Oxidation of Nhydroxycarbamic esters, ROCON HOH, with tetraethylammonium or sodium periodate in the presence of conjugated dienes gave N-alkoxycarbonyl-3,6-dihydro-2H-l ,2-oxazinesf formed apparently by cycloaddition of transient C-nitrosoformate esters, ROCONO, with the dienes. Cleavage of various N-alkoxycarbonyl derivatives under mild conditions is exemplified. The cycloadduct (1 Oa) of benzyl nitrosoformate and 9,lO-dimethylanthracene decomposed in benzene at 80 "C in the presence of thebaine (3) to give the corresponding adduct (4a) of thebaine, together with 9,lO-dimethylanthracene.Similarly, the adduct (6b) of 2,2,2-trichloroethyl nitrosoformate and cyclopentadiene, when heated with ergosteryl acetate, gave the corresponding adduct (8b) of the steroid. Benzyl and t-butyl azidoformates decomposed in dimethyl sulphoxide at 11 5-1 30 "C in the presence of thebaine to give the adducts (4a) and (4c) of benzyl and t-butyl nitrosoformate and the alkaloid. The other, major products were the sulphoximides (20a) and (20c). Alkoxycarbonylnitrenes, therefore, attack dimethyl sulphoxide either on sulphur, to give sulphoximides, or on oxygen, to give nitrosoformates. Benzyl nitrosoformate, generated thermally from either the adduct (10a) or (6a), reacted with triphenylphosphine to give, apparently, benzyloxycarbonylnitrene which attacked the solvent, benzene, to form Nbenzyloxycarbonylazepine (24a). The adduct (6c) of t-butyl nitrosoformate and cyclopentadiene behaved likewise to give the azepine (24c). The reaction of (6c) with triphenylphosphine in benzene or dichloromethane gave a small quantity of 5,5-dimethyloxazolidin-2-one (26), a known cyclisation product of t-butyloxycarbonylnitrene.Oxidation of hydroxamic acids, RCONHOH, is believed to generate transient C-nitrosocarbonyl compounds, RCONO, which may be trapped in situ by conjugated dienes to form Nacyldihydroxazines.' We find that oxidation of N-hydroxycarbamic esters (1) proceeds similarly (Scheme 1) to afford the oxazine derivatives (2) having readily removable, N-protecting groups of the type employed in peptide chemistry (Scheme 2).

Various allylic and homoallylic alcohols have been treated successively with phosgene and hydroxylamine to form the corresponding Nhydroxycarbamic esters (3). Oxidation of these hydroxamic acid derivatives with sodium or tetraethylammonium periodate in the presence of cyclopentadiene gave the cycloadducts (5) derived from transient allylic and homoallylic C-nitrosoformate esters (4) and cyclopentadiene. When the cycloadducts were heated in benzene at 80 "C, or toluene at 11 1 "C, they dissociated and the C-nitrosoformate esters underwent intramolecular 'ene' reactions to give hydroxamic acid derivatives having newly formed, five-six-or seven-membered heterocyclic rings.In earlier papers we presented evidence that oxidation of the hydroxamic acids, RCONHOH and ROCONHOH,3 produces transient nitrosocarbonyl compounds, RCONO and ROCONO, which may be trapped efficiently in situ as their cycloadducts with conjugated dienes. Similarly, oxidation of N-hydroxyureas4*' in the presence of conjugated dienes gives N-

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97-4; 1,1'-[(chloro)(chlorodithio)methylene]bisbenzene, 79991-72-3; l,l'-[(chloro)(chlorodithio)methylene]bis[4-methoxybenzene], 79991-73-4; benzophenone, 119-61-9; bis(4-methoxyphenyl)-

acid in 20 mL of methylene chloride. To the reaction mixture, which was allowed to warm to 20 °C over a period of 40 min, was then added 0.5 g of dimethyl sulfide followed (after 5 min) by a solution of 0.3 g (2.4 mmol) of p-anisidine in 10 mL of methylene chloride. The mixture was stirred at room temperature for 12 h, filtered, and the filtrate was concentrated and passed over basic alumina (elution with chloroform) to give 0.136 g (32%) of 6d as a brown solid: mp 108-109 °C; NMR 3.94 (s, 3 H, OCH3), 7.05 (d, 2 H), 7.35 (t, 1 ), 8.18 (d, 2 H), 8.96 (d, 2 H).