A-allenic alcohols

Alcaide, Aknendros and coworkers developed a combination of a 3,3-sigmatropic rearrangement of the methanesulfonate of an a-allenic alcohol to give a 1,3-bu-tadiene which is intercepted by a dienophile present in the molecule to undergo an intramolecular Diels-Alder reaction [83]. Thus, on treatment of 4-236 with CH3S02C1, the methanesulfonate was first formed as intermediate, and at higher temperature this underwent a transposition to give 4-237 (Scheme 4.51). This then led directly to the cycloadduct 4-238 via an exo transition state. 

The cyclopropanation of a-allenic alcohols 616 gave methylenecyclop-ropanes 617a, b and spiropentanes 618a, b in different proportions depending on the carbenoid reagent used (Scheme 88) [163,4b]. 

Air-stable palladium(O) catalyst, [(Cy3P)2Pd(H)(H20)]BF4, catalyses carbonylation of propargylic alcohols to generate dienoic acids and esters (equation 167)286. Since propar-gyl alcohols are obtained from carbonyl compounds by acetyhde addition reactions, this sequence constitutes a three-carbon homologation. a-Allenic alcohols are converted to tt-vinylacrylic acids under similar conditions (equation 168)287. 

With the proper choice of reaction conditions, diastereoselective synthesis of a-allenic alcohols 69 and 70 from propargylic epoxide 68 was achieved [80, 81], With RMgBr and 5 mol% of CuBr/2PnBu3, anti allenic alcohols 69 are obtained with up to 100% diastereoselectivity. On the other hand, syn allenic alcohols 70 can be prepared with 88-96% diastereoselectivity with RMgCl, Me3SiCl and 5mol% CuBr (Scheme 3.36). 

The reduction of a-allenic alcohols with lithium aluminum hydride afforded 1,3-dienes 376 via the intramolecular transfer of hydride from aluminum to the central carbon atom of the allene moiety [172]. 

In the same research group the cationic hydridopalladium complex [Pd(H)(H20)(PCy3)2] [BF4] has been shown to catalyze the hydroxycarbony-lation of triple bonds. As a representative example the dehydration occurring to give the dienoic acid is displayed in Scheme 3 [35]. The same cationic complex is able to activate a carbon oxygen bond in a-allenic alcohols to provide dienoic acids but with the COOH group in the branched position (Scheme 3) [36]. 

An improved procedure for the synthesis of a-allenic alcohols in good yields and with approximately 90% e.e. was reported by Olsson and Claesson (56). (- )-(S)-3-Butyne-2-ol (25) was converted into the monotetrahydropyranyl derivatives 26a to c, which gave on reduction with LAH in ether or THF the chiral allenes 27a to c (Scheme 4). The absolute configurations of 27b and c were... 

The same authors also studied the alkylation of alkynyl epoxides for formation of optically active a-allenic alcohols under kinetic resolution conditions (Scheme 8.29) [54]. 

Various types of diethylphosphono-substituted a-allenic alcohols have been transformed to the corresponding 4-(diethylphosphono)-2,S-dihydrofurans by treatment with a catalytic amount of silver(I) nitrate under a nitrogen atmosphere <99S463>. 

Lithium, 157 a-Allenic Alcohols Methoxyallene, 177 3-Trimethylsilyl-1 -propyne, 329... 

The AgNC>3-induced reaction of (3-lactam-tethered a-allenic alcohols 41 gave, with concomitant acetonide cleavage, the spirocyclic dihydrofurans 42 in quantitative yields (Scheme 13) [59]. 

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. 

For azetidine-2,3-diones, a-allenic alcohols are obtained as essentially regio- and diastereoisomerically pure products (Scheme ll).115... 

Pomet, J., Randrianoelina, B. Action of 1-trimethylsilyl-2-butyne on carbonyl derivatives in the presence of catalysts synthesis of a-allenic alcohols or chloroprenic derivatives. Tetrahedron Lett. 1981,22, 1327-1328. 

Allenic alcohols.3 The reaction of allylsilanes with carbonyl compounds catalyzed by this salt to give homoallylic alcohols (9, 455-446) has been extended to a synthesis of a-allenic alcohols. Use of TiCl4 as catalyst results in a 2-chloro-l,3-diene. 

A highly stereoselective preparation of iodovinyl-substituted oxiranes is accomplished by iodine addition to a-allenic alcohols followed by treatment of the resulting 3,4-diiodo-2-en-l-ols with strong base <93JOCl653>. 

Highly substituted alkylidenecyclopropanes 130, 131 and 133, 134 were made accessible by a regioselective hydroxy-directed cyclopropanation of a-allenic alcohols 129 and 132 using sa-marium/diiodomethane. ... 

The reduction of 4-aryl-3-butyn-1 -ols 4.6 is stereoselective towards the -a,p-unsaturated alcohol only in Et20. In THF, a mixture of E- and Z-isomers is formed [KJ2] (Figure 4.3). The presence of a leaving group a to the triple bond induces the formation of a-allenic alcohols 4.7 [HH3] (Figure 4.3). An allenic alcohol is also formed from conjugated allylic alcohol 4.8 (Figure 4.3). 

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). "... 

Propargylic organoboranes derived from the corresponding lithium reagents react with aldehydes and certain ketones with high regioselectivity to give trimethylsilyl-substituted a-allenic alcohols (Scheme... 

The high regioselectivity in the formation of a-allenic alcohols from boron reagents at low temperature is markedly different from that seen with the titanium reagent derived from 1-trimethylsilyl-l-bu-tyne, in which exclusive formation of 3-alkynic alcohols is observed. This result was explained by a rapid exchange between the allenic and alkynic structures, as shown in equation (2). The alkynic structure is thermodynamically less stable and kinetically more reactive than that of the allenic form. At lower temperature, the rate of equilibrum becomes faster than the subsequent reaction with aldehydes and thus the alkynic species becomes the major reaction form. 

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