Three-membered heterocycles with one

SYNTHESIS OF THREE-MEMBERED HETEROCYCLES WITH ONE BORON ATOM 340... 

The parent system of the three-membered heterocycles with one oxygen atom is oxirene [1] the corresponding saturated heterocycle is oxirane. 

Section 3.1 is a brief overview Section 3.2 deals with six-membered heterocycles, including those with more than one heteroatom in the ring Section 3.3 deals with five-membered heterocycles with one heteroatom Section 3.4 deals with five-membered heterocycles with more than one heteroatom in the ring Section 3.5 covers small (three- and four-membered) and large (>six) ring heterocycles. 

With this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocycles. The crucial consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkane or heterocycloalkene (see p 2). Various aromaticity criteria apply to heteroarenes, and as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocycles with one oxygen atom is furan. 

The parent compounds of the monocyclic five-membered heterocycles with one heteroatom are aromatic. When considering the three most important systems only, it appears that the aromaticity increases as follows furan < pyrrole < thiophene (< benzene). This sequence also applies to the respective benzo[Z ] condensed systems. 

The importance of five-membered heterocycles with one heteroatom, of the benzo and dibenzo condensed systems and of the partially or completely reduced compounds as natural products, pharmaceuticals, and starting materials or auxiliaries for syntheses is much greater than for three- or four-membered heterocycles, apart from oxirane. 

Three systems were pyrazoles are condensed to five-membered rings with one heteroatom exist (300-302). For pyrrolopyrazoles, a fourth iso-merer (303) is also possible. When the pyrazole ring is fused to a heterocyclic system in which the heteroatom is tetravalent, several isomeric structures can be drawn. Derivatives of 300-303 can be prepared either... 

Reactions related to cyclopropanation can also be carried out with (phosphino) (silyl)carbenes (I). For example, benzaldehyde reacts with la at 0 °C leading to the corresponding epoxide, again as only one diastereomer. Even more striking are the reactions with benzonitrile and tert-butylphosphaalkyne that lead initially to azirine and phosphirene. Both three-membered heterocycles subsequently undergo ring expansion reactions affording azaphosphete and diphosphete, respectively (Scheme 8.18). This reaction is a new route for the synthesis of heterocyclobutadienes, and this demontrates the usefulness of (phosphino)(silyl)carbenes (I) for the synthesis of novel species. 

Five- and Six-Membered Ring Heterocycles with One, Two, or Three Sulfur Atoms. A summary of the various sulfur heterocycles formed from aldehydes in the presence of hydrogen sulfide and the corresponding analytical data are presented in Table I. Examples of aldehydes used in this reaction include propionaldehyde, butyraldehyde and caproaldehyde. 

No systematic studies on vibrational spectroscopy of five-membered heterocycles with three or more heteroatoms and at least one tetracoordinated silicon atom were reported. 

No systematic experimental studies on thermodynamic properties of five-membered heterocycles with three or more heteroatoms and at least one tetracoordinated silicon atom were reported. The aromaticity of the fully conjugated germadisiloleanion 21, a heavy congener of the cyclopentadienyl anion, was deduced mainly from its NMR spectroscopic and structural parameters <2005JA13143>. 

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