Allenic alcohols, stereoselective

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],... 

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]. 

When treated with catalytic amounts of acid,404 403 silver salts406"410 or mercury salts,409,410 allenic alcohols will cyclize to ethers (equations 230-253). By proper choice of reaction conditions, the yields of products obtained in these cyclizations are generally high. Interestingly, the stereoselectivity observed in equation (253) is proposed to arise from a chair conformation of the intermediate.408... 

Cyclization of a variety of y-allenic alcohols with silver nitrate proceeds by 5-exo cyclization to form 2-alkenyltetrahydrofurans (equation 87).205c 206 Little stereoselectivity is seen in cyclizations of secondary alcohols. Cyclization by intramolecular oxypalladation/methoxycarbonylation or oxymercuration followed by transmetallation and methoxycarbonylation also showed no stereoselectivity (equation 88 and Table 24, entries 1 and 2).50 207 However, cyclization of the corresponding r-butyldimethylsilyl ether derivatives with mercury(II) trifluoroacetate followed by transmetallation/methoxycarbonylation pro-... 

Table 24 Effect of O-Silylation on Stereoselectivity of Cyclization of y-Allenic Alcohols (Equation 88)...

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>. 

Hegedus has reported the transmetalation of 370 with n-BuSnCls, which presumably generates the coordinated stannane 371 in situ for a stereoselective anti-SE reaction leading to the allenic alcohol 372 (Scheme 5.2.79). 13... 

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). 

Vinyl triflates can be coupled efficiently to various organotin compounds. Further details of this chemistry have now appeared including efficient preparations of trimethylsilyldienes (Scheme 33).71 Two other routes to silyldienes are outlined in Scheme 34. Reduction of allenic alcohols (21) gave moderate yields of the dienyl products, although the stereoselectivity of the process leaves much to be desired. The second method, involving hydroboration of the allene (22) is much more stereoselective and enables either isomer of the final product to be obtained depending on the elimination conditions used. A key application for such dienes is in Diels-Alder reactions, where the products have vinylsilane functionality for further modification. This type of chemistry has now been explored with 2,3-bis(trimethylsilyl)buta-1,... 

Conjugated dienes. Acetylenes of type (1) can be converted into 1,3-dienes (3) by way of a-allenic alcohols (2). This synthesis is not stereoselective, but yields are satisfactory. ... 

The [2,3] Wittig rearrangement is a useful reaction involving a [2,3] sigmatropic shift, and has been used to obtain homopropargylic alcohols from allylic propargylic ethers with high stereoselectivity. The reaction has been modified by Marshall to provide a useful route to optically active allenic alcohols. 

A series of Irisnbstituted and 1,3-disnbstituted a-allenic alcohols were carbonylated under the reaction conditions. The reaction of 1,3-disubstituted a-allenic alcohols was stereoselective, exclusively affording the -isomer.f ... 

A general and stereoselective rearrangement of allenic alcohols to ( , )-l,3-dien-2-yl triflates and chlorides has been reported (Scheme 211). ° ... 

Ti(lV) catalyses the stereoselective preparation of silyl substituted 1,3-aienes from allylsilanes and RCHO, or from el-allenic alcohols on reduction, and while 2,3-disilylbuta-1,3-dienes are formed from ot-8r vinylsiLanes, attempts to prepare 2-Me2Si-buta-1,3-diene from... 

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

Hydrozirconation of monosubstituted allenes offers easy access to allylzircono-cene chlorides, which react with carbonyl compounds to afford homoallylic alcohols in a highly regio- and stereoselective manner (Scheme 16.68) [73-75],... 

A stereoselective synthesis of the enantiomerically enriched allenic hydrocarbons was described in 2001 (Scheme 18.11) [37]. For example, hydrostannylation of the chiral propargylic alcohol 28 (obtained with 82% ee by enantioselective reduction of... 

Stereoselective Claisen-type [3,3] sigmatropic rearrangements of propynyl vinyl ethers, which are intermediates in the reaction of propynyl alcohols with aldehydes or ortho esters leading to a-allenic aldehydes or esters, have been reported15,16. 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

Predictably, with enynes dihalocarbene addition occurs preferentially with the double bond (equation 17).63 64 The reaction of dichlorocarbene with allenes usually gives rise to spiro-linked dichlorocyclopro-panes (equation 18),65 but rearranged products are obtained in some cases (equation 19).65 66 Somewhat surprisingly, a measure of stereoselectivity has been observed in the formation of dichlorocyclopro-pylcarbinols from secondary allylic alcohols and dichlorocarbene (equation 20).67... 

The rigidity of the allene and its facile formation were imaginatively exploited in a synthesis of the vitamin E side chain alcohol, in which an overall transfer of 1,4 to 1,5 acyclic stereoselection process was achieved (Scheme 8).162... 

A highly regio- and stereoselective synthesis of oxepine derivatives via cyclization of bromoallenes bound with hydroxy group by a four-carbon atom tether in the presence of a palladium(O) catalyst and alcohol is developed (Equation 16). In this reaction, the bromoallenes act as an allyl dication equivalent, and the intramolecular nucleophilic attack takes place exclusively at the central carbon atom of the allene moiety <2004JA8744>. 

A new perspective was opened up recently when Denmark demonstrated diat with chirally modified phosphoryl-activated allenes an asymmetric induction could be effected. From easily generated allenyl phosphoramidates containing an optically active amino alcohol, the diastereomeric adducts (37) and (39) could be obtained by addition of dlyl alcohol. When the separated adducts were employed in the carb-anionic Claisen rearrangement, a remarkable asymmetric induction (90 10) could be achieved with preferential formation of the diastereomers (38) or (40) respectively, whereas in a thermal reaction no stereoselection was observed (Scheme 63). Another example of an asymmetric induction in Claisen rearrangements is reported by Welch. ... 

Altenbach used the Michael addition of sodium methyl malonate to allene (206) for a dia-stereoselective spiroannulation to a steroid (equation 72). Or, in imaginative work by Okamura, allenyl sulfoxides were transformed into enantiomerically pure hydrocarbons by pericyclic reactions like electrocyclic ring closure (equation 73) or intramolecular cycloaddition (equation 74). Note that the starting materials (propargylic alcohols) are readily accessible as single enantiomers. 

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