Rawal’s diene

Notes See similarity with Chan s.2 Danishefsky s and Rawal s Diene. 

Rawal s diene has been shown to be about 25 times more reactive than Danishefsky s diene See Diels-Alder Reaction. 

Among the more reactive and synthetically useful dienes are doubly and triply activated alkoxy- and amino-substituted dienes, such as ( )-l-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (Danishefsky s diene),( )-l-(dimethylamino)-3-(fert-butyldimethylsilyloxy)-1,3-butadiene (Rawal s diene)-, and 1,3-dimethoxy-l-(trimethylsilyloxy)-1,3-butadiene (Brassard s diene). As illustrated below, the cyclo-addition products arising from these dienes can either be hydrolyzed or treated with fluoride ion to remove the silyl group, which is followed by (3-elimination to provide conjugated cyclohexenones. 

Hetero-Diels-Alder Reaction Using Rawal s Diene... 

Similarly, the ethoxycarbonyl-protected 2-nitroindole derivative 57a also participated in Diels-Alder reactions with Danishefsky s diene (52), as well as Rawal s diene (59), to produce the 3-hydroxycarbazole products 58a and 60a in synthetically useful yields (Scheme 16). However, the sulfonyl-protected analog 57b did... 

The study of dienophilic character of monosubstituted nitro-aromatic pentaheterocycles was carried out employing 2-nitrofuran la 2,5-dimethyl-3-nitrofuran lb, l-tosyl-2-nitropyrrole 2a, l-tosyl-3-nitropyrrole 2b, 2- and 3- nitrothiophene 3a, 3b, and 2- and 3-nitroselenophene 4a, 4b. In addition, isoprene 5, l-N-acetyl-N-propyl-l,3-butadiene 6, 1-methoxy-3-trimethylsiloxy-l,3-butadiene (Danishefsky s diene) 7 and l-diethyl-amino-3-ferf-butyldimethyl-silyloxy-1,3-butadiene (Rawal s diene) 8 were selected as dienes, covering an attractive sp>ectre of nucleophilic character (Figure 1). 

Then, the dienophilicity of la, 2a, 3a and 4a was investigated in the reactions with 1-N-acetyl-N-propyl-l,3-butadiene 6, Danishefsky s diene 7 and Rawal s diene 8. The use of dienes 7 and 8 allowed analyze the influence of bulky substituents on the yields and regiochemistry of the cycloadditions under thermal conditions. 

The reactions of la,. 2a, 4a with Rawal s diene afforded benzofuran 17 with very good yield, tosylindole 20, and benzoselenophene 24 (82%, 50%, and 50%, respiectively), (Figure 5). (Della Rosa, et al, 2007, 2005)... 

Fig. 5. Diels-Alder reactions of aromatic 2- nitroheterocycles with Rawal s diene.

By using a variant of Rawal s azadiene Diels-Alder cycloaddition chemistry [103,104], it was possible to synthesize several Kornfeld ketone analogues with substitution patterns that are difficult to otherwise obtain. Thus, heating a mixture of furan 280a and Rawal s diene (281) in CH3CN at reflux for 2h resulted in the formation of a 2 1-mixture of diastereomeric amines 282 that was immediately treated with HF at room temperature to unmask the enone 283 (Scheme 13.64) [105,106]. The crude reaction mixture was then heated at reflux in toluene for 30 min to effect... 

---