Reactions Involving Oxygen and Sulfur

The versatility of the Diels-Alder reaction becomes especially obvious, when considering the hetero-variants. One or more of the carbon centers involved can be replaced by hetero atoms like nitrogen, oxygen and sulfur. An illustrating example is the formation of the bicyclic compound 31, by an intramolecular hetero-Diels-Alder reaction ... 

In some instances, especially with the oxygen and sulfur heterocycles, the overall reaction leading to a substituted product does not involve an SEAr mechanism but proceeds by an addition followed by elimination sequence, as outlined for the bromination of coumarin in equation (4). The choice of experimental conditions can affect the outcome of the reaction, as illustrated in the formation of (114) and (115) in Section 3.2.1.4.7. 

The original procedure for the Nef reaction involved the aqueous sulfuric acid hydrolysis of the salts (170) obtained by the treatment of primary and secondary nitroalkanes with sodium hydroxide (equation 30). In contrast, if a neutral primary nitro derivative is treated in hot concentrated mineral acid, the corresponding carboxylic acid (172) is formed via the hydroxamic acid (171 equation 31). This reaction is known as the Meyer reaction and was first described in 1873. Protonation of salts of nitroalkanes occurs preferentially at the oxygen atom to give the aci form (173 equation 32). 

The vicinal hydroxyl group can be replaced by any function transformable to alkoxide under very basic conditions as, for example, acetate. More surprising is the obviously practical preparation starting from tron -ditosylates (reaction 7.4). This reaction occurs at 0°C, yet it involves heterolytic cleavage between the oxygen and sulfur atoms which is not at all simple to carry out by an intermolecular route. We can observe that the tosylate at C-3 is displaced leading to a-D-allo epoxide 7.18. Here again we see the inertia of position 2 relative to the substitution. 

It is well established that reactions involving oxygen dissolved during handling operations carried out in contact with air are generally slow. These reactions are catalyzed by iron and copper ions. Sulfur dioxide acts as an irreversible antioxidant. It has been shown (Ribereau-Gayon et al., 1976) that a free SO2 concentration of around 100 mg/1 is necessary to provide full protection,... 

At first, we prepared some starting trialkoxysilanes with oxygen- and sulfur-containing functional groups. Their synthesis involved the well-known reaction of primary amines with iso(thio)cyanates [see Scheme (5)]. Usually, this reaction does not lead to formation of by-products. It can often proceed at room temperature and allows one to obtain a... 

---