H-1-NMR analysis of intra- and intermolecular H-bonds of alcohols in DMSO:Chemical shift of hydroxy groups and aspects of conformational analysis ofselected monosaccharides, inositols, and ginkgolides

The interpretation of H-1-NMR chemical shifts, coupling constants, and coef ficients of temperature dependence (delta(OH),J(H,OH), and Delta delta(OH)/ Delta T values) evidences that, in (D,)DMSO solution, the signal of an OH g roup involved as donor in an intramolecular H-bond to a hydroxy or alkoxy g roup is shifted upfield, whereas the signal of an OH group acting as accept or of an intramolecular H-bond and as donor in an intermolecular H-bond to (D,)DMSO is shifted downfield. The relative strength of the intramolecular H-bond depends on co-operativity and on the acidity of OH groups. The acidi ty of OH groups is enhanced when they are in an antiparallel orientation to a C-O bond. A comparison of the H-1-NMR spectra of alcohols in CDCl3 and ( D-6)DMSO allows discrimination between weak and strong intramolecular H-bon ds. Consideration of IR spectra (CHCl3 or CH2Cl2) shows that the rule accor ding to which the downfield shift of delta(OH) for H-bonded alcohols in CDC l3 parallels the strength of the H-bond is valid only for alcohols forming strong intramolecular H-honds. The combined analysis J/(H,OH) and delta(OH) values is illustrated by the interpretation of the spectra of the epoxyalc ohols 14 and 15 ( Fig 3). H-Bonding of hexopyranoses, hexulopyranoses, alky l hexopyranosides, alkyl 4,6-O-benzylidenehexopyranosides, levoglucosans, a nd inositols in (D-6)DMSO was investigated. Fully solvated non-anomeric equ atorial OH groups lacking a vicinal axial OR group (R=H or alkyl, or (alkox y)alkyl) show characteristic J(H,OH) values of 4.5-5.5 Hz and fully solvate d non-anomeric axial OH groups lacking an axial OR group in beta-position a re characterized by J(H,OH) values of 4.2-4.4 Hz(Figs. 4-6). Non-anomeric e quatorial OH groups vicinal to an axial OR group are involved in a partial intramolecular H-bond (J(H,OH)=5.4-7.4 Hz), whereas non-anomeric equatorial OH groups vicinal to two axial OR form partial bifurcated H-bonds (J(H.OH) =5.8-9.5 Hz). Non-anomeric axial OH groups form partial intramolecular H-b onds to a cis-1.3-diaxial alkoxy group las in 29 and 41: J(H,OH)= 4.8-5.0 H z). The persistence of such a H-hond is enhanced when there is an additiona l H-bond acceptor, such as the ring O-atom (43-47: f(H.OH)= 5.6-7.6 Hz: 32 and 33: 10.5 - 11.3 Hz). The (partial) intramolecular H-bonds lead to an up field shift (relative to the signal of a fully solvated OH in a similar sur rounding) for the signal of the H-donor. The shift map also be related to t he signal of the fully solvated, equatorial HO-C(2), HO-C(3), and HO-C(4) o f beta-D-glucopyranose (16: 4.81 ppm) by using the following increments: -0 .3 ppm for an axial OH group, 0.2-0.25 ppm for replacing a vicinal OH by an OR group, cn. 0.1 ppm for replacing another OH by an OR group, 0.2 ppm for an antiperiplanar C-O bond, -0.3 ppm if a vicinal OH group is (partially) H-bonded to another OR group. and -0.3 to -0.6 for both OH groups of a vici nal diol moiety involved in (partial) divergent H-bonds. Flip-flop H-honds are observed between the diaxial HO-C(2) and HO-C(4) of thr inositol 40 (J( H.OH)=6.4 Hz, delta(OH)=5.45 ppm) and levoglucosan (42; J(H,OH)=6.7-7.1 Hz. delta(OH)=4.76-4.83 ppm: bifurcated H-bond): the former is completely pers istent and the latter to ca. 40%. A persistent, unidirectional H-bond C(1)- OHO-C(10) is present in ginkgolide B and C, as evidenced by strongly differ ent delta(OH) and Delta delta(OH)/Delta T values for HO-C(1) and HO-C(10) ( Fig. 9). In the absence of this H-bond. HO-C(1) of 52 resonates 1.1 - 1.2 p pm downfield, while HO-C(10) of ginkgolide A and of 48-50 resonates 0.5-0.9 ppm upfield.