5,6-Diphenyl-1,3,4,7-tetra-p -tolyl-1,3,3a,7a-tetrahydropentaleno[1,2-c ]furan

: The reaction of ( Z )-5-phenyl-1,3-di-p -tolylpent-2-en-4-yn-1-ol ( 1 ) with trimethylsilyl chloride in dichloromethane at ambient temperature gave a dimeric ether compound 2 in 30% yield. Subse-quently, heating 2 in toluene under refluxing temperature rendered the title compound quantitatively. The structure of this tricyclic-fused compound was characterized using NMR, mass spectroscopy, and X-ray crystallography. This unique linear tricyclic fused furan framework is reported for the first time.


Introduction
Derivatives of cyclopenta[a]pentalenes (I) are a family of tricyclic compounds composed of three fused cyclopentane rings, and such a skeleton is found in natural products [1][2][3][4][5].However, the corresponding heterocycles, such as pentaleno [1,2-c]pyrrole (II) or pentaleno [1,2-c]furan (III), are less studied (Figure 1).Amongst, Lycopalhine A with an aza-heterocycle is the only natural product found in fawcettiminetype Lycopodium alkaloid [6,7].For the furan derivative III, it has never been reported either in natural products or in synthetic targets.Several synthetic approaches and reactions leading to derivatives of I and II have been developed in the past [1][2][3][4][5][8][9][10].In a previously work, we investigated whether treatment of pent-1-en-3-yn-1-ol (IV) with anilines in the presence of Lewis acid provided the tricyclic compound VI directly (Scheme 1) [11].Presumably, the substitution of aniline with IV followed by dimerization took place to give V, which then underwent cascade cyclization to yield the tricyclic pentaleno [1,2-c]  Several synthetic approaches and reactions leading to derivatives of I and II have been developed in the past [1][2][3][4][5][8][9][10].In a previously work, we investigated whether treatment of pent-1-en-3-yn-1-ol (IV) with anilines in the presence of Lewis acid provided the tricyclic compound VI directly (Scheme 1) [11].Presumably, the substitution of aniline with IV followed by dimerization took place to give V, which then underwent cascade cyclization to yield the tricyclic pentaleno [1,2-c] Based on the above observations, we envisioned that if one can have an ether analog VIII, then it might undergo a similar cascade cyclization, which would eventually lead to the desired furan derivative X (Scheme 2).Here, we would like to demonstrate this idea and to obtain the tricyclic pentaleno[1,2-c]furan molecule.Scheme 2. Proposed approach leading to the target molecule.

Synthesis of Dihydropentaleno[1,2-c]furan 3
Scheme 3 illustrates the synthetic approach leading to the target molecule 3. The synthetic sequence commenced from the readily available pent-2-en-4-yn-1-ol 1, which was prepared according to our previously reported procedure by the addition of phenylacetylide to (E)-1,3-di-p-tolylprop-2-en-1-one followed by acid-catalyzed rearrangement [11,12].Treatment of 1 with trimethylsilyl chloride at room temperature provided the dimeric ether 2 in 30% yield [13].The reactant was totally consumed, giving various products, as indicated using a TLC analysis.Attempts to improve the yield of 2 was in vain even with the use of various Lewis acids such as TiCl4, BF3, and Me3SiBr.Upon chromatographic purification, compound 2 was obtained as viscous liquid in 30% yield.Thermal heating of 2 in toluene quantitatively rendered the target molecule 3 as a brown solid.Unlike the aza-analog VI, compound 3 did not undergo dehydrogenation reaction to form a fully conjugated system.Based on the above observations, we envisioned that if one can have an ether analog VIII, then it might undergo a similar cascade cyclization, which would eventually lead to the desired furan derivative X (Scheme 2).Here, we would like to demonstrate this idea and to obtain the tricyclic pentaleno[1,2-c]furan molecule.Based on the above observations, we envisioned that if one can have an ether analog VIII, then it might undergo a similar cascade cyclization, which would eventually lead to the desired furan derivative X (Scheme 2).Here, we would like to demonstrate this idea and to obtain the tricyclic pentaleno[1,2-c]furan molecule.Scheme 2. Proposed approach leading to the target molecule.

Synthesis of Dihydropentaleno[1,2-c]furan 3
Scheme 3 illustrates the synthetic approach leading to the target molecule 3. The synthetic sequence commenced from the readily available pent-2-en-4-yn-1-ol 1, which was prepared according to our previously reported procedure by the addition of phenylacetylide to (E)-1,3-di-p-tolylprop-2-en-1-one followed by acid-catalyzed rearrangement [11,12].Treatment of 1 with trimethylsilyl chloride at room temperature provided the dimeric ether 2 in 30% yield [13].The reactant was totally consumed, giving various products, as indicated using a TLC analysis.Attempts to improve the yield of 2 was in vain even with the use of various Lewis acids such as TiCl4, BF3, and Me3SiBr.Upon chromatographic purification, compound 2 was obtained as viscous liquid in 30% yield.Thermal heating of 2 in toluene quantitatively rendered the target molecule 3 as a brown solid.Unlike the aza-analog VI, compound 3 did not undergo dehydrogenation reaction to form a fully conjugated system.

Synthesis of Dihydropentaleno[1,2-c]furan 3
Scheme 3 illustrates the synthetic approach leading to the target molecule 3. The synthetic sequence commenced from the readily available pent-2-en-4-yn-1-ol 1, which was prepared according to our previously reported procedure by the addition of phenylacetylide to (E)-1,3-di-p-tolylprop-2-en-1-one followed by acid-catalyzed rearrangement [11,12].Treatment of 1 with trimethylsilyl chloride at room temperature provided the dimeric ether 2 in 30% yield [13].The reactant was totally consumed, giving various products, as indicated using a TLC analysis.Attempts to improve the yield of 2 was in vain even with the use of various Lewis acids such as TiCl 4 , BF 3 , and Me 3 SiBr.Upon chromatographic purification, compound 2 was obtained as viscous liquid in 30% yield.Thermal heating of 2 in toluene quantitatively rendered the target molecule 3 as a brown solid.Unlike the aza-analog VI, compound 3 did not undergo dehydrogenation reaction to form a fully conjugated system.Based on the above observations, we envisioned that if one can have an ether analog VIII, then it might undergo a similar cascade cyclization, which would eventually lead to the desired furan derivative X (Scheme 2).Here, we would like to demonstrate this idea and to obtain the tricyclic pentaleno[1,2-c]furan molecule.Scheme 2. Proposed approach leading to the target molecule.

Synthesis of Dihydropentaleno[1,2-c]furan 3
Scheme 3 illustrates the synthetic approach leading to the target molecule 3. The synthetic sequence commenced from the readily available pent-2-en-4-yn-1-ol 1, which was prepared according to our previously reported procedure by the addition of phenylacetylide to (E)-1,3-di-p-tolylprop-2-en-1-one followed by acid-catalyzed rearrangement [11,12].Treatment of 1 with trimethylsilyl chloride at room temperature provided the dimeric ether 2 in 30% yield [13].The reactant was totally consumed, giving various products, as indicated using a TLC analysis.Attempts to improve the yield of 2 was in vain even with the use of various Lewis acids such as TiCl4, BF3, and Me3SiBr.Upon chromatographic purification, compound 2 was obtained as viscous liquid in 30% yield.Thermal heating of 2 in toluene quantitatively rendered the target molecule 3 as a brown solid.Unlike the aza-analog VI, compound 3 did not undergo dehydrogenation reaction to form a fully conjugated system.

Characterization
Mass spectrum of compound 3 shows a [M + H] + ion at m/z = 659.328,which is in consistent with the molecular formula of C 50 H 42 O.Besides the signals for aromatic region, the 1 H NMR of 3 in CDCl 3 illustrates four signals corresponding to the protons on the furan ring.Among them, three sets of signals do show coupling interactions to each other, and there are signals at δ 5.33 (d, J = 7.7 Hz, H-1), 4.69 (dd, J = 7.7 Hz, 6.0 Hz, H-7a), and 4.27 (d, J = 6.0 Hz, H-3a), indicating that these protons are seated in cis fashion (Figure 2).On the other hand, a shift at δ 5.51 appears to be a singlet, which is assigned to be H-3 trans to the above-mentioned protons (Figure 2 and Figure S1 in Supplementary Materials).Based on 1 H NMR assignment, the relative configuration along the furan ring is concluded.Nevertheless, this observation is further confirmed using X-ray crystallography (see Section 2.3).In addition, four singlets due to the methyl groups of tolyl moieties were observed at δ 2.37, 2.29, 2.14, and 2.12, respectively.All these information readily support the structure proposed.
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Characterization
Mass spectrum of compound 3 shows a [M + H] + ion at m/z = 659.328,which is in consistent with the molecular formula of C50H42O.Besides the signals for aromatic region, the 1 H NMR of 3 in CDCl3 illustrates four signals corresponding to the protons on the furan ring.Among them, three sets of signals do show coupling interactions to each other, and there are signals at δ 5.33 (d, J = 7.7 Hz, H-1), 4.69 (dd, J = 7.7 Hz, 6.0 Hz, H-7a), and 4.27 (d, J = 6.0 Hz, H-3a), indicating that these protons are seated in cis fashion (Figure 2).On the other hand, a shift at δ 5.51 appears to be a singlet, which is assigned to be H-3 trans to the above-mentioned protons (Figures 2 and S1 in Supplementary Materials).Based on 1 H NMR assignment, the relative configuration along the furan ring is concluded.Nevertheless, this observation is further confirmed using X-ray crystallography (see Section 2.3).In addition, four singlets due to the methyl groups of tolyl moieties were observed at δ 2.37, 2.29, 2.14, and 2.12, respectively.All these information readily support the structure proposed.

Crystallography
The solid state structures of 3 was determined using a single-crystal X-ray diffraction analysis to reveal the structural details including the stereochemistry.ORTEP plot of 3 is shown in Figure 3A, and the relevant structural parameters are summarized in Table S5.The molecule comprises three fused five-member rings, namely furan ring (O1, C1, C2, C9, C10; A ring), cyclopentene ring (C2, C3, C4, C8, C9; B ring), and cyclopentadiene ring (C4, C5, C6, C7, C8; C ring).All bond lengths and bond angles in 3 are in normal ranges, as expected (Table S3 in Supplementary Materials).
The furan ring adopts an envelope conformation, which is supported by the observation of both torsional angles of O1-C10-C9-C2 and O1-C1-C2-C9 in [14.63( 14)°] and [26.83( 14)°], respectively (Table S5 in Supplementary Materials).The two fused cyclo-pentene/pentadiene rings (B and C rings) are almost coplanar (Figure 3B), as evidenced by the smaller torsional angles around both B and C rings (Table S5 in Supplementary Materials).Hydrogen atoms at C1, C2, and C9 are all pointed to the same side, i.e., cis to each other, whereas the hydrogen at C10 is seated to the opposite side.This is consistent with NMR spectroscopic analysis.

Crystallography
The solid state structures of 3 was determined using a single-crystal X-ray diffraction analysis reveal the structural details including the stereochemistry.ORTEP plot of 3 is shown in Figure 3A, and the relevant structural parameters are summarized in Table S5.The molecule comprises three fused five-member rings, namely furan ring (O1, C1, C2, C9, C10; A ring), cyclopentene ring (C2, C3, C4, C8, C9; B ring), and cyclopentadiene ring (C4, C5, C6, C7, C8; C ring).All bond lengths and bond angles in 3 are in normal ranges, as expected (Table S3 in Supplementary Materials).
The furan ring adopts an envelope conformation, which is supported by the observation of both torsional angles of O1-C10-C9-C2 and O1-C1-C2-C9 in [14.63(14) • ] and [26.83 (14) • ], respectively (Table S5 in Supplementary Materials).The two fused cyclopentene/pentadiene rings (B and C rings) are almost coplanar (Figure 3B), as evidenced by the smaller torsional angles around both B and C rings (Table S5 in Supplementary Materials).Hydrogen atoms at C1, C2, and C9 are all pointed to the same side, i.e., cis to each other, whereas the hydrogen at C10 is seated to the opposite side.This is consistent with NMR spectroscopic analysis.

General
All the chemicals were commercially purchased and used without further purification.Flash chromatography was performed using silica gel 230-400 mesh.1,3,5-triarylpent-2-en-4-yn-1-ol 1 was prepared according to the reported procedure [11,12]. 1 H and 13 C NMR were recorded in a 400 MHZ spectrometer in CDCl3 referenced to TMS.Melting points were determined on a Fargo MP-1D instrument.Unless otherwise noted, all the reactions were performed without any special precautions.

General
All the chemicals were commercially purchased and used without further purification.Flash chromatography was performed using silica gel 230-400 mesh.1,3,5-triarylpent-2-en-4-yn-1-ol 1 was prepared according to the reported procedure [11,12]. 1 H and 13 C NMR were recorded in a 400 MH Z spectrometer in CDCl 3 referenced to TMS.Melting points were determined on a Fargo MP-1D instrument.Unless otherwise noted, all the reactions were performed without any special precautions.

Crystal Structure Determination
Crystals suitable for X-ray determination were obtained for 3 using recrystallization from dichloromethane and hexane at room temperature.Cell parameters were determined using a Bruker AXS D8 VENTURE, PhotonIII_C28 diffractometer.Crystal  [13] and refined using the SHELXL-97 program [14] using full-matrix least-squares on F2 values.The X-ray crystallographic data for 3 have been deposited in the Cambridge Crystallographic Data Center with CCDC reference number 2341418.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html(accessed on 19 March 2024), or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Conclusions
We demonstrated a method for the synthesis of 5,6-diphenyl-1,3,4,7-tetra-p-tolyl-1,3,3a,7a-tetrahydropentaleno[1,2-c]furan, which is a compound composed of three fused five-member rings.The structure of the obtained compound was fully characterized using spectroscopic methods and X-ray single crystallography.Compound 3 is a derivative of pentalenes, which can be used as ligands for transition metal complexes or building blocks for organic synthesis.

Scheme 2 .
Scheme 2. Proposed approach leading to the target molecule.

Figure 2 .
Figure 2. 1 H NMR assignment for protons on the furan ring (numbers given for carbons are according to the nomenclature).

Figure 2 .
Figure 2. 1 H NMR assignment for protons on the furan ring (numbers given for carbons are according to the nomenclature).