Three-membered rings ring formation

Apparent nucleophilic attack on large, fully unsaturated rings may occur by way of attack on a valence tautomer, such as the reaction of oxepin with azide ion. Attack on the oxanorcaradiene valence tautomer leads to ring opening of the three-membered ring, and formation of 5-azido-6-hydroxy-l,3-cyclohexadiene (Section 5.17.2.2.4). 

Reduotion. Catalytic reduction of cyclopropenone commonly leads to reduction of both the double bond and the three-membered ring with formation of a dialkyl ketone ... 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

Tertiary amines have been shown to react with isocyanates ia an analogous fashion to form ureas (41—43). Similarly, a2iridines (three-membered rings containing nitrogen) are found to react with isocyanates to yield cycHc ureas. Tertiary amines have also been shown to form labile dipolar 1 1 adducts with isocyanates reminiscent of salt formation. In contrast, formaldehyde acetal aminals form iasertion products with sulfonyl isocyanates (44,45). 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Beyond the scope of this chapter, but similar in mode of formation and reactivity to the title compounds, are some three-membered rings with three heteroatoms. Oxadiaziridine... 

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. 

A-Carbamoyldiaziridines can open the three-membered ring and recyclize through the carbamoyl nitrogen, as demonstrated in a benzoyl isocyanate adduct (74JOC3198) and in the formation of (140) (79JOC3935). 

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. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

Compound (223), the 6a,7a-methano homolog of dienone (174), is extraordinarily stable on irradiation in dioxane, while the 6, 7 -isomer (224) isomerizes readily to the bicyclohexenone (225). The apparent failure of (223) to react may be attributed to the high steric strain which would be introduced by the two adjacent three-membered rings on ring B upon 1,5-bridging. This could cause reversal of primary product formation [cf. (226)] rather than ensuing formation of the corresponding zwitterion and... 

In the last 10 years many three-membered rings with two hetero atoms have been synthesized. Their formation occurs surprisingly smoothly with an ease comparable to that of the three-membered rings with one hetero atom. The oxaziranes (I), diaziridines (2), and diazirincs (3) have become, in a very short time, classes of compounds with an extensive literature. 

The oxaziranes are in the majority of cases distillable liquids with boiling points somewhat higher than the corresponding Sehiff s bases. During distillation, temperatures above 100 C should be avoided. In distinction to the isomeric nitrones, the less polar oxaziranes are usually noncrystalline. They have a characteristic unpleasant smell and are nonbasic. Attempts to force salt formation results in fission of the three-membered ring. ... 

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. 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

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. 

Actually, thiirane dioxides (17) have so far never been isolated in these reactions cis- and trans-olefins were the main products, and all attempts to obtain the three-membered ring system and prevent the loss of SO 2 failed. Hence, the method can be used only for the in situ formation of intermediates. 

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