From heterocyclic precursors

Ring expansion of activated aziridines (43) with sulfur ylides also provides a synthesis of azetidines (75JOC2990, 58BSF345, 81CC417). The highly reactive sulfonium methylide (44 R = R = H) undergoes further reaction with the azetidines (46), but the reaction is satisfactory for substituted methylides. The less reactive sulfoxonium methylide (45 R = R = H) 

A number of 2-acylazetidines have been prepared by reaction of 1,3-dihaloacyl compounds with amino derivatives (Section 5.09.2.3.l(m)). This is illustrated for azetidine 2-carboxylic acid (56), the only known naturally occurring azetidine. Ring expansion of activated aziridines (43) and contraction of 4-oxazolines (55) has also found limited use (Section 5.09.2.3.2(f) and Hi)). 

3-Acylazetidines were first synthesized by cyclization of 2-acyl substituted 3-bromo-propylamines with base (69JOC310). 

Ring expansion of haloalkyloxiranes provides a simple two-step procedure for the preparation of azetidin-3-ols (Section 5.09.2.3.2(f)) which can be extended to include 3-substituted ethers and O-esters (79CRV331 p. 341). The availability of 3-hydroxyazetidines provides access to a variety of 3-substituted azetidines, including halogeno, amino and alkylthio derivatives, by further substitution reactions (Section 5.09.2.2.4). Photolysis of phenylacylamines has also found application in the formation of azetidin-3-ols (33). Not surprisingly, few 2-0-substituted azetidines are known. The 2-methoxyazetidine (57) has been produced by an internal displacement, where the internal amide ion is generated by nucleophilic addition to an imine. 

Af-Halogeno-, fV-nitroso- and hence fV-amino-azetidines have been prepared from azetidines by reaction with positive halogen reagents and nitrosating agents, respectively (Section 5.09.2.2.3). 

---

Several approaches to unfused azetes have appeared, the most widely used being extrusion of small molecules from heterocyclic precursors. 

Bis(tnfluoromethyl -substituted phosphoranes are obtained on reaction of bis(tnfluoromethyl)-substituted heterodienes and P(ni) species [74J, 742 143] In the case of sulfur- and selemum-contaimng phosphoranes, the corresponding heterodienes are generated in situ from heterocyclic precursors [137] (equation 32)... 

Several derivatives of racemic amino-sugars have been synthesized from heterocyclic precursors. Thus, the four isomeric methyl 3-amino-3,4-dideoxy-DL-pentopyranosides have been derived from dihydro-(4 )-pyran, as indicated in Scheme 29, furfural has been converted into derivatives of 6-acetamido-6-deoxy-DL-mannopyranose, ° and 3-cyanopyridine has been transformed into the racemic nojirimycin (5-amino-5-deoxy-DL-glucopyranose) derivative (122) by the sequence of reactions outlined in Scheme 30. ... 

---