Of three-member rings

Figure 7-2. Strain energies [kj/mol] of three-membered ring systems.

Structural isomerization of three-membered rings with exocyclic unsaturation (Scheme 21) has been found with aziridineimines (Section 5.04.3.13) and diaziridineimines (Section 5.08.3.2.4). This involves an acyclic intermediate, while large ring systems prefer to isomerize through bicyclic ones. The 1,2- to 1,3-diazepine conversion is an example (Scheme 22) (Section 5.18.2.2). 

STRUCTURE OF THREE-MEMBERED RINGS WITH TWO HETEROATOMS... 

There was less agreement between calculated and experimental energy values. The use of 6-3IG, the best procedure in energy calculations of three-membered rings, yielded a value too low by more than 40 kJ moF in the case of diazirine bond separation energy was calculated as -45 kJ moF the experimental value is +0.4 kJ moF . Vibrational correction and extrapolation to 0 K would reduce this difference by several kJ moF . 

The heat of formation (332 kJ moF ) was calculated to be 376 kJ moF Since a correction of about 35 kJ moF is not unusual in calculations of three-membered rings, a corrected value of 340 kJ moF appears reasonable. 

The three types of three-membered ring are also accessible by photoisomerization of open chain 1,3-dipoles nitrones, azomethinimines and linear diazo compounds respectively. All three-membered rings prepared prior to 1967 were included in a comprehensive review . 

Since syntheses of three-membered rings with two heteroatoms almost always proceed under mild conditions with good yields and from simple starting materials, they are useful intermediates in synthesis. The easy formation of N—O and N—N bonds offers synthetic potentialities. Furthermore, the three-membered ring may serve as a protecting group during additional operations of substitution. 

This chapter will deal exclusively with three-membered rings containing the hetero atoms O, S and N, and fused to the steroid skeleton. Because of the conformational requirements in steroids, not all of the usual methods of synthesis of three-membered rings are applicable to the fused ring system. For the synthesis of steroids to which an aziridine, oxirane or thiirane is attached either in the side chain or at a ring position but not directly fused to the nucleus, the methods discussed in this chapter, as well as others, are applicable. 

Here a typical property of three-membered rings wdth two hetero atoms is shown and this property is also found in the diaziridines. Only with the oxaziranes which are substituted by aryl groups in the 3-position does the hydrolysis by acids occur according to Eq. (14) with formation of an aromatic aldehyde and alkyl hydroxylamine. 

Oshima and coworkers reported the preparation of allylsilanes such as 148 by ringopening reactions of three-membered rings such as aziridines 147 with silylalu-minium reagents as shown in Scheme 2.38 [58]. 

D. The Synthesis of Three-membered Ring Sulfones and Sulfoxides.. . ... 

The chemistry of three-membered rings containing oxidized sulfur starts with the work of Staudinger and Pfenninger5 (equation 1). 

TABLE 2. Calculated total energies (au) and atomic charges of three-membered ring sulfone and sulfoxide and their acyclic counterparts (after Reference 25)... 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

Selected additional reactions, transformations, or rearrangements of three-membered ring sulfones and sulfoxides will follow. The criteria for selection is the direct or indirect involvement of the functional groups in the reaction. 

The unique characteristics of three-membered ring sulfoxides and sulfones raise the question Are the major features observed in the three-membered ring series extended into the still small and strained four-membered ring series, or will the latter be more reminiscent of the larger ring and acyclic sulfoxides and sulfones ... 

Inductive constants 510, 511 Inductive effects 391, 392 Infrared spectroscopy 515 of sulphones 110 of sulphoxides 118, 119 of three-membered ring sulphones and sulphoxides, 393, 394... 

There is evidence that relief of ring strain is not the only factor responsible for the high rates of ring-opening of three-membered rings Di Vona, M.L. Illuminati, G. Lillocci, C. J. Chem. Soc., Perkin Trans. 2,1985, 1943 Bury, A. Earl, H.A. Stirling, C.J.M. J. Chem. 

Zimmermann SC, Lawless LJ (2001) Supramolecular Chemistry of Dendrimers. 217 95-120 Zwanenburg B, ten Holte P (2001) The Synthetic Potential of Three-Membered Ring Aza-Heterocycles. 216 93-124... 

The Synthetic Potential of Three-Membered Ring Aza-Heterocycles... 

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