Reactions with Nitrosoalkenes as Heterodienes

The hetero Diels-Alder reaction of nitrosoalkenes with electron-rich olefins has been known for a long time [363]. A detailed mechanistic study carried out by Reissig et al. has given evidence that this inverse electron demand cycloaddition is a concerted process [364]. Recent ab initio calculations dealing with the reaction between ethylene and nitroso ethylene strongly corrobate this view [365]. In this work, Jursic and Zdravkovski have also investigated the influence of BH3 as Lewis acid catalyst. However, cycloadditions of nitrosoalkenes already pro- 

The array of dienophiles amenable to these hetero Diels-Alder reactions is not limited to enol ethers and enamines since allylsilanes and simple alkenes have also been successfully employed [370, 371]. More recently, it has been shown that methoxy allenes such as 4-41 undergo formation of 6H-l,2-oxazines 4-43 upon cycloaddition to nitrosoalkenes such as 4-34 and subsequent tauto-merisation of the intermediate exo-methylene compound 4-42 (Fig. 4-9) [372, 373]. In these studies, 4-43 proved to be a versatile synthetical intermediate allowing oxidative demethylation or reductive removal of the methoxy group as well as nucleophilic substitutions after the generation of an azapyrylium ion [372 - 374]. Furthermore, ring contraction reactions of these oxazines leading to pyrroles [373] and y-lactames [375] are known. 

Indoles may as well serve as dienophilic compounds for hetero Diels-Alder reactions with nitrosoalkenes. However, the resulting adducts are not stable and undergo further conversion to oximes which represent useful intermediates for the straightforward synthesis of tryptophane derivatives [376,377]. 

In order to carry out asymmetric cycloadditions of nitrosoalkenes, Reissig et al. have introduced chiral enol ethers derived from terpenes [378] and from the glucose derivative 4-46 [379]. Using these compounds, considerable asymmetric induction has been obtained thus, the 5,6-dihydro-4H-l,2-oxazine 4-45 was formed by hetero Diels-Alder reaction of 4-34 with chiral 4-44 in good diastereo-selectivity (Fig. 4-10) [379]. 

Other current investigations concerning cycloadditions of nitrosoalkenes are directed towards employment of more complex dienophiles, e. g. NjM-bis-tri-methylsilyl enamines [380, 381] or 2,5-dihydrooxepines [382]. Furthermore, interest focuses on exploring the scope of subsequent reactions of the cycloadducts, such as stereoselective halogenation [383], ds-dihydroxylation [384] and numerous reductive [385] as well as acid or transition metal induced [386] transformations of 5,6-dihydro-4H-l,2-oxazines. 

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