Synthesis of the [11]Cyclacene Framework by Repetitive Diels–Alder Cycloadditions

The Diels–Alder cycloaddition between bisdienes and bisdienophile incorporating the 7-oxa-bicyclo[2.2.1]heptane unit are well known to show high diastereoselectivity that can be exploited for the synthesis of molecular belts. The related bisdiene 5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octene is a valuable building block for the synthesis of photoprecursors for acenes, but it has not been employed for the synthesis of molecular belts. The present work investigates by computational means the Diels–Alder reaction between these bisdiene building blocks with syn-1,4,5,8-tetrahydro-1,4:5,8-diepoxyanthracene, which shows that the diastereoselectivity of the Diels–Alder reaction of the etheno-bridged bisdiene is lower than that of the epoxy-bridged bisdiene. The reaction of the etheno-bridged bisdiene and syn-1,4,5,8-tetrahydro-1,4:5,8-diepoxyanthracene in 2:1 ratio yields two diastereomers that differ in the orientation of the oxa and etheno bridges based on NMR and X-ray crystallography. The all-syn diastereomer can be transformed into a molecular belt by inter- and intramolecular Diels–Alder reactions with a bifunctional building block. The molecular belt could function as a synthetic intermediate en route to a [11]cyclacene photoprecursor.


Introduction
The Diels-Alder (DA) reaction developed into a powerful synthetic method since its discovery more than 100 years ago [1][2][3][4][5][6]. As a member of the class of pericyclic reactions, this cycloaddition is particularly valuable due to its stereospecificity and the often observed high diastereoselectivity. This last feature is highly desirable and turned out to be the key to the successful synthesis of molecular belts [7][8][9][10]. In a series of classic papers, Stoddart et al. reported their results of the detailed analysis of the diastereoselectivity of DA reactions involving epoxy bridged bisdiene 1a and bisepoxy bridged bisdienophile 2 (Scheme 1) [7,[11][12][13][14][15].
The DA reaction of a 1:1 mixture of 1a and 2 exclusively yields the syn/endo-H isomer 3a out of four possible diastereoisomers due to stereoelectronic effects, as discussed by Stoddart et al. (Scheme 1b) [7]. We follow their nomenclature and use the descriptors syn and anti for the relative configurations of the endoxide bridges and exo and endo for the relative configurations of the hydrogen atoms of the bisdiene after DA reaction.
First envisioned by Heilbronner [16], fully conjugated molecular belts consisting solely of linearly fused benzene rings, cyclacenes ([n]CA, where n is the number of benzene rings), have not yet been synthesised although related systems have been achieved, for example, by Gleiter et al. [17,18] and by Itami et al. [19]. Seminal work by Stoddart [7,[11][12][13][14][15], Schlüter [9,20], and Corey [8,21] demonstrated the significant resistance of the belts toward full conjugation. More recent attempts by on-surface synthesis using a tetraepoxy precursor further demonstrated the challenge in achieving full conjugation [10]. Synthesis of conjugated molecular belts and structurally related molecules has been reviewed in the literature [22][23][24][25][26]. Computational analyses of the strain energies arrive at values around 123 kcal mol −1 for [10]CA and 103 kcal mol −1 for [12]CA, indicating that the final reaction step in a CA synthesis will result in a substantial energy penalty due to the buildup of strain [27][28][29].
The resistance of cyclacene formation also under the conditions of on-surface synthesis is remarkable as the combination of ultrahigh vacuum, submonolayer coverage, and catalytically active single crystal transition metal surface is employed. These conditions previously allowed the production of acenes [30][31][32][33][34], the highly reactive linear analogues of cyclacenes [35]. Acenes could be obtained up to undecacene under matrix isolation conditions by photocleavage of α-diketone bridges. These were installed synthetically by DA reactions involving bisdiene 1b that is closely related to 1a employed by Stoddart. If this shows similar diastereoselectivity, then it should be possible to employ it in the synthesis of molecular belts as well. However, the high diastereoselectivity of 1a is ascribed to the distortion of the diene unit from planarity, which causes the 'lower' side to be more reactive, thus favoring the formation of syn diastereomers [7]. The larger etheno bridge of 1b causes less distortion of the diene moieties according to single-crystal X-ray crystallography [36] and thus reduced diastereoselectivity is anticipated.
Indeed, Wegener and Müllen reported that the DA reaction of bisdiene 1b with the closely related monodienophile 1,4-dihydro-1,4-epoxynaphthalene gave two diastereomers in a ratio of 4:1 under high-pressure conditions [37,38]. The major isomer was identified as having all bridges syn standing with hydrogen atoms at the newly formed bonds in endo position with respect to the epoxy bridge. The minor isomer had the corresponding H atoms also in endo position, but one epoxy bridge was anti to the other bridges, as evidenced by X-ray crystallography [37,38]. This observation confirms the reduced diastereoselectivity in DA reactions of bisdiene 1b and bisdienophile 2 under high-pressure conditions. The reaction did not result in cyclic materials, according to Müllen, but in polymeric materials for which elucidation of the stereochemistry was impossible [37,38].
We here report a computational analysis of the DA reactions involving bisdienes 1a and 1b with bisdienophile 2 that confirm the reduced diastereoselectivity in their Diels-Alder reaction. We demonstrate that diastereomer 6a can be obtained from 1b and 2 (Scheme 1c) with sufficient diastereomeric excess to employ it in the synthesis of a molecular belt, as described in the second part of this paper.

Computations
The computational analysis employed the M062X functional and the 6-311+G** basis set. The effects of the solvent toluene were taken into account with a polarisable continuum model for geometry optimisation and subsequent computation of harmonic vibrational frequencies (see SI for comparison with other computational methods and Tables S6 and S7 for energy data). The Diels-Alder reaction between the bisdienes 1 and the bisdienophile 2 in a 1:1 ratio can result in four diastereomeric products 3a-d (from 1a) and 4a-d (from 1b) (see Scheme 1b).
The syn/endo-H isomer 3a, the sole reaction product observed by Ashton et al. [7], is the least stable diastereomer, while the anti/exo-H isomer 3d is the most stable one. The energy difference (∆∆G • (298.15 K) = 1.9 kcal mol −1 ) between these isomers is small. Among the diastereomers, 4 the syn/endo-H product 4a is the most stable one, while the syn/exo-H isomer 4b is the least stable. The free energy difference between these isomers, ∆∆G • (298.15 K) = 0.9 kcal mol −1 , is smaller than in the case of 3.
Since two transition states (TS) are conceivable for the formation of each diastereomer, eight TS each are relevant for the formation of 3 and 4. The expected TS could each be the diastereomers, 4 the syn/endo-H product 4a is the most stable one, while the syn/exo-H isomer 4b is the least stable. The free energy difference between these isomers, ΔΔG°(298.15 K) = 0.9 kcal mol −1 , is smaller than in the case of 3.
Since two transition states (TS) are conceivable for the formation of each diastereomer, eight TS each are relevant for the formation of 3 and 4. The expected TS could each be located on the corresponding potential energy surfaces. Among diastereomers 3, the least stable 3a is formed with the lowest energy barrier (ΔG ‡ = 24.8 kcal mol −1 ) ( Figure 1). The second-lowest barrier (ΔG ‡ = 27.8 kcal mol −1 ) is associated with the formation of the anti/endo-H isomer 3c. Most importantly, the energy difference between the lowest and the second-lowest barrier (formation of 3a and 3c) is ΔΔG ‡ = 3.0 kcal mol −1 . The computations also show that the subsequent DA reaction of the syn/endo-H adduct 3a, with its remaining dienophile and a second equivalent of bisdiene in a syn/endo-H fashion to give 5 has almost the same barrier (ΔG ‡ = 24.7 kcal mol −1 ) as the first syn/endo DA reaction. The large energetic preference of the syn/endo-H cycloaddition mode is in accord with the observation of the diastereoselective DA reaction between 1a and 2 by Ashton et al. [7].  as computed at the M062X/6-311+G**/toluene level of theory. Distances between reacting carbon atoms are given in Å (blue), enthalpies (normal print), and free energies (italics) at 298.15 K are given in kcal mol −1 . Note that for the formation of each diastereomer, there is another transition state that is higher in energy and that is not depicted for clarity: Red, oxygen; dark gray, carbon; light gray, hydrogen.
The desired syn/endo-H isomer 4a and the anti/endo-H isomer 4c are almost isoenergetic. As a consequence, the barriers of their formations are more much more similar, although the formation of the isomer 4a still is preferred by ∆∆G ‡ = 1.9 kcal mol −1 . The barrier height for the formation of 4a is lower than that of 3a by 1.0 kcal mol −1 , indicating that the endoxide bridge of 1a reduces reactivity. Formation of the syn/exo-H (4b) and anti/exo-H (4d) isomers are associated with barriers that are higher than that for 4a by 7.5 kcal mol −1 and 6.2 kcal mol −1 , respectively, which excludes the corresponding stereochemical modes of reaction. As in the case of 3a, the subsequent DA reaction of 4a with the second equivalent of bisdiene 1b has a similar barrier to the first one for both the syn/endo (∆G ‡ = 24.3 kcal mol −1 to give 6a) and the anti/endo (∆G ‡ = 25.8 kcal mol −1 to give 6b) fashion (see Figure S65). In consequence, the computations suggest that the 2:1 DA reaction will likely give a mixture of twofold syn/endo and syn/endo plus anti/endo diastereoselectivity.
To summarise, the results of the computations of the DA reactions involving 1a and 2 are in full accord with the observations of Ashton et al. [7] regarding the high diastereoselectivity of the DA reaction. The computations suggest that the diastereoselectivity is reduced in the DA reaction of 1b and 2, in accord with the reports by Wegener and Müllen [37,38]. The formation of a molecular belt was not observed by them, but we reason that it should be possible to achieve if the proper diastereomer 6a can be isolated and employed in subsequent DA reactions.

Synthesis
The required bisdiene 1b and bisdienophile 2 were synthesised, as reported in [7,39]. The DA reaction of 2 with 2.2 equivalents of 1b in boiling toluene resulted in the formation of isomers 6a and 6b (Scheme 2). Integration of aromatic protons of the crude product reveals the ratio of diastereomers 6a:6b to be 1.9:1. and free energies (italics) at 298.15 K are given in kcal mol −1 . Note that for the formation of each diastereomer, there is another transition state that is higher in energy and that is not depicted for clarity: Red, oxygen; dark gray, carbon; light gray, hydrogen.
The desired syn/endo-H isomer 4a and the anti/endo-H isomer 4c are almost isoenergetic. As a consequence, the barriers of their formations are more much more similar, although the formation of the isomer 4a still is preferred by ΔΔG ‡ = 1.9 kcal mol −1 . The barrier height for the formation of 4a is lower than that of 3a by 1.0 kcal mol −1 , indicating that the endoxide bridge of 1a reduces reactivity. Formation of the syn/exo-H (4b) and anti/exo-H (4d) isomers are associated with barriers that are higher than that for 4a by 7.5 kcal mol −1 and 6.2 kcal mol −1 , respectively, which excludes the corresponding stereochemical modes of reaction. As in the case of 3a, the subsequent DA reaction of 4a with the second equivalent of bisdiene 1b has a similar barrier to the first one for both the syn/endo (ΔG ‡ = 24.3 kcal mol −1 to give 6a) and the anti/endo (ΔG ‡ = 25.8 kcal mol −1 to give 6b) fashion (see Figure  S65). In consequence, the computations suggest that the 2:1 DA reaction will likely give a mixture of twofold syn/endo and syn/endo plus anti/endo diastereoselectivity.
To summarise, the results of the computations of the DA reactions involving 1a and 2 are in full accord with the observations of Ashton et al. [7] regarding the high diastereoselectivity of the DA reaction. The computations suggest that the diastereoselectivity is reduced in the DA reaction of 1b and 2, in accord with the reports by Wegener and Müllen [37,38]. The formation of a molecular belt was not observed by them, but we reason that it should be possible to achieve if the proper diastereomer 6a can be isolated and employed in subsequent DA reactions.

Synthesis
The required bisdiene 1b and bisdienophile 2 were synthesised, as reported in [7,39]. The DA reaction of 2 with 2.2 equivalents of 1b in boiling toluene resulted in the formation of isomers 6a and 6b (Scheme 2). Integration of aromatic protons of the crude product reveals the ratio of diastereomers 6a:6b to be 1.9:1. The stereochemistry of isomers 6a and 6b was determined by NMR spectroscopy and single-crystal X-ray crystallography. According to the Karplus equation, the coupling constant with a dihedral angle around 90° is close to zero, compared to dihedral angles of 0° and 180°. For the exo-H-isomers, the strong coupling would be expected, as opposed to no or weak coupling in the endo-H-isomers. In the COSY NMR spectra of 6a ( Figure S5) and 6b ( Figure S11), no coupling can be observed for the corresponding atoms. The NOESY Scheme 2. Diels-Alder reaction of 1b and 2. Reaction conditions and isolated yields (a) 2.2 equivalent of 1b, 1 equivalent of 2, toluene, reflux, 18 h; (b) equimolar ratio of 1b and 2, toluene, reflux, 18 h. 4c could not be isolated.
The stereochemistry of isomers 6a and 6b was determined by NMR spectroscopy and single-crystal X-ray crystallography. According to the Karplus equation, the coupling constant with a dihedral angle around 90 • is close to zero, compared to dihedral angles of 0 • and 180 • . For the exo-H-isomers, the strong coupling would be expected, as opposed to no or weak coupling in the endo-H-isomers. In the COSY NMR spectra of 6a ( Figure S5) and 6b ( Figure S11), no coupling can be observed for the corresponding atoms. The NOESY NMR spectra, however, show an interaction between the corresponding atoms (for 6a: Figure S6, for 6b: Figure S12). An assignment of the 1 H and 13 C NMR signals to the corresponding atoms of 6a (Table S1) and 6b (Table S2) and the computed dihedral angles between the corresponding protons for the four diastereomers 4a-d ( Figure S21) can be NMR spectra, however, show an interaction between the corresponding atoms (for 6a: Figure S6, for 6b: Figure S12). An assignment of the 1 H and 13 C NMR signals to the corresponding atoms of 6a (Table S1) and 6b (Table S2) and the computed dihedral angles between the corresponding protons for the four diastereomers 4a-d ( Figure S21) can be found in the Supplementary Materials. The conclusions based on NMR spectroscopy were confirmed by single-crystal X-ray crystallography ( Figure 2).  Figure S64), indicating a rearrangement of 8b. It appears that the higher temperatures employed in this reaction, compared to those for the synthesis of 4a, resulted in significant thermal degradation. Unfortunately, we were not able to find milder conditions for this transformation to proceed in better yields.  Figure S64), indicating a rearrangement of 8b. It appears that the higher temperatures employed in this reaction, compared to those for the synthesis of 4a, resulted in significant thermal degradation. Unfortunately, we were not able to find milder conditions for this transformation to proceed in better yields. We, therefore, sought a different way to construct a molecular belt. Ashton et al. [7] showed that the final DA reaction can proceed under mild conditions as it is an intramolecular reaction. We thus resorted to a more reactive dienophile than 2 willing to sacrifice the high diastereoselectivity. For this purpose, we chose an aryne that we generated from 10. Macrocycle 12 was obtained from 6a and 10 in an overall yield of 7% upon heating a mixture of 6a and 10 with a fluoride source, which generates the aryne from 10 to react with 6a in a Diels-Alder reaction to afford compound 11a (Scheme 4). This in turn undergoes an intramolecular Diels-Alder reaction to yield 12. Compound 10 was obtained in a yield of 66% after treating commercially available 9 with furan and CsF in MeCN for 1 h at 45 °C (Scheme 5). The products 11a and 11b could be obtained and characterised after treatment of 6a and 10 with KF and 18-crown-6 for 24 h at room temperature. All the ether and olefin bridges in the target molecule 11a are syn to each other. Additional side products were formed during the reaction, which were more polar than 11a and 11b, and were not isolated or characterised. The formation of 2:1 adducts 13, with different orientations of the newly installed endoxide bridges would be expected. We, therefore, sought a different way to construct a molecular belt. Ashton et al. [7] showed that the final DA reaction can proceed under mild conditions as it is an intramolecular reaction. We thus resorted to a more reactive dienophile than 2 willing to sacrifice the high diastereoselectivity. For this purpose, we chose an aryne that we generated from 10. Macrocycle 12 was obtained from 6a and 10 in an overall yield of 7% upon heating a mixture of 6a and 10 with a fluoride source, which generates the aryne from 10 to react with 6a in a Diels-Alder reaction to afford compound 11a (Scheme 4). This in turn undergoes an intramolecular Diels-Alder reaction to yield 12. Compound 10 was obtained in a yield of 66% after treating commercially available 9 with furan and CsF in MeCN for 1 h at 45 • C (Scheme 5).
The products 11a and 11b could be obtained and characterised after treatment of 6a and 10 with KF and 18-crown-6 for 24 h at room temperature. All the ether and olefin bridges in the target molecule 11a are syn to each other. Additional side products were formed during the reaction, which were more polar than 11a and 11b, and were not isolated or characterised. The formation of 2:1 adducts 13, with different orientations of the newly installed endoxide bridges would be expected. We, therefore, sought a different way to construct a molecular belt. Ashton et al. [7] showed that the final DA reaction can proceed under mild conditions as it is an intramolecular reaction. We thus resorted to a more reactive dienophile than 2 willing to sacrifice the high diastereoselectivity. For this purpose, we chose an aryne that we generated from 10. Macrocycle 12 was obtained from 6a and 10 in an overall yield of 7% upon heating a mixture of 6a and 10 with a fluoride source, which generates the aryne from 10 to react with 6a in a Diels-Alder reaction to afford compound 11a (Scheme 4). This in turn undergoes an intramolecular Diels-Alder reaction to yield 12. Compound 10 was obtained in a yield of 66% after treating commercially available 9 with furan and CsF in MeCN for 1 h at 45 °C (Scheme 5). The products 11a and 11b could be obtained and characterised after treatment of 6a and 10 with KF and 18-crown-6 for 24 h at room temperature. All the ether and olefin bridges in the target molecule 11a are syn to each other. Additional side products were formed during the reaction, which were more polar than 11a and 11b, and were not isolated or characterised. The formation of 2:1 adducts 13, with different orientations of the newly installed endoxide bridges would be expected. Since compounds 11a and 11b could not be differentiated by NMR and MS, both compounds were heated in boiling toluene for 2 h, respectively, with 11a affording 12 as product and 11b remaining unchanged. This indicates that 11a has all the bridges on the same side. The chemical transformation from 11a to 12 can be proven by the changes in the 1 H and 13 C NMR spectra. For CS symmetric compound 11a all the required 13 C signals can be observed, but due to the very similar chemical shifts of the bridgehead carbon atoms (C-8) and (C-12), their assignment is interchangeable (Table 1). Likewise, the diastereotopic hydrogen atoms at (C-6) and (C-14) can only be assigned pairwise ( Figure S49). The same is true for a number of 13 C and 1 H signals of 12, but the required amount of 13 C resonances for CS symmetric 12 is observed ( Table 1). The terminal carbon of the diene of 11a can easily be identified with a DEPT-135 measurement, which shows a negative signal at δC 101.2 (C-1). After the intramolecular DA reaction to 12, the signal is shifted to δC 31.2 (C-1). The methylene protons of the diene unit of 11a at δH 4.86 and 4.70 (H-1) are transformed to diastereotopic protons in 12, which can be found in the range between 2.5 and 2.0 ppm (H-1) ( Figure S50). The protons for the dienophile at δH 6.91 (H-24) in 11a are shifted to the range between 1.36 and 1.20 ppm (H-24) in 12. Analogously a shift takes place from δC 143.0 (C-24) to δC 46.3/45.8 (C-24). The assignment to 11a and 12 are in agreement with the 2D NMR and DEPT spectra and with the high-resolution APCI measurements. Since compounds 11a and 11b could not be differentiated by NMR and MS, both compounds were heated in boiling toluene for 2 h, respectively, with 11a affording 12 as product and 11b remaining unchanged. This indicates that 11a has all the bridges on the same side. The chemical transformation from 11a to 12 can be proven by the changes in the 1 H and 13 C NMR spectra. For C S symmetric compound 11a all the required 13 C signals can be observed, but due to the very similar chemical shifts of the bridgehead carbon atoms (C-8) and (C-12), their assignment is interchangeable (Table 1). Likewise, the diastereotopic hydrogen atoms at (C-6) and (C-14) can only be assigned pairwise ( Figure S49). The same is true for a number of 13 C and 1 H signals of 12, but the required amount of 13 C resonances for C S symmetric 12 is observed ( Table 1). The terminal carbon of the diene of 11a can easily be identified with a DEPT-135 measurement, which shows a negative signal at δ C 101.2 (C-1). After the intramolecular DA reaction to 12, the signal is shifted to δ C 31.2 (C-1). The methylene protons of the diene unit of 11a at δ H 4.86 and 4.70 (H-1) are transformed to diastereotopic protons in 12, which can be found in the range between 2.5 and 2.0 ppm (H-1) ( Figure S50). The protons for the dienophile at δ H 6.91 (H-24) in 11a are shifted to the range between 1.36 and 1.20 ppm (H-24) in 12. Analogously a shift takes place from δ C 143.0 (C-24) to δ C 46.3/45.8 (C-24). The assignment to 11a and 12 are in agreement with the 2D NMR and DEPT spectra and with the high-resolution APCI measurements.

Properties of Macrocycle 12
Since we were not able to obtain single crystals suitable for X-ray crystallography, we computed the structure of 12 ( Figure 3). The distances between the internal endo hydrogen atoms range between 3.3-5.3 Å. The molecular belt 12 is significantly more stable than precursor 11a as indicated by the heat of reaction for cyclisation, ∆H • (298.15 K) = −42.5 kcal mol −1 .

Properties of Macrocycle 12
Since we were not able to obtain single crystals suitable for X-ray crystallography, we computed the structure of 12 ( Figure 3). The distances between the internal endo hydrogen atoms range between 3.3-5.3 Å. The molecular belt 12 is significantly more stable than precursor 11a as indicated by the heat of reaction for cyclisation, ΔH°(298.15 K) = −42.5 kcal mol −1 . The molecular belt 12 decomposes both in toluene solution during heating within a few hours and in substance during storage in a refrigerator within a few weeks to unidentified products. The molecular belt 12 decomposes both in toluene solution during heating within a few hours and in substance during storage in a refrigerator within a few weeks to unidentified products.

X-ray Crystallographic Data
Suitable crystals for 6b were grown from a solution of chloroform and toluene at 5 • C. Data were collected on a Bruker SMART APEX II instrument equipped with a fine focus sealed tube and TRIUMPH monochromator using MoK α radiation (λ = 0.71073 Å). The data collection strategy was determined using COSMO [42] employing ωand φ scans. Raw data were processed using APEX [43] and SAINT [44] corrections for absorption effects were applied using SADABS [45].
Suitable crystals for 6a were grown from a solution of dichloromethane and hexane at room temperature. Data collections were therefore performed on a Bruker APEX DUO instrument equipped with an IµS microfocus sealed tube and QUAZAR optics for MoK α radiation (λ = 0.71073 Å). The Data collection strategy was determined using COSMO [42] employing ωscans. Raw data were processed using APEX [46] and SAINT [47], corrections for absorption effects were also applied using SADABS [45].
The structures were solved by direct methods and refined against all data by fullmatrix least-squares methods on F 2 using SHELXTL [48] and Shelxle [49]. All atoms (except hydrogen) were refined anisotropically. All methyl groups were refined as rigid and rotating (difference Fourier density optimisation) CH 3 groups with U iso(H) = 1.5U eq(C) and all other hydrogen atoms were placed in calculated positions and refined using a riding model with U iso(H) = 1.2U eq(C) for aromatic and U iso(H) = 1.5U eq(C) for all other hydrogen atoms.
The X-ray crystal structures of compounds 6a and 6b were uploaded to the Cambridge Crystallographic Data Centre with the deposition numbers CCDC 2076991 for 6a and CCDC 2076992 for 6b. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, accessed on 19 May 2021.

Computational Chemistry
The computations were performed using the M062X [50] functional along with the 6-311+G** basis set. Although the DA reaction is not very sensitive to solvent polarity, we took into account the effect of the toluene solvent in our computations. For this purpose, the polarisable continuum model using the integral equation formalism variant (IEFPCM) [51], as implemented in Gaussian 16 [52], was employed. Computations of harmonic vibrational frequencies confirm the nature of stationary points as minima or saddle points.

Conclusions
In summary, the computational investigation of the diastereoselectivity of the Diels-Alder reaction of strained bisdiene 1a with strained bisdienophile 2 is in agreement with the experimental observations reported by Stoddart et al. [7,11,[13][14][15]. The computations suggest that the geometrically more flexible bisdiene 1b should undergo the DA reaction with bisdienophile 2 with lower diastereoselectivity, but the concave all-syn diastereomer 6a required for construction of a molecular belt should be dominant. This is confirmed experimentally since 6a is the dominant 2:1 cycloaddition product, while 6b is a minor by-product. The concave 6a can be chemically transformed into a molecular belt 12 by two subsequent DA reactions. The novel belt 12 could function as a synthetic intermediate for the synthesis of a photoprecursor for [11]cyclacene.
Supplementary Materials: Charts of NMR spectra ( 1 H, 13 C, selected COSY and NOESY), mass spectra, chromatograms, computational method evaluation, relative energies, figures of transition states discussed in the text, Cartesian coordinates of stationary points are available online.