2- 1 - azetidin

The reaction of the vinylcyclopropanedicarboxylate 301 with amines affords an allylic amine via the 7r-allylpalladium complex 302[50]. Similarly, three-membered ring A -tosyl-2-(l,3-butadienyl)aziridine (303) and the four-mem-bered ring azetidine 304 can be rearranged to the five- and six-membered ring unsaturated cyclic amines[183]. 

Serratia mane seem 3,4-dehydro-Pro prodegradation , thiazoline-4-carboxylate/ azetidine-2-carboxylate (transduction) 75 145... 

Azetidines under analogous reaction conditions to those above result in six-membered ring formation. However, diketene (472), an oxetan-2-one, offers considerable promise for five-membered heterocycle formation. With hydroxylamine the 3-methylisoxazolin-5-one (473) was formed. Phenylhydrazine gave the corresponding 3-methyl-l-phenylpyrazolin-5-one. 

Azetidine N-oxides produce isoxazolidines by a thermal ring expansion (77AHC(21)207, 75GEP2365391), and nitrosobenzenes react with alkenes to provide isoxazolidines (77AHC(21)207, 79IZV1059). 

N-Inversion in azetidine and azetidin-2-one is rapid, even at —77 and -40 °C, respectively (B-73NMR144). Again, halo substituents on nitrogen drastically slow the inversion rate, so that Af-chloro-2-methylazetidine can be separated into two diastereomers (b-77SH(1)54). Substituent effects on N-inversion are much the same as in the aziridines Af-aryl and N- acyl... 

Scheme 1 H NMR shifts and coupling constants of azetidine derivatives...

Azetidine N shifts are similar to those of the aziridines. Unsubstituted azetidine has its N resonance (relative to anhydrous ammonia) at 25.3 p.p.m., and N-r-butylazetidine shows the signal at 52 p.p.m. (80JOC1277). 

Four-membered heterocycles prefer to cleave, upon ionization, into two fragments, each containing two of the ring atoms. Further cleavages commence from these initial fragments (Scheme 5). Specific details can be found as follows azetidines (B-71MS296), oxetanes... 

Electrophiles, such as C—Hal functions, contained in side chains may be well positioned for interaction with ring heteroatoms. Thus, Af-t-butyl-2-tosyloxymethylaziridine in ethanol displaces tosylate ion from the side chain, and nucleophilic opening of the resulting azabicyclobutanonium ion by solvent gives 3-hydroxy- and 3-ethoxy-azetidine (Section 5.09.2.3.2). 

While these rearrangements are used most often to prepare large rings, it should be noted that the expansion of cyclopropane derivatives to azetidines is also practical (Scheme 6 Section 5.09.3.3.3.a). 

Addition of trichloromethide ion to azirine (210) generates aziridine (230). When this aziridine was treated with base, cyclization and rearrangement occurred and the azetidine (233) was isolated (73JA2982). 

Vibrational spectra including Raman data of 3,3-dimethyldiaziridine and its hexadeutero compound were recorded in the gas phase and in the crystalline state. Assuming C2 symmetry and employing isotopic shifts and comparison with azetidine, a classification of bands which regarded 33 normal modes could be given (75SA(A)1509). 

In view of the uneven attention which azetidines, azetines and azetes have received and because of their lack of chemical similarity, they are treated separately in this chapter. Furthermore, because of the considerable literature on azetidin-2-ones, these have been dealt with in their own right, rather than as derivatives of azetidine. 

Azetidine (1) is a colourless, mobile liquid, b.p. 62.5 C/747 mmHg (56JA4917), which is completely miscible with water. Its density 4 = 0.8412 and refractive index d = 1.4278 (37HCA109). Table 1 gives b.p. and m.p. data for other representative azetidines. 

Azetidine itself has been studied by electron diffraction, which reveals a non-planar structure (Figure 1) (73CC772). The enhanced length of the bonds reflects the strain in the ring and the angle between the CCC and CNC planes of 37° is similar to that found for cyclobutane (35°), but quite different from that for oxetane (4°). 

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