Propargyl-type acetylenic alcohols

Acetylenic alcohols, usually of propargylic type, are frequently intermediates in the synthesis, and selective reduction of the triple bond to a double bond is desirable. This can be accomplished by carefully controlled catalytic hydrogenation over deactivated palladium [56, 364, 365, 366, 368, 370], by reduction with lithium aluminum hydride [555, 384], zinc [384] and chromous sulfate [795], Such partial reductions were carried out frequently in alcohols in which the triple bonds were conjugated with one or more double bonds [56, 368, 384] and even aromatic rings [795]. 

Isomerization of chiral propargyl alcohols.1 The isomerization of chiral alcohols of the type RCHOHC,=C(CH2)nCH3 to terminal acetylenic alcohols, RCHOH(CH2)n + 1C=CH, in the presence of KAPA occurs with no significant loss of enantiomeric purity. Evidently, formation of the alkoxide suppresses racemization. Retention of configuration is observed even when the triple bond moves through several methylene groups. 

Promoters are not necessary for the polymerization reaction of VDF. Their use could increase the reaction rate and decrease the reaction time. Suitable promoters include reducing agents such as oxidizable sulfoxy compounds, that is, sulfur compounds which contain a sulfur-oxygen bond. Examples of these compounds include sodium bisulfite, sodium sulfite, sodium hydrosulfite, sodium thiosulfite, and ammonium bisulfite. The rate of addition of promoters ranges Ifom 0.001 % to 5% by weight, based on the amount of monomer. Other types of promoters include acetylenic alcohols, propargyl alcohol, nickel carbonyl, and iron carbonyl. 

Several helical polycyclic aromatic hydrocarbons bearing aryl substituents at the most sterically hindered position were synthesized in an efficient three-step cascade reaction. The initial benzannulated enediynes were synthesized by the reaction of appropriate lithium acetylenides with an aryl-rert-butyl ketone. This was followed by reduction of the resultant acetylenic propargyl type alcohol with triethylsilicon hydride. This method turned out to be particularly successful for the synthesis of helical molecules. The reaction of ketones 3.588 and 3.590 with the lithium derivative of l-ethynyl-2-(2-penylethynyl)benzene 3.545 or related binaphthyl derivative followed by reduction and three-step sequence of cascade reactions led to polycyclic aromatic compounds 3.589 and 3.591, respectively, in a good yield (Scheme 3.48) [294, 295]. 

CYCLIZATION OF SOME 1,3-BUTADIENES PRODUCED FROM ACETYLENIC ALCOHOLS OF THE PROPARGYL TYPE... 

We also consider new achievements in some classical cycloaddition reactions such as the Diels-Alder condensation with acetylenic dienophiles, [2-1-2] cycloadditions with acetylene component leading to creation of cyclobutene ring, and new results in cyclobutene syntheses by [2-1-2] acyclization of phosphorus containing 1,3-butadiene derivatives synthesized starting with propargyl-type alcohols. 

The reactions involving the generation of acetylenic bonds or extension of unsaturated bonds are extremely useful in chemical synthesis of unsaturated alcohols, carboxylic acids, and poly-yne acetals [Eq. (47) 125]. For the preparation of essential fatty acids, extension of propargyl units, followed by hydrogenation in the presence of Lindlar catalyst, will result in the efficient formation of ail c/x-type fatty acids [Eqs. (48) and (49) 126,127]. 

Brinkmeyer and Kapoor have now found that acetylenic ketones, RC= CCOR, are reduced by LiAlH4 and 1 at — 78° with the highest enantiomeric selectivity observed to date. Thus CH3Ce CCOCH2CH(CH3)2 is reduced to the corresponding (R)-alcohol with an enantiomeric excess of 827 . Similar asymmetric reductions were observed with seven other ketones of this type. Propargylic ketones are readily available by reaction of lithium acetylides with aldehydes followed by Jones oxidation of the propargylic alcohols. 

In the synthesis of propargylic alcohols, we saw the reaction of an alkynyl nucleophile (either the anion RC=CNa or the Grignard RC CMgBr, both prepared from the alkyne RC CH) with a carbonyl electrophile to give an alcohol product. Such acetylide-type nucleophiles will undergo Sn2 reactions with alkyl halides to give more substituted alkyne products. With this two-step sequence (deprotonation followed by alkylation), acetylene can be converted to a terminal alkyne, and a terminal alkyne can be converted to an internal alkyne. Because acetylide anions are strong bases, the alkyl halide used must be methyl or 1° otherwise, the E2 elimination is favored over the Sn2 substitution mechanism. 

While acetylenes add directly to aldehydes and ketones to give rise to the propargylic systems 39, which lend themselves for hydrogenation, the vinyl anions of type 38 lead directly to the corresponding aUylic alcohols 40. 

---