1.3- Dioxolane ring

The exocyclic 1,3-dioxolane ring is much more vulnerable to acid hydrolysis than the ring connected with the acetal group. Partial deprotection of the side-chain is easily achieved by treatment with sulfurie acid. 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

Thermal decomposition of the Meisenheimer zwitterion 88 gave rise to the 2-hydroxy-7-oxa-4-azoniaspiro[3.51-nonane picrate salt 90 by cleavage of the 1,3-dioxolane ring followed by intramolecular cyclization (Scheme 9) <2000RJOC706>. 

Cationic Copolymerization of 1,3>5-Trioxane with 1,3-Dioxolane (Ring-Opening Copolymerization)... 

It has been found that the 1,3-dioxolane ring corresponds to approximately two oxyethylene units with regard to effect on the critical micelle concentration and adsorption characteristics [42]. Thus, surfactant type I in Fig. 14 should resemble ether sulfates of the general formula R-(0CH2CH2)20S03Na. This is interesting since the commercial alkyl ether sulfates contain two to three oxyethylene units. 

The introduction of methyl groups into both the five- and six-membered rings generally causes a rate decrease. This effect is particularly noticeable when all the carbons of the 1,3-dioxolan ring are fully substituted. Thus 2-hydroxy-2,4,4,5,5-pentamethyl-l,3-dioxolan reacts about 20 times more slowly than 2-hydroxy-1,3-dioxolan. 

The insertions of PTC-generated dichlorocarbene into CH bonds of 1,3-dioxane and 1,3-dioxolane rings represent examples of C-alkylation of heterocyclic nucleophiles.119,120 The mechanism is different from those just considered because a carbanion on the heterocyclic ring probably is not formed during insertion. 

SAMPLE SOLUTION (a) The cyclic acetals derived from ethylene glycol contain a five-membered 1,3-dioxolane ring. 

Acetals and ketals protect two hydroxyls at a time by forming either a 1,3-dioxane or a 1,3-dioxolane ring (Scheme 2.12). The most common ketal and acetal protecting groups are isopropylidene or acetonide and the benzylidene-type groups with... 

On the basis of kinetic studies on the polymerization of 80 with 1,3-dioxolan-2-ylium ion, an Ac2 mechanism (bimolecular addition on a carbenium ion) has been proposed for this polymerization. This mechanism assumes the bimolecular reaction of the cyclic dioxacarbenium ion with the monomer as the rate-determining step and predicts the preparation step to be nonstereospecific, that is, the formation of a polymer consisting of cis- and frans-2,4-linked 1,3-dioxolane rings. 

This reaction is thermodynamically controlled, because the polymer containing 1,3-dioxolane rings converts itself to a polyether when allowed to stand at room temperature for several days or heated at 80 °C for a few hours in the presence of an acid catalyst. Similar double ring-opening polymerizations were observed for 2,6,7-trioxabicyclo[2.2.2]octane and its derivatives [86, 87] and for spiro orthoesters and spiro orthocarbonates as well (see Sects. 6 and 7). 

The 1,3-dioxolane ring is usually inert to catalytic hydrogenation unless forcing conditions are employed. Hydrogenolysis over rhodium or palladium catalysts in the presence of an acid catalyst gives mainly hydroxy ethers as shown in equation (8). Raney nickel cleaves the oxathiolane ring (Scheme 9) and converts 1,3-benzoxathioles into phenol derivatives. 

The 1,3-dioxolane ring is found in a major antifungal drug, ketoconazole (123). Ketoconazole is a broad spectrum, orally active antifungal agent and is used to treat a wide variety of superficial or deep fungal infections. Various workers, especially in Russia, have explored the use of simple oxathiolanes as radioprotectants. For example, the survival rate of mice irradiated with lethal doses of X- or -y-rays was 40% when they were pretreated with 2,2-dimethyl-l,3-oxathiolane. Other oxathiolanes were less active. 1,3-Oxathiolane, administered intravenously to dogs 2 or 3 times daily for 2-4 days before chronic irradiation with polonium-210, increased their life expectancy from 9 months to 3-7 years and alleviated radiation sickness. Some blood indicators were also restored to normal. 

In the thermodynamically controlled isopropylidenation of propane-1,2,3-triols, it is the 5-membered 1,3-dioxolane ring that is preferred (see section 3.2.1). However in the case of propane-2-acylamino-l 3-diols, the thermodynamically favoured product is the 1,3-dioxane ring [Scheme 8,162].356... 

Okoumal 126, a powerful ambergris odorant, has two stereogenic centers in the 1,3-dioxolane ring and therefore there are four stereoisomers of 126a-d. The four stereoisomers of disila-okoumal 125 were obtained by chromatographic separation of the racemate prepared by Scheme 24. 

The commonest type of acetal in this class has the 1,3-dioxolane ring fused cis to a furanose ring. Formation of 1,2-acetals of a-D-glucofuranose has been proved for reactions of o-glucose with acetone, " cyclohexanone, "... 

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