Oxaziridines, spiro

Oxaziridines unsubstituted at nitrogen as well as some iV-acylated oxaziridines offer synthetic potentialities due to their ability to transfer their nitrogen function to nucleophiles (Section 5.08.3.1.4). The simplicity of preparation of some aziridines from alkenes and the Spiro oxaziridine (S2) equals the simplicity of epoxidation. Aziridine (299), for example, is obtained by simple heating of indene with (52) in toluene (74KGS1629). 

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

Photolysis of nitro-steroids 225 yields the aci-nitronate at 254 nm131. This in turn gives various products, among them are ketone 226 and hydroxamic acid 227 (equation 105) which could be formed from the intermediate anions of the Af-hydroxyoxaziridines, with a possible participation of gem-hydroxynitroso transient (or its anion see Scheme 10). For comparison, IV-butyl spiro-oxaziridine 228 in ethanol is photolysed at 254 nm (equation 106) to give 7V-butyl lactam 229 (50%) and the ketone 230 (25%). The former process is a well-known photoprocess of oxaziridine131. 

An interesting intramolecular variant of this epoxidation procedure is represented in the reaction of the unsaturated oxaziridine 167, which undergoes highly stereoselective oxygen transfer through a spiro transition state to provide the epoxyaldehyde 1 <99TL4453>. 

Adamantan 2-Nitroso- E16a, 971 (NH-OH NO) Adamantan-<2-spiro-3)-oxaziridin E14a/2, 531 (Keton/NH3/ Persaure)... 

Irradiation of spiro oxaziridines gives rise to various cyclic lactams as summarized in Table 1. 

Table 1 Irradiation of spiro oxaziridines to cyclic lactams...

Photorearrangement of a spiro-oxaziridine such as (95) to the lactam (96) has been employed in alkaloid synthesis. Oxaziridines have also been proposed as intermediates in the photorearrangement of oximes. The lactams (97) and (98), for example, have been prepared in this way by irradiation of either (Z)- or (E)-dihydromayurone oxime (99). ... 

The photochemistry o oxaziridines has been independently investigated. Regiospecific photorearrangements of spiro-oxazirid-ines derived from 2-methylindan-1-one, 2-methyltetral-1-one and 1-methyltetral-2-one have been reported.The oxaziridines (65), for example, are converted on irradiation in ethanol into the lactams (66). A one-photon pathway resulting in ring cleavage and amide formation has also been observed in the gas-phase irradiation... 

Key Words Iminium, Oxaziridinium, Oxaziridine, Ketiminium, Oxone, Tetra-phenylphosphonium monoperoxysulphate, Isopinocampheylamine, Alkene, Epoxide, Enantiomeric excess. Asymmetric synthesis, Organocatalysis, 2-(2-Bromoethyl)benzaldehyde, Levcromakalim, Dihydroisoquinolinium, Spiro, Azepinium, Benzopyran, Dielectric constant, Binol. 2008 Elsevier B.v. 

Two transition states have been proposed for the epoxidation of alkenes by dioxiranes and oxaziridines, the spiro and the planar (Fig. 5.8). In the spiro transition state, the alkene approaches the oxaziridinium moiety in such a way that the axis of the carbon-carbon double bond is perpendicular to the carbon-nitrogen bond axis. In the planar transition state, the two components approach one another so that their axes are parallel to one another, and they and the oxygen atom are in the same plane. 

The spiro transition state is now generally accepted as the mechanism in operation during both dioxirane- and oxaziridine-mediated epoxidation. This conclusion is supported by theoretical and computational studies [31-33]. 

Oxaziridines are of interest in their own right and as intermediates in various photorearrangements. It has been shown that the nature of the nitrogen substituent does not affect the regioselectivity of lactam formation on irradiation of spiro-oxaziridines. The oxaziridine (80) has been proposed without any real evidence as an intermediate in the photoinduced transformation of tra 5-canadine iV-oxide... 

The results of separate photochemical studies of oxaziridines have been published. The azahomoadamantanes (98) can be prepared in 85—95% yield by irradiation of the spiroadamantane-oxaziridines (99).77 The photorearrangement of substituted spiro-oxaziridines and the effect of solvent on these rearrangements have also been examined.78... 

There are several reaction sequences that use a chiral auxiliary to produce chiral, nonracemic GABOB, but are rather di arate in their approaches. In one, cyclobuta-none 6.5553 reacted with R-phenethylamine to give an imine, and this was oxidized with m-chloroperoxybenzoic acid to give spiro-oxaziridine derivative 6.59.54 Photolysis of 6.59 led to a mixture of lactams 6.60 and 6.61. Separation of these... 

A large proportion of Davis s work has been involved in the elucidation of the transition state employed in the transfer of the oxygen from the oxaziridine to the olefin substrate. Davis favoured the planar transition state and was at the time supported by theoretical calculations however, more recent calculations favour the spiro transition state [6]. Davis has also described the asymmetric oxidation of enolate anions by chiral oxaziridines, which led to a-hydroxyketones with enanti-oselectivities of up to 95% ee [56, 57], Silyl enol ethers have also been reported to give epoxides when treated with oxaziridines, but the instability of these compounds is too great to allow isolation [37,58,59], To date, only Davis has reported successful isolation of a-silyloxy epoxides [60],... 

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