Preparation alkynes

Addition of hydrogen halides to alkynes preparation of alkyl dihalides and tetrahalides... 

Addition of water to alkynes preparation of aldehydes and ketones... 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Oxidation of alkynes preparation of diketones and carboxylic acids... 

Corey s synthesis of leukotrienes, human metabolites that control many important natural defence reactions like inflammation, involves the lithium derivative of an alkyne prepared by deprotonation with the very strong base butyllithium. The tosyl derivative of a primary alcohol reacts with this lithium derivative and a perfectly normal S>j-2 reaction follows. The alkyne provides the carban-ion (Chapter 8) for the displacement of the tosylate. 

Preparation of Alkynes by Alkylation of Alkynyl Anions The other major alkyne preparation is based on the easy accessibility of nucleophilic carbanions from terminal alkynes (Section 13-2). So, a wide variety of internal alkynes may be made from any terminal alkyne, via the sehenie... 

How are alkynes prepared Alkynes react as Br0nsted-Lowry bases in acid-base reactions with acids such as HBr or with aqueous acid. Alkynes react as Lewis bases with dihalogens such as Brg and in hydroboration reactions (see Chapter 10). Alkynes often come from reactions involving other alkynes, using the fact that a terminal alkyne is a weak acid, as introduced in Chapter 6 (Section 6.2.6). Treatment of a terminal alkyne with a base such as sodium... 

Brown et al [8] have devised a general, convenient, and simple synthesis of straight-chain alcohols from internal alkynes. Long-chain internal alkynes, prepared by Eiters procedure [9] by metalating 1-alkynes, followed by treatment with alkyl halides, are isomerized to 1-alkynes on treatment with potassium-3-aminopropylamide (KAPA) [10] in 1,3-diaminopropane (APA). KAPA is prepared by the quantitative reaction of potassium hydride with excess of (APA) [10]. This difunctional superbase produces exceptionally rapid migration of internal C=C to the terminal C=C position. The terminal alkynes thus obtained are subjected to dihydroboration with 2 equiv of 9-BBN. The dibora intermediate on alkaline hydrogen peroxide oxidation provides 61-80% yield (Table 6.4) [8] of the corresponding alcohols (Eq. 6.2). 

In addition to the alkynes found in nature, many synthetic alkynes (prepared in the laboratory) are of particular interest. One such example is ethynylestradiol, found in many birth control formulations. This synthetic oral contraceptive elevates hormone levels in women and prevents ovulation. The presence of the triple bond renders this compound a more potent contraceptive than its natural analogue, which lacks a triple bond. The effect of the triple bond is attributed to the additional structural rigidity that it imparts to the compound (as described in the box on molecular rigidity). 

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