Intermolecular Diels-Alder Cycloadditions of Furfural-Based Chemicals from Renewable Resources: A Focus on the Regio- and Diastereoselectivity in the Reaction with Alkenes.

A recent strong trend toward green and sustainable chemistry has promoted the intensive use of renewable carbon sources for the production of polymers, biofuels, chemicals, monomers and other valuable products. The Diels-Alder reaction is of great importance in the chemistry of renewable resources and provides an atom-economic pathway for fine chemical synthesis and for the production of materials. The biobased furans furfural and 5-(hydroxymethyl)furfural, which can be easily obtained from the carbohydrate part of plant biomass, were recognized as "platform chemicals" that will help to replace the existing oil-based refining to biorefining. Diels-Alder cycloaddition of furanic dienes with various dienophiles represents the ideal example of a "green" process characterized by a 100% atom economy and a reasonable E-factor. In this review, we first summarize the literature data on the regio- and diastereoselectivity of intermolecular Diels-Alder reactions of furfural derivatives with alkenes with the aim of establishing the current progress in the efficient production of practically important low-molecular-weight products. The information provided here will be useful and relevant to scientists in many fields, including medical and pharmaceutical research, polymer development and materials science.

1. Introduction

To date, the development of efficient technologies for catalytic or biocatalytic conversion of renewable plant biomass into viable targeted products remains one of the most important and challenging tasks for modern chemical science [1,2,3,4,5]. The primary advantage of biorefining based on renewable carbon sources over traditional refining using exhaustible resources is the realization of a carbon-neutral cycle, leading to zero total carbon emissions into the environment during chemical production and consumption. Biobased furans—furfural (FF) and 5-(hydroxymethyl)furfural (HMF)—can be obtained by acid-catalyzed dehydration of carbohydrates and are recognized as “platform chemicals”. As expected, the key role of biobased technologies is to replace the key existing products of oil-based refinement with renewables [4,6,7]. The tremendous synthetic potential explains the unprecedented scale of research in the fields of synthesis and application of furanic platform chemicals for the production of biofuels, chemicals, polymers and other industrially important products, which was evidenced by the increasing number of relevant publications (partially since 2010, Figure 1) and was highlighted in many recent reviews [7,8,9,10,11,12,13,14,15,16,17,18,19,20].

Figure 1

Number of publications mentioning biobased furans per year. Source: Scopus. Keyword: “furfural”.

One of the focused reactions of furan chemistry is the [4+2]-cycloaddition, well known as the Diels-Alder (DA) reaction, in the classic mechanism based on the interaction of the highest occupied molecular orbital of furanic diene (HOMOdiene) and the lowest unoccupied molecular orbital of dienophile (LUMOdienophile). The DA reaction may proceed with high efficiency under solvent-free and/or noncatalytic conditions, representing the ideal example of a “green” process characterized by a 100% atom economy and a low to moderate E-factor [21,22]. Intermolecular furan/alkene DA reactions have a high potential for application in fine organic synthesis, biomedical areas, materials sciences, polymers and bio-organic chemistry (Figure 2) [23,24,25,26,27,28,29,30].

Figure 2

Diels-Alder cycloaddition with biobased furans as an approach towards practically important products. Summarizing and analyzing scientific data about the regio- and diastereoselectivity of intermolecular Diels-Alder cycloadditions between furfural derivatives and alkenes was a general aim of this review.

The direct Diels-Alder reaction of FF or HMF with common alkenes is thermodynamically unfavorable [31,32,33], but this type of cycloaddition can be performed after decreasing the HOMO–LUMO gap through reduction of the aldehyde group into more donor functionality. Another approach is redox-neutral chemical activation through modification of aldehyde into acetal or hydrazone with the possibility of aldehyde deprotection. In general, the nature of the substituent at the C2 position in the furan ring strongly affects reactivity in DA cycloadditions; furans with electron-donating groups are well-suited as substrates, while electron-poor furans display low reactivity [34,35].

In the case of highly active dienophiles, DA adducts may be formed under noncatalytic conditions; for other substrates, catalysis by Lewis acids is usually needed. Reactions of furans with alkene dienophiles are often characterized by facile retro-DA (rDA) reactions due to the low reactivity of furan as a diene that leads to low diastereo- and regioselectivity of the cycloaddition (Scheme 1). The orbital HOMOdiene and LUMOdienophile energy difference seems to control the diastereomer distribution [32,36]. Charge interactions between diene and dienophile favor orthoselectivity, while steric hindrance promotes metaselectivity but without strong kinetic or thermodynamic preference for a single regioisomer [32,37].

Scheme 1

Possible regio- and diastereomers in Diels-Alder cycloaddition of C-2-substituted furans with mono-substituted alkenes.

Information about the selectivity of DA reactions is helpful to scientists in many fields, including medical and pharmaceutical research, polymer development and materials science. The regio- and diastereoselectivity of DA cycloaddition are important parameters for the high-yielding synthesis of chemically pure products, especially in the development of drugs, because diastereomers may exhibit different biological activities [38]. The endo- and exo-DA adducts have different steric properties and convert to furan and alkene components at different temperatures, which may be important in the development of various dynamic systems [39,40]. Moreover, the stereo structure of cyclic alkenes may influence the reactivity in ring-opening metathesis polymerization used for the synthesis of stereoregular polymers [41]. This difference for furan-derived oxanorbornanes was clearly demonstrated by Kilbinger and coworkers. They showed in several examples that furan/maleimide DA adducts react quickly and selectively with the G3 catalyst, resulting in the formation of monomolecular carbene complexes that display low reactivity with the second molecule of oxanorbornane (both endo or exo) due to unfavorable steric factors (Scheme 2a). In contrast, exo-oxanorbornane counterparts undergo efficient homopolymerization under the same reaction conditions (Scheme 2b) [41].

Scheme 2

Different reactivities of endo (a) and exo (b) furan-derived oxanorbornanes in Ru-catalyzed ring-opening metathesis polymerization. R = H, Me, n-propyl or n-pentyl. G3 = third generation Grubbs catalyst.

Several approaches may be used to increase the regio- and diastereoselectivity of DA reactions: fine-tuning of steric and electronic properties of dienes or dienophiles; variation of reaction conditions such as temperature, time, type of solvent and pressure; and catalysis by Lewis acids. Generally, for furan/alkene cycloadditions, exo isomers are more stable and form under thermodynamic control of the reaction (at high temperature), while endo isomers are kinetically preferred [36,42,43,44].

In this review, we summarized the recent literature about the regio-, stereo- and diastereoselectivity of intermolecular Diels-Alder (IMDA) cycloadditions of simple furfural derivatives with alkenes used for the synthesis of cyclic aliphatic or aromatic products. Some aspects, such as the influence of a catalyst or solvent, the type of diene and dienophile and, in some cases, comparison with other furanic substrates, were highlighted. Several reviews have covered the synthetic potential of biobased furans for the production of biofuels, chemicals and materials [10,11,15,18,30,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59], as well as the mechanisms and selectivity of DA cycloadditions [60,61,62,63,64]. These discussions will not be repeated here. Instead, a dedicated survey of the literature focused on the selectivity of IMDA cycloadditions of FF derivatives with alkenes (which has not been previously reported) will be provided here.

2. Selectivity of Diels-Alder Cycloaddition with Furfural Derivatives as Substrates

2.1. 2-Methylfuran

2-Methylfuran (2-MF) is the simplest 2-substituted furan produced by the reduction of the aldehyde group in FF. The selectivity of IMDA reactions of 2-MF with common cyclic and acyclic alkenes is presented in Table 1 and Table 2. Noncatalytic reactions of 2-MF with maleic or citraconic anhydride led to cycloadducts with exo configurations even at room temperature (Table 1, entries 1–3). The current literature provides scarce information about the selectivity of reactions of 2-MF with maleimides under kinetic conditions. In the case of maleimides reacting with 2-MF at room temperature, the formation of >20% endo isomer was observed (entry 4), while at temperatures more than 60 °C, exclusive formation of the exo isomer was found for most maleimides (Table 1). However, in a water medium for some N-substituted maleimides, the content of endo isomers was higher even under high temperature (entries 8, 10). For N-carboxyethyl maleimide reacting with furan, 2-MF or 2,5-dimethylfuran, the best exoselectivity was obtained in the case of furan, while 2,5-dimethylfuran showed the best endoselectivity under kinetic conditions (entries 16–19) [65]. The cycloadduct of 2-MF with N-phenyl maleimide was isolated in a pure, optically active form with 90% ee using dynamic enantioselective crystallization by continuous suspension in heptane or hexane solution with glass beads at 80 °C in the presence of trifluoroacetic acid (TFA) to accelerate the deracemization (entry 13) [44].

Table 1

IMDA cycloadditions of 2-MF with cyclic alkenes.

№	Dienophile	Conditions	Selectivity	Yield of DAAdducts (%),[Ref.]
1	X = O	Neat, RT, N2, 24 h	Exo	91, [66]
2	X = O	Neat, RT, 10–15 °C, 2–3 h	Exo	96 (crude), [67]
3	Citraconicanhydride	CH2Cl2, RT, 15 kbar	Exo (ortho/meta 1:1)	65 1, [68]
4	X = NH	Et2O, RT, 3 days	Endo/exo 2	21 (for endo), [69]
5	X = NH	THF, reflux, 4 h	Exo	94, [70]
6	X = NMe	Toluene, 90 °C	Exo	92, [71]
7	X = NMe	Et2O, 90 °C	Exo	66, [72]
8	X = NEt	H2O, 65 °C	Endo/exo 1.4:1	100, [73]
9	X = N(tBu)	H2O, 65 °C	Exo	100, [73]
10	X = NPh	H2O, 65 °C	Endo/exo 1.6:1	100, [73]
11	X = NPh	4:1 Toluene/benzene, RT, 1.1 GPa	Endo/exo 1.66:1	85, [74]
12	X = NPh	CDCl3, 60 °C	Exo with traces of endo	90, [44]
13	X = NPh	Hexane or heptane, TFA, glass beads, 80 °C, 5–8 days 3	(-)-Exo, 86–90 ee	80, [44]
14	X = NPhF5	Neat, reflux	Exo	50, [75]
15		THF, 65 °C	Exo	64, [70]
16	X = NCH2CH2COOH	CHCl3, 38 °C, 5 days	Endo/exo 28:72	100, [65]
17 4	X = NCH2CH2COOH	CHCl3, 38 °C, 5 days	Exo	100, [65]
18 5	X = NCH2CH2COOH	CH2Cl2, RT, overnight	Endo/exo 78:22	100, [65]
19 5	X = NCH2CH2COOH	CH3CN, 60 °C, 6 h	Endo/exo 22:78	100, [65]

1 Yield of DA adduct after hydrogenation. 2 Ratio of diastereomers was not provided. 3 The reaction was conducted under dynamic enantiomeric crystallization conditions. 4 Furan as a substrate. 5 2,5-Dimethylfuran as a substrate.

Table 2

IMDA cycloadditions of 2-MF with acyclic alkenes.

№	Dienophile	Conditions	Selectivity	Yield of Adducts (%), [Ref.]
1	Dimethyl maleate	HfCl4, CH2Cl2, −30 °C	Endo/exo 84:16	94, [76]
2	Dimethyl maleate	HfCl4, CH2Cl2, −50 °C	Endo/exo > 98:2	82, [76]
3		22 °C		90, [77]
4		Ar = 1-naphthyl (cat.),CH2Cl2, −78 °C	99 de, 99 ee	99, [78]
5		Ar = 1-naphthyl (cat.),CH2Cl2, −78 °C	99 de, 99 ee	74, [78]
6		(cat.), CH2Cl2, −78 °C	Ortho (endo/exo 94:6), 98 ee (for endo isomer)	74, [79]
7	Benzyl acrylate	HfCl4, CH2Cl2, −30 °C	Endo/exo 28:72 (mixture of regio isomers)	84, [76]
8	Benzyl acrylate	HfCl4, CH2Cl2, −50 °C	Endo/exo (31:69) (mixture of regio isomers)	85, [76]
9	Acrylonitrile	ZnI2, neat, 50 °C	N.d.	69, [80]
10	Acrylonitrile	Neat, 60 °C	Ortho 66 (endo/exo 61:39), meta 34 (endo/exo 56:44)	69, [31,32]
11	1-Cyanovinyl acetate	ZnI2, neat, 0 °C, 8 days	Ortho (endo/exo 1:1) 2	52, [81]
12	1-Cyanovinyl acetate	ZnI2, neat, 20 °C, 26 h	Ortho endo 2	17, [81]
13	1-Cyanovinyl acetate	ZnI2, neat, RT, 24 h	Ortho (endo/exo 3:1) 2	30, [82]
14	1-Cyanovinyl acetate	MgI2, neat, RT, 24 h	Ortho (endo/exo 4:1) 2	57, [82]
15	2-Chloroacrylonitrile	ZnI2, neat, 0 °C	Ortho/meta 10:1 (mixture of endo/exo)	91 1, [83]
16		Benzene, reflux	Endo/exo 1,1:1	70, [84]
17		Eu(fod), RT	Endo/exo 2,1:1	80, [84]
18		Eu(fod), RT	Endo/exo ~2,8:1	80, [84]
19		Benzene, reflux	Endo/exo ~1:1	73, [84]
20	Itaconic anhydride	Neat, 23 °C	Ortho (endo:exo)/meta (endo:exo) 3:1/11:8 3	13 4, [85]

1 Yield of DA adduct after hydrogenation. 2 Endo- and exoconformation with regard to the position of the OAc group. 3 Structure of regio- and diastereomers in DA cycloaddition of C-2-substituted furans with itaconic anhydride are provided in Scheme 5. 4 Was detected by NMR. N.d.—not determined.

An important possible application of 2-MF is the protection of double bonds in functionalized alkenes against nucleophiles using the DA reaction. For example, modification of the 2-MF/maleimide DA adduct by alkylation or a Mitsunobu reaction, followed by thermal deprotection, was used for the synthesis of N-alkylated maleimides (Scheme 3) [69,70].

Scheme 3

Synthesis of N-substituted maleimides from 2-MF and maleimide using the DA approach.

Representative reactions of 2-MF with acyclic alkenes containing one or two electron-withdrawing groups (EWGs) are covered in Table 2. High endoselectivity was obtained for the HfCl4-catalyzed reaction of 2-MF with dimethyl maleate at low temperatures (Table 2, entries 1, 2). However, under the same conditions, benzyl acrylate showed exoselectivity for cycloaddition (entries 7, 8). An adduct of 2-MF and trans-4,4,4-trifluorocrotonic acid formed with high regio- and diastereoselectivity (entry 3). An enantioselective version of DA reactions with some fluorinated alkene dienophiles was implemented using chiral oxazaborolidine organocatalysts, which affords corresponding chiral oxabicyclic products with high yields and selectivity (entries 4–6). In the case of acrylonitrile reacting with 2-MF, regio- and diastereoselectivity was poor even in the presence of Lewis acid catalysts (entries 9, 10). Orthoadducts of 2-MF with 1-cyanovinyl acetate or 2-chloroacrylonitrile that are favored over meta-isomers due to electronic reasons were obtained under kinetic conditions with high regioselectivity (entries 11–15). A shift towards endo-products was found for reactions of 2-MF with allenic esters in the presence of Eu(fod) as the catalyst (entries 16–19).

2.2. Furanic Acetals

With rare exceptions, furfural does not react with dienophiles, but the introduction of aldehyde groups by DA reaction may be performed using an acetalization strategy that reduces the electron-withdrawing character of the carbonyl group. Table 3 highlights the results of reactions of furanic acetals with cyclic and linear alkenes. Literature data about the stereoselectivity of reactions of furanic acetals with cyclic alkenes are scarce. Predominant formation of endoadducts under kinetic conditions was detected by NMR when N-methyl maleimide was used as a dienophile (entry 1). For reactions of furfural acetals with mono-substituted acyclic alkenes, regioselectivity significantly depended on the type of substrates and reaction conditions. For dioxolane acetal reacting with methyl vinyl ketone, methyl acrylate or acrolein at 60 °C, a mixture of regio- and stereoisomers was obtained with predominant meta- and endoselectivity. In the case of acrylonitrile reacting with furanic acetals, the selectivity of cycloadditions was poor even in the presence of Lewis acid catalysts (entries 5–9). For the ZnCl2-catalyzed reaction of ethylthioacetal with acrylonitrile at 30 °C, 91% orthoselectivity and moderate endoselectivity were observed (entry 10). According to DFT calculations, the regioselectivity of reactions of furanic acetals with alkenes is a result of two opposite factors: charge interactions between the furan and alkene favor orthoselectivity, while steric factors promote metaselectivity [32].

Table 3

IMDA cycloadditions of furfural acetals with alkenes.

№	FurfuralAcetal	Dienophile	Conditions	Selectivity	Yield of Adducts (%), [Ref.]
1		N-Methylmaleimide	CH2Cl2, 23 °C	Endo/exo 87:13	N.d., [86]
2		Methyl vinyl ketone	Neat, 60 °C	Ortho 13 (endo/exo 74:26),meta 87 (endo/exo 65:35)	36, [32]
3		Methyl acrylate	Neat, 60 °C	Ortho 33 (endo/exo 87:13),meta 67 (endo/exo 77:23)	40, [32]
4		Acrolein	Neat, 60 °C	Ortho 38 (endo/exo 71:29),meta 62 (endo/exo 43:57)	28, [32]
5		Acrylonitrile	Neat, 60 °C, 120 h	Ortho 48 (endo/exo 72:28),meta 52 (endo/exo 42:58)	76, [32]
6		Acrylonitrile	ZnCl2, neat, 60 °C	Ortho 50 (endo/exo 70:30),meta 50 (endo/exo 56:44)	75, [32]
7		Acrylonitrile	ZnI2, neat, 60 °C	Ortho 53 (endo/exo 70:30),meta 67 (endo/exo 60:40)	75, [31]
8		Acrylonitrile	ZnCl2, neat, 60 °C	Ortho 43 (endo/exo 85:15),meta 57 (endo/exo 56:44)	68, [32]
9		Acrylonitrile	ZnCl2, neat, 60 °C	Ortho 39 (endo/exo 67:33), meta 61 (endo/exo 54:46)	67, [32]
10		Acrylonitrile	ZnCl2, neat, 30 °C	Ortho 91 (endo/exo 66:33),meta 9 (endo/exo 53:47)	73, [32]
11		Acrylonitrile	ZnCl2, neat, 60 °C	Ortho 53 (endo/exo 60:40), meta 47 (endo/exo 54:46)	81, [32]
12		Acrylonitrile	ZnCl2, neat, 60 °C	Ortho 52 (endo/exo 62:38),meta 48 (endo/exo 56:44)	85, [32]

N.d.—not determined.

2.3. Functionalized Furfural Derivatives

Mild reduction of the aldehyde group in FF is a path to important furanic building blocks furfuryl alcohol (FA) and furfuryl amine (FAM), which are widely used for the development of functional or dynamic molecular and biomolecular systems. Examples of possible areas of applications include but are not limited to the synthesis of biologically active compounds [87,88,89,90], oxanorbornane-based amphiphiles [91,92,93,94], supramolecular systems [95], self-assemblies [96], self-healing polymers and other dynamic systems [28].

The diastereoselectivity of DA reactions of FA, FAM and some common derivatives with cyclic and acyclic alkenes is shown in Table 4, Table 5 and Table 6. Preferable formation of exoadducts was observed for reactions of maleic and citraconic anhydrides with selected furanic substrates even at low temperatures (Table 5 and Table 6), except for the vinylated derivative of FA, which showed preferable endoselectivity (Table 5, entries 5–10).

Table 4

IMDA cycloadditions of FA with alkenes.

№	Dienophile	Conditions	Selectivity	Yield of Adducts (%), [Ref.]
1 1	Maleimide	Ethyl acetate, 24 °C	Endo/exo 96:4	87, [33]
2 2	Maleimide	Ethyl acetate, 24 °C	Endo/exo 97:3	42, [33]
3	N-Me-maleimide	Et2O, 90 °C	Endo/exo 21:79	43, [72]
4	N-Bn-maleimide	CH3CN, 35 °C	Endo/exo 70:30	75, [96]
5	N-Propargylmaleimide	CH3CN, 35 °C	Endo/exo 80:20	72, [96]
6	N-(2-Hydroxymethyl)maleimide	Ethyl acetate, 80 °C	Exo	76, [97]
7	N-(2-Hydroxyethyl)maleimide	Benzene, reflux	Exo	86, [98]
8	N-(3-Hydroxypropyl)maleimide	Toluene, 80 °C	Endo/exo 30:70 3	77, [99]
9	N-(4-Hydroxyphenyl)maleimide	Acetone, 55 °C	Exo	71, [40]
10	N-(4-Hydroxyphenyl)maleimide	Acetonitrile, 35 °C	Endo/exo 80:20	N.d., [40]
11	N-(p-Methoxyphenyl)maleimide	CH3CN, 40 °C, 18 h	Mostly exo	89, [100]
12	N-(p-Nitrophenyl)maleimide	CH3CN, 60 °C	Endo/exo 70:23	52, [100]
13	BMI 4	Toluene, 75–80 °C, 2 days	Mostly exo	92, [101]
14	Acrylonitrile	Neat, 60 °C	Ortho 56 (endo/exo 69:31), meta 44 (endo/exo 56:44)	81, [32]
15		Neat, RT, 96 h	N.d.	66, [37]

1 2,5-bis(Hydroxymethyl)furan (BHMF) as a substrate. 2 2,5-bis(Acetoxymethyl)furan (BAMF) as a substrate. 3 Slowly transformed to the exo isomer over a period of several months. 4 4,4′-bis(Maleimido)diphenylmethane. N.d.—not determined.

Table 5

IMDA cycloadditions of FA derivatives with cyclic alkenes.

№	R	Dienophile	Conditions	Selectivity	Yield of Adducts(%), [Ref.]
1	Allyl	N-Me-maleimide	Toluene, 50 °C, 24 h	N.d.	65 (endo), [103]
2	Allyl	N-Ph-maleimide	Toluene, 50 °C, 24 h	N.d.	26 (exo), [103]
3	Bn	Maleic anhydride	Toluene, RT, 3 days	Exo	43, [91]
4	Bn	Citraconic anhydride	15 kbar, CH2Cl2, 60 h	Exo (ortho/meta 5:7)	31 1, [68]
5	Vinyl	Maleic anhydride	Et2O, 22‒24 °C, 48 h	Endo	72, [104]
6	Vinyl	Maleic anhydride	Et2O, 35 °C, 48 h	Endo/exo 8:1	66, [104]
7	Vinyl	Maleic anhydride	THF, 22‒24 °C, 90 h	Endo/exo 8:1	66, [104]
8	Vinyl	Maleic anhydride	MeCN, 22‒24 °C, 48 h	Endo/exo 4:1	68, [104]
9	Vinyl	Maleic anhydride	Toluene, 22‒24 °C	Endo/exo 12:1	64, [104]
10	Vinyl	Maleic anhydride	Toluene, 80 °C	Endo/exo 4:1	66, [104]
11	Vinyl	N-Ph-maleimide	Et2O, 22‒24 °C	Endo/exo 1:2.8	47, [104]
12	Vinyl	N-Ph-maleimide	Toluene, 80 °C	Endo/exo 4:1	66, [104]
13	Ac	Maleic anhydride	Et2O, 25 °C, 7 days	Exo	34, [105]
14	Ac	Maleic anhydride	Toluene, RT, 97 h	Exo	74, [88]
15	Ac	Citraconic anhydride	15 kbar, CH2Cl2, 60 h	Exo (ortho/meta6:5)	59 1, [68]
16	Ac	N-Me-maleimide	CH2Cl2, 23 °C	Endo/exo 77:23	N.d., [86]
17	Ac	N-Dodecylmaleimide	THF, 23 °C	Endo/exo 64:36	N.d., [86]
18	Ac	N-Ph-maleimide	CH2Cl2, 23 °C	Endo/exo 65:35	N.d., [86]
19	Ac	N-(p-Nitrophenyl)maleimide	CH2Cl2, 23 °C	Endo/exo 55:45	N.d., [86]
20	Ac	N-(p-Methoxyphenyl)maleimide	CH2Cl2, 23 °C	Endo/exo 67:33	N.d., [86]
21	Ac	N-(Methoxy-2-propyl)maleimide	CH2Cl2, 23 °C	Endo/exo 76:24	N.d., [86]
22	Ac	N-(2-Methoxyethyl)maleimide	CH2Cl2, 23 °C	Endo/exo 75:25	N.d., [86]
23	Bz	Maleic anhydride	Toluene, 80 °C, 456 h	Exo	46, [88]
24 2	Bz	Maleic anhydride	Et2O, 24 °C, 24 h	Endo	84, [106]
25	Bz	N-Me-maleimide	CH2Cl2, 23 °C	Endo/exo 70:30	N.d., [86]
26	Bz	N-Dodecylmaleimide	THF, 23 °C	Endo/exo 63:37	N.d., [86]
27	COiBu	N-Pr-maleimide	CHCl3, 55 °C	Endo/exo 60:40	N.d., [107]
28	COiBu	N-iBu-maleimide	CHCl3, 55 °C	Endo/exo 45:55	N.d., [107]
29	COiBu	N-tBu-maleimide	CHCl3, 55 °C	Endo/exo 51:49	N.d., [107]
30	COiBu	N-Bn-maleimide	CHCl3, 55 °C	Endo/exo 44:56	N.d., [107]
31	COiBu		CHCl3, 55 °C	Endo/exo 26:74	N.d., [107]
32	COiBu	N-(2-Methylphenyl)-maleimide	CHCl3, 55 °C	Endo/exo 67:33	N.d., [107]
33	COiBu	BMI	CHCl3, 55 °C	Endo/exo 19:81	N.d., [107]
34	COtBu	N-Me-maleimide	CH2Cl2, 23 °C	Endo/exo 71:29	N.d., [86]
35	COtBu	N-Dodecylmaleimide	THF, 23 °C	Endo/exo 62:38	N.d., [86]

1 Yield of DA adduct after hydrogenation. 2 BHMF dibenzoate as a substrate. N.d.—not determined.

Table 6

IMDA cycloadditions of FAM derivatives with cyclic alkenes.

№	R	Dienophile	Conditions	Selectivity	Yield of Adducts (%),[Ref.]
1	Ac	Maleic anhydride	Et20, 23 °C	Exo	100, [109]
2	Ac	Maleimide	H3BO3/PEG-400, 90 °C	Exo	84, [110]
3	Ac	N-Ph-maleimide	H3BO3/PEG-400, 90 °C	Exo	78, [110]
4	Ac	N-(4-Chlorobenzyl)maleimide	H3BO3/PEG-400, 90 °C	Exo	92, [110]
5	Boc 1	Maleic anhydride	Toluene, 50 °C	Exo	94, [111]
6	Boc 1	Thiomaleic anhydride	Benzene, RT	Exo	68, [112]
7	Boc 1		EtOAc, reflux	Endo/exo (1:3.4)	85, [113]

1 tert-Butyloxycarbonyl.

The adduct of FA with maleic anhydride (1-exo) is unstable and undergoes irreversible intramolecular cyclization during storage or warming, yielding the corresponding thermodynamically stable lactone 2-exo (Scheme 4) [102].

Scheme 4

Formation of lactone 2-exo after DA reaction of FA with maleic anhydride.

The diastereoselectivity of the reactions with N-alkyl- and N-benzyl-substituted maleimides was in accordance with typical kinetic profiles demonstrating a shift towards endo- and exo-products under kinetic or thermodynamic conditions, respectively (Table 4, Table 5 and Table 6). However, this relationship was disrupted for some N-aryl maleimides reacting with various furanic substrates under both kinetic and thermodynamic conditions. For example, the diastereoselectivity of the cycloaddition of vinyl-substituted FA and N-Ph-maleimide shifted from a 1:2.8 endo/exo ratio under kinetic conditions to Et2O to a 4:1 endo/exo ratio in toluene at 80 °C (Table 5, entries 11, 12).

Information about the regio- and diastereoselectivity of functional FF derivatives with acyclic alkenes is scarce. A mixture of regio- and diastereoisomers with approximately equal distribution was detected after the noncatalytic reaction of FA with acrylonitrile (Table 4, entry 14). A mixture of regio- and diastereomers with ortho (endo:exo)/meta (endo:exo) 2:1/8:6 ratio was formed from itaconic anhydride reacting with FA acetate (Scheme 5) [85]. However, unfavorable thermodynamic parameters of cycloaddition with this dienophile were overcome using FA as a substrate, where proximal (ortho) DA adducts undergo further intramolecular cyclization, shifting the reaction equilibrium towards metastable lactone 5, which was isolated in 94% yield (Scheme 5) [85].

Scheme 5

Diels—Alder reactions of FA and FA acetate with itaconic anhydride.

Overall, the diastereoselectivity of DA reactions of alkenes with FF derivatives containing donor substituents at the C2 position is not always predictable, because it strongly depends on the structure of both the diene and dienophile. More predictable diastereoselective construction of functionalized oxabicyclic structures may be performed using HMF-derived 2,5-disubstituted furans that predominantly react with cyclic alkenes with high endoselectivity (Table 4, entries 1–2; Table 5, entry 24) [33,43,106,108].

Examples of DA reactions of furfural derivatives containing acceptor-type substituents with alkenes are rare. After the reaction of 2-furoic acid with β-alanine-substituted maleimide, only a small amount of one isomer was detected at 40 °C after 128 h [26]. Interestingly, a very low equilibrium constant for this reaction was observed in DMF media, while the equilibrium constant in water was at least two orders of magnitude greater. This difference was explained by the statement that water has a significant effect on the entropy of the reaction. The model reaction of methyl furoate with 1,6-bis(N-maleimido)hexane was investigated by NMR. Only approximately 20% conversion was detected after 4 days at 70 °C in a DMSO-d6 medium [35]. However, despite the low reactivity of furans with acceptor substituents, dynamic materials containing furanic ester-[35] or oxime-[114] functionalized polymers and maleimide functionalities showed moderate self-healing efficiency based on the DA reaction.

Bruijnincx and coworkers reported a new strategy for the direct introduction of furans containing aldehyde groups into DA cycloaddition [34]. Reactions of furanic aldehydes with water-soluble maleimides at 60 °C in a water medium led to the formation of DA adducts with good selectivity (Table 7). In the case of furfural, good exoselectivity of cycloaddition was achieved, while for some HMF derivatives, endoselectivity was preferable. In-water formation of the DA adduct was also detected for 2-acetylfuran, which reacts with N-methylmaleimide with the formation of only the exoadduct (entry 9). DFT calculations showed that the formation of furan/maleimide DA adducts through hydration of the aldehyde group is thermodynamically possible if hydration occurs both prior to (which increases the rate of the forward DA reaction) or after the cyclization step (which decreases the rate of the retro-DA reaction) [34].

Table 7

Direct DA reaction of furanic aldehydes with maleimides in water medium.

№	Furanic Substrate	R1	Products, Selectivity 1
1	R = H	Me	6a (endo/exo 18:40), 7a (endo/exo 1:3)
2	R = H	H	6b (endo/exo 8:30), 7b (endo/exo 0:0)
3	R = H	Et	6c (endo/exo 8:28, 7c (endo/exo 1:6)
4	R = H	nPr	6d (endo/exo 1:7), 7d (endo/exo 1:11)
5	R = H	Ph	6e (endo/exo 0:1), 7e (endo/exo 1:5)
6	R = Me	Me	6f (endo/exo 3:8), 7f (endo/exo 0:3)
7	R = CH2OH	Me	6g (endo/exo 37:13), 7g (endo/exo 0:0)
8	R = CH2OMe	Me	6h (endo/exo 7:5), 7h (endo/exo 3:3)
9	2-Acetylfuran	Me	7i (endo/exo traces:32)

Reaction conditions: H2O, 60 °C, 16 h. 1 Determined by 1H NMR (data were obtained from reference [34]).

3. Regioselectivity in the Synthesis of Aromatics Using the IMDA Reaction of Furfural Derivatives with Alkenes

The dehydration of furan/alkene adducts is an important sustainable approach to accessing renewable aromatic chemicals (Scheme 6) [7,30,37,115,116,117]. Utilization of HMF-derived C6 renewable furans (especially 2,5-dimethylfuran or 2,5-furandicarboxylic acid) provides access to para-substituted aromatics (as a route towards “green” polymers) and various polysubstituted aromatic products (Scheme 6) [116]. The presence of only one substituent in furfural increases the diversity of possible aromatic products to ortho- and meta-xylylene derivatives as well as various 1,2,3-trisubstituted compounds (Scheme 6).

Scheme 6

Aromatization of furan/alkene DA adducts as a route towards biobased aromatics.

Several approaches were used for the construction of aromatic rings using furan/alkene DA reactions starting from furanic, oxanorbornene or oxanorbornane furfural-derived compounds. For some furanic and alkene substrates, dehydration occurs spontaneously following the DA reaction stage. The tandem Diels-Alder cycloaddition/dehydration reaction of 2-MF with ethylene is an important approach to renewable toluene (Table 8). This type of DA cycloaddition is thermodynamically difficult and therefore requires the use of a catalyst, high temperature and pressure. Heterogeneous Brønsted-acidic catalysts, mainly zeolites or MOFs, are beneficial for these reactions [118]. Significant problems include side reactions such as the formation of furanic dimers (benzofurans), larger oligomers, products of furan hydrolysis and other reactions [115,118,119,120]. The introduction of acrylic acid instead of ethylene in reactions with 2-MF over zeolites or using ionic liquid catalysts showed good efficiency in the formation of aromatics [121]. Fast pyrolysis of a mixture of 2-MF and propylene using various zeolites under continuous flow conditions gives a mixture of monocyclic and polycyclic aromatic hydrocarbons with low selectivity [122].

Table 8

Synthesis of toluene by DA reaction of 2-MF with alkenes.

№	R	Conditions	Products Yield (%), [Ref.]
1	H	H-BEA zeolite, heptane, 62 bar, 250 °C	Toluene (46%), [119]
2	H	H-Beta-22 zeolite, 300 °C, 20 h	Toluene (50%), [123]
3	COOH	Bi-BTC, 160 °C, 24 h	Toluene (65%), 2-methyl benzoic acid (23%), [121]
4	COOH	[Emim]NTf2, Sc(OTf)3, 15 °C, 0.5 h	Toluene (12%), 2-methyl benzoic acid (2%), 3-methyl benzoic acid (9%), [124]
5	COOH	[BSO3HMIm]HSO4, 100 °C, 2h	Toluene (12%), 2-methyl benzoic acid (30%), 3-methyl benzoic acid (3%), [125]

Furfural dimethyl hydrazone reacts with active dienophiles such as maleic anhydride or maleimides, yielding corresponding arene derivatives through noncatalytic in situ DA cycloaddition followed by spontaneous dehydration (Table 9) [126,127,128]. One-pot synthesis of arenes starting from furfural using a hydrazine strategy was carried out with good yields in water (entries 7–11) [129].

Table 9

Preparation of phthalimides from furfural using a hydrazine strategy.

№	Substrates	Conditions	Yield of Aromatic Product, [Ref.]
5	2-Furaldehyde dimethylhydrazone,maleic anhydride	CHCl3, RT	94, [126]
6	2-Furaldehyde dimethylhydrazone,N-Et-maleimide	CHCl3, RT	90, [126]
7	2-Furaldehyde, N,N-dimethylhydrazine, N-Et-maleimide	H2O, 50 °C	97, [129]
8	2-Furaldehyde, N,N-dimethylhydrazine, maleimide	H2O, 50 °C	86, [129]
9	2-Furaldehyde, N,N-dimethylhydrazine, N-cyclopropylmaleimide	H2O, 50 °C	80, [129]
10	2-Furaldehyde, N,N-dimethylhydrazine, N-Ph-maleimide	H2O, 50 °C	73, [129]
11	2-Furaldehyde, N,N-dimethylhydrazine, N-(4-Methylbenzyl)maleimide	H2O, 50 °C	68, [129]

Acid-catalyzed dehydration of furan-derived oxanorbornenes to aromatic products requires strong reaction conditions and therefore may be used only for a narrow range of substrates. Renewable 3-methylphthalic anhydride (MPA) was obtained using acid-catalyzed dehydration of the corresponding 2-MF-derived DA adduct 8 with only 48% maximum yield (Scheme 7) [130]. An important problem in this synthetic approach is the facile retro-DA reaction, which is forced to carry out these transformations at industrially non-practical temperatures (−30 °C and lower) [124,125]. A novel approach to MPA synthesis that overcomes the problem of the rDA reaction is the introduction of oxanorbornane 9 (which is unable to recycle) instead of 8 into the aromatization stage (Scheme 7) [67,131,132]. Aromatization of 9 by solid acid catalysts led to MPA with 67% maximum yield. Some important byproducts, such as 2-methyl benzoic acid and 3-methyl benzoic acid, were also formed during this reaction, and their ratio depended on the catalyst used [67,131]. Higher selectivity of aromatization was achieved by oxidative dehydrogenation of 9 into phthalate 10 using a silicomolybdic acid catalyst in diethyl carbonate (Scheme 7) [132].

Scheme 7

Synthesis of arenes by aromatization of 2-MF-derived tricycles.

The deprotonation of DA adducts formed from 2-(furan-2-yl)-1,3-dioxolane and acrylonitrile by CH3ONa/DMSO superbase affords aromatic products at 30 °C with high total yield and a good ortho/meta ratio (Table 10, entries 1, 2) [31]. The study of kinetic features of the aromatization stage showed that the meta-adduct is more reactive than the ortho-isomer, which made it possible to isolate pure meta-adducts from the reaction mixture at 50% conversion, with subsequent regeneration of the ortho-isomer. Aromatization of DA adducts by BuONa/DMSO superbase was also efficient for 2-MF and methyl group-protected FA but showed a low yield of aromatics in the case of unprotected FA (Table 10, entries 3–5) [31].

Table 10

Preparation of aromatics by base-catalyzed dehydration of acrylonitrile-derived oxanorbornenes.

№	Oxanorbornene	Yield of DA Adducts 1	Yield of Aromatic Products 1
1	R = dioxolane acetal	76 (ortho/meta ~1:1) 2	84 (ortho/meta ~1:1.5)
2	R = dioxolane acetal	76 (ortho/meta ~1:1)	86 (ortho/meta ~1:1.8) 3
3	R = Me	53 (ortho), 13 (meta)	97 (ortho), 62 (meta) 4
4	R = CH2OEt	36 (ortho), 18 (meta)	94 (ortho), 100 (meta) 4
5	R = CH2OH	47 (ortho), 26 (meta)	21 (ortho), 42 (meta) 4

1 Data were obtained from reference [31]. 2 After 120 h of the reaction. 3 CH3ONa as a base. 4 Relative to the corresponding ortho- or meta-DA cycloadduct.

Recently, a new dynamic kinetic trapping strategy was developed for the construction of “drop-in” phthalide systems using tandem IMDA/lactonization and then aromatization reactions (Scheme 8) [37]. The first stage of this process is the reversible formation of unstable adducts (mixture of regio- and stereoisomers) of FA (11a–c) or BAMF (14) with acrylates substituted by EWGs (HFIP, TFE or 4NP) at an oxygen atom. The role of EWG in the dienophile was the activation of both double bonds for the IMDA reaction and the carbonyl group towards diastereoselective intramolecular cyclization and into a more thermodynamically stable exo-lactone (the next step). The last aromatization stage was performed using an Ac2O/strong acid mixture yielding phthalides 13 or 16 with maximum 98% and 60% yields, respectively.

Scheme 8

Synthesis of phthalides from furanic alcohols using a dynamic kinetic trapping strategy. HFIP = 1,1,1,3,3,3-hexafluoroisopropyl. TFE = 2,2,2-trifluoroethyl. 4NP = 4-nitrophenyl.

4. Conclusions

The IMDA reactions of biobased furans with alkene dienophiles are an important strategy for accessing practically important products, such as fundamental building blocks, fine chemicals, biologically active compounds or various organic and hybrid dynamic systems. Based on the literature highlighted in this review, we can assume that the problem of low regio- and stereoselectivity, which significantly reduces the synthetic potential of furan/alkene DA cycloaddition in fine organic synthesis and materials development, is still not solved for many functional furfural derivatives and alkene substrates. The reactivity of furfural-derived acceptor furans towards common alkenes, as well as the synthesis and aromatization of DA adducts of functional furfural derivatives with acyclic alkenes, are very poorly represented in the current literature. However, these types of reactions are important sustainable approaches towards functional aliphatic or aromatic products and therefore require further scientific investigations.

Rapid progress in this area can be anticipated, taking into account emerging trends in sustainable development towards the incorporation of bioderived chemicals and materials into the chemical industry. The focus of this review clearly shows that selectivity issues are far from solved and do not match current requirements. More studies are needed to develop practical and easy-to-use procedures to achieve high selectivity in reactions involving simple bioderived furanic starting materials.