Four-membered ring heterocycles

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

As we have seen previously (see Section II,A,3), the four-membered ring heterocycle 20 is a precursor of germene 15 that has been characterized by its dimerization and by insertion into the Ge-S bond of 2029 (Scheme 7). However, the formation of 63 could also arise by insertion of 21 into a Ge-C bond of the 1,3-digermacyclobutane 62, and 64 could be formed from germene 15 and a four-membered ring (Me2Ge-S)2 intermediate.29 The six-membered ring derivative 61 has also been observed. 

C. Four-Membered Ring Heterocycles 1. Oxetanes and Spiro-oxetanes... 

Only a few papers on the formation of compounds with small rings have been published. One example is the [2 + 2]-cycloaddition of electron-rich enamines to Schiff bases under high pressure (1.4 GPa) (87JOC365). The reaction leads to substituted azetidines (1). Four-membered ring heterocycles, thietane derivatives (4), are formed by interaction of sulfene (2) with enamines (3) (86CB257 93JOC3429). 

The following four-membered ring heterocycles 1-5, which contain one oxygen and one sulfur atom, are known. Of these the 1,2-oxathietanes are quite common with numerous examples of their oxidised forms, namely the /3-sultines and the /3-sultones, reported over the decades. These systems will form the major part of this chapter. 1,2-Oxathietes, 1,3-oxathietanes, and their S-oxides are still quite rare but examples have been reported and will be highlighted within the chapter where appropriate. 

Figure 2 X-ray diffraction data for four-membered ring heterocycles containing one oxygen and one sulfur atom.

The reactivity of the four-membered ring heterocyclic molecule Cp2Ti(GH2SiMe2NSiMe3) as a single-source precursor to titanium-based ceramic thin films has been studied under atmospheres of nitrogen, argon, and... 

Dipole from Four-Membered-Ring Heterocycles.258... 

Keywords Five-membered rings Four-membered rings Heterocycles containing unusual heteroatoms Rings with more than six members Six-membered rings Three-membered rings... 

Across C=S bonds Allenes undergo a thermal and a photochemical [2+2] cycloaddition reaction with thioketones to give four-membered ring heterocycles. For example, irradiation of tetramethylallene and thiobenzophenone in benzene affords the [2+2] cycloadduct 246 in 64 % yield. As the result of the thermal reaction the linear adduct 247 was formed in 36 % yield... 

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