Abstract
Substituted 4,4,5,5-tetraethoxy-2-pentyn-1-ols undergo stereospecific reduction to allylic and homoallylic alcohols under the right conditions. Hydrogenation over Lindlar's catalyst gave the corresponding (Z)-allylic alcohols in excellent yield provided potassium carbonate was added. Reduction was also achieved with lithium aluminum hydride, but the product appeared to be solvent and temperature dependent. In THF at -15 o C the corresponding (E)-allylic alcohols were formed, in better than 70% yield from secondary propargylic alcohols, but below 60% from tertiary ones; in refluxing diethyl ether the products were the corresponding 1-substituted derivatives of homoallylic alcohol (E)-4,5,5-triethoxypent-3-en-1-ol, obtained in 93% yield in the best case.
Highlights
In spite of the compound’s polar and bulky 1,1,2,2-tetraethoxyethyl group (TEE) the corresponding propargylic alcohols (2) are obtained when TEB acetylide is generated with base in the usual way and subsequently treated with an aldehyde or a ketone
Quite conceivable that conversion of 2 to the corresponding (Z)- and (E)-allylic alcohols, by standard hydrogenation over Lindlar’s catalyst and by treatment with lithium aluminum hydride, respectively, can be hampered by various effects caused by TEE
If there are voluminous substituents close to the triple bond, the reaction rate may drop considerably and in some cases the conversion even fails completely. This is for instance the case with di-t-butylacetylene, which does not give the corresponding alkene at all when exposed to hydrogen under standard Lindlar conditions.[10] (If the catalyst is changed to Raney nickel, hydrogenation takes place, but the corresponding alkene, 2,2,5,5tetramethylhex-3-ene, obtained in 49% yield, has the E and not the Z configuration.10) if other functional groups are present next to the carbon-carbon multiple bond, most notably hydroxyl groups, halogen atoms, and carbon-sulfur single bonds, hydrogenolysis may occur to such an extent that the method becomes unsuitable.[11,12]. On this basis it was quite reasonable to expect that propargylic alcohols 2, with the large TEE group on one side of the triple bond and a 1-hydroxyalkyl group on the other would react sluggishly, and initially that turned out to be the case
Summary
We reported a high-yield synthesis of 3,3,4,4-tetraethoxybut-1-yne (1), denoted TEB.[1,2,3] In spite of the compound’s polar and bulky 1,1,2,2-tetraethoxyethyl group (TEE) the corresponding propargylic alcohols (2) are obtained when TEB acetylide is generated with base in the usual way and subsequently treated with an aldehyde or a ketone. Alcohols 2 are somewhat special due to the TEE moiety which is sterically demanding, has a large number of oxygen atoms that can engage in complex formation, and contains ethoxy groups that can act as a leaving group under the right reaction conditions.
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