Five-membered Oxygen-containing Rings

Five-membered Oxygen-containing Rings. The reaction of aldehydes with 1,1-dicyano-oxirans (77) gives 1,3-dioxolans (79). The authors suggest, by 

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In contrast with unreactive, unfunctionalised terminal alkenes, allylic and homoallylic ethers (22, 24) and alcohols (20) from which the product organolithiums (21, 23, 25) can be chelated in a (preferably) five-membered, oxygen-containing ring, carbolithiate rapidly and cleanly.23 Coordination overrides any preference for the lithium to be bonded to the primary carbon, but cannot overcome the unfavourability of forming a tertiary organolithium - 26 gives 27, but 28 cannot be carbolithiated. Coordination to sulfur in similar thioethers 29 works too. 

Bloch, R., and Seek, M., Optically active five-membered oxygen-containing rings. A synthesis of (-t-)-eldanolide. Tetrahedron, 45, 3731, 1989. 

Among the heterocyclic compounds, there are aromatic, e.g. pyridine, as well as nonaromatic, e.g. tetrahydrofuran, compounds. Similarly, there are saturated (e.g. tetrahydrofuran) and unsaturated (e.g. pyridine) heterocyclic compounds. Heterocycles also differ in their ring sizes, e.g. pyridine has a six-membered ring, whereas tetrahydrofuran is a five-membered oxygen-containing heterocyclic compound. 

Derivatives with five- and six-membered oxygen-containing rings have been isolated when vindoline and deacetylvindoline were treated under superacid conditions.686 Formation of the unexpected oxolane derivatives (R = Ac 4%, R = H 18%) was accounted for by rearrangement through ethyl and hydride shifts (Scheme 5.66). 

The cyclic hemiacetal that has a five-membered ring is called a furanose. This name is derived from that of tire five-membered, oxygen-containing heterocyclic compound fu-ran. The hemiacetal with a six-membered ring is known as a pyranose after the heterocycle pyran. 

The thiopyranofurans contain a six-membered sulfur heterocyclic ring and a five-membered oxygen heterocyclic ring (Table 10). Another common name for these isomers is furothiopyrans. Not much chemical literature exists regarding the thiopyranofuran compounds. 

Furfural (1), derived from annually renewable agricultural byproducts, is an important industrial chemical manufactured and used throughout the world. It is the feedstock for a number of derivative chemicals generically known as furans -the structural characteristic of which is the five-membered oxygen-containing heterocyclic ring. Furfuryl alcohol (2) is the most important derivative of commerce, where it is used primarily in synthesis of adhesive polymers. 

Figure 11.17. Examples of unsaturated fatty acids. Large numbers of such alkenes both as ( )- and (Z)-isomers exist. The intermediate in the oxidation of arachidonic acid to (only one shown member of the) prostaglandins is part of a cascade of reactions to an entire family of compounds that vary in oxidation state. Leukotrienes (three conjugated double bonds and no five-membered ring) and thromboxanes with six-membered oxygen-containing rings (oxa-cyclohexane, pyran, oxane) are also derived from C20 polyalkenes.

This reaction was first described by Gabriel in 1910 (40), when he warmed an acylaminoketone (197a) with an equimolecular amount of phosphorus pentasulfide. The reaction (Scheme 103) is similar to the preparation of other five-membered oxygen- and sulfur-containing rings from 1,4-dicarbonyl compounds. 

Benzyl carbamates have been used to form both five- and six-membered nitrogen-containing rings. The selectivity for N over O nucleophilicity in these cases is the result of the nitrogen being able to form a better ring size (5 or 6 versus 7 or 8) than the carbonyl oxygen. 

The syntheses of five-membered heterocycles containing three oxygen or sulfur atoms in the 1,2,3-positions can be classified into three groups (i) one-component syntheses, in which a suitable acyclic molecule is cyclized by linkage of two heteroatoms (ii) two-component syntheses, in which the ring is built up either by a [3 -f 2] process or a [4 -1-1] process and (iii) synthesis by transformation of another heterocyclic ring. 

Table 4 Ring syntheses of five-membered heterocycles containing three oxygen or sulfur atoms from non-...

The syntheses of monocyclic five-membered heterocycles containing three oxygen or sulfur atoms can be classified into three groups, according to the number of building blocks required to form the ring in a one-step process ... 

Table 10 Ring Syntheses of Monocyclic Five-membered Heterocycles Containing Three Oxygen or Sulfur Atoms from Non-heterocyclic Compounds... 

Nitrogen- and Oxygen-containing Rings.- Five-membered Rings. 

Five-membered Nitrogen- and Oxygen-containing Rings. Huisgen et al. have generated benzonitrile-4-nitroben lide (102) in small equilibrium concentrations. This is the first representative of a class of nitrile ylides and... 

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