DBU/AgOTf Relay‐Catalysis Enabled One‐Pot Synthesis of 1,3‐Dihydroisobenzofurans and Its Conversion to Indanones

A DBU/AgOTf relay‐catalyzed one‐pot reaction of 2‐alkynylbenzaldehydes and α‐diazo esters for the efficient construction of 1,3‐dihydroisobenzofuran derivatives has been documented. This protocol can tolerate a wide range of substrates and its scalability is demonstrated by the gram‐scale reaction. Moreover, the obtained 1,3‐dihydroisobenzofurans can be converted to indanone derivatives by a AgOTf‐catalyzed ring recombination process. Both of the resulting products, 1,3‐dihydroisobenzofurans and indanones, are important structural units of a variety of pharmaceuticals and natural products, and their structures are clearly confirmed by single crystal X‐ray diffraction analysis.

Keywords: 1,3-Dihydroisobenzofurans; Indanones; One-pot synthesis; 2-Alkynylbenzaldehyde; α-Diazo esters

Introduction

1,3‐Dihydroisobenzofurans are a class of prominent substructure which widely present in numerous pharmaceuticals and natural products.[1] For example, xylarinol B, a nutritional sugar substitute that is most suitable for diabetics, has a rare 1,3‐dihydroisobenzofuran skeleton.[1a] In addition, some 1,3‐dihydroisobenzofuran derivatives can be used as inhibitors of a variety of biological enzymes, such as peptide deformylase,[1b] glutathione S‐transferase[1c] (Figure 1). In the past few decades, strategies for the synthesis of 1,3‐dihydroisobenzofurans have been intensively developed, most of which involve the use of transition‐metals such as Rh, Pd, Au, Ag, Cu and Mn through a cyclization process.[2–7] Besides, some metal‐free synthetic methods of 1,3‐dihydroisobenzofurans have also been disclosed.[8–10] On the other hand, indanones constitute the core structure of many natural products, biologically active molecules, and functional materials.[11,12] It has been proved to have a wide range of pharmacological activities, such as anti‐bacterial, antiviral, anti‐HIV[11a–c] and anticancer activities[11d] (Figure 1). In recent years, a series of methods have been developed to synthesize indanone and their derivatives,[13–17] which are mainly cyclization reactions involving transition‐metal‐catalyzed C−H activation[14] or cross‐coupling,[15] radical cascade reaction[16] and acid‐catalyzed intramolecular cyclization.[17] Despite these major advances, due to the importance of 1,3‐dihydroisobenzofuran and indanone derivatives in biology, pharmacy and synthesis, the development of straightforward and practical methods to synthesize these scaffolds is still highly desirable.

2‐Alkynylarylaldehydes have been widely used as versatile building blocks for the construction of cyclic compounds in organic synthesis.[18] Generally, 2‐alkynylbenzaldehydes could react with nucleophilic reagents via a cascade 6‐endo‐dig cyclization/addition sequence or addition/5‐exo‐dig cyclization sequence to form various six or five membered ring compounds.[18d] Diazo compounds are also remarkably versatile synthons that have broad applications in organic synthesis and are nucleophilic.[19] Accordingly, the synthesis of β‐hydroxy α‐diazo carbonyl compounds by base or acid catalyzed Aldol‐type condensation of aldehydes with diazoacetates has been extensively studied.[20] However, reports on reactions between diazo compounds with 2‐alkynylbenzaldehydes to synthesize heterocyclic compounds are rare.[21] In 2015, we reported a silver‐catalyzed three‐component reaction of 2‐alkynylbenzaldehydes, amines, and diazo compounds for the synthesis of 3‐benzazepines.[22] Inspired by the above reports and as our continuing interest in synthetic transformation of diazo compounds,[23] we wish to report herein a one‐pot DBU/AgOTf relay‐catalyzed cascade addition‐cyclization reaction of 2‐alkynylbenzaldehyde and α‐diazoacetates for the construction of aforementioned important 1,3‐dihydroisobenzofurans. The reaction is easy to scale‐up and has broad substrate scope and functional group compatibility. Moreover, the efficient conversion of resulting 1,3‐dihydroisobenzofurans to indanones through AgOTf‐catalyzed ring recombination was also achieved (Scheme 1).

Results and Discussion

We began our studies by optimizing reaction conditions of the one‐pot addition‐cyclization reaction between 2‐(phenylethynyl)benzaldehyde 1 a and ethyl diazoacetate 2 a. The reaction were conducted by stirring a mixture of 1 a (0.2 mmol), 2 a (0.4 mmol) and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU, 10 mol%) in DMSO (1 mL) at room temperature for 12 h, then AgOTf (10 mol%) and MeCN (1.5 mL) were added and the stirring continued for additional 24 h. To our delight, the desired 1,3‐dihydroisobenzofuran 3 a was isolated in 65% yield (Table 1, entry 1). The structure of 3 a was unambiguously confirmed by single‐crystal X‐ray diffraction analysis.[24] Replacing DBU by DABCO resulted in a decreased yield of 35%, and the use of inorganic bases such as NaOH and K2CO3 instead of DBU showed that no desired product detected (Table 1, entries 2–4). Various metal salts such as AgOAc, AgNO3, Ag2CO3, Cu(OTf)2, Sc(OTf)2 and protonic acid such as trifluoromethanesulfonic acid (TfOH) were then examined, but they are all not as efficient as AgOTf (Table 1, entries 5–10). The screening of solvents revealed that the use of DMSO, MeCN, DCM, DCE and PhMe as a single solvent for both of the two steps gave 3 a in trace to 55% yields (Table 1, entries 11–15). It was found that DMSO favored the first step, and the 1:1.5 mixed solvent of DMSO and MeCN is the best choice for the second step (Table 1, entries 16–18).

1 TableOptimization of the conditions for two‐step one‐pot synthesis of 1,3‐dihydroisobenzofuran. [a]

entry	catalyst 1	catalyst 2	solvent 1	solvent 2	yield (%)[b]
1	DBU	AgOTf	DMSO	MeCN	65
2	DACO	AgOTf	DMSO	MeCN	35
3	NaOH	AgOTf	DMSO	MeCN	ND[c]
4	K2CO3	AgOTf	DMSO	MeCN	ND
5	DBU	AgOAc	DMSO	MeCN	33
6	DBU	AgNO3	DMSO	MeCN	trace
7	DBU	Ag2CO3	DMSO	MeCN	trace
8	DBU	Cu(OTf)2	DMSO	MeCN	ND
9	DBU	Sc(OTf)2	DMSO	MeCN	ND
10	DBU	TfOH	DMSO	MeCN	ND
11[d]	DBU	AgOTf	DMSO		52
12[d]	DBU	AgOTf	MeCN		55
13[d]	DBU	AgOTf	DCM		28
14[d]	DBU	AgOTf	DCE		25
15[d]	DBU	AgOTf	PhMe		trace
16[e]	DBU	AgOTf	DMSO	MeCN	61
17[f]	DBU	AgOTf	DMSO	MeCN	59
18[g]	DBU	AgOTf	DMSO	MeCN	63

1 [a] Reaction conditions: 1 a (0.2 mmol), 2 a (2.0 equiv.) and catalyst 1 (10 mol%) were mixed and stirred in solvent 1 (1 mL) at r.t. for 12 h, then, catalyst 2 (10 mol%) and solvent 2 (1.5 mL) were added and the stirring continued for additional 24 h. [b] Isolated yields. [c] No detected. [d] Solvent (2.5 mL). [e] MeCN (1.0 mL). [f] MeCN (0.5 mL). [g] MeCN (2.0 mL).

With the optimized conditions established, we subsequently explored the substrate scope of the one‐pot addition‐cyclization reaction by employing various 2‐alkynylbenzaldehydes 1 and diazoacetates 2 (Table 2). To our delight, the reaction could tolerate a series of different substituents on the arylethynyl group (R2) of 2‐alkynylbenzaldehydes 1, including electron donating groups such as para‐methyl, para‐methoxy, ortho‐methoxy and electron withdrawing groups such as para‐bromine, para‐chlorine, meta‐chlorine, para‐fluorine, para‐trifluoromethyl, para‐cyano, para‐nitro groups (3 b–3 k). Notably, 2‐(cyclopropylethynyl) and 2‐(2‐thiophenylacetyl) were also suitable for this transformation, and the corresponding 1,3‐dihydroisobenzofuran product 3 l and 3 m could be obtained in 45% and 73% yield, respectively. Substrates bearing a 4‐fluorine, 4‐methyl, 5‐chlorine and 5‐methoxy group on the phenyl ring linked to aldehyde reacted with ethyl diazoacetate 2 a smoothly to afford the corresponding products 3 n, 3 o, 3 p and 3 q in yields of 59%, 78%, 54% and 68%, respectively. When the phenyl group of 2‐alkynylbenzaldehyde is replaced by naphthyl group, it is also compatible in the reaction. A variety of α‐diazo esters bearing butyl, isobutyl, 2‐methylallyl, cyclohexyl, phenethyl and 2‐phenylpropyl group underwent the one‐pot addition‐cyclization reaction smoothly to afford the corresponding 1,3‐dihydroisobenzofurans (3 s–3 x) in 41%–84% yields. However, when 2‐alkynylbenzaldehydes with terminal alkyne, TMS protected alkyne, heterocyclic substituted alkyne or long chain alkyl alkyne was used, we did not detect the corresponding cyclization products. In addition, we have tested the substitution of aldehydes by ketones and alkynes by olefins in 2‐alkynylbenzaldehyde 1. Unfortunately, they proved incompatible with the reaction.

2 [a] Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol) and DBU (10 mol%) were mixed and stirred in DMSO (1 mL) at r.t. for 12 h, then, AgOTf (10 mol%) and MeCN (1.5 mL) were added and the stirring continued for additional 24 h. [b] Isolated yields.

To demonstrate the synthetic scalability of this one‐pot addition‐cyclization protocol, the reaction was carried out on a gram‐scale. The reaction of 1 a (5.0 mmol), 2 a (10.0 mmol) and DBU (10 mol%) were mixed and stirred in DMSO (25 mL) at room temperature for 12 h, then AgOTf (10 mol%) and MeCN (37.5 mL) were added and the stirring continued for additional 24 h to produce the desired product 3 a in 62% yield (0.99 g) (Scheme 2).

To further expand the utility of this synthetic method, we made our next efforts to explore the conversion of obtained 1,3‐dihydroisobenzofurans 3 to bioactive important indanones 4 through a silver‐catalyzed ring recombination process. A series of reactions were conducted using 3 a as model substrate to optimize the reaction conditions (Table 3). Pleasingly, an isomerization reaction was observed in the presence of AgOTf (10 mol%) at 100 °C in THF, and indaone 4 a was obtained in 50% yield (Table 3, entry 1). The structure of the product 4 a was unambiguously confirmed by single‐crystal X‐ray diffraction analysis.[24] Switching the catalyst from AgOTf to other transition metals, such as AgF, AgOAc, Ag2CO3 AgTFA, AgNO3, AgSbF6, or [Rh(OAc)2]2, all resulted in low efficiency of conversion (Table 3, entries 2–8). TfOH was also examined, resulted in decreased efficiency (Table 3, entry 9). Further screening the reaction media showed that the reaction only occurred in dioxane, but a decreased yield was detected (Table 3, entries 10–16). Either a lower temperature (80 °C) or a higher temperature (120 °C) gave lower yields of 4 a (Table 3, entries 17–18).

3 TableOptimization of the conditions for synthesis of indanones. [a]

entry	catalyst	solvent	temp. (°C)	yield (%)[b]
1	AgOTf	THF	100	50
2	AgF	THF	100	ND[c]
3	AgOAc	THF	100	trace
4	Ag2CO3	THF	100	trace
5	AgTFA	THF	100	ND
6	AgNO3	THF	100	ND
7	AgSbF6	THF	100	ND
8[d]	[Rh(OAc)2]2	THF	100	ND
9	TfOH	THF	100	31
10	AgOTf	Dioxane	100	43
11	AgOTf	MeCN	100	NR[e]
12	AgOTf	PhMe	100	NR
13	AgOTf	DCE	100	ND
14	AgOTf	DMSO	100	ND
15	AgOTf	DMF	100	ND
16	AgOTf	DMAc	100	ND
17	AgOTf	THF	80	35
18	AgOTf	THF	120	41

3 [a] Reaction conditions: 3 a (0.2 mmol), catalysts (10 mol%), solvent (2 mL), T, 12 h. [b] Isolated yields. [c] No detected. [d] 2 mol% [Rh(OAc)2]2. [e] No reaction.

The generality of above silver‐catalyzed ring recombination of 1,3‐dihydroisobenzofurans 3 to indanones 4 was next investigated. As shown in Table 4, the alkyl substituents attached to the ester group have little effect on the reaction efficiency, affording the desired indanones 4 a–4 c in 49%–86% yields. 6‐Methyl or 5‐methoxy 1,3‐dihydroisobenzofuran was also suitable substrates for the reaction, yielding the corresponding ring recombination products 4 d and 4 e in good yields. However, substrates bearing an electron‐withdrawing group such as 5‐fluoro or 6‐chloro only produced a small amount of products and inseparable by‐products were formed. The electron‐withdrawing groups such as trifluoromethyl, fluorine and chlorine, as well as electron‐donating groups such as methyl and methoxy on R2 were well tolerated in the reaction, assembling the desired indanone derivatives 4 f–4 j in 42%–93% yields. Notably, 3‐(cyclopropylmethylene) and 3‐(thiophen‐2‐ylmethylene) isobenzofuran were also suitable for this transformation, and provided the corresponding indanone products 4 k and 4 l in medium yield.

4 [a] Reaction conditions: 3 (0.2 mmol), AgOTf (10 mol%), THF (2 mL), 100 °C, 12 h. [b] Isolated yields.

To understand the reaction mechanism, a series of mechanistic experiments were carried out on the current addition‐cyclization reaction. Firstly, the step‐by‐step experiments indicated that the addition‐cyclization reaction was proceeded involving diazo compounds 5 (scheme 3a). Deuterium labeling experiments were subsequently conducted in order to gain insights into proton source of the reaction (scheme 3b). When the reaction of 1 a and 2 a was performed in the presence of 10 equivalent of D2O, deuterium incorporation was observed in the α‐position of ester group, and no deuterium incorporation was observed in the α‐position of phenyl. This result indicated that H2O may have participated in the diazo decomposition process rather than the cyclization process. The silver‐catalyzed ring recombination of 3 a was also conducted with the addition of 10 equivalent of D2O, 4 a‐D (66% of D) was obtained in 55% yield, revealing water is involved in the transformation. Further control experiments showed that diazo decomposition did not occur before the cyclization process (scheme 3c), thus excluding the mechanism that diazo decomposition occurs first followed by AgOTf‐catalyzed cyclization of possible intermediates 6. By performing above reactions in the presence of radical scavengers, we ruled out the possibility that free radical pathway was involved in these transformations (see the supporting information for details).

On the basis of the above experiments and previous literature reports,[20,22] a plausible mechanism for the addition‐cyclization and ring combination processes is proposed in scheme 4. Initially, 2‐(phenylethynyl)benzaldehyde 1 a and ethyl diazoacetate 2 a undergo DBU‐catalyzed Aldol‐type addition to give 2‐ethynylphenyl β‐hydroxy‐α‐diazo compound 5, which is then coordinated with AgOTf to generated intermediate A. Intermediate A subsequently undergoes AgOTf‐catalyzed 5‐exo‐dig cyclization to provide intermediate B, followed by protonation transfer leading to generation of cyclic diazo compound C. C is decomposed by AgOTf to form the metal‐bound carbene intermediate D with the extrusion of nitrogen. O−H insertion of D with H2O gives alcohol intermediate E and regenerated the silver catalyst. E eliminates a molecule of water to afford product 3 a. For the AgOTf‐catalyzed ring recombination process, coordination of 1,3‐dihydroisobenzofuran 3 a with AgOTf gives intermediate F. Activation of the alkene moiety by AgOTf generates oxonium G. G undergoes nucleophilic addition and protonation with H2O to give intermediate H together with the regenerated catalyst.[25] Ring open of H affords zwitterionic intermediate I, which subsequently cyclizes via an intramolecular nucleophilic addition to produce intermediate J. After dehydration, the ring recombination product 4 a is finally formed.

Conclusion

In summary, we have developed a one‐pot method for effective construction of 1,3‐dihydroisobenzofuran derivatives from 2‐alkynylbenzaldehydes and diazo esters through DBU/AgOTf relay‐catalyzed tandem addition‐cyclization sequence. In addition, the resulting 1,3‐dihydroisobenzofurans can convert to indanone derivatives by AgOTf‐catalyzed ring recombination, which is rarely studied in literature reports. This novel strategy provides an efficient and practical route to pharmaceutically important 1,3‐dihydroisobenzofurans and indanones with broad substrate scope and good functional group tolerance.

Experimental Section

Representative experimental procedure for the synthesis of 1,3‐dihydroisobenzofuran 3 a: To a 10 mL test tube equipped with a magnetic stir bar, 2‐(phenylethynyl)benzaldehyde 1a (0.2 mmol), ethyl diazoacetate 2 a (0.4 mmol) and DBU (0.02 mmol) were mixed and stirred in DMSO (1 mL) at room temperature for 12 h, and then AgOTf (0.02 mmol) and MeCN (1.5 mL) were added and the stirring continued for additional 24 h. Upon completion of the reaction, H2O (15 mL) was added and extracted with EtOAc (3×15 mL). The combined organic layer was washed with brine (3×5 mL), dried over Na2SO4 and concentrated under reduced pressure to afford a crude product. Purification by column chromatography (petroleum ether/ethyl acetate=20/1) on silica gel afforded the desired product 3 a as a yellow solid (38.0 mg, 65% yield).

Representative experimental procedure for the synthesis of indanones 4a: To a 10 mL test tube equipped with a magnetic stir bar, 2‐((Z)‐3‐((Z)‐benzylidene)isobenzofuran‐1(3H)‐ylidene)acetate 3 a (0.2 mmol) and AgOTf (0.02 mmol) were mixed and stirred in THF (2 mL) at 100 °C for 12 h. Upon completion of the reaction, the solvent was evaporated under vacuum, the crude product was purified by column chromatography on silica gel with petroleum ether/ethyl acetate (20/1) as the eluant, giving the pure product 4 a as an orange solid. (29.2 mg, 50% yield).

Acknowledgements

We are grateful for the financial support by the National Natural Science Foundation of China (Nos. 22161024 and 21901091), the Science and Technology Planning Project of Yunnan Province (No. 2019FD048).

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

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By Jiazhuang Wang; Hongtai Huang; Haotian Gao; Guiping Qin; Tiebo Xiao and Yubo Jiang

Reported by Author; Author; Author; Author; Author; Author