Allyl alcohols, alkynyl

Scheme 82 Cycloisomerization of alkynyl allyl alcohols to cyclic enals [148]...

Isomerization of p-Alkynyl Allylic Alcohols to Furans Catalyzed by Silver Nitrate on Silica Gel 2-Pentyl-3-Methyl-5-heptylfuran. 

It has been shown by Marshall and Sehon that in the presence of AgNOs absorbed on silica gel, /3-alkynyl allylic alcohols, which are more accessible than allenyl ketones, also undergo cycloisomerization to furans (Equation 19) <1995JOC5966>. Similarly, 3-trifluoromethylfurans can be prepared utilizing a Pd(ii) catalyst <2000JOC2003>. 

Marshall, J.A., and DuBay. W.J.. Synthesis of the pseudopterane and furanocembrane ring systems by intraannular cyclization of P and y-alkynyl allylic alcohols, J. Org. Chem., 59, 1703, 1994. Kodama, M., Shiobara, Y., Sumitomo. H.. Fukuzumi, K., Minami, H., and Miyamoto, Y, Synthesis of macrocyclic terpenoids by intramolecular cyclization. Part 10. Total synthesis of methyl ceriferate-1, Tetrahedron Lett., 27, 2157. 1986. 

ISOMERIZATION OF p-ALKYNYL ALLYLIC ALCOHOLS TO FURANS CATALYZED BY SILVER NITRATE ON SILICA GEL, 76, 263... 

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... 

Ortho ester rearrangement of allylic alkynyl alcohols. The allylic alkynyl alcohols (1) and (4) undergo stereoselective ortho ester Claisen rearrangement when refluxed with triethyl orthoacetate (propionic acid catalysis). In both cases... 

Furans. Isomerization of allenones, alkynyl allylic alcohols, and allenylcarbinols to give furan derivatives is catalyzed by AgNOj/silica gel (10 examples, 73-99%). 

Using this remarkably effective catalyst alkynyl allylic alcohol (e.g. 265) are converted into furans 266 very efficiently. Similarly, allenic alcohols (e.g. 267) can be converted into 2,5-dihydrofurans (e.g. 268) by this relative of a 5-e rfo-dig process <93JOC7180 94JOC324>. At a higher oxidation level, conjugated allenic ketones 269 can be similarly cyclized, effectively by carbonyl hydration and a final dehydration to the furans 270 <92JOC3387 94JOC7169>. 

In 2002, a novel aminocyclopentadienyl ruthenium chloride complex was introduced by Park s group involving a new mode of catalytic racemisation which allowed use of the more reactive isopropenyl acetate as an acyl donor and much less lipase. This catalytic system was particularly efficient for the DKR of various aliphatic or aromatic alcohols as shown in Scheme 4.9. Not only simple alcohols, but also functionalised alcohols such as allylic alcohols, alkynyl alcohols, diols, hydroxyl esters and chlorohydrins were successfully transformed into the corresponding chiral acetates. ... 

The Ag-catalyzed cycloisomerization of isomeric-to-118, -alkynyl allylic alcohols 120, was first investigated by Marshall. While various Ag salts provided high yields of trisubstituted furans 121, performing cycloisomerization reaction with the catalytic... 

Scheme 12.2 Pd(0)-catalyzed Alder-ene cydoisomerization of alkynyl allyl alcohols to give cyclic y,5-enals.

The Rh-BINAP-catalyzed intramolecular Alder-ene cycloisomerization is very rapid and, therefore, sequentially Rh-catalyzed sequences for the efficient enantioselective generation of five-membered carbo- and heterocycles were envisioned. Korber et al. [21] reported the enantioselective rhodium-catalyzed cycloisomerization of alkyl and (hetero)aryl alkynyl allyl alcohols for the generation of aldehyde-bearing chiral 4-alkyl 3-alkylidene THFs and tetrahydro-furanones, which were converted into a,P-unsaturated carbonyl side chains in a one-pot manner via a concluding Wittig olefination in good yields. 

Based on the initial Alder-ene cycloisomerization of alkyl and (hetero)aryl-substituted alkynyl allyl alcohols 13, the rhodium(I)-BINAP complex subsequently can be employed in the reduction of the primary products with hydrogen, furnishing 2,7-dioxabicyclo[3.2.1]octanes 14 in the sense of a sequentially Rh-catalyzed one-pot process (Scheme 12.7) [22]. 

Using less polar, hexane-based solvent mixture, more readily available alkynyl allylic alcohols can also be isomerized to the corresponding furans upon exposure to 10% AgN03 on silica. Extension of this approach to alkynyl bromopropenoic acids allows formation of a-bromo lactones suited for further cross-coupling reactions. Cyclization of a-alkynyl carbonyl substrates proved to be an efficient method for the preparation of substituted fura-nopyrimidine nucleosides (eq 13). ... 

In 2002, we reported that monomeric Ru catalyst 5 had a good racemization activity at room temperature and excellent compatibility with isopropenyl acetate [23]. We thus accomplished the first DKR of secondary alcohols, at room temperature by combining 5 with Novozym 435 or lipase PS-C in the presence of isopropenyl acetate (Scheme 5.15). A wide range of secondary alcohols including simple alcohols, allylic alcohols, alkynyl alcohols, diols, hydroxyl esters, and chlorohydrins were transformed to their acetates with good delds and excellent enantiomeric excesses in the DKR using 5 (Chart 5.12) [24]. 

Despite the great success of the transmetalation process in the enantiose-lective arylation of ketones, its extension to allylation or alkynylation reactions failed, providing the corresponding tertiary alcohols with enantiomeric excesses never higher than 50% ee. On the other hand, more success has been found in the alkenylation of ketones. The process started with the hydrozirconation of terminal alkynes to give the corresponding alkenylzirconium intermediates, which were transmetalated by reaction, in this case, with various ketones in the presence of the HOCSAC ligand. This protocol tolerated the presence of other carbon-carbon multiple bonds on the alkyne, as well as different functionalities and achieved excellent results for alkyl ketones, a,(3-unsaturated ketones and even dialkylketones, as shown in Scheme 4.22. 

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