Bioinspired Binding and Conversion of Linear Monoterpenes by Polyaromatic Coordination Capsules

Linear monoterpenes, versatile reaction biosubstrates, are bound and subsequently converted to various cyclic monomers and oligomers with excellent selectivity and efficiency, only in natural enzymes. We herein report bioinspired functions of synthetic polyaromatic cavities toward linear monoterpenes in the solution and solid states. The cavities are provided by polyaromatic coordination capsules, formed by the assembly of Pt(II) ions and bent bispyridine ligands with two anthracene panels. By using the capsule cavities, the selective binding of citronellal from mixtures with other monoterpenes and its preferential vapor binding over its derivatives are demonstrated in water and in the solid state, respectively. The capsule furthermore extracts p-menthane-3,8-diol, with high product- and stereoselectivity, from a reaction mixture obtained by the acid-catalyzed cyclization of citronellal in water. Thanks to the inner and outer polyaromatic cavities, the catalytic cyclization-dimerization of vaporized citronellal efficiently proceeds in the acid-loaded capsule solid and product/stereoselectively affords p-menthane-3,8-diol citronellal acetal (∼330% yield based on the capsule) under ambient conditions. The solid capsule reactor can be reused at least 5 times with enhanced conversion. The present study opens up a new approach toward mimicking terpene biosynthesis via synthetic polyaromatic cavities.


■ INTRODUCTION
Highly selective binding and conversion of biosubstrates are hallmarks of natural enzymatic reactions. 1These reactions occur in the isolated hydrophobic pockets of large protein assemblies under ambient aqueous conditions. 2 Inspired by such functional biocavities, a large number of synthetic molecular tubes, cages, and capsules have been developed with the goal to mimic or even surpass such enzymatic reactions in vitro. 3,4However, the functions of the previous artificial systems are usually limited to either selective binding or conversion of the substrates, and generally studied in organic solvents under narrow conditions without reusability.Water-soluble synthetic cavities with dual functionality, i.e., displaying both selective binding and conversion abilities, are still rare to this date.
Linear monoterpenes are versatile reaction substrates in nature.They are bound and subsequently cyclized to various monomers and oligomers in protein pockets, with excellent selectivity and efficiency. 2 These cyclic terpenes represent the largest class of natural products with the majority of them displaying bioactive properties. 5While there have been several reports on synthetic receptors capable of tightly binding linear monoterpenes, their binding selectivities are only low to moderate, owing to the flexible chain structures and scarce presence of interactive groups (i.e., −OH, C�O, and C� C). 6,7 As for synthetic terpene reactors, the Raymond and Shionoya groups reported catalytic cyclization of citronellal using a Ga(III)-based coordination capsule and Pd(II)-based macrocycle solid, respectively, in water with high conversion (>70%) yet moderate stereoselectivity. 8The metal-hinges are essential to construct well-defined cavities yet inactive for the catalytic reactions.Efficient catalytic cyclization of geraniol and citronellal was achieved by the Tiefenbacher and other groups employing a hydrogen-bonded capsule in CHCl 3 (∼100% conversion). 9,10Nevertheless, control over the productselectivity of more complex cyclizations remains difficult.In addition, selective cascade cyclization-dimerization of linear monoterpenes has seldom been accomplished by the previous artificial systems, both in the solution and solid states. 11,12o develop a biomimetic synthetic cavity with both receptor and reactor functions for linear monoterpenes, we herein used polyaromatic coordination capsules 1 (Figure 1), 13 capable of selectively binding various biomolecules (e.g., caffeine, sucrose, and androgen) in water, through multiple CH-π, π−π, and/or hydrogen-bonding interactions. 13,14The capsule is prepared by the assembly of Pt(II) ions and bent bispyridine ligands with two anthracene panels.Regardless of the side chains (R = R′ = OCH 2 CH 2 OCH 3 for 1a and R = OCH 3 , R′ = H for 1b), a large polyaromatic cavity (∼1.3 nm and ∼580 Å 3 ) and four small windows (∼0.3 × 0.3 nm 2 ) are provided both in solution and in the solid state.The assembled structures are stable enough even under acidic conditions, as compared with the isostructural derivatives (e.g., M = Pd(II), Ni(II), Cu(II), and Zn(II)).13a,b With the aid of these cavity features, here we report (i) highly selective binding (≥90%) of citronellal (CAL) by polyaromatic capsule 1a from a mixture of linear/ cyclic monoterpenes in water, (ii) preferential vapor binding capability (60−99% selectivity, >2 equiv.based on 1b) of capsule solid (1b) n toward CAL over other monoterpenes, (iii) product-selective extraction (>99% yield, 75% stereoselectivity) of p-menthane-3,8-diol (PMD) by the capsule from a product mixture derived from acid-catalyzed CAL cyclization in water, (iv) the efficient catalytic cyclization-dimerization of vaporized CAL into p-menthane-3,8-diol citronellal acetal, with high product/stereoselectivity (∼330% yield based on 1b, ∼80% stereoselectivity), in the acid-loaded capsule solid under ambient conditions, and (v) the catalyst reusability (5 times) for the cyclization-dimerization reaction with gradually enhanced conversion (up to 1.4-fold).We therefore achieved both receptor and reactor functions toward linear monoterpenes utilizing a single unified synthetic design based on a polyaromatic cavity.

Selective Binding of Citronellal from Mixtures in Water
We initially revealed the selective binding ability of capsule 1a, bearing hydrophilic, long side chains (R = R′ = OCH 2 CH 2 OCH 3 ; Figure 1b), toward (S)-(−)-citronellal (CAL) in water from mixtures with its closely related derivatives.Unlike mono/bicyclic monoterpenes with rigid and unreactive frameworks studied previously, 15 CAL is a linear monoterpene, with a terminal formyl group and an internal double bond, found in plants such as citronella, lemon, and lemon grass. 2 CAL provides a relatively high reactivity against acid and generates a complex mixture of e.g., isopulegol and p-menthane-3,8-diol in solution. 16Synthetic receptors capable of selectively binding CAL and its acid-catalyzed cyclic products have been uncommon so far. 6apsule 1a bound one molecule of CAL in a quantitative fashion within 1 h through simple mixing of 1a and CAL (4.0 equiv.) in water at room temperature, as confirmed by 1 H NMR, ESI-TOF MS, and other analyses (Figures S1−S6 of the Supporting Information). 17The efficient capture stems from typical host−guest hydrophobic effect and CH-π interactions. 13When a complex mixture of 1a (1.0 mg, 0.26 μmol), CAL, geraniol (GOL), (−)-β-citronellol (COL), and myrcene (MRC; 4.0 equiv.each based on 1a) was stirred in D 2 O (0.5 mL) under the same conditions, host−guest complexes 1a• CAL and 1a•GOL were formed with 90 and 10% selectivity, respectively, with no free 1a detectable (Figures 2a and S7− S13a).In the 1 H NMR spectrum, the three methyl signals of CAL within 1a appeared at −2.69, −1.97, and −1.75 ppm, which were highly upfield-shifted (Δδ max = −4.04ppm) relative to free CAL in D 2 O, because of the anisotropy effect of the polyaromatic framework of 1a (Figure 2c−e).The binding selectivity was estimated on the basis of the integral ratio of inner capsule signals H a at ∼6.0 ppm.The ESI-TOF MS   S13b). 17Other monoterpenes present in the mixture, like more hydrophilic COL and more hydrophobic MRC, 18 were hardly bound by 1a and therefore the observed high selectivity toward CAL is most probably derived from effective CH•••O� C-based hydrogen-bonding interactions between the slightly acidic pyridyl α-hydrogens (H f ) of 1a and the carbonyl group of CAL.The host−guest hydrogen-bonding interactions within 1a were supported by FT-IR analysis (Figure S5b). 17inding selectivity of 1a toward CAL was further clarified by a competitive experiment with bicyclic and monocyclic monoterpenes.Stirring a complex mixture of CAL, (−)-camphor (CMP), (−)-borneol (BNL), and (−)-menthol (MTL) in water under the same conditions described above led to the quantitative formation of 1a•CAL (>99% selectivity and yield, Figure 2b).The selectivity was confirmed by the 1 H NMR and ESI-TOF MS analyses of the product solution (Figures 2f and  S14). 17These spectra are virtually identical to those of 1a•CAL obtained from 1a and CAL (Figure 2d).The 1 H DOSY NMR of 1a•CAL showed the host and guest signals with the same diffusion constant (Figure S2).The binding constant (K a ) of 1a toward CAL was estimated to be >3 × 10 5 M −1 , based on that toward CMP obtained by previously reported ITC studies. 15On the basis of these studies, the synthetic polyaromatic cavity of capsule 1a was shown to provide unusual binding ability toward a monoterpene, featuring both a formyl group and a linear flexible framework, in water through effective CH•••O�C hydrogen-bonding, hydrophobic effect, and CH-π interactions.

Selective Vapor Binding of Citronellal from Mixtures
High volatility is one of the characteristic properties differentiating monoterpenes from the majority of biosubstrates.In contrast to host−guest studies in solution, 6 those in the solid state targeting volatile linear monoterpenes have been relatively rare to this date.To assess solid-state binding selectivity toward vaporized CAL, capsule 1b without long side chains (R = OCH 3 and R′ = H; Figure 1b) was employed to increase the hydrophobicity of the space between the capsules in the solid state.Thus, the capsule solid possesses large polyaromatic surfaces (BET surface area: 172 m 2 g −1 ) to effectively interact with substrates via not only the interior but also the exterior of the capsule framework. 21Competitive vapor binding experiments eventually revealed that porous capsule solid (1b) n binds CAL (more than 2 equiv.)preferentially over other linear and cyclic monoterpenes even under ambient temperature and pressure.The binding preference is independent from the intrinsic vapor pressures of the monoterpenes (Figure S15). 18AL and geraniol (GOL) (40 equiv.each based on 1b) were separately put in a sealed glass vessel (50 mL) including pale yellow solid (1b) n (0.6 mg, 0.19 μmol) at room temperature for 2 h (Figure 3a).After removal of the substrates outside the solid under vacuum (480 Pa, 1 h, room temperature), the binding capability of (1b) n toward the substrate vapor was estimated by 1 H NMR analysis.The bound CAL and other monoterpenes in the capsule solid remained intact even under the harsh conditions, due to effective host−guest interactions inside the solid.The proton spectrum of resultant solid (1b) n •(CAL) x •(GOL) y in CD 3 CN showed sharp signals derived from dissociated 1b, CAL, and GOL (Figure S16−21), owing to the lack of host−guest interactions in nonaqueous solution.The signal integrals indicated that a total of 2.6 molecules of the substrates based on 1b were bound by the solid with 82% selectivity for CAL (Figure 3b,e).The CAL-binding selectivity of solid (1b) n decreased against COL yet increased against MRC under the same conditions (−22% and +17% yield, respectively; Figure 3c,e and S22). 17These results suggest that the capsule solid can incorporate the monoterpenes into polyaromatic spaces inside and between the capsules, which slightly alters the binding selectivity relative to that of 1a in water.Thus, it is noteworthy that the amount of the bound linear monoterpenes increased by ∼2−3-fold (based on the capsule), as compared with that of cyclic ones in our previous work, 15 under similar conditions.
The vapor binding ability of solid (1b) n toward CAL was higher than toward cyclic monoterpenes, i.e., CMP, BNL, and MTL.Solid product (1b) n •(CAL) x •(CMP) y was obtained by placing (1b) n , CAL, and CMP in a 1:40:40 ratio in a sealed glass vessel (50 mL) at room temperature for 2 h.The 1 H NMR spectrum of the product elucidated that 2.7 monoterpenes based on 1b (i.e., n = 1.0, x = 2.2, y = 0.5) are captured by the capsule solid and the selectivity for CAL was estimated to be 80% (Figure 3d,e and S23).The vapor binding selectivity of (1b) n toward CAL was virtually unchanged even from mixtures of CAL and BNL as well as CAL and MTL (≤ ± 3%; Figure 3e and S24−S25).

Selective Extraction of Acid-Catalyzed Cyclization Products in Water
We next focused on the acid-catalyzed cyclization of citronellal (CAL) in solution and the binding properties of capsule 1a (R = R′ = OCH 2 CH 2 OCH 3 ) toward the complex reaction mixture. 15CAL is readily converted to various cyclic monoterpenes, including mixed stereoisomers of isopulegol (IPG) and p-menthane-3,8-diol (PMD; Scheme 1), under acidic aqueous conditions. 16Therefore, the strict stereocontrol of the cyclization reaction remains difficult even under various conditions. 8We herein alternatively elucidated the productselective as well as stereoselective extraction of PMD (i.e., its equatorial isomer eq-PMD) by 1a from the reaction mixture in water.

Catalytic Cyclization-Dimerization of Vaporized Citronellal
Taking advantage of the efficient and selective binding ability of capsule solid (1b) n (R = OCH 3 and R′ = H), we for the first time set out to develop a reactor function of the capsule solid toward vaporized CAL.Monoterpene conversion in the vaporized state was envisioned to circumvent certain problems observed in solution, particularly suppression of multistep and intermolecular reaction pathways. 16Solid (1b) n is composed of amorphous polyaromatic assemblies bearing well-defined spherical cavities within the capsules and randomly distributed spaces between (1b) n . 21To generate reactive cation species from CAL in the polyaromatic cavities, acid-loaded capsule solid (1b) n •(TS) x was used for the present studies, because the capsule framework as well as its metal-hinges provide no catalytic activity.In sharp contrast to the previous acidcatalyzed cyclization of CAL in solution, 8−10 the acidic porous solid prompted catalytic cyclization-dimerization of vaporized CAL and yielded p-menthane-3,8-diol citronellal acetal (MCA) with high efficiency and product/stereoselectivity under ambient conditions. 22olid (1b) n •(TS) x was obtained by instant mixing (10 s) and solvent evaporation of 1b (0.6 mg, 0.2 μmol) and ptoluenesulfonic acid (TS; 0.7 equiv.) in diethyl ether at room temperature. 17The incorporation of TS into solid (1b) n was confirmed by 1 H NMR analysis. 22The sequential vapor binding of CAL and its cyclization-dimerization occurred within (1b) n •(TS) x at room temperature under ambient pressure (Figure 5a).Pale yellow solid (1b) n •(TS) x (0.2 μmol based on 1b) and CAL (1.5 mg, 10 μmol) were placed separately in a closed glass vessel (50 mL) for 6 h.range of 1−5 ppm, besides the methoxy signals derived from free capsule 1b (Figures 5b,d and S37).Tiny proton signals of unreacted CAL (<10% based on 1b) and p-menthane-3,8-diol (PMD) were also found in the NMR spectrum.The GC-MS spectrum of the acetonitrile solution showed a single prominent peak, derived from a monoterpene dimer, at 15.1 min GC retention time (Figure 5e), largely shifted compared to that of CAL at 8.1 min (Figure 5c).After isolation of main product MCA by gel permeation chromatography (GPC), 17 its structure and stereochemistry were revealed by the combination of MS, NMR, and molecular modeling studies as follows (Figures S38−S46).The product mass was confirmed by ESI-TOF MS analysis and estimated to be 308.27Da, indicating the formation of a dimeric compound (C 20 H 36 O 2 ; Figure S45). 17The 13 C NMR spectrum of MCA indicated a product containing 20 inequivalent carbon atoms, including one acetal carbon, two carbons connected to -OR groups, six methyl carbons, and nine carbons derived from a 2,6-dimethylhept-5enyl group (Figure 5f).The 1 H NMR and 1 H− 1 H COSY analyses suggested the formation of two fused six-membered rings including an acetal unit (Figure S39).The combination of Nuclear Overhauser effects (NOEs) in the 1D NOESY spectra (i.e., correlation of protons H3−H10, H10−H1′, and H1′−H3; Figures 5g and S40), besides molecular modeling studies, revealed the formation of MCA-a as a single isomer.Capsule 1a (as well as 1b) provides a large enough, inner cavity to bind one molecule of MCA-a in a quantitative fashion (Figures S47−S49).
From the 1 H NMR integrals of product solid (1b) n •(TS) x • (MCA-a) y dissolved in CD 3 CN, the yield of cyclic dimer MCA-a was estimated to be 332% (TON = 3.3) based on capsule 1b.The minor formation of isomer MCA-b (87%; Figure 5h) and PMD (60%) without MCA-c/d was also observed in the polyaromatic cavities of (1b) n •(TS) x .The substrate-based yields of products MCA-a, MCA-b, and PMD were calculated to be 14, 4, and 1% based on CAL, respectively, owing to the gas/solid-state reaction under ambient conditions.Weak signals for MCA-b and PMD were observed in the 1 H NMR spectrum and GC chart (Figure 5d,e).These results clarified that the selective cyclizationdimerization of CAL took place within solid (1b) n •(TS) x in a catalytic fashion, most likely triggered by the concentration effect as well as size and shape complementarity.In sharp contrast, neither cyclization nor dimerization of GOL and COL was observed within (1b) n •(TS) x (Figure S50). 17oth the well-defined polyaromatic cavity of solid (1b) n and the loading of organic acid TS are essential for the present catalytic system.The metal-hinges and polyaromatic frameworks of 1b have no catalytic activity toward CAL.In contrast, the treatment of vaporized CAL with solid TS without (1b) n yielded MCA-a in 108% virtually based on 1b under the same conditions as described above.This control experiment revealed that the reactivity of solid (1b) n •(TS) x for the cyclization-dimerization of CAL is 3.1-fold higher than the background reaction (Figure 6a).Solid (1b) n •(TS) x with a 1:0.7 1b/TS ratio acted as a superior catalyst, as compared with those with 1:0.4,1:1.2, and 1:2 1b/TS ratios (Figure S51a).The catalytic activities of capsule solids (1b) n , loading benzoic, benzenesulfonic, or benzenephosphonic acids (0.7− 1.2 equiv.based on 1b), toward CAL were lower than that of (1b) n •(TS) x (<0.2-fold; Figure S51b).As compared with the background reactivity, other acid-loaded solids comprising TS and common organic macrocycles, such as γ-cyclodextrin (γCD), cucurbit [6]uril (CB6), and a pillar [5]arene derivative (P5A; Figure 6b), showed only similar to lower reactivities (i.e., 0.3 to 1.1-fold; Figures 6a and S52−S53).In these reactions, MCA-b was yielded as a byproduct in ∼20% yield (based on the GC data; Figure S54), indicating poor reactivity on the outer surface of acid-loaded solids.The importance of the inner and outer polyaromatic cavities was further supported by using an analogous blocked capsule including  one molecule of fullerene C 60 (C 60 ).Acid-loaded 1:1 host− guest solid (1b′•C 60 ) n •(TS) x , prepared from capsule 1b′ (M = Pd(II)), C 60 , and TS, afforded MCA-a and MCA-b in 163% (TON = 1.6) and 39% yield based on 1b′, respectively, under the same conditions (Figure 6a). 17It should be noted that, under neat conditions without (1b) n , the acid-catalyzed reaction of CAL using trifluoroacetic acid gave rise to a complex mixture, including a tiny amount of MCA (7% based on CAL; Figure S38).Transformation of volatilized monoterpenes was thus successfully shown to be an important strategy for efficiently accessing otherwise difficult-to-obtain product species.

Reusability of the Capsule Solid for the Catalytic Cyclization-Dimerization Reaction
Porous capsule solid (1b) n could be reused for the acidcatalyzed cyclization-dimerization of vaporized CAL at least five times with gradually enhanced conversion.After the first reaction under the same conditions (room temperature, 6 h) described above, the resultant solid (1b) n •(TS) x •(MCA-a) y was washed with diethyl ether and CH 3 CN to selectively remove and isolate MCA-a from the cavity, recovering a mixed solid consisting of (1b) n •(TS) x and (1b) n .The product structure and yield were analyzed by 1 H NMR spectroscopy.After the reloading of TS into the capsule solid, regenerated solid (1b) n • (TS) x was subjected to the next reaction cycle, to confirm the reusability of the capsule solid.The 1 H NMR spectra of the extracted solutions displayed that the yields of cyclic dimer MCA-a increased gradually and reached 457% based on 1b (TON = 4.6, 19% based on CAL) at the fifth reaction cycle (1.4-foldenhancement based on 1b; Figures 7 and S55−S56).
The stereoselectivity of product MCA-a remained intact throughout the reusability studies.The observed enhanced conversion is most probably caused by the gradual accumulation of p-toluenesulfonate ions in the capsule solid, through the counterion exchange with the nitrate ions (Figure S57), which could alter the space features between the capsules.In each cycle, MCA-b and PMD formed in 70−133 and 98−136% yields, respectively (based on 1b; Figures 7 and  S56).Whereas the detailed reaction mechanism is unclear, 23 time-dependent 1 H NMR study suggested a rapid stepwise cascade reaction from CAL to MCA in the capsule solid, due to the observation of minor PMD signals during the reaction (Figure S37b).No product inhibition by MCA-a was observed, probably due to the high volatility and flexibility of CAL.The high inertness and sufficient structural stability of the polyaromatic capsule framework under the repeated, acidcatalyzed reaction conditions was supported by the 1 H NMR spectra (Figure S55), successfully preventing alkylation of the host, an undesired side reaction observed in one of the previous catalytic systems. 9nally, the cyclization-dimerization reactions from a vapor mixture of CAL and geraniol (GOL) were examined using acid-loaded capsule solid (1b) n •(TS) x and its reused species under ambient conditions.After separately putting solid (1b) n • (TS) x (0.2 μmol based on 1b), CAL (8.7 μmol), and GOL (8.7 μmol) in a closed vessel for 6 h, the 1 H NMR analysis of the resultant solid revealed the formation of cyclic dimers MCA (165% based on 1b) and cyclic monomers PMD (93%; Figure S58). 24The same reaction using acid-loaded, reused solid (1b) n •(TS) x gave rise to MCA (210%) and PMD (117%).Notably, the bound GOL (0.5−0.6 equiv.based on 1b) showed neither conversion nor decomposition.Accordingly, the present cyclization-dimerization of CAL proceeded sufficiently in the acid-loaded capsule solid, even from the mixed vapor of CAL and its derivative.

■ CONCLUSIONS
Inspired by functional biocavities with highly selective binding and conversion of substrates, we have revealed new cavity functions of polyaromatic coordination capsules toward highly volatile and flexible, linear monoterpenes in the solution and solid states.In aqueous solution, the capsule preferentially bound one molecule of citronellal in the cavity, from complex mixtures with other linear/cyclic monoterpenes.From a reaction mixture of its acid-catalyzed cyclization, the capsule quantitatively bound p-menthane-3,8-diol with high equatorialstereoselectivity to generate the corresponding 1:1 host−guest complex.We expect that such binding functions will lead to the facile extraction of minor yet valuable terpenes from natural plants, which contain a wide range of derivatives.In the solid state, the capsule furthermore exhibited preferential vapor binding ability toward citronellal (more than two molecules) from mixtures with other linear/cyclic monoterpenes under ambient conditions.Most importantly, the acid-loaded capsule solid, facilely prepared from the polyaromatic capsule and an organic acid, promoted the catalytic cyclization-dimerization reaction of vaporized citronellal with high efficiency, selectivity, and reusability.The herein demonstrated dual functionality of both selective binding and efficient conversion has rarely been demonstrated with other synthetic cavities reported previously.In contrast to flexible biocavities composed of large protein assemblies and rigid synthetic cavities composed of inorganic frameworks, the present polyaromatic cavities provide both flexible and rigid features as well as biological interactions, such as hydrogen-bonding, hydrophobic effect, and CH-π interactions.Therefore, we expect that the present study will promote novel approaches toward bioinspired terpene biosynthesis in both solution and vapor states using synthetic polyaromatic cavities.

■ METHODS
The following analytical instruments and software were used in this study.NMR: Bruker AVANCE III HD 500 (500 MHz) and AVANCE III 400 (400 MHz, TMS (δ = 0.00 ppm) in CDCl

Figure 1 .
Figure 1.(a) Schematic representation of the binding and conversion of linear monoterpene(s) via a synthetic polyaromatic cavity with high efficiency and selectivity.(b) Polyaromatic coordination capsule 1 and its crystal structure (R = R′ = H for clarity).

Figure 2 .
Figure 2. Solution-state selective binding of CAL by 1a from (a) a mixture of CAL, GOL, COL, and MRC, and (b) a mixture of CAL, CMP, BNL, and MTL in water. 1 H NMR spectra (500 MHz, D 2 O, r.t.) of (c) 1a and (d) 1a•CAL, and products from mixtures of 1a with (e) CAL, GOL, COL, and MRC and (f) CAL, CMP, BNL, and MTL in water (red circle: proton signals derived from bound CAL).(g) ESI-TOF MS spectrum (H 2 O) of products from mixtures of 1a with CAL, GOL, COL, and MRC in water.

Figure 3 .
Figure 3. (a) Solid-state selective binding of vaporized CAL by solid (1b) n from a vapor mixture with GOL. 1 H NMR spectra (500 MHz, CD 3 CN, r.t.) of products after the competitive vapor binding of (b) CAL and GOL, (c) CAL and MRC, and (d) CAL and CMP by solid (1b) n at r.t. for 2 h.(e) Host−guest ratios and guest % of the products after competitive vapor binding for CAL (red bar) and other monoterpenes (blue bar).
Scheme 1. Formation of a Mixture Including Stereoisomers of PMD and IPG from CAL under Acidic Conditions

Figure 6 .
Figure 6.(a) The relative reaction enhancement of TS-loaded host and host−guest solids for the conversion of vaporized CAL into MCA-a, based on the background reaction.(b) Structures of organic hosts γCD, CB6, and P5A.

Figure 7 .
Figure 7. Reusability of solid (1b) n •(TS) x for the conversion of CAL to MCA.