Tetrahydrofuran fragmentation

Volatile products of combustion CO, COj, hydrocarbon fragments, tetrahydrofuran Dzieciol, M, Intern. J. Environ. Anal. Chem., 89, 8-12, 881-89, 2009. 

Chemical modification of monensin, polyether ionophoric antibiotic with bound tetrahydropyran, two tetrahydrofuran, and octahydrospiro-2,2 -furopyran fragments 97YZ583. 

Discovery and development of a novel immunosuppressant. Tacrolimus hydrate (FK-506), macrolide antibiotic with piperidine and tetrahydrofuran fragments 97YZ542. 

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

Rh K-edge EXAFS analysis7 showed that (3) exists in tetrahydrofuran (THF) at — 20 °C, but fragments at temperatures >0°C to give [Rh(COCH3)I3(CO)(solvent)] . 

Reactions in which a reagent is cloven (i) The termination reactions of the type (IX) in which an anionic fragment is abstracted from the anion, so that the chain-carrier is neutralised. This type of termination has been claimed to occur in many systems for example, in the polymerization of tetrahydrofuran by PF5 a terminal F from PF 6 was indeed found [119]. 

The same basic strategy was applied to the synthesis of the smaller fragment benzyl ester 28 as well (Scheme 4). In this case, aldehyde 22 prepared from (S)-2-hydroxypentanoic acid [9] was allylated with ent-10 and tin(IV) chloride, and the resulting alcohol 23 was converted to epimer 24 via Mitsunobu inversion prior to phenylselenenyl-induced tetrahydrofuran formation. Reductive cleavage of the phenylselanyl group, hydrogenolysis of the benzyl ether, oxidation, carboxylate benzylation, and desilylation then furnished ester 28. 

This radical cyclization strategy was utilized for the synthesis of the smaller fragment silyl ether 54 as well (Scheme 8). Evans aldol reaction of the boron eno-late derived from ent-32 with aldehyde 33, samarium(III)-mediated imide methyl ester conversion, and protecting group exchange led to tosylate 51. Elaboration of 51 to ketone 53 was achieved under the conditions used for construction of the second tetrahydrofuran moiety of 49 from 46. A highly diastereoselective reduc-... 

Access to the corresponding enantiopure hydroxy esters 133 and 134 of smaller fragments 2 with R =Me employed a highly stereoselective (ds>95%) Evans aldol reaction of allenic aldehydes 113 and rac-114 with boron enolate 124 followed by silylation to arrive at the y-trimethylsilyloxy allene substrates 125 and 126, respectively, for the crucial oxymercuration/methoxycarbonylation process (Scheme 19). Again, this operation provided the desired tetrahydrofurans 127 and 128 with excellent diastereoselectivity (dr=95 5). Chemoselective hydrolytic cleavage of the chiral auxiliary, chemoselective carboxylic acid reduction, and subsequent diastereoselective chelation-controlled enoate reduction (133 dr of crude product=80 20, 134 dr of crude product=84 16) eventually provided the pure stereoisomers 133 and 134 after preparative HPLC. 

In addition to his total synthesis of pamamycin-607 (lb) (see Schemes 14-17), Kang also communicated an alternative synthesis of a larger fragment surrogate of lb [51]. Again, iodoetherifications of y-triethylsilyloxy alkenes were utilized as key transformations to control the 2,5-c/s-disubstitution of the two tetrahydrofuran moieties. However, whereas his total synthesis of lb involved a two-directional formation of both heterocycles in a single operation, the alternative route depicted in Schemes 31 and 32 is characterized by sequential... 

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