Heterocyclic compounds tetrahydrofuran

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

Another useful class of reactions which has been successfiilly demonstrated as a continuous SCF process are etherfication/cyclisation reactions, again using acid catalysts [52]. Examples include the formation of asymmetric esters and heterocyclic compounds. Scheme 12.2 shows the intramolecular cyclization of 1,4-butanediol over Amberlyst-15 at 120 bar, 170 °C, which produces the heterocyclic compound tetrahydrofuran (THE). 1,4-Butanediol has a melting point of 16 °C, and so, although not strictly required, a small amount of methanol may be added to the reactants to aid solubility during decompression of the SCF. Adding a cosolvent to the system may also improve solubility in SCCO2 [13]. 

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). 

Tetracblorotbiopbene under Single-Ring Heterocyclic Sulfur Compounds Tetraethyl Lead under Lead Compounds Tetrahydrofuran under Noncyclic Aliphatic or Aromatic Ethers Thallium... 

Cyclic compounds that have one or more of atoms other than carbon, e.g. N, O or S (hetero-atoms), in their rings are called heterocyclic compounds or heterocycles, e.g. pyridine, tetrahydrofuran, thiophene and so on. 

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. 

A-12. Draw the structure of the sulfur-containing heterocyclic compound that has a structure analogous to that of tetrahydrofuran. 

Atoms other than carbon and hydrogen that appear in organic compounds are called heteroatoms. Cyclic organic compounds that contain one or more heteroatoms are called heterocycles. Heterocyclic compounds are the largest class of organic compounds and can be either aromatic (such as pyridine, pyrrole, and furan) or nonaromatic (such as piperidine, pyrrolidine, and tetrahydrofuran). 

Among the O-containing heterocyclic ligands, tetrahydrofuran (THF) 376 is the subject of permanent interest [13]. As a result of its interaction (3.18) with metal salts, the molecular solvent complexes of the type 298 (Sec. 2.2.4.4) and 577 are formed, and can be used as precursors to obtain coordination compounds with other ligand systems ... 

Allylic alcohols can serve as 7t-allyl cation precursors to act as electrophiles in Sn reactions with a tethered O-nucleophile giving rise to the formation of spiroannulated tetrahydrofurans <2000TL3411>. Michael acceptors are also suitable electrophiles for the cyclization to tetrahydrofuran rings <2003T1613>. The Tsuji-Trost allylation has found widespread application in the synthesis of carbo- and heterocyclic compounds. Allylic substitution has been employed in the stereoselective synthesis of 2-vinyl-5-substituted tetrahydrofurans <2001H(54)419>. A formal total synthesis of uvaricin makes twofold use of the Tsuji-Trost reaction in a double cyclization to bis-tetrahydrofurans (Equation 73) <20010L1953>. 

Actually, of course, we have already encountered numerous heterocyclic compounds cyclic anhydrides (Sec. 20.9) and cyclic Mdes (Sec. 20.14), for example lactones (Sec. 20.15) and lactams (Problem 28.3, p. 891) cyclic acetals of dihydroxy alcohols (Problem 23, p. 651) the solvents dioxane and tetrahydrofuran (Sec. 17.9). In all these, the chemistry is essentially that of their open-chain analogs. 

An efficient method for the preparation of mono-heterocyclic compounds such as substituted tetrahydrofurans with high stereoselectivity via intramolecular SN2 O-cyclization of alkoxides has been reported. This is a nucleophilic displacement with a simultaneous allylic rearrangement (Scheme 321).1114... 

Fractions 3 and 4 from the tar trap tar, which primarily contained polycyclic aromatic hydrocarbons and their alkylated derivatives, were combined and subfractionated on a silica gel column. In sequential elution, toluene eluted 46 percent of the mass (subfraction A), propanol eluted 48 percent (subfraction B), methanol eluted 3 percent (subfraction C) and 3 percent was recovered by washing the silica gel with tetrahydrofuran (subtraction D). The toluene subfraction was found by GC/MS analysis to consist of a series of PAH, both parent compounds and numerous methylated derivatives. In addition, the heterocyclic compound dibenzothiophene was present as a major peak. Tentative identifications are listed in Table IX. Compounds whose identifications were confirmed by GC retention time comparisons with standards are noted. Although the identities of many of the individual PAHs have not been confirmed, it is clear that most of the subtraction is PAH in nature. 

Monosaccharides occur as substances with a free carbonyl group (acyclic compounds) and as cyclic hemiacetals, also called lac-tols. Trioses are exclusively acyclic substances, tetroses and higher monosaccharides exist predominantly in five- and six-membered and, exceptionally, also in seven-membered cyclic structures. They can therefore be regarded as substances derived from oxolane (tetrahydrofuran), oxane (tetrahydropyran) or oxepane, and are thus actually heterocyclic compounds. Acyclic forms, which exist in constitutional equilibrium with cyclic forms, occur in zigzag conformers, as well as alditols (see Section 4.3.1.1.1). 

There are many reports on the use of SO2 as the reductant to initiate the polymerization. For example, Polish workers [245] studied the emulsion polymerization of the styrene-SOj system using cumene and pinene hydroperoxide. Gomes and Lourdes [246] investigated the liquid SOa-cumene hydroperoxide system. Ghosh et al. used the SO2 in combination with heterocyclic compounds by pyridine, tetrahydrofuran, and N-N -dimethylformaniide for photopolymerization [247-249] as well as aqueous polymerization [250]. 

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... 

The conversion of furans by oxidative acetylation or methoxylation to 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofurans respectively, and their subsequent hydrogenation to the corresponding tetrahydrofurans, provides a useful source of protected 1,4-dicarbonyl compounds capable of conversion inter alia into the other five-membered heterocycles [Pg.142]

Other advances include the construction of seven- and nine-membered rings via the analogous [4-1-3] and [6-1-3] cycloadditions with dienes and trienes respectively. Heterocycles, such as tetrahydrofurans and pyrrolidines, are accessible using carbonyl compounds and imines as substrates. The following discussion is organized around these recent discoveries. It serves to illustrate the versatility and the high degree of selectivity which are some of the distinctive features of the Pd-TMM chemistry. 

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