Allenes, with alcohols tetrahydrofurans

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

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. 

The metallacycle S6 (Equation 1.62) is also a a-bonded tautomer of a jr-allylruthe-nium complex. As such, it can be envisioned that it might be possible to capture this species with a nucleophile as illustrated in Equation 1.62, path b [58, 59]. Tethering a judiciously placed alcohol to the allene partner does indeed lead to such a product with excellent regioselectivity (Equation 1.69) [60], Both tetrahydrofurans and -pyrans form with equal efficiency. Interestingly, reduction followed by add treatment forms the spiroketal efficiently and reasonably atom economically (Equation 1.69). 

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... 

In 2006, Widenhoefer reported an effective gold(I)-catalyzed protocol for the exo-hydroalkoxylation of y- and 6-hydroxy allenes to form 2-vinyl tetrahydrofurans and 2-vinyl tetrahydropyrans, respectively [104]. For example, treatment of 1-phenyl-5,6-heptadienol with a catalytic 1 1 mixture of [P(f-Bu)20-biphenyl]AuCl and AgOTs in toluene at room temperature led to isolation of 2-phenyl-6-vinyltetrahydropyran in 96% yield as a 7.2 1 mixture of diastereomers (Eq. (12.33)). This gold(I)-catalyzed hydroalkoxylation protocol tolerated substitution at the terminal allenyl carbon atoms and along the alkyl chain that tethered the hydroxy group to the allenyl moiety and was also effective for the S-exo hydroalkoxylation of y-hydroxy allenes. Alcaide and Almendros have shown that gold(III) also catalyzes the S-exo hydroalkoxylation of y-allenyl alcohols in modest yields (Eq. (12.34)) [105]. 

Toste and coworkers have developed effective gold(I)-catalyzed protocols for the intramolecular enantioselective hydroalkoxylation of y- and 8-hydroxy allenes employing chiral, enantiomerically pure silver salts [107]. For example, treatment of y-hydroxy allene 66 with a catalytic 1 2 mixture of the achiral bis(gold) complex (dppm)Au2Cl2 [dppm = bis(diphenylphosphino)methane] and chiral silver phos-phonate Ag-(J )-67 in benzene at room temperature led to isolation of 2-alkenyl tetrahydrofuran 68 in 90% yield with 97% ee (Eq. (12.36)). A combination of chiral bis(gold) complex with a chiral silver salt proved effective for terminally unsubstituted allenyl alcohols. For example, reaction of 5,6-heptadienol catalyzed by a mixture of [(S,S)-DIPAMP]Au2Cl2 [DIPAMP = l,2-ethanediylbis[(2-methoxyphenyl) phenylphosphine] and Ag-(J )-67 gave 2-vinyltetrahydropyran 69 in 96% yield with 92% ee (Eq. (12.37)). 

Dihydrofurans and Tetrahydrofurans.—Silver (i)-catalysed cyclizations of allenic alcohols lead to 2,5-dihydrofurans in moderate yields (Scheme 23). ° Dimethylsulphonium methylide reacts smoothly with o- and p-hydroxy-benzylidene ketones to yield 2,5-dihydrofurans. " Quinone methides, as inter-... 

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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