Four-membered ring systems 5-lactones

Four-membered Ring Sulphur Heterocycles.— Thietanimines are a rare class of compounds for which only two examples have previously been described. This year has seen the publication of two papers describing the synthesis of this ring system. In the first of these, deprotonation of /3-lactones with LDA and reaction of the resulting anion with phenylisothiocyanate gave the N-phenylthietanimine (197). The key step in this process is the recyclization (195) - (196), which proceeds with inversion at C(4) (Scheme 27). The second... 

It will be seen that the enediolic system can theoretically be written in the isomeric 2-keto (II) or 3-keto (III) forms and these in turn are seen to be derived from the 2-keto and the 3-keto acids IV and V, respectively (compare with benzoin which reacts with iodine in an analogous fashion to L-ascorbic acid). Consequently the synthesis of L-ascorbic acid and of its analogs has consisted in devising methods for the formation of 2-keto or 3-keto hydroxy acids followed by their enolization and lactonization. Four main methods are available for the synthesis of analogs of L-ascorbic acid containing the characteristic five-membered unsaturated enediolic ring. 

Another alkaloid with the oxazolidine system is samandaridine (XVII), C21H31NO3, also a secondary amine. Chemical reactions (9) and IR-spectra indicate that it possesses a five-membered lactone ring. Its structure was also elucidated by X-ray analysis. Samandaridine hydrobromide crystallizes from methanol in monoclinic prisms its space group is C2 with four molecules in the unit cell. The cell constants are a = 14.50 A, b = 6.15 A, c = 22.52 A, = 94° (10). 

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