Three novel 5(6-->7)abeoabietane-type diterpenes from the bark of Taiwania cryptomerioides

Three new 5(6-->7)abeoabietane diterpenes with the uncommon skeleton of fused 6-5-6 rings were isolated from the bark of Taiwania cryptomerioides designated as taiwaniaquinone F (8), taiwaniaquinol C (9) and taiwaniaquinol D (10). Meanwhile, two known compounds, taiwaniaquinones A (1) and D (4), were also obtained. Their structures were determined principally from spectral evidence.     

Synthesis of the core tetrasaccharide of Trypanosoma cruzi glycoinositolphospholipids: Manp(alpha1-->6)-Manp(alpha1-->4)-6-(2-aminoethylphosphonic acid)-GlcNp(alpha1-->6)-myo-Ins-1-PO4

[structure: see text] Synthesis of the core tetrasaccharide Manp(alpha1-->6)-Manp(alpha1-->4)-6-(2-aminoethylphosphonic acid)-GlcNp(alpha1-->6)-myo-Ins-1-PO4, found in glycoinositolphospholipids of Trypanosoma cruzi parasites, is described. The key building block, 6-O-(2-azido-3-O-benzyl-6-O-((2-benzyloxycarbonylaminoethyl)phosphonic acid benzyl ester)-2-deoxy-alpha-D-glucopyranosyl)-1-di-O-benzylphosphoryl-4,5-O-isopropylidene-2,3-O-(D-1,7,7-trimethyl[2,2,1]bicyclohept-6-ylidene)-D-myo-inositol, was synthesized using a partially protected glucosyl D-camphorinositolphosphate and a (2-benzyloxycarbonylaminoethyl)phosphonic acid derivative in a regioselective phosphonate esterfication. Elongation with ethyl 2-O-benzoyl-3,4,6-tri-O-benzyl-alpha-D-mannopyranosyl-(1-->6)-2,3,4-tri-O-benzyl-1-alpha-D-thiomannopyranoside using dimethyl(methylthio)sulfonium trifluoromethanesulfonate gave a fully protected tetrasaccharide which was successfully deprotected subsequently with sodium methoxide, sodium in liquid ammonia, and aq hydrochloric acid to give title compound.     

Application of the anomeric samarium route for the convergent synthesis of the C-linked trisaccharide alpha-D-Man-(1-->3)-[alpha-D-Man-(1-->6)]-D-Man and the disaccharides alpha-D-Man-(1-->3)-D-Man and alpha-D-Man-(1-->6)-D-Man

Studies are reported on the assembly of the branched C-trisaccharide, alpha-D-Man-(1-->3)-[alpha-D-Man-(1-->6)]-D-Man, representing the core region of the asparagine-linked oligosaccharides. The key step in this synthesis uses a SmI(2)-mediated coupling of two mannosylpyridyl sulfones to a C3,C6-diformyl branched monosaccharide unit, thereby assembling all three sugar units in one reaction and with complete stereocontrol at the two anomeric carbon centers. Subsequent tin hydride-based deoxygenation followed by a deprotection step produces the target C-trimer. In contrast to many of the other C-glycosylation methods, this approach employes intact carbohydrate units as C-glycosyl donors and acceptors, which in many instances parallels the well-studied O-glycosylation reactions. The synthesis of the C-disaccharides alpha-D-Man-(1-->3)-D-Man and alpha-D-Man-(1-->6)-D-Man is also described, they being necessary for the following conformational studies of all three carbohydrate analogues both in solution and bound to several mannose-binding proteins.     

Chemoenzymatic synthesis of n-hexyl and O-beta-D-xylopyranosyl-(1-->6)-beta-D-glucopyranosides

Direct beta-glucosidation between 1,6-octanediol (5) and D-glucose (3) using the immobilized beta-glucosidase (EC 3.2.1.21) from almonds with the synthetic prepolymer ENTP-4000 gave a mono-beta-glucoside (6) in 61.4% yield, which was converted into the n-hexyl beta-D-glucopyranoside (1) by means of a chemoenzymatic method. The coupling of the n-hexyl beta-D-glucopyranoside congener (13) and 2,3,4-tri-O-acetyl-beta-D-xylosyl congener (14), followed by deprotection, afforded the synthetic n-hexyl O-beta-D-xylopyranosyl-(1-->6)-beta-D-glucopyranoside (2), which was identical to the natural 2 with respect to the spectral data and specific rotation.     

Luminescence spectroscopy of matrix-isolated atomic manganese: excitation of the "forbidden" a6D(J)<-->a6S transitions

Laser-induced excitation spectra recorded for the electric-quadrupole 3d(6)4s a6D(J)<--3d(5)4s2a6S(5/2) transitions of atomic Mn, allow assignment of the red emission features, previously observed in Mn/RG (RG=Ar, Kr and Xe) matrices with resonance 3d(5)4s4pz6P(5/2)<--3d(5)4s2 a6S(5/2) excitation, to the metastable a6D(9/2) state. Narrow excitation bands recorded for the red site in the Mn/Kr system allow identification of all five spin-orbit levels (J=1/2, 3/2, 5/2, 7/2 and 9/2) in the a6D state. The coincidence of the lowest energy excitation band and the observed 585.75 nm (17,072 cm(-1)) emission band of atomic Mn in Kr matrices, yielded a definitive assignment of this emission to a transition from the J=9/2 spin-orbit level. Temperature dependent emission scans lead to the identification of the zero phonon line for the a6D(9/2)-->a6S(5/2) transition at 585.75 nm. The identified matrix-shift of +20 cm(-1) allows an assessment of the extent of the ground state stabilization in the red (secondary) site of atomic Mn isolation in solid Kr. Emission produced with direct a6D state excitation yielded both the 585.75 and 626 nm features. The former band arises for Mn atoms occupying the red site--the latter from blue site occupancy in solid Kr. The excitation linewidths recorded for these two sites differ greatly, with the blue site yielding a broad featureless profile, in contrast to the narrow, structured features of the red site. The corresponding red site a6D(J)<-->a6S(5/2) transitions in Ar and Xe matrices are broader than in Kr--a difference considered to originate from the site sizes available in these hosts and the interatomic Mn x RG potentials. The millisecond decay times recorded for the red emission bands in the Mn/RG systems are all much shorter than the 3 s value predicted for the gas phase a6D(9/2)-->a6S(5/2) transition. This enhancement allows optical pumping of the forbidden a6D(J)<-->a6S transitions with low laser powers when atomic manganese is isolated in the solid state. However all the emission decays are complex, exhibiting triple exponential decays. This behavior may be related to the dependence of the excitation linewidths on the J value, indicating removal of the J degeneracy due to weak matrix-induced, crystal field splitting.     

Synthesis of various sulfoforms of the trisaccharide beta-d-GlcpA-(1-->3)- beta-d-Galp-(1-->3)-beta-d-Galp-(1-->OMP) as probes for the study of the biosynthesis and sorting of proteoglycans

A straightforward preparation of various sulfoforms of the trisaccharide 4-methoxyphenyl O-(sodium beta-d-glucopyranosyluronate)-(1-->3)-( beta-d-galactopyranosyl)-(1-->3)-beta-d-galactopyranoside (1), namely its 6a- and 4a-monosulfate, 6b- and 4b-monosulfate and 6a,6b-disulfate derivatives, is reported for the first time. These compounds, which are partial structures of the linkage region of proteoglycans, will serve as probes for the study of the biosynthesis and sorting of these macromolecules. A key trisaccharide derivative, in which the two similar d-Gal units were differentiated at C-4,6 with 4,6-benzylidene and 4,6-di-tert-butylsilylene acetals, respectively, was used as a common intermediate. Both acetal groups showed excellent orthogonality, and allowed the preparation of all target compounds in high yield. Noteworthy is the possibility to prepare the 6a- and 6b-monosulfated and the 6a,6b-disulfated species through a one-pot regioselective procedure starting from a tetrol precursor.     

Synthesis of alpha-(2-->5)Neu5Gc oligomers

A facile synthesis of the sialic acid oligomers alpha-(2-->5)Neu5Gc (1) is presented. Monosaccharides 2-4 with suitable functionality were used as the building blocks. After selective removal of the paired carboxyl and amine protecting groups, the fully protected oligomers were assembled through consecutive coupling of the building blocks by well established peptide coupling techniques. By this approach, fully protected oligomers as large as an octasaccharide were synthesized. Deprotection of these fully protected oligomers was conducted in two steps (LiCl in refluxing pyridine and 0.1 n NaOH) to afford the desired products in high yield. Enzymatic degradation of the octamer with neuraminidase, monitored by capillary electrophoresis (CE), was also accomplished. The stepwise exo-cleavage adducts were all well separated and identified in the CE spectrum. The strategy described here for solution-phase synthesis also provides the basis for future solid-phase synthesis of poly-alpha-(2-->5)Neu5Gc.     

Direct chemical synthesis of the beta-D-mannans: the beta-(1-->2) and beta-(1-->4) series

The direct syntheses of a beta-(1-->2)-mannooctaose and of a beta-(1-->4)-mannohexaose are reported by means of 4,6-O-benzylidene-protected beta-mannosyl donors. The synthesis of the (1-->2)-mannan was achieved by means of the sulfoxide coupling protocol, whereas the (1-->4)-mannan was prepared using the analogous thioglycoside/sulfinamide methodology. In the synthesis of the (1-->4)-mannan, the glycosylation yields and stereoselectivities remain approximately constant with increasing chain length, whereas those for the (1-->2)-mannan consist of two groups with the formation of the tetra- and higher saccharides giving yields and selectivities consistently lower than those of the lower homologues. The decrease in yield after the trisaccharide in the (1-->2)-mannan synthesis is attributed to steric interference by the n-3 residue and is consistent with the collapsed, disordered structure predicted by early computational work. The consistently high yields and selectivities seen in the synthesis of the (1-->4)-mannan are congruent with the more open, ordered structure originally predicted for this polymer. The lack of order in the structure of the (1-->2)-mannan, as compared to the high degree of order in the (1-->4)-mannan, is also evident from a comparison of the NMR spectra of the two polymers and even from their physical nature: the (1-->2)-mannan is a gum and the (1-->4)-mannan is a high melting solid.     

Structural determination of a new 2(3 --> 20)abeotaxane with an unusual 13beta-substitution pattern and a new 6/8/6-ring taxane from Taxus cuspidata

A new 2(3 --> 20)abeotaxane with an unusual 13beta-substitution pattern and a new 6/8/6-ring taxane were isolated from the methanol extract of the needles of Taxus cuspidata. The structures were established as 2alpha,7beta-diacetoxy-5alpha,10beta,13beta-trihydroxy-2(3 --> 20)abeotaxa-4(20), 11-dien-9-one (1) and 2alpha,5alpha,7beta,9alpha,13alpha-pentahydroxy-10beta-acetoxytaxa-4(20),11-diene (2) on the basis of 1D and 2D NMR spectral data and high-resolution FAB-MS analyses.     

Multiple pathways in the synthesis of new annelated analogues of 6-benzyl-1-(ethoxymethyl)-5-isopropyluracil (emivirine)

Condensation of 3-(3,5-dimethylphenyl)-2-oxocyclopentanecarboxamide (11) with oxalyl chloride and condensation of ethyl 2-benzylamino-5-methyl-3-phenylcyclopent-1-enecarboxylate (17a) with trimethylsilyl isothiocyanate gave 7-(3,5-dimethylphenyl)-6,7-dihydro-5H-cyclopenta[e][1,3]oxazine-2,4-dione (12) and 1-benzyl-5-methyl-7-phenyl-2-thioxo-1,2,3,5,6,7- hexahydrocyclopentapyrimidin-4-one (18a), respectively. Acid catalyzed ring-closure of 6-(4-methyl-1-phenylpent-3-enyl)-2-thioxo-2,3-dihydro-1H-pyrimidin-4-one (26) and radical mediated ring-closure of 1,3-bis(benzyloxymethyl)-5-bromo-6-(1-phenylbut-3-enyl)-1H-pyrimidine-2,4- dione (32a) gave 5,5-dimethyl-8-phenyl-5,6,7,8-tetrahydro-1H-quinazoline-2,4- dione (28) and 1,3-bis(benzyloxymethyl)-5-methyl-7-phenyl-1,5,6,7- tetrahydrocyclopentapyrimidine-2,4-dione (33), respectively. Annelated emivirine analogues 7-(3,5-dimethylphenyl)-1- ethoxymethyl-1,5,6,7-tetrahydrocyclopentapyrimidine-2,4-dione (4), 1-ethoxymethyl-5,5-dimethyl-8-phenyl-5,6,7,8-tetrahydro-1H-quinazoline- 2,4-dione (5) and 1-ethoxymethyl-5-methyl-7-phenyl-1,5,6,7- tetrahydrocyclopentapyrimidine-2,4-dione (6) were obtained in few steps from 12, 28 and 18a/33, respectively. These new analogues can be considered as conformationally locaTed analogues of emivirine. However, the compounds 4 6 showed lower activities against HIV-1 than emivirine and it is concluded that the locked conformation disfavours activity against HIV-1.     

A hydration study of (1-->4) and (1-->6) linked alpha-glucans by comparative 10 ns molecular dynamics simulations and 500-MHz NMR

The hydration behavior of two model disaccharides, methyl-alpha-D-maltoside (1) and methyl-alpha-D-isomaltoside (2), has been investigated by a comparative 10 ns molecular dynamics study. The detailed hydration of the two disaccharides was described using three force fields especially developed for modeling of carbohydrates in explicit solvent. To validate the theoretical results the two compounds were synthesized and subjected to 500 MHz NMR spectroscopy, including pulsed field gradient diffusion measurements (1: 4.0. 10(-6) cm(2). s(-1); 2: 4.2. 10(-6) cm(2). s(-1)). In short, the older CHARMM-based force field exhibited a more structured carbohydrate-water interaction leading to better agreement with the diffusional properties of the two compounds, whereas especially the alpha-(1-->6) linkage and the primary hydroxyl groups were inaccurately modeled. In contrast, the new generation of the CHARMM-based force field (CSFF) and the most recent version of the AMBER-based force field (GLYCAM-2000a) exhibited less structured carbohydrate-water interactions with the result that the diffusional properties of the two disaccharides were underestimated, whereas the simulations of the alpha-(1-->6) linkage and the primary hydroxyl groups were significantly improved and in excellent agreement with homo- and heteronuclear coupling constants. The difference between the two classes of force field (more structured and less structured carbohydrate-water interaction) was underlined by calculation of the isotropic hydration as calculated by radial pair distributions. At one extreme, the radial O em leader O pair distribution function yielded a peak density of 2.3 times the bulk density in the first hydration shell when using the older CHARMM force field, whereas the maximum density observed in the GLYCAM force field was calculated to be 1.0, at the other extreme.     

The C-disaccharide alpha-C(1-->3)-mannopyranoside of N-acetylgalactosamine is an inhibitor of glycohydrolases and of human alpha-1,3-fucosyltransferase VI. Its epimer alpha-(1-->3)-mannopyranoside of N-acetyltalosamine is not

The radical C-glycosidation of (-)-(1S,4R,5R, 6R)-6-endo-chloro-3-methylidene-5-exo-(phenylseleno)-7-ox abi cyclo[2. 2.1]heptan-2-one ((-)-4) with 2,3,4, 6-tetra-O-acetyl-alpha-D-mannopyranosyl bromide gave (+)-(1S,3R,4R, 5R,6R)-6-endo-chloro-5-exo-(phenylseleno)-3-endo-(1',3',4', 5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-7-oxabi cyc lo[ 2.2.1]hept-2-one ((+)-5) that was converted into (+)-(1R,2S,5R, 6R)-5-acetamido-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl)-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-10) and into (+)-(1R,2S,5R, 6S)-5-bromo-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-19). Ozonolysis of (+)-10 and further transformations provided 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-galac tos e (alpha-C(1-->3)-D-mannopyranoside of N-acetylgalactosamine (alpha-D-Manp-(1-->3)CH(2)-D-GalNAc): 1). Displacement of the bromide (+)-19 with NaN(3) in DMF provided the corresponding azide ((-)-20) following a S(N)2 mechanism. Ozonolysis of (-)-20 and further transformations led to 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-talose (alpha-C(1-->3)-D-mannopyranoside of N-acetyl D-talosamine (alpha-D-Manp-(1-->3)CH(2)-D-TalNAc): 2). The neutral C-disaccharide 1 inhibits several glycosidases (e.g., beta-galactosidase from jack bean with K(i) = 7.5 microM, alpha-L-fucosidase from human placenta with K(i) = 28 microM, beta-glucosidase from Caldocellum saccharolyticum with K(i) = 18 microM) and human alpha-1, 3-fucosyltransferase VI (Fuc-TVI) with K(i) = 120 microM whereas it 2-epimer 2 does not. Double reciprocal analysis showed that the inhibition of Fuc-TVI by 1 displays a mixed pattern with respect to both the donor sugar GDP-fucose and the acceptor LacNAc with K(i) of 123 and 128 microM, respectively.     

Solution conformations of mycothiol bimane, 1-D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins and 1-D-GlcNAc-alpha-(1 --> 1)-L-myo-Ins

Mycothiol is an abundant small molecular weight thiol found only in actinomycetes, which include mycobacteria. Mycothiol biosynthetic and detoxification enzymes are novel and unique to actinomycetes, thereby representing potential antimycobacterial targets. To better guide inhibitor design, we have determined by NMR the solution conformations of mycothiol bimane (MSmB) and the pseudodisaccharide 1-D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins (D-GI), molecules that represent the natural substrates for the mycothiol-dependent detoxification enzyme mycothiol-S-conjugate amidase (MCA) and the mycothiol biosynthetic enzyme D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins deacetylase (AcGI deacetylase), respectively. Comparison of the mean structure of MSmB and the energy-minimized structures of two competitive spiroisoxazoline-containing MCA inhibitors shows striking similarities between these molecules in the region of the scissile amide bond of MSmB and provides structural evidence that those inhibitors are substrate mimics. Owing to our earlier finding that AcGI deacetylase will not deacetylate the unnatural isomer 1-d-GlcNAc-alpha-(1 --> 1)-L-myo-Ins (L-GI), the solution conformation of L-GI was also determined. The interglycosidic bond angles for all three compounds are comparable. When considered together with the observation that a simplified cyclohexyl thioglycoside mycothiol analogue is a good substrate for MCA, it appears that the stereochemistry of the inositol ring is critical for deacetylase function, superceding the importance of the full complement of hydroxyl groups on the "nonreducing" ring.     

Stereoselective Synthesis of （Z）-5-（Trideca-4-enyl）resorcinol and Gibbilimbols A-D

(Z)-5-(Trideca-4-enyl)resorcinol (1) and gibbilimbols A-D(2-5) were synthesized in 47%-60% yields over 6 steps from commercially available starting materials. The Wittig reaction of various alkyl phosphonium bromides with appropriate aldehydes in the presence of potassium tert-butoxide (t-BuOK) in anhydrous THF solution at room temperature served as the key step, and the result showed that only (Z)-configuration olefins were formed by this procedure. The synthesis of the (Z)-5-(trideca-4-enyl)resorcinol (1) was reported for the first time.     

Aldolase-catalyzed synthesis of beta-D-galp-(1-->9)-D-KDN: a novel acceptor for sialyltransferases

[reaction: see text] beta-D-Galp-(1-->9)-D-KDN, a disaccharide component of the cell wall of Streptomyces sp. MB-8, was synthesized from beta-D-Galp-(1-->6)-D-Manp and pyruvate using a sialic acid aldolase. The obtained KDN-containing compound was a novel acceptor for bacterial sialyltransferases. Unusual alpha2,3- and alpha2,6-linked sialyltrisaccharides and a tetrasaccharide were synthesized using a one-pot two-enzyme system containing a Neisseria meningitidis CMP-sialic acid synthetase and a Pasteurella multocida sialyltransferase or a Photobacterium damsela alpha2,6-sialyltransferase.     

Conformation of glycomimetics in the free and protein-bound state: structural and binding features of the C-glycosyl analogue of the core trisaccharide alpha-D-Man-(1 --> 3)-[alpha-D-Man-(1 --> 6)]-D-Man

The conformational properties of the C-glycosyl analogue of the core trisaccharide alpha-D-Man-(1 --> 3)-[alpha-D-Man-(1 --> 6)]-D-Man in solution have been carefully analyzed by a combination of NMR spectroscopy and time-averaged restrained molecular dynamics. It has been found that both the alpha-1,3- and the alpha-1,6-glycosidic linkages show a major conformational averaging. Unusual Phi ca. 60 degrees orientations for both Phi torsion angles are found. Moreover, a major conformational distinction between the natural compound and the glycomimetic affects to the behavior of the omega(16) torsion angle around the alpha-1 --> 6-linkage. Despite this increased flexibility, the C-glycosyl analogue is recognized by three mannose binding lectins, as shown by NMR (line broadening, TR-NOE, and STD) and surface plasmon resonance (SPR) methods. Moreover, a process of conformational selection takes place, so that these lectins probably bind the glycomimetic similarly to the way they recognize the natural analogue. Depending upon the architecture and extension of the binding site of the lectin, loss or gain of binding affinity with respect to the natural analogue is found.     

Photochemistry of Thiophen-2(5H)-ones

Mechanistic evidence for the light-induced ring opening of thiophen-2(5H)-ones 1 in alcohols affording α, β-unsaturated mercapto esters 2 is presented. Regio-and stereochemical aspects of the ring closure of alkenylthio (type 3 ) radicals 15 and 17 to S-heterocycles 16 and 18 , of 3-thiahex-5-enyl radicals 4 to (tetrahydrothien-3-yl)methyl radicals 6 and of (2,3-dihydrothien-3-yl)methyl radicals 30 (type 7, but-3-enyl radicals) to cyclopropane-methyl radicals 29 are discussed. Irradiation (λ 350 nm) of 1 in cyclohexane in the presence of 2,3-dimethylbut-2-ene affords [2 + 2] cycloadducts 14 albeit in very low yields.     

Intramolecular 1,8-hydrogen atom transfer. Stereoselectivity of the hexopyranos-5'-yl radical reactions in Hex p-(1-->4)-Hex p disaccharide systems

The stereoselective reduction of hexopyranos-5'-yl radicals in alpha-D-Hex p-(1-->4)-D-Hex p disaccharide models is described. These radicals are generated from a 6-O-yl radical located in the other monosaccharic unit through a 1,8-hydrogen atom transfer. The reaction, which is strongly influenced by steric and stereoelectronic effects, permits in some cases the transformation of alpha-D-Hex p-(1-->4)-D-Hex p into beta-L-Hex p-(1-->4)-D-Hex p disaccharides in a single step with high diastereoselectivity.     

Synthesis of alpha-D-Galp-(1-->3)-beta-D-Galf-(1-->3)-D-man,a terminal trisaccharide of Leishmania type-2 glycoinositolphospholipids

The synthesis of alpha-D-Galp-(1-->3)-beta-D-Galf-(1-->3)-D-Man, present in the type-2 glycoinositolphospholipids and in the core of the lipophosphoglycan of Leishmania, is described. The glycosyl aldonolactone approach, followed by reduction of the lactone with diisoamylborane, was utilized for the introduction of the internal galactofuranosyl unit and the trichloroacetimidate method for the O-glycosidation reaction. A high-yield synthesis of the beta-D-Galf-(1-3)-D-Man unit, also present in the lipopeptidophosphoglycan of Trypanosoma cruzi, is reported.     

Intramolecular 1,8-hydrogen-atom transfer reactions in (1-->4)-O-disaccharide systems: conformational and stereochemical requirements

The stereochemical and conformational factors controlling the intramolecular hydrogen-atom transfer (HAT) reaction between the two pyranose units in a (1-->4)-O-disaccharide when promoted by a primary 6-O-yl radical are studied. Models with alpha-D-Glcp-(1-->4)-beta-D-Glcp, alpha-L-Rhamp-(1-->4)-alpha-D-Galp or alpha-D-Manp-(1-->4)-beta-L-Gulp skeletons led exclusively to the abstraction of the hydrogen from H--C-5' and the formation, through a nine-membered transition state, of a 1,3,5-trioxocane ring system in a stable boat-chair conformation. Notwithstanding, derivatives of alpha-L-Rhamp-(1-->4)-alpha-D-Glcp or alpha-D-Manp-(1-->4)-alpha-D-Galp exclusively abstract the hydrogen from H--C-1' through a seven-membered transition state and, therefore, lead to an interglycosidic spiro ortho ester.