Oxygen, heterocyclic compounds tetrahydrofuran

Then, the oxygen cannot be terminal. One such structure is the heterocyclic compound tetrahydrofuran [Fig. 9-37(a)]. 

There are several reactions (bromination, oxymercuration, epoxidation) of olefinic hydroxy and epoxy compounds in which the oxygen function can react intramolec-ularly with an intermediate formed at the double bond, leading to an oxygen-containing heterocyclic compound (tetrahydrofuran or tetrahydropyran) (Scheme 6). 

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. 

Although oxirane (oxacyclopropane) and oxetane (oxacyclobutane) react with Grignard and organolithium reagents to form alcohols, tetrahydrofuran (oxacyclopentane) is so unreactive that it can be used as the solvent in which these organometallic compounds are prepared. Explain the difference in reactivity of these oxygen heterocycles. 

The stereoselective construction of substituted tetrahydrofurans as enantiomerically pure form is of great interest because many biologically active compounds have such oxygen heterocycles. Fujita et al. developed the synthesis of tetrahydrofuran-3-yl carboxylates 26 via intramolecular oxygenatirm of but-3-enyl carboxylates using lactic acid-derived chiral X -iodanes 25 (Scheme 11) [49, 50]. The enr o-selectivity achieved in this case contrasts with the cxo-selectivity observed in the reaction with conventional oxidizing reagents. The products 26 are obtained in up to 64% ee. 

The strain energies of these five-membered heterocycles are relatively small with values of 23.5, 24.8 and S.SkJmoF estimated for tetrahydrofuran, pyrrolidine and tetrahy-drothiophene respectively (74PMH(6)199). The closeness of the values for the two former compounds reflects the almost identical covalent radii of oxygen (0.66 A) and nitrogen (0.70 A) atoms. The sulfur atom with a much larger covalent radius of 1.04 A causes a... 

Tetrahydrofuran — A heterocyclic ether compound, also known as tetramethylene oxide, 1,4-epoxybutane, oxacyclopentane. This five-membered ring comprises four carbon atoms and one oxygen atom, is an aprotic polar - solvent, and is fully miscible in other solvents such as water, alcohol, and ether. It also can be used as... 

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