Dienyl alcohols, preparation

Trost used the dihydroxylation methodology extensively in the total synthesis of (+)-parvi-florin (68) [96]. Both bistetrahydrofuran elements were prepared from 8, tn-dienyl alcohol 71. To avoid overoxidation the reaction was stopped before complete conversion, giving triol 72 with 94% ee in 71 % yield. Protection as the acetonide afforded 73 and subsequent hydrogenation furnished saturated alcohol 74. The construction of 75 was accomplished by Julia olefination using derivatives of 73 and 74. Diene 75 was thenregioselectively dihydroxylated at the internal... 

Brown and co-workers developed a novel homoallenyl boronate reagent 169 based on diisopropyl tartrate for the stereoselective homoallenylation of aldehydes 170. The reagent 169 was prepared via homologation of the corresponding allenyl boronate or the alkylation of halomethyl boronate with allenyl Grignard similar to those reported in Scheme 26. The allyl boronate 169 upon reaction with aldehydes furnished the dienyl alcohols 172 with high ee (Scheme 28) <1996JOC100>. 

A boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

A useful building block 519 for vitamin D synthesis was prepared by reaction of the alkenyloxirane 517 [195], Under usual conditions of the elimination, both the dienyl alcohol 519 and the unsaturated ketone 520 were formed in equal amounts. Radinov and co-workers discovered an interesting and remarkable effect of the addition of fluorinated alcohols, such as l,3-bis-(l,l,l,3,3,3-hexafluoro-2-hydroxypropyl)benzene or perfluoro-t-butyl alcohol as a proton source, on the chemoselectivity, and the desired 519 was obtained with nearly complete chemos-electivity with these additives [195], They suggest that rapid protonation of the intermediate alkoxide 518 with these alcohols seems to be important. 

DIENYL METHACRYLATES Prepared By Transesterification of the Corresponding Alcohol ... 

Both dienyl- and 7r-allylmetal complexes of Fe,492 Mo,493,494 and W374 have also been prepared and undergone similar reactions with oxygen nucleophiles. One pertinent example is the reaction of 7r-allylmolybdenum complexes with internal alcohols leading to THFs (Equation (128)) 493 additional examples of this chemistry have appeared in a... 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

An a-cyclopropylvinyl cation, l-cyclopropyl-3-methylbuta-l,2-dienyl cation 38, has been prepared by Siehl by ionizing the corresponding acetylenic alcohol with SbF5 (equation 32)68. 

A number of alkenylcyclopropanes have been prepared by elimination of bromide, either from a side chain or from the cyclopropane ring. The first known example is the synthesis, in 51 % yield, of (buta-l,3-dienyl)cyclopropane (2) by the reaction of(4-bromobut-l-enyl)cyclopropane (1) at 150°C with a solution of the sodium salt of 2-butylcyclohexanol in the alcohol. ... 

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,... 

Nucleophilic attack by bicarbonate onto a dienyl complex 10.118 with an ester substituent also proceeded regioselectively. Starting from the resolved complex, methyl shikimate 10.122 could be prepared in optically active form using this reaction (Scheme 10.30). Protection of the alcohol and decomplexation gave the free diene 10.121, which underwent selective dihydroxylation trans to the TBS ether. Desilylation gave the natural product ester 10.122. 

Preliminary studies demonstrated that acyclic dienyl esters were unsuitable substrates for the RCM that provide eight-membered lactones. It was envisioned that the strain in the TS could be reheved using a silicon tether, as the silicon atom is larger and has more polarizable soft d-orbitals that allow for more facile bond distortion. Hoye and Promo had previously demonstrated that a nine-membered silaketal is accessible in excellent yield and with 95 5 selectivity for the Z-isomer, thereby illustrating proof-of-concept (Table 8.2) [15]. The unsymmetrical bix-alkoxysilane 66 was prepared via the sequential silylation of the homoallyUc alcohols 65 and 64 with dichlorodimethylsilane (Scheme 8.13). 

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