converted to the corresponding oximes 13.3a−i using hydroxylamine hydrochloride salt (Scheme 13). It was found that the cyclization reaction was efficient when groups such as Br, Cl, OMe, and Me are present on the aromatic ring (Table 7). Here it is important to note that Timoney and co-workers also previously synthesized53 the bicyclic sultone 13.2a in relatively low yield (overall 30%). Spiro sultones are of interest in view of their potential for biological activity. In 1992, Camarasa et al. discovered a family of new drugs, [1-[2′,5′-bis-O-(tert-butyldimethylsilyl)-β-Dribofuranosyl]thymine]-3′-spiro-5″-(4″-amino-1″,2″-oxathiole 2″,2″-dioxide) (TSAO), which exhibited a potent and selective inhibition of HIV-I replication in vitro.54,55 Camarasa and co-workers successfully demonstrated the synthesis of pyrimidine nucleosides containing TSAO analogues and studied their anti-HIV-1 activities.56 The ulose derivative 14.1 was first treated with sodium cyanide in a twophase ethyl ether/water system in the presence of NaHCO3 followed by mesylation to give the cyano mesylate derivative 14.2 in 78% yield. Hydrolysis of the 1,2-O-isopropylidene
Table 5. Cyclization of the γ-Sultones 7.4a−e 7.4 a b c c d e
R1 H t-Bu H H t-Bu t-Bu
R2 CH3 CH3 H H H H
R3 H H H H H CH3
yield (%)
d.e.a,b (%)
d
81 90d 29d 72e 64e 52e,f
78 80 78 54
(≥98) (≥98) (≥98) (≥98)
e.e.c (%) ≥98 ≥98 ≥98 ≥98 ≥98 ≥98
a Determined by 13C NMR spectroscopy. bThe value in parentheses denotes d.e. after flash column chromatography or HPLC. c Determined by HPLC on chiral stationary phase (daicel AD, nheptane/2-propanol 95:5). e.e. = enantiomeric excess. dCyclization conditions: 2% TFA in CH2CI2, reflux, 20 h. eCyclization conditions: 20% TFA in CH2CI2, reflux, 24 h. fProduct mixture also contained 21% of δ-sultone.
2.2.3. Nucleophilic Addition Reactions. In 2007, Bachand et al. demonstrated a new strategy for the synthesis of 2-alkylpropane γ-sultones starting from α-bromomethyl ketones.51 The reaction was initiated by generating the lithium salt of ethyl methanesulfonate with LiHMDS in THF at −78 °C, and the resulting anion was reacted with α-bromomethyl ketones (12.1a−e) to afford, after warming to −50 °C, the corresponding α,β-unsaturated γ-sultones (12.2a−e) (Scheme Scheme 8. Enanatioselective Synthesis of γ-Sultones
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Scheme 10. Synthesis of Sultones by RCM Strategy
Scheme 11. Synthesis of Bicyclic Sultones 11.3a−l by [3 + 2]-Cycloaddition Reactions
Scheme 12. Synthesis of β-Alkylpropane γ-Sultones
Table 7. Summarized Results of Bicyclic Sultone Oxime Synthesis yield (%)
Table 6. Reaction of α-Bromomethyl Ketones with Lithioethylmethanesulfonate and the Preparation of Propane γ-Sultones
a
12.1a−e
R
12.2a−e (%)
12.3a−e (%)
a b c d e
1-adamantyl t-butyl α,α-dimethylbenzyl phenyl ethyl
97 75 ND 70 55
98 89 40a 96 59
compound
13.1
13.2
13.3
13.2
13.3
a b c d e f g h l
H 5-Br 5-CI 3,5-dichloro 3,5-dibromo 3-OMe 4-OMe 5-OMe 4-Me
H 6-Br 6-CI 6,8-dichloro 6,8-dibromo 8-OMe 7-OMe 8-OMe 7-Me
H 6-Br 6-CI 6,8-dichloro 6,8-dibromo 8-OMe 7-OMe 8-OMe 7-Me
76 58 53 47 49 62 62 63 61
93 66 62 58 58 76 76 77 81
(dimethylamino)pyridine (DMAP) gave the 2′,5′-bis-O-silylated nucleosides 14.7a,b,c in 24−32% overall yields (Scheme 14). In a subsequent report, the authors also investigated the synthesis of purine and purine-modified nucleosides containing TSAO analogues.57 Camasara and co-workers also extended their work by the application of the Stille coupling reaction to the synthesis of iodo 3′-spiro sultone nucleosides.58 The iodo precursor 15.2 was prepared in excellent yield by the reaction of the TSAO derivative 15.1 with elemental iodine and ceric ammonium nitrate (CAN) in the presence of triethylamine. Pd-catalyzed cross-coupling reactions of 15.2 with different stannanes were accomplished by the addition of copper(I) iodide as cocatalyst and AsPh3 as ligand, providing the desired sultones 15.3 in moderate to very good yields (Scheme 15). This methodology provided an efficient and straightforward route for the synthesis of 3″-substituted TSAO derivatives bearing differently substituted alkenyl, allyl, aromatic, and heteroaromatic groups, which were evaluated for inhibitory effects on HIV-1 and HIV-2 activities.59 Camarasa and co-workers also described the synthesis of novel adamantane spiro sultones, which were evaluated as antivirals.60 Treatment of 2-adamantanone 16.1 with sodium
Yield over two steps.
Scheme 13. Synthesis of Bicyclic Sultone Oximes
group of 14.2 with aqueous trifluoroacetic acid followed by reaction with acetic anhydride/pyridine afforded a mixture of the two anomeric forms (α and β) of the diacetate derivative 14.3 in 95% yield. Condensation of 14.3 with different heterocyclic bases (thymine, uracil, and 5-ethyluracil) using trimethylsilyl triflate as condensing reagent gave the 14.4a,b,c derivatives in good yields. Treatment of cyano mesylate derivatives 14.4a,b,c with Cs2CO3 afforded the spiro derivatives 14.5a,b,c, which were fully deprotected by saturated methanolic ammonia solution, giving the nucleosides 14.6a,b,c. Finally reactions of 14.6a,b,c with tert-butyldimethylsilyl chloride in the presence of a catalytic amount of 45344
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Scheme 14. Synthesis of TSAO Analogues
tetracyclic sultones by palladium-catalyzed intramolecular cyclization.61 When the compound 17.1a was heated with Pd(PPh3)4 as catalyst, KOAc as base, and tetra-n-butylammonium bromide (TBAB) as additive in dimethylformamide (DMF) for 1 h, the tricyclic sultone 17.2a was formed in 90% yield (Scheme 17). Here it is important to note that, with other palladium catalysts, such as Pd(OAc)2 and Pd(PPh3)2Cl2, the successive decomposition of the starting materials occurred, and in the case of PdCl2 no reaction was observed. With these optimized conditions we prepared a series of polycyclic sultones in 71−90% yields (Table 8). Majumdar et al. reported tricyclic sultone derivatives starting from unactivated vinylic systems by the implementation of the palladium-catalyzed intramolecular Heck reaction.62 The intramolecular Heck reaction of the 2′-vinylphenyl-2-bromobenzene sulfonates 18.1 was carried out using Pd(OAc)2 as catalyst, KOAc as base, and tetra-n-butylammonium bromide (TBAB) as promoter in dry DMF at 80 °C under nitrogen atmosphere, giving the sultones 18.2 in 79−91% yield (Scheme 18). Using the same methodology they also extended their work to the synthesis of polycyclic sultones.63 2.2.5. Miscellaneous Reactions. Recently, Rogachev and co-workers synthesized64 a series of 4,6-diaryl-substituted 1,3dienic δ-sultones (19.4a−g) by the heterocyclization of arylalkynes (19.1a−g) with sulfur trioxide (19.2 or 19.3) (Scheme 19). The influence of temperature and substituents on the reactivity of the substrate was studied. The same group previously reported the synthesis of 1,3-dienic δ-sultone 4,6diphenyl-[1,2]oxathiine 2,2-dioxide by the heteroannulation of phenylacetylene with sulfuric acid or dioxane sulfur trioxide.65,66 Motherwell and co-workers discovered67,68 an unusual rearrangement of homopropargyl arenesulfonates 20.1 under radical reaction conditions [Bu3SnH, AIBN (azobisisobutyronitrile), benzene, 80 °C, 15 h] leading to the formation of α,βunsaturated δ-sultones 20.2. Subsequently, Zhang and Pugh performed69 the same reactions in 10 h and observed the
Scheme 15. Synthesis of 3″-Substituted TSAO Derivatives 15.3 via Stille Coupling Reaction of the Iodo Sultone 15.2
Scheme 16. Synthesis of Adamantyl Spiro Sultone
Scheme 17. Pd-Catalyzed Synthesis of Tricyclic Sultones
cyanide followed by mesylation of the resulting cyanohydrin with mesyl chloride provided the α-mesyloxynitrile 16.2, which underwent intramolecular cyclization to give the adamantane spiro sultones 16.3 upon treatment with base. Subsequent hydrolysis of the spiroadamantane 16.3 led to the corresponding spiro β-keto-γ-sultone 16.4 in good yield (Scheme 16). 2.2.4. Pd-Catalyzed Intramolecular Cyclization Reactions. Recently, we investigated the synthesis of tricyclic and 5345
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Table 8. Synthesis of Polycyclic Sultones
Scheme 20. Synthesis of β-Aryl Cyclic α,β-Unsaturated δSultones
Scheme 18. Pd-Catalyzed Synthesis of Tricyclic Sultones
Scheme 19. Synthesis of δ-Sultones from Arylalkynes
In 2005, Braddock and Peyralans described70 a general route for the synthesis of 5-, 6-, and 7-membered silasultones (21.2a−f) in moderate-to-good yields by the dehydrative cyclization of siloxane disulfonic acids (21.1a−f) via vacuum sublimation (Scheme 21). When the dehydrative cyclizations were performed in refluxing toluene by azeotropic removal of
formation of cyclic α-tributyltin-substituted δ-sultone 20.3 along with α,β-unsaturated cyclic δ-sultones 20.2 (Scheme 20). 5346
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Scheme 21. Synthesis of Silasultones via Dehydrative Cyclization
Scheme 23. Chemoselective Reduction of β-Sultones
water, mixtures of silasultones were obtained, along with unreacted starting materials. Very recently, Velázquez and co-workers demonstrated the synthesis of a series of pyridine and pyrazine ring-fused bicyclic sultones and investigated their cytostatic and antiviral activities.71 The β-amino-γ-sutones (22.1a,b) react with a variety of bis-electrophilic reagents, giving the pyridosultones (22.3, 22.4, and 22.5) and pyrazinosultones (22.8), even though the amino group has poor nucleophilicity (Scheme 22).
Scheme 24. Asymmetric Synthesis of α-Phenyl-γ-HeteroSubstituted Isopropyl Sulfonates
3. SYNTHETIC APPLICATIONS OF SULTONES Sultones have emerged as valuable heterocyclic intermediates because they undergo cleavage of their carbon−oxygen bond in the presence of nucleophiles and can therefore be used as sulfoalkylating agents.4−8 Propane-1,3-sultone (2) is an extremely reactive sulfoalkylating agent, and there are many reports covering its use in organic transformations.72−83 There have also been several new developments in the synthesis of sultones that have been applied to the total synthesis of natural products. Therefore, the synthetic applications of sultones can be divided into two parts: (i) sultones in general organic synthesis and (ii) sultones in natural product synthesis. Scheme 22. Synthesis of Pyrido- And Pyrazinosultones
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Scheme 25. Asymmetric Synthesis of γ-Azido Isopropyl Sulfonates
Scheme 27. Desulfurization of Sultones
3.1. Sultones in General Organic Synthesis
Many groups have utilized sultones in asymmetric as well as general organic transformations. Recently, Koch and Peters described84 chemoselective reductions of β-sultones (4.3) at three different sites: C−Cl, C−O, or S−O bond, which allowed the formation of β-hydroxysulfinic acids (23.1), γ,γ-dichloroallylsulfonic acids (23.2), and γ-monochloroallylsulfonic acids (23.4), respectively (Scheme 23). Enders and Iffland investigated the asymmetric synthesis of α-phenyl-γ-heteroatom-substituted isopropyl sulfonates via diastereoselective ring-opening of γ-sultones.85 The ringopening of sultone 24.1 with various heteroatom nucleophiles was performed in DMF or DMSO as solvent under anhydrous conditions. The resulting sodium and potassium sulfonates 24.2 could be converted into their corresponding isopropyl esters in a two-step sequence. Protonation with methanolic HCl gave the sulfonic acids 24.3 and subsequent treatment with triisopropyl orthoformate in dichloromethane provided the desired isopropyl sulfonates 24.4 in good overall yields (65−86%) and excellent diastereo- and enantiomeric excesses (d.e. = 94−96%, e.e. ≥ 98%) (Scheme 24). Diastereoselective ring-opening of γ-sultones for the asymmetric synthesis of homotaurine derivatives was also developed by Enders and Harnying.86 The ring-opening of the sultones (R,R)-25.1a−f was performed with an excess of sodium azide in DMF under anhydrous conditions. The reactions were completed after stirring at 60 °C for 2 h yielding the corresponding sodium sulfonates 25.2a−f, which were converted to isopropyl sulfonates in a two-step sequence. Protonation with methanolic HCl and subsequent treatment
with triisopropyl orthoformate in refluxing CH2Cl2 provided the desired γ-azido isopropyl sulfonate (R,S)-25.4a−f in 74− 98% overall yields (Scheme 25). Moreover, Enders et al. reported the diastereoselective hydrolysis of α,γ-substituted γ-sultones for the asymmetric synthesis of γ-hydroxy sulfonates.87 Refluxing of diastereo- and enantiomerically pure sultones 26.1 in a solution of H2O− acetone (1:2) for 3 days led to the corresponding sulfonic acids 26.2. The sulfonic acids 26.2 were directly converted into the corresponding methyl sulfonates 26.3 with diazomethane. The hydroxy methyl sulfonates 26.3 were obtained in very good yields (88−91%) and excellent diastereo- and enantiomeric excesses (d.e., e.e. ≥ 98%, Scheme 26). Using similar procedures they extended their work to the synthesis of α,γsubstituted γ-alkoxy methyl sulfonates by the diastereospecific alcoholysis of enantiopure α,γ-substituted γ-sultones.88 γSultones are sometimes also used as alkylating reagents.89 The synthetic elaboration of sultones has been developed by Metz and co-workers by the desulfurization of sultones.90−93
Scheme 26. Hydrolysis of Enantiopure γ-Sultones for the Synthesis of the Corresponding γ-Hydroxy Methyl Sulfonates
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Scheme 28. Synthesis of Oxabicyclic Compounds from an Hydroxyl-Containing α,β-Unsaturated δ-Sultone
sultones (−)-27.1, 27.4, and 27.7 gave the hydroxy dienes (−)-27.3, 27.6, and 27.9, respectively, performed by alkylation with (iodomethyl)trimethylsilane followed by fluoride-induced β-elimination of the resultant silyl compound with tetrabutylammonium fluoride (Scheme 27). Metz and co-workers also investigated the conjugate reduction of hydroxyl-containing α,β-unsaturated δ-sultones by Red-Al [sodium bis(2-methoxyethoxy)aluminumhydride] followed by desulfurization for the synthesis of oxabicyclic compounds.95 The sultones 28.1a,b were hydrogenated to give the saturated derivatives 28.2a,b, which were deprotonated with methyllithium, effecting a smooth β-elimination with concomitant ring-opening to furnish the vinyl sultones 28.3a,b in high yield. The sultone 28.3a was treated with Red-Al and afforded diastereomeric mixtures of saturated heterocycles 28.4a and 28.5a, which were converted to the corresponding ketones 28.6a and 28.7a by Dess-Martin periodinane (DMP) oxidation. Upon treatment of ketones 28.6a and 28.7a with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a single ether 28.8a was produced via a domino process consisting of a βelimination with loss of sulfur dioxide, followed by an intramolecular oxy-Michael addition of the resultant alkoxy cyclohexenone. Similarly, ketone 28.5b gave the cis-fused oxabicyclic compound 28.6b as a single diastereomer under these conditions (Scheme 28). Recently, Novikov and co-workers reported the reduction of δ-sultones (29.1a-c) by SmI2/DMPU (N,N′-dimethylpropyle-
Scheme 29. SmI2 Reduction of Sultones
Scheme 30. Diels−Alder/retro-Diels−Alder Reaction of a Sultone
Recently they reported a new method for the rapid synthesis of highly substituted methylenecyclohexenes with concomitant desulfurization of sultones.94 A two-step desulfurization of 5349
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Scheme 31. Total Synthesis of β-Santalol (31.5) from the Camphenesultone 31.2
target β-santalol (31.5) in a straightforward fashion (Scheme 31). Metz and co-workers developed the total synthesis of 1,10seco-eudesmanolides ivangulin (32.2),99 eriolanin (32.3), and eriolangin (32.4).100−102 Starting from 2-acetylfuran, ivangulin (32.2) was accessed by means of an intramolecular Diels− Alder reaction of a furan-derived vinylsulfonate, a radical cyclization onto a dienylsultone, and a reductive cleavage of a sultone as key steps (Scheme 32). For the synthesis of the more highly oxygenated natural products eriolanin (32.3) and eriolangin (32.4), an alkoxide-directed 1,6-addition to a dienyl sultone and a desulfurization with simultaneous methylenation featured key roles. These sultone routes feature excellent control of the relative configurations of the stereogenic center located on the side-chains. Here it is relevant to note that very recently Metz and co-workers have also reported the total synthesis of two other cytotoxic 1,10-seco-eudesmanolides.103 Moreover, Metz and co-workers also demonstrated the application of the sultone route in the total synthesis of pamamycin-607104 (33.1) by coupling the methyl ester (33.4) of the larger fragment 33.2 and the methyl ester 33.5 of smaller fragment 33.3. The methyl esters 33.4 and 33.5 were synthesized from the tricyclic sultones 33.6 and 33.7, respectively.105−109 Treatment of hydroxyalkylfurans 33.8 and 33.9 with vinylsulfonyl chloride afforded the tricyclic sultones 33.6 and 33.7, respectively, via tandem esterification/cycloaddition in high yields (Scheme 33). In a similar manner, they also succeeded in the total synthesis of pamamycin-621A and pamamycin-635B110 as well as pamamycin-649B.111 By applying the sultone chemistry Metz et al. were also able to synthesize the monomeric subunits, methyl nonactate 34.4, of naturally occurring macrotetrolides 34.5.112−114 Ozonolysis of the sultone 34.1 with eliminative workup using acetic anhydride and pyridine yielded the hemiacetal 34.2 in good yield. A Lewis acid-catalyzed exchange of the hydroxy group in 34.2 with a phenylthio group in 34.3 set the stage for a chemoselective reductive cleavage of both C−S bonds and subsequent alkene reduction in one operation by Raney nickel, leading to the desired methyl nonactate (34.4) with high diastereoselectivity (Scheme 34). A further synthetic application employing a sultone as a key intermediate in an enantioselective synthesis of the unusual sesqiterpenoid alcohol (−)-myltaylenol has been described by Winterfeldt and co-workers.115,116 Intramolecular Diels−Alder reaction of the vinyl sulfonate 35.1 led to the δ-sultone 35.2 in excellent yield. Subsequent oxidative desulfurization by treatment of the lithiated sultone 35.2 with molecular oxygen
Scheme 32. Total Synthesis of 1,10-seco-Eudesmanolides
neurea) for the synthesis of lactones (29.2a−c) (Scheme 29).96 The reactions gave good yields when freshly prepared SmI2 (by the reaction of samarium metal with mercury iodide in THF) were used for the transformations. The domino Diels−Alder/retro-Diels−Alder reaction of δsultone with alkyne was studied by Rogachev and co-workers.97 When the domino Diels−Alder/retro-Diels−Alder reaction of 1,3-dienic δ-sultone (30.1) with acetylene (30.2) was performed under high pressure (1300 MPa) in dichloromethane at room temperature for 3 days, the m-terphenyl dicarboxy derivative (30.3) was obtained in 67% yield (Scheme 30). 3.2. Sultones in Natural Product Synthesis
In 1983, Solas and Wolinsky reported the use of the camphenesultone 31.2 derived from d-10-camphorsulfonic acid24,78 as the starting material for the total synthesis of βsantalol (31.5).98 Alkylation of the sultone 31.2 with the tetrahydropyranyl ether of 2-bromoethanol followed by ringopening of the resulting sultone 31.3 with phenyllithium and desulfurization with sodium−mercury amalgam furnished the desired monoprotected diol 31.4, which was elaborated to the 5350
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Scheme 33. Total Synthesis of Pamamycin-607
Scheme 34. Synthesis of Methyl Nonactate
Scheme 35. Enantioselective Synthesis of (−)-Myltaylenol
desulfonation with Bu3SnLi for the preparation of bakuchiol (36.4) (Scheme 36). Cossy and co-workers also described the racemic synthesis of the originally proposed structure of marine natural product mycothiazole via an unsaturated sultone intermediate, generated by ring-closing metathesis as a key step.118−120 The unsaturated sultone 37.2 was obtained by ring-closing metathesis of diallyl sulfonate 37.1 using Grubbs’ second-generation catalyst. It was subjected to alkylation, followed by deprotonation with n-BuLi and subsequent addition of ICH2MgCl, providing the homoallylic conjugated (Z)-dienol (37.4). From this homoallylic conjugated (Z)-dienol (37.4), they successfully synthesized the originally proposed structure of mycothiazole (37.5) in racemic form (Scheme 37).
provided the desired hydroxy ketone 35.3, which was elaborated to the target (−)-myltaylenol (35.4) in a straightforward fashion (Scheme 35). Recently, Novikov and co-workers developed117 the enantioselective synthesis of bakuchiol by the desulfonation of δ-sultone as the key step. The δ-sultone 1.2 was prepared from (−)-citronellol using diazosulfonate C−H insertion, which was converted to the key intermediate 36.3 by the diisobutylaluminum hydride (DIBAL) reduction followed by
4. BIOLOGICAL ACTIVITIES OF SULTONES Sultones are demanded scaffolds in medicinal chemistry research.7 Biological studies on sultones are mainly concerned with their toxicological, skin sensitization, and antiviral activities. Propane-1,3-sultone (2) presents itself as an extremely potent local and systemic carcinogen.121 It is easily transported 5351
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Scheme 36. Enantioselective Synthesis of Bakuchiol
Scheme 37. Racemic Synthesis of the Originally Proposed Structure of Mycothiazole
Figure 3. Anti-HCMV and anti-VZV sultones.
Figure 4. Structure of TSAO derivatives as anti-HIV-1 agents.
toward the central nervous system is in accordance with the experimental induction of DNA damage in the brain.124 The skin-sensitizing property of sultones came to light in the mid-1970s, when the cause of a 1968 outbreak of contact dermatitis in Scandinavia was traced to 2-chloro-γ-sultones and α,β-unsaturated γ-sultones that had been formed as contaminants in a batch of ether sulfate used to formulate dishwashing liquids.125 Recently, Lepoittevin and co-workers extensively studied the skin-sensitizing properties of hex-1-ene- and hexane-1,3-sultones.126,127 The efficiency of skin sensitization of some sultones are shown in Figure 2.128 Velázquez and co-workers discovered that 4-benzyloxy-γsultone derivatives (38) (Figure 3) have a selective inhibitory activity against human cytomegalovirus (HCMV) and varicellazoster virus (VZV) replication in vitro.129,130 Moreover, Camarasa and co-workers reported that [2′,5′-bisO-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]-3′-spiro-5″-(4″amino-1″,2″oxathiole-2″,2″-dioxide) nucleosides (TSAO) containing a sultone entity in their spiro moiety (39) (Figure 4) were potent and selective non-nucleoside reverse transcriptase inhibitors (NNRTIs) active against human immunodeficiency virus type 1 (HIV-1).131−134
to systemic target sites, does not require metabolic activation,122 and reacts spontaneously with biological macromolecules.123 The preferred experimental organotropism of 2
5. CONCLUSION This review focuses on the recent (mainly the period 2000− 2011) developments of sultones. Emphasis has been given to their syntheses, biological activities, and synthetic applications. Biological activities of sultones are mainly concerned with
Figure 2. Skin sensitization potential of sultones. 5352
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REFERENCES
toxicological, skin sensitization, and antiviral properties. Using sultones as key intermediates, many natural products have been synthesized. We hope that this review will be useful to those who have an interest in sultones.
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AUTHOR INFORMATION Corresponding Author
*Tel.: 04 91 28 83 09. Fax: 04 91 67 09 44. E-mail: shovanku@ gmail.com. Notes
The authors declare no competing financial interest. Biography
Shovan Mondal was born in Madai Pidi (Burdwan), West Bengal, India. He received his B.Sc. from the Burdwan University, West Bengal, India, in 2001 and completed his M.Sc. in pure chemistry in 2003, from Visva-Bharati, Santiniketan, India, with a brilliant academic record. He qualified the National Eligibility Test (NET), jointly conducted by CSIR & UGC (New Delhi, Govt. of India). He finished his Ph.D. degree in 2010 under the supervision of Professor K. C. Majumdar at the Kalyani University, West Bengal, India, and worked as postdoctoral research fellow (with CSIR fellowship, Govt. of India) with Prof. Majumdar at the same university for one more year. Dr. Mondal is currently a postdoctoral research associate in Aix-Marseille Université, Marseille Cedex 20, France, with joint supervision of Dr. Malek Nechab and Prof. Michèle Bertrand. His research interests surround asymmetric synthesis with memory of chirality (specially in enediyne rearrangement), synthesis of heterocyclic compounds of biological interest, and heterocycles containing liquid crystalline compounds. He has coauthored 25 publications so far including one book chapter on thiophene containing natural product synthesis and one chemical review on recent developments in the synthesis of fused sultams.
ACKNOWLEDGMENTS The author is thankful to Université de Provence for his postdoctoral grant. S.M. is indebted to his parents and his beloved wife Swarnali for their unconditional support. The author takes this opportunity to express his deepest gratitude to Prof. K. C. Majumdar (doctoral supervisor, University of Kalyani, west Bengal, India), Dr. Malek Nechab (postdoctoral supervisor, Aix-Marseille Université, France), and Prof. Michèle Bertrand (postdoctoral supervisor, Aix-Marseille Université, France) for their constant encouragement in synthetic organic chemistry. 5353
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Chemical Reviews
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dx.doi.org/10.1021/cr2003294 | Chem. Rev. 2012, 112, 5339−5355