Oxolane rings

The competition between attack by solvent at C-6 (intermediate 78) and attack at C-5 by the hydroxyl group on C-2 (intermediate 77) explains these results, which show the extent to which closure of an oxolane ring is favored. In principle, a third (unobserved) possibility exists, namely, closure of a six-membered, oxygen-containing heterocycle as a result of opening of the epoxide at C-6 by the hydroxyl group on C-2. 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

The n.mj. spectrum67 of dianhydro-xylitol23 and its methyl ether confirmed the chemical evidence that it has a 4-membered and a 5-membered ring, and hence is l,4 3,5-dianhydro-xylitol. This conclusion was further supported by the i.r. spectrum of the methyl ether, which has bands at 980 cm"1 (oxetane ring) and at 1050 cm"1 (oxolane ring).67... 

The following order of oxidizability or selectivity was formulated primary OH = cis OH on the oxolane ring > trans OH. If the resulting anhydroaldonic acid can lactonize, the secondary hydroxyl group on the lactonic ring is oxidized. 

The three l,4 3,6-dianhydrohexitols show the shielding effect of the cts-fused oxolane ring on their di-O-sulfonyl derivatives. As C-2 and C-5 of 1,4 3,6-dianhydro-D-mannitol 2,5-di-p-toluene-sulfonate and 2,5-dimethanesulfonate are open to attack from the exo direction, they react readily with such nucleophilic reagents as benzoate in /V,iV-dimethylfoimainide,94 acetate in acetone,93 thioacetate,95 phthalimide in A/,JV-dimethylformamide,93 and iodide in acetic anhy-dride.59<6) Inversion occurs at these positions and the L-iditol configuration results, as pointed out in the several examples in Table V. However, hydroxide ion may merely saponify,92 with retention of configuration.93... 

An intramolecular reaction (i.e., the displacement of a mesylate in the presence of cesium fluoride), a key step in the total synthesis of octosyl acid A [Eq. (3)], has been repotted in preliminary form [10]. Equation (4) describes ring-closures to oxolane rings, the... 

Epoxy sugars are frequently used as starting compounds in the synthesis of sugar derivatives (compare Section IV) such as halo, amino, azido, thio, deoxy, and branched-chain derivatives. The oxirane ring is in general more reactive than the oxetane or oxolane ring. It is opened with nucleophiles under base or acid catalysis. On the other hand, the oxirane ring remains unattacked under the conditions of catalytic debenzylation on palladium,... 

These compounds have a similar steric arrangement of the oxolane ring as do ordinary derivatives of furanoses. Exocyclic oxirane-ring closure proceeds readily as compared to endocyclic epoxides because the required trans-orientation of the reacting groups is adopted without significant steric hindrance. 

Since the oxolane ring is relatively stable toward acids and bases, the reactivity of these anhydro sugars is predetermined mainly by the presence of a free or potential aldehyde group. 

Acid hydrolysis of 2,3-anhydrohexopyranosides yields free 3,6-anhydro-hexoses (for example, 184 185), which suggests that the hydrolysis of the glycoside bond precedes the formation of the oxolane ring.525,526... 

Rule 4 in aliphatic substituents, halogen atoms show an inductive effect, decreasing with increasing distance of the halogen from the tri-oxolane ring (Table XI). 

AnEryt, the flve-membered chelate ring is flat. An intramolecular hydrogen bond between the 01 atom of the AnEryt ring and an en ligand is responsible for the conformation of the complex (the conformation of the oxolane ring is distorted in the direction of Eqi)-... 

Taking into account the different possible orientations of the two oxolane rings, relative to the apical ligand, for the three compounds 82, 83 and 84, the existence of syn/syn isomers in addition to syn/anti and anti/anti isomers must be considered. Although there is some confusion in the literature [92,93,94,95,96,97] about the correct interpretation of the NMR data of silicate complexes, there is no doubt that the three above-mentioned compounds exist in solution. In O Fig. 19, Si and (DEPT-135) NMR spectra of 82 and 83 are shown. 

An oxolane ring is readily available through an intramolecular reaction of a carbene or carbenoid with an oxygen atom or a C—H bond, particularly... 

A useful synthetic method was recently elaborated for oxolane ring creation by a [3-f-2]-cycloaddition reaction of a carbonyl ylide with a multiple carbon-carbon bond. 

It follows from the H-n.m.r. spectra of the 1,6-anhydrohexofuranose peracetates (see Table IV) that their 1,3-dioxane ring exists in the alCa4 conformation99,113,119,121,793 and their oxolane ring favors the E0 conformation,119 although, for the D-g alacto triacetate (271), the E4 conformation of the oxolane ring was assumed.119,783 For l,6-anhydro-/3-... 

Such a wide range of / values indicates that the oxolane ring may possibly be distorted. 

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