Alkynes from alcohols

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Among the simplest syntheses of this type are those of tetrahydro-quinolines or -iso-quinolines based on Friedel-Crafts cyclizations. The use of side-chain halides is shown by the synthesis of 1,2,3,4-tetrahydroisoquinolines (158) (71CC799), and of 3,4-dihydroquinol-2-ones (159) (27CB858). Electrophilic carbon atoms can be developed from secondary or tertiary alcohols, or from alkenes or alkynes. In the synthesis of the tetrahydroisoquinoline... 

The high synthetic utility of alcohols 38 stems from the fact that terminal alkynes are among the most versatile functional groups for the further elaboration of a carbon skeleton. Asymmetric synthesis of alcohols 38 from aldehydes with the concurrent formation of the two stereogenic C atoms has been accomplished mainly by two methods. The first features synthesis of chiral nonracemic allenylmetal compounds from the corresponding chiral nonracemic propargyl alcohols and addition of the former to aldehydes [26] and the second method in-... 

The position of the triple bond can alter the reactivity of the alkynes. Acidic alkynes react with certain heavy metal ions, e.g. Ag+ and Cu+, to form precipitation. Addition of an alkyne to a solution of AgNOs in alcohol forms a precipitate, which is an indication of hydrogen attached to the triple bonded carbon. Thus, this reaction can be used to differentiate terminal alkynes from internal alkynes. 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

From this perspective, Teles used [AuMePPh3] as a catalyst and methanesulfonic acid as a co-catalyst when adding alcohols to alkynes, and achieved excellent results [14]. The scope of this reaction covered both internal and terminal alkynes. In the case of symmetrical alkynes, the only product formed in the presence of excess alcohol came from dialkoxylation. 

Fatty acids from fatty alcohols, up to C5 benzene from nonpolar organic compounds Aqueous solutions of formaldehyde alkynes from alkanes Polar materials such as water, alcohols,... 

Hydroxyisocoumarins are accessible from 3-hydroxybenzyl ketones using directed lithiation as the key step (95G 111) and isocoumarins have been prepared from 2-iodobenzoic acid and alkynes through a Pd-mediated annulation (95JOC3270, 3711). The cyclisation of tertiary alcohols derived from the reaction of o-cyanobenzyllithium with aldehydes and ketones yields 3,4-dihydroisocoumarins (95S1102). 

Alkynes from halides also alkynes from alkynes Esters from alcohols also esters from carboxylic acids Alkyls from alkenes (Conjugate Reduction)... 

For a long time" , it has been known that there is rearrangement of 1-alkynes to 2-alkynes under basic conditions (for instance by alcoholic potassium hydroxide or powdered potassium hydroxide) at 175 C. It has been shown that in fact an equilibrium mixture is obtained . It contains isomeric compounds, mainly the starting 1-alkyne and isomeric 1,2-alkadiene and 2-aIkyne, but the latter, more stable from thermodynamic considerations, is predominant. Therefore the method could be used to prepare 2-alkynes from corresponding 1-alkynes. The reverse reaction is possible disubstituted acetylenes can be converted to sodium derivatives of 1-alkynes by sodium or sodamide . 1-Alkynes are recovered by hydrolysis. We have tried to apply both reactions to prepare acetylenes deuterated in defined positions with high isotopic purity. 

An access to chiral allylic alcohols starts from hydrozirconation of 1-alkynes which is... 

Via tandem alcohol oxidation/alkyne hydration/aldol condensation sequence from benzylic alcohol+alkyne... 

Anti-Markovnikov addition of H2O to olefins is of enormous importance in view of the production of linear alcohols directly from alkenes. It is a general phenomenon, however, that reaction of water with olefins and alkynes, as in the previous example, gives products of Markovnikov addition. The first anti-Markovnikov hydration of terminal alkynes (Scheme 42) with transition metal catalysis was reported in 1998 (227). A series of aliphatic and aromatic alkynes... 

Fragments 2 and 3 are not real because each carbon (marked in red) has only three bonds. As noted before, there must be a protocol that converts these fragments into real molecules. Disconnection of the indicated bond in 4 can be correlated with two real molecules 5 and 6. A reaction in Chapter 18 prepared the indicated bond of an alkyne-alcohol (4) from fragments 5 and 6 (Section 18.3.2). Note the similarity of 5 to 2 and of 6 to 3. In terms of a disconnection, this simply means that the bond in 4 is disconnected because that bond can be formed in a synthesis by an acyl addition of the anion of 5 to 6. Acyl addition... 

Elimination of halogen from a,P-dihalides in a variety of circumstances has been observed and can be used to prepare alkenes and alkynes from vicinyl dihalides. The process is usually of limited utility since the dihalides themselves are often prepared from the unsaturated compounds. As shown in Scheme 7.42, both erythro (meso) and threo ( )-2,3-dibromobutane diastereomers undergo stereospecific elimination of bromine to their respective (E)- (or trans) and (Z)- (or cis) 2-butenes on treatment with iodide anion (T) in ethanol (ethyl alcohol, CH3CH2OH). 

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