Allene-alcohols

Some allenic alcohols can be prepared in analogous waysfrom acetylenic epoxides or tetrahydropyrans ... 

Synthesis of allenic alcohols hy 1., 2-substitution of chlorine on ether groups by hydride... 

To a mixture of 100 ml of THF and 0.10 mol of the epoxide (note 1) was added 0.5 g Of copper(I) bromide. A solution of phenylmagnesium bromide (prepared from 0.18 mol of bromobenzene, see Chapter II, Exp. 5) in 130 ml of THF was added drop-wise in 20 min at 20-30°C. After an additional 30 min the black reaction mixture was hydrolysed with a solution of 2 g of NaCN or KCN and 20 g of ammonium chloride in 150 ml of water. The aqueous layer was extracted three times with diethyl ether. The combined organic solutions were washed with water and dried over magnesium sulfate. The residue obtained after concentration of the solution in a water-pump vacuum was distilled through a short column, giving the allenic alcohol, b.p. 100°C/0.2 mmHg, n. 1.5705, in 75% yield. 

Apparatus. 500-ml round-bottomed, three-necked flask with a gas inlet tube, thermometer and a gas outlet for the preparation of chlorotetrahydropyran 1-1 four--necked, round-bottomed flask with a gas inlet tube, a dropping funnel, a mechanical stirrer and a thermometer, combined with a gas outlet for the preparation of HC=CMgBr and its reaction with chlorotetrahydropyran 1-1 three-necked, round--bottomed flask with a dropping funnel, combined with a gas inlet, a mechanical Stirrer and a thermometer, combined with a gas outlet for the conversion into the allenic alcohol. 

CarefuT disti11 ation of the residue afforded the allenic alcohol, b.p. 55°C/15 mmHg, 23 20... 

Allenic alcohols couple with allyl indium reagents at 140°C to give allylic alcohol products. Similarly, (o-hydroxy lactones couple with organoindium reagents. 

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 bromination of the optically pure alkadienephosphonic acids proceeds with similar stereoselectivity [55], while in the case of allenic alcohols complete racemi-zation of the product occurred (Scheme 16) [56],... 

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]. 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

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. 

Allenic alcohols,3 In the presence of Sml2 and Pd[P(C6H3),]4, sec- and tert-propargylic acetates add to ketones to give allenic alcohols as the only or major product. A mixture of allenic and homopropargylic alcohols is formed from reactions of primary propargylic acetates. 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

In 1974, Vermeer et al. described formation of allenic alcohols 61 by the reaction of alkynyl epoxides 60 with Grignard reagents in the presence of 10mol% of Cul (Scheme 3.33) [71]. In the absence of Cul, a complicated mixture of products was obtained. Furthermore, the Cu-catalyzed reactions exhibited higher yields and higher selectivity than analogous reactions of alkynyl epoxides with lithium dialkylcup-rates [72], This method was applied to a reaction of allylmagnesium bromide with an alkynyl epoxide [73]. 

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 synthesis of chiral liquid-crystalline allenes was reported by Tschierske and co-workers (Scheme 4.10) [14]. An asymmetric reduction of 41 with Alpine borane was a key step to an enantioenriched allene 44. After removal of the silyl group, the allenic alcohol was etherified by the Mitsunobu method to give 45, the first liquid-crystalline allene derivatives. 

Scheme 4.26 [2,3] Wittig rearrangement for asymmetric synthesis of allenic alcohol 102.

Spino and Frechette reported the synthesis of non-racemic allenic alcohol 168 by a combination of Shi s asymmetric epoxidation of 166 and its organocopper-mediat-ed ring-opening reaction (Scheme 4.43) [74]. Reduction of the ethynyl epoxide 169 with DIBAL-H stereoselectively gave the allenic alcohol 170, which was converted to mimulaxanthin 171 (Scheme 4.44) [75] (cf. Section 18.2.2). The DIBAL-H reduction was also applied in the conversion of 173 to the allene 174, which was a synthetic intermediate for peridinine 175 (Scheme 4.45) [76], The SN2 reduction of ethynyl epoxide 176 with DIBAL-H gave 177 (Scheme 4.46) [77]. 

Scheme 4.49 Asymmetric synthesis of allenic alcohols from cyclic carbonates or sulfites...

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

To prepare the other cross-conjugated allene, 4-methylene-l,2,5-hexatriene ( 2-allenyl-1,3-butadiene ) (23), the allene alcohol 215 was first converted into the phosphate 216, that readily underwent an SN2 -type substitution with allenylmagnesium bromide to yield the target hydrocarbon as a highly reactive allene derivative (Scheme 5.32) [76],... 

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]. 

TBA-F (lM in THF, 0.2 ml) is added to Me,SiCH2C=CH (0.56 g, 4 mmol) and the carbonyl compound (4 mmol) in THF (10 ml) at -5° toO°C. On completion of the reaction (Table 6.14), the mixture is washed sequentially with methanolic HCl and aqueous Na2C03. Et20 is added and the filtered solution is dried (K2CO,) and fractionally distilled to yield the allenic alcohol. 

Evans, Landor, and Regan found that lithium bismenthoxyaluminum hydride (13) reduced alk-2-en-4-yn-l-ols (14) to optically active allenic alcohols (52,53). 

The reaction is thermodynamically controlled and was postulated to involve an achiral, delocalized anion 15, which cyclizes to a somewhat strained 7-membered ring complex 16 capable of existing in isomeric forms. The more stable form could be hydrolyzed to the predominant ( + )-(S)-allenic alcohol 17 (Scheme 1)... 

A synthetic method for the preparation of chiral a- and p-allenic alcohols starting with a chiral acetylenic amine 20 and involving an LAH reduction as a key step was reported by Claesson and Mosher (54). This is illustrated in Scheme 3. Based on the stereochemistry of 20 to 24, it can be deduced that the attack... 

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... 

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