Radical tetrahydrofuryl

Scheme 55, Eq. 55a) [119]. A plausible mechanism is depicted in Scheme 55 and involves radical addition of the 2-tetrahydrofuryl radical to the aldehyde followed by a rapid reaction of the alkoxyl radical with Et3B. Triethylborane has a crucial role since by reacting with the alkoxyl radical it favors the formation of the condensation product relative to the -fragmentation process (back reaction). A similar reaction with tertiary amines, amides and urea is also possible (Eq. 55b) [120]. 

Tetrahydrofuran (12) adds to propadiene when heated in the presence of DTBP to 160°C to afford a minor fraction of diadduct in addition to 71% of a mixture of monoaddition products 13 and 14 (Scheme 11.8). The reaction proceeds via the nucleophilic 2-tetrahydrofuryl radical (not shown) that adds with a low a-selectivity to propadiene (la), thus leading after hydrogen atom trapping to a 66 34 ratio of functionalized heterocydes 13 and 14 [59]. 

Terazosin Terazosin, l-(4-amino-6,7-dimethoxy-2-quinazohnyl)-4-(2-tetrahydrofuroyl)-piperazine (12.2.13), only differs from prazosin in that the furyl radical is replaced with a tetrahydrofuryl radical. It is synthesized in exactly the same manner except using l-(2-tetrahydrofuroyl)piperazine instead of l-(2-furoyl)piperazine [48-51]. 

Generally, potassium persulfate in the presence of Ag+ is used for the Hunsdiecker type radical decarboxylation of carboxylic acids in water. (Bu4N+)2S208 (P) is soluble in THF, and a sulfate anion radical [i] is formed under refluxing conditions. Thus, refluxing treatment of / (tetrabutylammonium) persulfate (P) in the presence of alcohol in THF provides tetrahydrofuryl-protected alcohol (4), through the abstraction of a-H from THF by sulfate anion radical [I], followed by oxidation to a tetrahydrofuryl cation, as shown in eq.12.3 [28]. 

Togo and Yokoyama developed a general and efficient method for the synthesis of C-nucleosides employing radical coupling pathways.53 -b Several of these C-nucleosides have been synthesized by ionic pathways but they require many steps and suffer a lack of generality. The thiohydroxamates derived from pentose or 2-tetrahydrofuryl carboxylic acid, Scheme 31, gave the corresponding C-nucleoside derivatives 77 in the presence of an appropriate heteroaromatic compound.53b... 

Much the same sequence leads to a protease inhibitor that incorporates a somewhat more complex fiiryl function-linked oxygen heterocychc. This fused bis(tetrahydrofuryl) alcohol (16) was designed to better interact with a pocket on the viral protease. The first step in preparing this intermediate consists of reaction of dihydrofuran (13) with propargyl alcohol and iodosuccinimide to afford the iodoether (14). Free radical displacement of the iodine catalyzed by cobaloxime leads to the fused... 

For the unsubstituted pyridinium cation, reaction at the 2- and at the 4-position is predicted according to the theory. Indeed, reaction of protonated pyridine with the tcrt-butyl radical at low conversions (<30%) afforded selectively the ortho- and para-substituted derivatives without any alkylation at the meta position [16], It turned out that the ortho-para ratio is highly solvent dependent. If the reaction is conducted in H2O, the para product is formed as the major compound (see 10, ortho. para — 23 77). The same reaction in benzene afforded mainly the ortho compound [ortho-.para = 11 29). The reversal of the selectivity can be explained by assuming a reversible initial radical addition, especially if the reaction is conducted in H2O [16]. Similar results were obtained for the reaction with the tetrahydrofuryl radical [16]. The alkylations are generally stopped at low conversions. Since the alkylated pyridinium cations are only slightly less electrophilic than the starting pyridinium cations, overalkylation competes at higher conversion. For example, ethylation of the pyridinium cation at 100% conversion afforded a mixture of mono-, double- and tri-ethylated pyridinium salts (—> 11a e) [17]. 

In the presence of a reactive monomer, the radicals formed by hydrogen abstraction initiate chain polymerization. However, in the absence of any other reactive species, the radicals can combine to form dimers. The species that absorbs the radiation (anthraquinone) and gets promoted to an excited state is termed the initiator, while the hydrogen donor (THF) is termed the co-initiator. The tetrahydrofuryl radical is the principal initiating species. ... 

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