Inversion, furanose rings

The conformational dynamics of furanose rings may be described by the mechanisms of pseudorotation and inversion. The former mechanism describes a continuous pathway of interconversion between twenty idealized non-planar (envelope, twist) conformers (Figures 1 and 2) that does not involve the planar form (e.g., - E4 — ... 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

The reaction of 2,3-0-isopropylidene-5-0-mesyl-D-lyxofuranose (8) with sodium methoxide proceeds similarly. The main product, methyl 2,3-0-isopropylidene-y3-L-ribofuranoside (10), is formed (with its an-omer) by opening of the furanose ring, elimination of the primary, 5-methanesulfonate anion (inversion not observable) by 0-4 alkoxide ion to form epoxide 9, and attack at C-l by methoxide to form an 0-1 alkoxide ion which attacks C-4 with inversion. Some attack at C-5 gives minor proportions of methyl 2,3-0-isopropylidene-/3-i>lyxo-pyranoside and its anomer (11). The formation of l,5-anhydro-2,3-0-isopropylidene-/J-D-lyxofuranose (l,4-anhydro-2,3-0-isopropylidene-/3-D-lyxopyranose), 12, occurs to only a minor extent55,56 (see Scheme 2). 

Figure 1. Schematic diagram of a p-a-p stretch, in which the a-anomeric nucleotide is fused to the flanking p-nucleotides via 3 -V and 5 -5 phosphodiester linkages. The T, 3 and 5 furanose ring positions are labeled B = nitrogenous base. a-Anomeric nucleotides differ from their natural p-counterparts by a single inversion of stereochemistry at the CT position.

When OH-6 is protected (as in educt 146, entry 6), a new, exciting type of MS[2,rO] reaction, unprecedented in the furanose series, appears Contraction of the five-membered ring, with inversion of configuration at C-2, to form the... 

Biological CBHs are usually pentoses (five carbon units) or hexoses (six carbon units) which can be found either in linear forms or as cyclic five-membered (furanose) or six-membered (pyranose) rings. Pyranoses have a relatively rigid chair skeleton with dominant C4 or conformations connected by ring inversion. On the other hand, furanoses are more flexible because of the possibility of... 

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