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&Organocatalysis

Organocatalytic Asymmetric Reactions of Epoxides:Recent Progress

Sara Meninno and Alessandra Lattanzi*[a]

Chem. Eur. J. 2016, 22, 3632 – 3642 Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3632

MinireviewDOI: 10.1002/chem.201504226

http://orcid.org/0000-0003-1132-8610http://orcid.org/0000-0003-1132-8610

Abstract: In this Minireview recent advances in the asym-metric reactions of meso and racemic epoxides promoted byorganocatalysts is reviewed. Organic promoters, such as

chiral phosphoric acids, amino- and peptidyl thioureas, andsulfinamides, have been successfully used for a variety ofenantioselective transformations of epoxides under catalyticconditions, involving direct nucleophilic attack at the oxirane

ring, base-catalysed b-eliminations and Brønsted acid cata-

lysed 1,2-rearrangements. Accordingly, highly valuable enan-

tioenriched 1,2-functionalised alcohols, carbonyl compoundsand nitroepoxides are attainable. Dual activation of the re-

agents, provided by the organocatalysts, appears to be themost recurrent strategy, potentially suitable to face otherunmet challenges in asymmetric ring-opening reactions ofepoxides.

Introduction

Ring-opening reactions represent an important tool in organic

synthesis to obtain, in a single step, functionalised linear or

cyclic molecules bearing multiple stereocentres at contiguousor distal carbon atoms.[1]

Epoxides are among the most studied class of compounds,given their ability to undergo ring-opening reactions due to

the significant strain of the three-membered ring. Generalpathways involving ring-opening reaction of epoxides are illus-

trated in Figure 1: a) nucleophilic attack to give 1,2-functional-

ised products; b) strong base catalysed b-elimination leading

to allylic alcohols; c) Lewis or Brønsted acid catalysed Mein-

wald rearrangement to give aldehydes and ketones; d) homo-lytic epoxide C¢O bond breaking under reductive conditions.The generated b-metaloxy radical species, afford alcohols ordifferent products deriving from classical reactivity of radicals.[2]

Although stereospecific substitution of s-bonds is not a pre-dominant strategy in asymmetric synthesis, over the last de-

cades, notable results have been achieved in the nucleophilicring-opening reactions of epoxides (pathway a).[3] As a generalremark, structural features of the epoxides and reaction condi-

tions affect the regio- and stereocontrol of the ring cleavage,

thus showing some limitations in the application of this tool. A

variety of differently substituted non-racemic epoxides, accessi-ble via asymmetric epoxidation of alkenes or alternative meth-

ods,[4] facilitates their employment to obtain highly valuable

and functionalised products, such as 1,2-amino alcohols, 1,2-diols, 2-hydroxy sulfides, 1,2-halohydrins, 1,2-cyanohydrins and

allylic alcohols, in a stereodefined manner. Unsurprisingly, nu-cleophilic ring-opening reaction of enantioenriched epoxides is

one of the most applied key steps in the total synthesis of nat-ural products.[3a, 4d,e]

Asymmetric transformation of epoxides that circumvent the

availability of chiral non-racemic epoxides, as the primarysource, are highly appealing synthetic alternatives to produce

2-functionalised alcohols, carbonyl compounds or enantioen-riched epoxides, difficult to access by classical methods.

Indeed, ready available achiral and racemic epoxides are usedas the starting material in asymmetric ring-opening (ARO) reac-

tions promoted by non-racemic catalysts (Figure 2).

These processes can be grouped into two categories:

I) The desymmetrization of meso-epoxides is the most inves-

tigated approach to obtain enantioenriched 2-functional-

ised alcohols.[5] The two enantiotopic carbon atoms of theoxirane are differentiated by the presence of a chiral non-

racemic Lewis acid, which through coordination with theepoxide oxygen facilitates nucleophile attack. This ap-

proach is particularly attractive as achiral compounds canbe manipulated to enantioenriched products in up to

quantitative yields. Several chiral ligand systems based onZrIV–trialkanolamines,[6] CrIII–[7] and CoIII–salen,[8] YbIII–, TiIV–and MgII–BINOL,[9] ScIII–bipyridine,[10] and Li–Ga(BINOL)2

[11]

have been developed for this scope with azide, amines, al-cohols, water, carboxylic acids, thiols, selenols, cyanide as

the nucleophiles (salen = N,N’-bis(salicylidene)ethylenedia-mine; BINOL = 1,1’-bi-2-naphthol). For the synthesis of hal-

Figure 1. General reactive pathways involving epoxide ring-opening.

Figure 2. Schematic approaches of catalytic asymmetric ring-opening reac-tion of I) meso-epoxides and II) racemic epoxides.

[a] Dr. S. Meninno, Prof. Dr. A. LattanziDipartimento di Chimica e Biologia “A. Zambelli”Universit di SalernoVia Giovanni Paolo II, 84084 Fisciano (Italy)E-mail : [email protected]

ORCID from one of the authors for this article is available on the WWWunder http ://dx.doi.org/10.1002/chem.201504226.

Part of a Special Issue “Women in Chemistry” to celebrate InternationalWomen’s Day 2016. To view the complete issue, visit : http://dx.doi.org/chem.v22.11.

Chem. Eur. J. 2016, 22, 3632 – 3642 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3633

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ohydrins, chiral non-racemic Lewis bases in the presenceof SiCl4, developed by Denmark, proved to be a conceptu-

ally different system for this challenging example of anARO reaction.[12] Likewise, a few examples were reported

for the ARO reaction of meso-epoxides with carbon-basednucleophiles.[3b, 13] A particularly useful application of the

ARO reaction exploits strong chiral lithium amides. Theydemonstrated the ability to catalyse b-elimination of meso-epoxides to enantioenriched allylic alcohols by means of

pathway b (Figure 1), an area which experienced signifi-cant improvement over the years.[14]

II) Kinetic resolution of racemic epoxides is a strategy of pri-mary importance especially when chiral non-racemic epox-

ides cannot be achieved through classical asymmetricmethods. To serve as an efficient process, the reaction

rates of the two enantiomers have to differ significantly in

the selectivity-determined step, involving diastereoisomer-ic transition states, as measured by the stereolectivity

factor (S = krel = kfast/kslow).[15] Taking into account that in an

ideal kinetic resolution the theoretical maximum of 50 %

yield of the less reactive epoxide is attainable, the controlof regioselectivity is a crucial point for the successful appli-

cation of this tool. ARO reactions of racemic terminal epox-

ides came out as the first impressive example, consideringeither the high regiocontrol experienced in their ring-

opening reactions and the lack of general asymmetricmethods to produce them.

Jacobsen and co-workers first demonstrated that CrIII–[16] and

CoIII–salen[17] complexes serve as highly efficient catalyst for the

kinetic resolution of a broad range of terminal epoxides ob-tained together with 2-functionalised alcohols in practically

useful yields and excellent level of enantioselectivity. These cat-alytic systems showed stereoselectivity S factor values higher

than 100, typical of enzymatic catalysis. In this respect, differ-ent hydrolases have been also successfully used in ARO reac-

tions of either meso- and racemic epoxides.[5b, 18]

This Minireview focuses on the recent debut of popular bi-functional organic promoters in asymmetric transformations ofmeso- and racemic epoxides, namely chiral phosphoric acids,amino- and peptidyl thioureas, and sulfinamides. The organo-

catalysed reactions of epoxides are divided into three sections:1) the desymmetrization of meso-epoxides; 2) kinetic resolution

of racemic epoxides; and 3) Meinwald or semi-pinacol rear-rangements involving 1,2-migration of a substituent from onecarbon atom to the other of the epoxide ring. Asymmetric

transformations of epoxides catalysed by either strong chirallithium amides and Lewis bases/SiCl4 systems have been previ-

ously reviewed and will not be considered here.[3a, 19]

Desymmetrization of meso-Epoxides

Intramolecular b-elimination to allylic alcohols

In 2002, inspired by the b-elimination reaction of meso-epox-

ides to allylic alcohols mediated by strong chiral organolithiumbases,[14] Pietrusiewicz and co-authors illustrated the first exam-

ple in which a bifunctional organocatalyst was used in the de-

symmetrization of meso-1-phenyl-3-phospholene oxide

1 (Scheme 1).[20]

The allylic alcohol 3, bearing also a stereogenic phosphorouscentre, was obtained in 41 % yield and 52 % ee. The best per-forming catalyst among the cinchona alkaloids tested, quini-

dine 2, was suggested to preferentially deprotonate the Ha hy-drogen atom anti to the epoxide oxygen. In the hydrogen-

bonded binary epoxide–quinidine complex, the interaction of

the catalyst OH group with the phosphoryl oxygen of the re-agent was important to selectively direct the deprotonation.

The opposite enantiomer of the product was recovered whenusing pseudoenantiomeric quinine catalyst. The role of the hy-

droxyl moiety proved to be critical as the O-acetyl quinidinewas found to be completely inactive.

Sara Meninno was born in Ariano Irpino(Italy). She received her M.Sc. in Chemistryfrom the University of Salerno in 2011. In2015, she earned her Ph.D. in Chemistry atthe University of Salerno under the supervi-sion of Prof. A. Lattanzi working on asymmet-ric oxyfunctionalisations and tandem reac-tions. At present, she is working as a postdoc-toral fellow on the development of new orga-nocatalytic asymmetric methodologies for theconstruction of heterocyclic compounds.

Alessandra Lattanzi was born in Rome (Italy).She graduated and received her Ph.D. from“La Sapienza” University of Rome. She hasbeen visiting scientist in the groups of Prof. V.K. Aggarwal (Sheffield, 1999–2000) and Dr.N. E. Leadbeater (London, 2001). Since 2005she has been Associate Professor at Universityof Salerno. Her research interests includeasymmetric organocatalysis, stereoselectivemetal-catalysed oxidations, and the descrip-tion of chiral structures through algebraicand geometrical methods.

Scheme 1. Quinidine-catalysed desymmetrization of phospholene epoxide 1.


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Although a substoichiometric loading of quinidine and longreaction time were necessary to achieve modest conversion,

this process paved the way to the employment of readily avail-able bifunctional organocatalysts in base-catalysed rearrange-

ments of meso-epoxides.More recently, Jørgensen and co-workers illustrated an ele-

gant example of b-elimination of meso-epoxycyclopentanones4 to 4-hydroxycyclopent-2-enones 5 catalysed by thiourea-de-rived cinchona alkaloids (Scheme 2).[21] The transformation is

particularly interesting in view of the usefulness of enantioen-riched compounds 5 as starting reagents for the synthesis ofprostaglandins.[22]

The outcome of the intramolecular b-elimination of epoxides4 was found to be dependent on epoxide substitution. Highenantioselectivity was achieved with organocatalysts 6 a–b,bearing the thiourea and the quinuclidine nitrogen placed at

different distances. In the case of compound 5 c, the absoluteconfiguration was confirmed by X-ray analysis, which helpedto formulate a transition state for the process.

The authors suggested that hydrogen bonding of the thiour-ea group with the carbonyl moiety enhances the acidity of a-

protons, thus easily and selectively deprotonated by the quinu-clidine nitrogen, according to an E1cb-like mechanism.

Interestingly, one-pot desymmetrization/Michael addition se-quences were reported by using meso-epoxycyclopentanone4 c and catalyst 6 b to obtain functionalised 4-hydroxycyclo-pentanone scaffolds in high stereocontrol (Scheme 3).

The highly diastereocontrolled formation of syn-diastereoiso-mer 7 could be explained invoking, a hydroxyl-directed ap-proach of the thiol, through hydrogen bonding coordination,to the most hindered face of the carbon–carbon double bond

of enantioenriched intermediate 5 c.[23] In the case of stericallydemanding malonate, the enolate approach to the less en-

coumbered face would preferentially lead to trans-derivative 8.

Nucleophilic ring-opening to 2-amino alcohols, 2-hydroxysulfides, O-protected 2-hydroxythiols and 1,2-diols

Peptides are becoming attractive catalysts in asymmetric syn-thesis thanks to their facile access and more importantly, the

great variety of structural diversity achievable in the final scaf-

fold.[24] However, the high level of functionalisation coupledwith their conformational complexity in solution, make both

the catalyst design and insights into the origin of enantioselec-tivity difficult, when compared to simpler bifunctional organo-

catalysts. A general tool to assist the screening and identifica-tion of the most effective peptide catalyst in different reactions

has been illustrated by Miller’s and Wennemers’ groups, taking

advantage of combinatorial chemistry.[25] Moreover, site selec-tive modifications of peptides,[26] coupled with molecular mod-

elling[27] of the best performing peptides, give useful hints toidentify the most active conformers involved in the catalysis.

Although applications of peptides in asymmetric catalysis havegrown comparatively far less rapidly than commonly known

organocatalysts, striking results, using catalytic loadings as low

as 1 mol %, appeared in the area of oxidation, carbon–carbonbond formation and esters hydrolysis.[24]

In 2014, Chimni and co-authors designed a chiral combinedthiourea peptidyl scaffold for the desymmetrization of meso-

epoxides with amines.[28] The idea was to exploit a dual activa-tion of the epoxide, provided by the thiourea group and the

nucleophile by a generic site in the chiral peptidyl moiety. Ac-

cording to the literature,[29] a peptide linker incorporating b-turn conformations, introduced with l-Pro-d-Pro and d-Pro-Glydipeptides, would have been expected to be the most effec-tive. After a preliminary screening of peptidyl thiourea catalysts

and reaction conditions on model reaction of cis-stilbene oxideand N-phenyl piperazine, catalyst 9 a, used at 10 mol % load-ing, was selected to explore the scope of the ARO reaction inCH3CN as solvent at room temperature (Scheme 4).

The 1,2-amino alcohols 12 were isolated in high yield andand up to 79 % ee, bearing meta- and para-halogen substitu-tion on the aromatic ring of either the piperazine and the ep-

oxide. ortho-Halogen- and nitro-substituted epoxides were notring-opened by the amine, likely due to steric reasons and del-

eterious competitive hydrogen-bonding ability of nitro groupwith respect to the oxirane oxygen with the thiourea moiety

of the catalyst. Simple N-alkyl-substituted piperazine, piperi-dine and primary or secondary alkyl amines afforded racemicproducts. By further screening, it was established that the ab-

solute configuration of the N-terminal residue of the catalystscontrolled the enantioselectivity of the product. Predictably,

the opposite enantiomer of compounds 12 are obtainablewhen using enantiomeric catalyst 9 b and a significant impact

Scheme 3. One-pot desymmetrization/Michael sequence to functionalised 4-hydroxycyclopentanones.

Scheme 2. Desymmetrization of meso-epoxycyclopentanones catalysed bythiourea-derived cinchona alkaloids (Ar = 3,5-(CF3)2C6H3, TIPS = triisopropylsil-yl).


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on the enantiocontrol was observed when using diastereoiso-meric catalyst 9 c, bearing opposite absolute configuration ofthe C-terminal residue (Scheme 4). It will be necessary to en-hance the catalytic efficiency and to extend the scope of this

rather challenging ARO reaction, possibly developing newhybrid thiourea–peptidyl organocatalysts.

The catalytic activity of a large number of transformations

mediated by BINOL-derived phosphoric acids has been ex-plained invoking a Lewis base/Brønsted acid catalysis.[30] DFT

studies supported concerted mechanisms, in which the orga-nocatalyst establishes simultaneous hydrogen-bonding interac-

tions with the electrophile and the nucleophile.[30a,b] This typeof bifunctional activation is well-suited for a ring-opening reac-tion, and has been successfully applied in the BINOL-derived

phosphoric acid mediated nucleophilic ARO reaction of meso-epoxides.

In 2013, Sun and co-authors reported the first example byusing mercaptobenzothiazoles.[31] A variety of potential nucleo-

philes, such as alcohols, phenols, amines and thiols, werechecked in the model ARO reaction of cyclohexene oxide in

the presence of a phosphoric acid without success. Only whenusing highly nucleophilic 5-methoxymercaptobenzothiazole 13was the desired alcohol derivative obtained. (R)-3,3’-Bis(2,4,6-triisopropylphenyl)-1,1’-binaphthyl-2,2’-diyl hydrogen phos-phate (TRIP) 14 a was found to be the most effective catalystin CH2Cl2 as the solvent (Scheme 5).

The process was affected by the nature of the starting epox-

ide and, accordingly, it was performed at low or at room tem-

perature using 2.5 mol % of the organocatalyst. Cyclic and acy-clic products 15 were recovered in moderate to good yieldand enantioselectivity.

The authors proposed the activation of epoxide through hy-

drogen-bonding with the acidic group of the TRIP-BINOL-de-rived phosphoric acid, whereas the tautomeric form of the nu-

cleophile would be involved in hydrogen-bonding interaction

with the basic phosphoryl oxygen (Figure 3).Although this process appears to be restricted to mercapto-

benzothiazoles as the nucleophiles, cleavage of the heteroaro-matic moiety of product 15, to give the corresponding 2-mer-capto alcohol without loss of the enantioselectivity, demon-

strated to be a synthetically useful derivatisation (Scheme 5).Kureshi and co-workers reported the ARO reaction of meso-

epoxides with anilines catalysed by an easily available enantio-merically pure sulfinamide 16 (Scheme 6).[32]

Cinchona alkaloids such as cinchonidine and quinidine didnot catalyse the reaction, whereas commercially available (S)-

and (R)-tert-butylsulfinamides led to both enantiomerically en-riched products 18 with modest yield, but an encouraginglevel of enantioselectivity. Improvements were achieved whenmodifying catalyst structure by introducing a chiral amide por-tion with a view to enhance either the hydrogen-bonding abili-

ty and exploit matching effects.Replacement of the sulfinamido group with a chiral secon-

dary amine moiety drastically decreased the level of enantiose-lectivity, showing the preferential role of sulfinamide chiralityin directing the absolute configuration of the final product.

Cyclic and linear meso-epoxides were smoothly transformedinto 1,2-amino alcohols in high yield and enantioselectivity

when using aniline as the nucleophile. Electron-withdrawingpara-substituted anilines proved to be unreactive as well as ali-

Scheme 4. ARO reaction of cis-stilbene oxides with N-aryl piperazines cata-lysed by peptidyl thiourea (Ar = 3,5-(CF3)2C6H3).

Scheme 5. ARO reaction of meso-epoxides with mercaptobenzothiazoles cat-alysed by TRIP-BINOL-derived phosphoric acid (Ar = 2,4,6-(iPr)3C6H2),TBS = tbutyldimethyl silyl.

Figure 3. Activation of the reagents by TRIP-BINOL-derived phosphoric acidin the ARO reaction of meso-epoxides with mercaptobenzothiazoles.


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phatic amines, which was ascribed to their low nucleophilicity

and strong basicity, respectively.These results showed that electronic effects are critical in

regulating the activation of the reagents by sulfinamide 16.NMR experiments support the formation of a hydrogen-bond-

ing network among catalyst, epoxide and aniline in the transi-tion state of the process (Scheme 6).[32]

A conceptually new approach for ARO reactions has been re-

cently devised by List and co-workers for the hydrolysis ofmeso-epoxides catalysed by BINOL-derived phosphoric acids.[33]

Inspired by the mechanism of action of epoxide hydrolases,[18]

a class of enzymes that do not depend upon metal cofactors,

they envisaged the organocatalyst could activate a carboxylicacid in a supramolecular assembly, which is responsiblefor the nucleophilicity enhancement of the carboxylic acid as

well as the acidity of the assembled heterodimer species(Scheme 7).[34]

To achieve this goal, new H8-BINOL-derived phosphoricacids, bearing sterically demanding groups at the 3,3’-positionof the BINOL-derived backbone, were designed. The compactstructure of this modified class of BINOL-derived phosphoric

acids, named “confined catalysts”, was conceived to imparthigher substrate selectivity by decreasing the size of the activesite.[30c] Under optimised conditions a variety of cyclic epoxides,

also bearing heteroatoms in the ring, reacted with benzoicacid in the presence of 10 mol % of catalyst 14 b at variabletemperatures, to give the mono-protected trans-1,2-diols inhigh yield and enantioselectivity. Interestingly, the ARO reac-

tion catalysed by compound 14 b showed to have a broadsubstrate scope including challenging acyclic small epoxidesand stilbene oxide.

A convenient one-pot sequential approach to enantiomeri-cally enriched trans-diols was developed starting from the cor-

responding Z-alkenes. Benzoic acid, a by-product generatedfrom the epoxidation of the alkene, was partially consumed as

reagent of the following ARO reaction (Scheme 8). The se-

quence illustrated in Scheme 8 can be considered a first com-

plementary chemical protocol to the known Sharpless asym-metric dihydroxylation of alkenes.[35]

The same group reported a valuable extension of the activa-tion strategy illustrated in Scheme 7, using thiocarboxylic acids

21 to obtain the corresponding 2-hydroxythioesters 22(Scheme 9).[36]

This transformation enabled a facile entry to the synthesis of

a variety of 2-hydroxy thioesters with excellent level of enan-tioselectivity when employing thiobenzoic acid. The usage of

Scheme 6. Sulfinamide-catalysed ARO reaction of meso-epoxides with ani-lines.

Scheme 7. ARO reaction of meso-epoxides with benzoic acid catalysed byH8-BINOL-derived phosphoric acid.

Scheme 8. One-pot sequential asymmetric synthesis of trans-diols from Z-al-kenes.

Scheme 9. ARO reaction of meso-epoxides with thiobenzoic acid catalysedby H8-BINOL-derived phosphoric acid.


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thioacetic acid was also feasible, although a slight decrease ofthe enantioslectivity was observed. By modifying the reaction

parameters, a cascade process has been developed, encom-passing a Brønsted acid catalysed intramolecular transesterifi-

cation reaction to the corresponding esters, bearing a freethiol group (Scheme 10).

Moreover, products 22 could be easily in situ deprotectedby hydrazine to give almost enantiopure 1,2-hydroxy thiols asexemplified in Scheme 10. The entire sequence stands for

a first example of formal asymmetric sulfhydrolysis reaction ofepoxides. It is interesting to point out that the ARO reaction of

meso-epoxides catalysed by Brønsted acid 14 b is the key stepto obtain by design, thio-protected alcohols 22 or O-protectedthiols 23 of the same absolute configuration.

Kinetic Resolution

As already mentioned, the majority of kinetic resolution exam-ples focused on terminal epoxides for two equally important

reasons: 1) the nucleophilic attack in ARO reaction occurs withhigh regioselectivity to the sterically less hindered position and

2) racemic terminal epoxides are easily obtainable from inex-pensive and broadly available terminal olefins. For the catalytic

kinetic resolution of racemic epoxides, mainly heteroatom-cen-tred (nitrogen and oxygen) and, to a far lesser extent, carbon-

centred nucleophiles have been used.[3a]

Besides water,[17a] azide,[16] anilines,[37] alkyl amines,[38] N-Bocprotected sulfonamides[39] and carbamates[17b] have been em-

ployed in the catalytic kinetic resolution. In the field of organo-catalysis, cornerstone research showed that achiral ureas[40]

and thioureas[41] are able to promote the regioselective ring-opening reaction of epoxides. In 2008, Schreiner and co-au-

thors developed a regioselective alcoholysis of styrene oxides

assisted by an organocatalytic system composed of twoBrønsted acids, N,N-bis-[3,5-bis-(trifluoromethyl)phenyl]-thiour-

ea 24 and mandelic acid 25 (Scheme 11).[41a]

The authors suggested a cooperative Brønsted acid catalysis,

in which the thiourea coordinates to the acid 25 throughdouble hydrogen-bonding interaction enhancing a-OH bond

acidity. The epoxide is activated by means of a single-point hy-drogen bond assisting nucleophilic attack at the benzylic posi-

tion.The first example of an ARO reaction through kinetic resolu-

tion of a racemic epoxide was illustrated by Jørgensen and co-workers in the thiourea-derived cinchona alkaloid 6 b catalysedrearrangement of compound 26 (Scheme 12).[21] Synthetically

useful compound 27 was isolated in good yield and 69 % ee.We have recently disclosed an asymmetric aminolytic kinetic

resolution (AKR) of racemic b-aryl-substituted a-nitroepoxides

with aniline.[42] The process, catalysed by thiourea-derived cin-chona alkaloid 6 c, allowed the first enantioselective synthesisof b-aryl-substituted a-nitroepoxides 28 (Scheme 13).

Racemic b-aryl-substituted a-nitroepoxides are well-suited

substrates to undergo a process of catalytic kinetic resolution.Indeed, in the 1970s, it was first demonstrated that they could

Scheme 11. Alcoholysis of styrene oxides promoted by two cooperative hy-drogen-bonding organocatalysts.

Scheme 12. ARO reaction of racemic epoxide 26 catalysed by amino thiour-ea 6 b.

Scheme 13. Aminolytic kinetic resolution of a-nitroepoxides (Ar = 3,5-(CF3)2C6H3) catalysed by amino thiourea 6 c.

Scheme 10. ARO reaction of meso-epoxides with thiobenzoic acid catalysedby H8-BINOL-derived phosphoric acid.


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be ring-opened with complete regioselectivity by different nu-cleophiles to give a-substituted ketones or heterocyclic com-

pounds.[43]

The substrate scope of AKR is broad for variously substituted

aromatic nitroepoxides 28, bearing either electron-donating orelectron-withdrawing groups. The epoxides were recovered in

acceptable yield and good to high enantioselectivity. Modestvalues of selectivity factors (3

for the cis-configured epoxide. The latter needed several hoursto achieve a good conversion to aldehyde and a partial hydro-

gen shift (3–5 % ketone) was also observed.The authors noticed that the enantiopurity of both product

(R)-33 a and the starting epoxide (S,R)-31 a increased over thetime. They rationalised these data proposing the mechanism il-

lustrated in Figure 5. Complexation between catalyst 32 and

starting material would occur to give diastereomeric matched

and mismatched transition-state structures. This would gener-ate an increase in ee values during the reaction, in a similar

way to a kinetic resolution of non-racemic starting material.[15]

An example of semipinacol rearrangement has been report-

ed by Tu and Cao, who developed an one-pot procedure for

the synthesis of various trioxygenated spirocycloalkanedionederivatives, bearing a tertiary and quaternary contiguous cen-

tres.[48] The entire process is based on an asymmetric epoxida-tion, catalysed by primary amine 34, followed by a semipinacolrearrangement (Scheme 17). Water as co-solvent was found to

be essential for the rearrangement to proceed. A series of vi-nylogous a-ketol substrates were checked in the asymmetric

epoxidation with H2O2 followed by in situ acidification.

This protocol proved to be highly efficient in terms of ste-reocontrol when using different substituents at the C-3’ posi-tion of the cyclobutanol moiety, including cyclic portions. Theone-pot reaction worked with the same efficiency also witha larger C3-cyclopentanol moiety. The rearrangement proceed-ed by means of a concerted SN2-type process, as attested bythe isolation of a single spiro-isomer.

The examples illustrated in Schemes 16 and 17 are sub-strate-controlled asymmetric rearrangements of chiral non-rac-

emic epoxides which allowed the formation of optically activecarbonyl compounds.

A first study on an asymmetric 1,2-rearrangement of racemicepoxides controlled by SPINOL-derived N-triflyl phosphoramide38, has been recently reported by Du and coauthors.[49] Thispromoter proved to be more effective than less sterically de-

manding BINOL-derived phosphoric acids tested in the dera-

cemisation of terminal epoxides 39 to chiral non-racemic alde-hydes (Scheme 18).

The latter were reduced in situ to afford more stable alco-hols 40, isolated in satisfactory yield and up to 50 % ee. Thepostulated reaction mechanism

would involve activation of the ep-oxide by the Brønsted acid with

consequent ring-opening, as de-picted in Figure 6. The carbocationformed would undergo an asym-metric 1,2-hydride shift, generatinga chiral non-racemic aldehyde. It is

likely to expect an improvement ofthis process by using confined C2-

symmetric BINOL-derived imidodi-phosphoric acids.[30c]

Summary and Outlook

Asymmetric reactions of epoxides, previously established withchiral metal-based systems, strong lithium amides and hydro-

lases, have recently stimulated the employment of some of themost effective bifunctional organocatalysts as players in the

ARO reaction of meso- and racemic epoxides with a variety ofnucleophiles. Notable and more general results have been ach-

Figure 5. Mechanistic hypothesis for the epoxide 1,2-rearrangement cata-lysed by silicon–thiourea Lewis acid.

Scheme 17. One-pot synthesis of spiro-cycloalkanediones by an organocata-lytic asymmetric epoxidation/semipinacol rearrangement.

Scheme 18. 1,2-Rearrangement of racemic terminal epoxides catalysed bya SPINOL-derived N-triflyl phosphoramide.

Figure 6. Hypothetical transi-tion state for the Brønstedacid catalysed 1,2-rearrange-ment of terminal racemic ep-oxides.


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ieved in the desymmetrization of meso-epoxides with BINOL-derived phosphoric acids when using thiols, anilines, benzoic

and thioacids. These catalysts also display potential applicationin asymmetric 1,2-rearrangements of epoxides. Hybrid peptidyl

thioureas are promising promoters in the challenging ARO re-action of cis-stilbene oxides with amines. Cinchona-derived thi-

oureas serve as catalysts in the desymmetrization of meso-ep-oxides proceeding through intramolecular b-elimination to

give allylic alcohols and in the kinetic resolution of a-nitroep-

oxides. As a general remark, simultaneous activation of the ep-oxide and the nucleophile provided by the organocatalysts has

been proposed to be the “leitmotif”. The purpose of this Mini-review was to highlight the underexplored potential of orga-

nocatalysed asymmetric reactions of epoxides and inspired re-searchers for further improvements. At present, the organoca-talysed ARO reactions of epoxides are limited to highly nucleo-

philic, heteroatom-centred reagents. The design of more so-phisticated organocatalysts suitable to extend the arena of

nucleophiles, including softer carbon-centred ones, will signifi-cantly expand application of this tool for the asymmetric syn-thesis of functionalised scaffolds.

Acknowledgements

Financial support was provided by MIUR and University of Sale-

rno. A.L. thanks the European COST Action CM0905-Organoca-talysis (ORCA).

Keywords: desymmetrization · epoxides · kinetic resolution ·organocatalysis · ring-opening reactions

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Received: October 20, 2015Published online on January 19, 2016


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