Tetrahydrofurans, from allenes

An iodine-mediated 6(0) " -exo-Trig cyclization has been used in the synthesis of (25, 4/ )-4-hydroproline from (S)-O-benzylglycidol219. Stereocontrolled 5(0) " -endo-Trig iodoetherification has provided an entry to traw.s-2.5-disubstituted tetrahydrofuran derivatives220. On treatment with iodine, trichloroimidates of primary a-allenic alcohols are converted into oxazolines with high stereoselectivity221. 

A similar cyclization to give cyclopropanone acetals 33 was accomplished with magnesium in tetrahydrofuran or zinc in hexamethylphosphoric triamide. However, considerable amounts of allenes were also isolated as byproducts. Acetals derived from linear 1,3-dibromo ketones gave a mixture of spiroacetals and allenes, while acetals derived from cyclic ketones, e.g. 2,6-dibromocyclohexan-l-one, afforded cyclopropanone acetals exclusively. In general, the more substituted the substrate, the greater the yield of cyclpropanone acetals. ... 

Diallenyltin dibromide, prepared by treatment of propargyl bromide with metallic tin in the presence of metallic aluminum in dry tetrahydrofuran, reacts with aldehydes and ketones to afford 3-alkynic alcohols selectively (Scheme 4). This result is different from a previous report in which an a-allenic alcohol was found to be a major product from a propargyltin reagent prepared from a tin(II) halide. " On the other hand, -alkynic and a-allenic alcohols were synthesized selectively by the reaction of aldehydes and ketones with bis(trimethylsilylpropargyl)tin diiodide using different solvent systems (Scheme S). "... 

The present procedure evolved from our previous work on the conversion of allenals, allenones, and allenylcarbinols to furans and 2,5-dihydrofurans with catalytic silver nitrate (AgN03) in acetone.5-10 It has also been shown that allenylcarbinols can be converted to 2,5-dihydrofuran under these conditions.11 0- and y-Alkynyl allylic alcohols can also be isomerized to furans under strongly basic conditions with potassium tert-butoxide in tetrahydrofuran-tert-butyl alcohol-18-crown-6 or hexamethylphosphoramide (KO-t-Bu in THF-t-BuOH-18-crown-6 or HMPA).12 The AgNOa/silica gel method is milder, faster, and more efficient than the previously reported procedures.13 Moreover, it offers the potential advantage of catalyst recovery and possible applicability to a flow system in which a packed column, protected from light, could serve as the reactor.4... 

Tandem coupling and cyclization. Functionalized allenes are converted to a-arylvinylated cyclopropanes and heterocycles including epoxides, tetrahydrofurans, and oxazolidinones. Furan derivatives are formed from allenyl ketones." ... 

Undeca-l,2-diene treated sequentially with equimolecular quantities of -butyl-lithium and -propyl iodide in tetrahydrofuran -> tetradeca-4,5-diene. Y 96% purity 95%. F. e., also synthesis of allenes from bromoallenes, s. G. Linstrumelle and D. Midielot, Chem. Commun. 1975, 561 alkylation of allenes via Li-diallyl-cuprates cf. Tetrah. Let. 1976, 275. 

In previons work (Allen et al., 1963 Tangpakdee et al., 1997), NR coagulated from fresh NR latex, just after tapping from Hevea brasiliensis, was found to be soluble in tolnene, cyclohexane, and tetrahydrofuran. In contrast, the mbber from latex preserved in the presence of ammonia contained about 30-70% gel fraction, which was insolnble in the solvents. The formation of the insoluble fraction would be concerned with the interactions of mbber and proteins, because the gel fractions are reported to be soluble in the solvents after the enzymatic deproteinization (Eng et al., 1994 Tangpakdee et al., 1997). If the interactions are physical but not chemical, it is possible to remove the proteins from the mbber after denaturation of the proteins with urea. Here, the removal of the proteins from fresh NR latex and preserved high-ammonia latex was investigated with mea in the presence of surfactant. 

Palladous acetate will catalyse the cyclization of 2-allylphenols to benzofurans, and a useful extension of this work is the synthesis of 2-vinyltetrahydrofurans by oxidation of yS-unsaturated alcohols (Scheme 16). The oxidation of various allenic alcohols with hydrogen peroxide led, depending upon substitution, to a new synthesis of 3-oxo-tetrahydrofurans and -tetrahydropyrans (Scheme 17) yields vary from zero to 60%. 

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