Three-membered ring systems preparation

Three-membered ring systems have also provided examples of aromatic and antiaromatic behavior. Despite the very substantial angle strain, Breslow and his collaborators have succeeded in preparing a number of cyclopropenyl cations (51).30 Cyclopropenone (52) has been isolated and is stable in pure form below... 

The main active ingredients of cannabis are cannabinol, cannabidiol and several isomers of tetrahydrocannabinol, of which delta-9-tetrahydro-cannabinol (THC) is probably responsible for most of the psychoactive effects of the various preparations. It is of interest to note that THC does not contain nitrogen in its three-membered ring system. The structure of THC is shown in Figure 15.8. 

Until now, there was only one example of a cyclotristannene—tetrakis(tri-fert-butyl-silyl)cyclotristannene 20 reported by Wiberg and coworkers in 1999 . The tide compound 20 was prepared by the reaction of t-Bu3SiNa with stable stannylenes Sn[N(SiMe3)2]2 or Sn(OBu-f)2 in pentane at room temperature (Scheme 7). It was isolated as dark, red-brown crystals in 27% yield. In agreement with the symmetrical structure of the molecule, 20 exhibits only two sets of signals in the H, and Si NMR spectra. The most important and informative was the Sn NMR spectrum, which showed both up-field (—694 ppm) and down-field (412 ppm) resonances. The last one is typical for the three-coordinated doubly-bonded tin atoms , whereas the first one can be assigned to an endocyclic Sn atom in a three-membered ring system . ... 

This chapter constitutes a walk on the trail of the fascinating and stimulating sulfur and additional heteroatom containing, three-membered ring systems which are the result of 25 years of effort by the chemical community. Therefore, although much has been accomplished thus far and the first members of these classes of compounds have been prepared, isolated, characterized, and studied, a substantial amount of work remains to be done in this area. The challenge is there, since many of these systems have been elusive and their existence has been inferred only indirectly or circumstantially. 

Three-membered ring systems continue to play a pivotal role in heterocyclic chemistry. These compounds are extremely valuable substrates tor the synthetic organic chemist, as versatile intermediates or as reagents with unique selectivity. Since a comprehensive review would be beyond the scope of this chapter, the following text is meant to provide a selective overview of the preparation and use of these three-membered heterocycles, drawn from the previous year s literature and with an emphasis on methodology. The organization is similar to that of previous years. 

Oxidation of thiirane and thiirene oxides to the dioxides is the best method to obtain the sulfones. Indeed, in the acyclic, or large-ring systems, the sequence sulfide- sulfoxide - sulfone is by far the easiest method to prepare sulfoxides and sulfones. The situation is different in the three-membered ring series Thus, oxidation of thiiranes to the oxides by either perbenzoic acid or m-chloroperbenzoic acid under mild conditions affords the corresponding thiirane sulfoxides in almost quantitative yield - . However, further... 

A New Improved Synthesis of Tricycle Thienobenzazepines Apphcation of chemistry recently developed by Knochel" combined with the well-described halogen dance (HD) reaction, allowed preparation of our key intermediate A in only three synthetic transformations (Scheme 6.4). In this respect, treatment of 2-bromo-5-methylthiophene with hthium diisopropylamide followed by dimethylformamide afforded aldehyde 11 in good yield, lodo-magnesium exchange with conunercial 4-iodo-3-nitro anisole followed by reaction with 11 afforded the thiophene catbinol 12. Dehydroxylation of 12 provided our key intermediate A which presented the requisite functionality to examine our approach to the construction of the seven-member ring system. 

Another possibility for the preparation of bicyclic systems such as III/47 from three-membered rings can be realized by a [2+2] cycloaddition of the cyclopropene, III/41, and an unsaturated molecule, III/46, such as alkene, alkyne, ketene, ketenimine, ketone, isocyanate, etc. A large number of examples of this reaction type have been reviewed recently [55]x). 

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