Part I: The synthesis of a stable nitronic ester: Part II: Thermal isomerization of the nitronic ester to a spiro-oxazirane

Abstract

Part I: The instability of nitronic esters has been well reported. The rapid decomposition of nitronic esters has been suggested to occur with the ester functioning as an internal base. A synthesis of a stable ester was devised with the hope of discovering new reactions and thus attaining a better understanding of this ephemeral class of compounds. This was to be accomplished by the synthesis of an ester of such a nature that the compound could not function as an internal base. The sodium salt of 3-nitrocamphor was treated with triphenylmethyl chloride in dichloromethane. Triphenylmethyl camphor-3-nitronate(I), m.p. 149-152°, was obtained. Treatment of the nitronic ester I with dilute sodium hydroxide afforded triphenylcarbinol and 3-nitrocamphor (after acidification). Reduction of I with platinum and hydrogen gave triphenylcarbinol and a mixture of epimeric amino alcohols which Gave an IR spectrum similar to that of β-aminoborneol. Mercury arc irradiation of I in dichloromethane produced triphenylcarbinol and 3-nitrocamphor. These three reactions of the ester establishes the structure of I and eliminates the possibility of carbon alkylation which would render II as the correct structure. The nitronic ester reacted with two other bases. With piperidine, triphenylcarbinol and the hydroxamic acid III were formed. Pyrrolidine reacted similarly. Part II: The thermal isomerization in xylene of triphenyl camphor-3-nitronate(I) afforded a spiro-oxazirane IV, m.p. 175-176°. In two other experiments a lower melting form of the oxazirane IV was isolated, m.p. 156.4-157°. The substance may be a stereoisomer or crystalline modification of the other. The oxazirane IV was reduced with platinum and hydrogen to an alcohol with the oxazirane ring intact. Treatment of IV with semicarbazide hydrochloride yielded a semicarbazone. Acid hydrolysis of IV gave isonitrosocamphor, benzophenone, and phenol. Isolation of isonitrosocamphor signified that the ester was converted to IV without skeletal rearrangement. Pyrolysis at 270-280° of IV yielded a compound with physical properties in agreement with those reported for the anhydride V. Resisting pyrolysis at temperatures below 255° and surviving catalytic hydrogenation, the oxazirane ring of IV has much greater stability than those of most other oxaziranes. The stability must be due to its inclusion within a rigid bicyclic system and due to the protection provided by bulky groups. The synthesis of an oxazirane by the isomerization of a stable nitronic ester provides a new route to oxaziranes. But more important than a method of synthesis is support for the concept that some reactions of primary and secondary nitro compounds may be interpretated on the basis of oxazirane intermediates.