A thorough experimental study of CH/π interactions in water: quantitative structure–stability relationships for carbohydrate/aromatic complexes

A dynamic combinatorial analysis of carbohydrate/aromatic complexes clarifies the structural determinants and origins of these important interactions in water.


General Information
Chemicals were purchased from commercial sources and were used without further purification. All solvents were purified by distillation over drying agents or by elution through a PURE SOLV purification system. Unless stated otherwise, reactions were carried out under a dry argon atmosphere in vacuum-flame dried glassware. Residual water was removed from starting compounds by repeated coevaporation with toluene. Analytical thin layer chromatography was carried out using pre-coated, aluminium backed plates (Merck Kieselgel 60 F254). Detection was by examination under UV light (254 nm) and then by charring with 10% sulfuric acid in ethanol or cerium-ammonium-molybdate.
Flash column chromatography was performed using silica gel [Merck,].
Extracts were concentrated in vacuo using both a Buchi rotary evaporator (bath temperatures up to 40 °C) at a pressure of 15 mmHg (diaphragm pump) and a high vacuum line at room temperature. 1 H NMR and 13 C NMR spectra were measured in the solvent stated at 500, 400 or 300 and 126, 101 or 75 MHz, respectively. Chemical shifts are quoted in parts per million from residual solvent peak and coupling constants (J) given in Hertz. Multiplicities are abbreviated as: b (broad), s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) or combinations thereof. An Agilent 6520 accurate-mass quadrupole time-of-flight (Q-TOF) mass analyzer was used for the HR-MS. Glycosyl donor was prepared from allyl 3-azido-3-deoxy-4,6-O-benzylidene α-D-altropyranose 1 by acetylation, allyl cleavage at the anomeric position, and trichloroacetimidate formation according to standard procedures. Starting glycosyl acceptors A1-I1 were prepared following well-established procedures, and cleavage of protecting hydroxyl groups was carried out according standard procedures. 2

General experimental procedure for mesylation:
A solution of the corresponding alcohol (1 mmol) in anhydrous CH 2 Cl 2 (10 mL) cooled to 0 ºC and under inert atmosphere was treated with triethylamine (3 mmol) and mesyl chloride (1.2 mmol). The mixture was stirred until TLC showed no starting alcohol was left and was then poured onto a saturated aqueous sodium bicarbonate solution. The organic phase was separated and the aqueous layer was extracted twice with CH 2 Cl 2 . The combined organics layers S6 Scheme 1. Alternative routes for the preparation of the 2-aryl-acetaldehydes library
In order to evaluate more precisely the stability of the different CH/π complexes considered in our study, we performed pairwise competition experiments following the same protocol.    Figure S1.-1D-(up), 2D-TOCSY (Bottom-Right) and 2D-HSQC (Bottom-Left) spectra acquired for compound A at 500 MHz, pH 7.5 and 298K.
S37 Figure S3.-Key region of 1D-NMR Spectra acquired for compounds B-1(Left) and H-1 (Right). NMR signals for protons involved in CH/π interactions with the aromatic ring are highlighted with coloured circles. Chemical shift perturbations promoted by the aromatic ring at the interacting D-pyranose units are shown above. These data indicate that complexation is mediated by the pyranose α-face in B-1 and by the βface in H-1. Figure S4.-We performed pairwise competitions with all possible Ref/disaccharide pairs so that the net free energy (ΔG) of the stacking complexes could be determined (upper panel). In addition, crosschecks were carried out with selected aldehydes and disaccharide pairs (lower panel). The obtained ΔΔG values were found to be fully consistent with the net interaction energies (ΔG) and errors represented in Table S1. NMR signals for the altrose anomeric proton in the final products, together with the product ratios (grey. In brackets) and the estimated free energy differences between alternative interaction modes (in black) are shown.

S38
S39 Figure S5.-Chemical shift perturbations promoted by the aromatic ring at the interacting D-pyranose unit in selected complexes. These values were taken as indicative of the interaction geometry.
S40 Figure S6.-Chemical shift perturbations promoted by the aromatic ring at the interacting D-pyranose unit in complexes B-1/B-3 (left) and C-1/C-3 (right). According to these data, the hydroxymethyl group (highlighted with a red ellipse) participates in contacts with the aromatic unit only for B complexes.
S41 Figure S7.-The magnitude of the energy penalty associated to the axial/equatorial inversion of a pyranose polar group (highlighted with a green ellipse) depends on the precise geometrical features of the complex, varying from 0.3 kcal/mol, (C-1 vs D-1 or C-3 vs D-3) to <0.1 kcal/mol (A-1 vs B-1 or A-3 vs B-3). Measured Δδ values indicate that for C and D complexes the aromatic ring is closer to the inverted position.  (ΔG exp ) measured for complexes formed by derivatives C, D or F with substituted nahpfthyl or phenyl rings (Right). Overall, the stability of the CH/π bonds increases with the electron-rich character of the aromatic unit. However, it can be observed that those complexes formed by densely oxygenated aromatic systems as 5-7 display reduced ΔG exp values. Interestingly, quantum mechanics calculations show that the OR groups present in 5-7 are not co-planar with the naphthyl unit (so that multiple conformations are feasible). Although merely speculative, this peculiarity might oppose binding by, imposing a significant entropic penalty on the recognition process (Left).