1.3- Oxathiolane

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

C3OS 0 S — — — 3,3-Diphenyl-1,2-oxathiolane 2,2-dioxide 5H-l,2-benzoxathiole 2,2-dioxide... 

For 1,3-dithiolanes the ring is flexible and only small energy differences are observed between the diastereoisomeric 2,4-dialkyl derivatives. The 1,3-oxathiolane ring is less mobile and pseudoaxial 2- or 5-alkyl groups possess conformational energy differences (cf. 113 114) see also the discussion of conformational behavior in Section 4.01.4.3. 

Dithiolanes are not affected by these conditions, but a 1,3-oxathiolane is cleaved (100% yield). 

Me2CH(CH2)20NO, CH2CI2, 25°, 15 min H2O, 63-93% yield." Isoamyl nitrite cleaves aromatic dithioacetals in preference to aliphatic dithioace-tals, and dithioacetals in preference to dithioketals. It also cleaves 1,3-oxathiolanes (1 h, 65-90% yield). 

O2, hv, hexane, Ph2CO, 2-5 h, 60-80% yield. 1,3-Oxathiolanes and dithiolanes are also cleaved by these conditions. 

For(n = 3) Me2CH(CH2)20N0, CH2CI2, reflux, 2.5 h, 65% yield. 1,3-Oxathiolanes are also cleaved by isoamyl nitrite. 

The section on the cleavage of 1,3-dithianes and 1,3-dithiolanes (pp. 203-205), should be consulted since many of the methods described there are also applicable to the cleavage of oxathiolanes. 

Cyclohexane-1,2-dione reacts with ethylene glycol (TsOH, benzene, 6 h) to form the diprotected compound. Monoprotected 1,3-oxathiolanes and 1,3-dithiolanes are isolated on reaction under similar conditions with 2-mercaptoethanol and eth-anedithiol, respectively. ... 

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