Formation and dienophilic reactions of transient C-nitrosocarbonyl compounds

Abstract

Oxidation of benzohydroxamic acid with tetraethylammonium periodate in the presence of the conjugated diene thebaine (1) gave the cycloadduct 6β,14β-(N-benzoylepoxyimino)-6,14-dihydrothebaine (2; R = Ph), in high yield. The corresponding N-acetyl derivative (2; R = Me) was prepared similarly using acetohydroxamic acid. Likewise, 2-benzoyl- and 2-acetyl-3,6-dihydro-2H-1,2-oxazine (5; R = Ph) and (5; R = Me) were obtained from buta-1,3-diene, and the N-benzoyl and N-acetyl derivatives of 9,10-epoxyimino-9,10-dihydro-9,10-di-methylanthracene (6; R = PhCO) and (6; R = Ac) from 9,10-dimethylanthracene (DMA). These reactions are believed to involve the formation of nitrosocarbonylbenzene or nitrosocarbonylmethane, respresentatives of a new class of transient, reactive species. The cycloadduct (6; R = Ac) decomposed in benzene at 60 °C in the presence of thebaine (1) to give thebaine adduct (2; R = Me) and DMA. First-order kinetics were observed for the release of DMA, consistent with slow dissociation of the adduct (6; R = Ac)followed by rapid capture of nitrosocarbonylmethane by thebaine. The related adduct (6; R = PhCO) behaved similarly. DMA adducts of the type (6) are valuable in studies on the reactions of nitrosocarbonyl compounds, especially with co-reactants sensitive to oxidation. Thus (6; R = Ac) and 1,3-diphenylisobenzofuran (7) reacted cleanly in benzene at 80°C to give the O-acetyloxime (10) of 1,2-dibenzoylbenzene, possibly via the N-acetylnitrone (9) derived from the initially formed cycloadduct (8).