Lindlar catalyst, and

Name the following alkynes, and predict the products of their reaction with (i) H2 in the presence of a Lindlar catalyst and (ii) HsO" " in the presence of HgSC ... 

The dianion derived from but-2-ynoic acid reacts with aldehydes to give 5-hydroxyalk-2-ynoates (539). Partial reduction over a Lindlar catalyst and acid-catalyzed cyclization of the resulting enoate gives the dihydropyran-2-one (78LA337). The route is exemplified by the synthesis of the naturally occurring massoia lactone (Scheme 200). In previous work (46JCS954) the hydroxyalkynoic acids themselves, obtained from epoxides and acetylene, were used. 

Starting with tropone 330, the azido compound 331 was first synthesized, as shown in Scheme 55. Then, compound 331 was chemoselectively reduced to unsaturated amine 332 by Lindlar catalyst, and this material was elaborated to the meso carbamate 333, ready for enzymatic asymmetrization. Treatment of 333 with Amano P-30 lipase in the presence of isopropenyl acetate resulted in formation of the enantiomerically pure (>98% ee) monoacetate 334, the common intermediate to both calystegines 337 and ent-337. Using conventional chemistry, elaboration of the functional groups within tropane 334... 

Bombykol was for the first time prepared by Butenandt using the Wittig reaction of 2-hexynylidene-triphenylphosphorane with 9-formylnonanoic ester. The resulting (Z)- resp. ( )-10-hexadecen-12-ynoic acid esters were resolved by urea, the ( )-isomer hydrogenized with Lindlar catalyst and reduced with lithium aluminium hydride. Further purification via the urea inclusion compound and low-temperature crystallization gave (10F,12Z)-10,12-hexadecadien- l-ol 186,187). 

The ester, ethyl p-apo-8 -carotenate (17 X = OEt) corresponding to p-apo-8 -carotenal is an important colourant which has been referred to earlier. Technically it has been derived from the Cjt compound, dehydro-p-CjT-carotenal, an intermediate in the synthesis of p-apo-8 -carotenal (Scheme 14b) by Wittig reaction with the phosphoran formed from the triphenylphosphonium salt of ethyl o-bromopropionate with sodium ethoxide (ref. 5,p459). The resultant dehydro compound was partially reduced with Lindlar catalyst and the product then themally isomerised to ethyl p-apo-8 -carotenate as shown in Scheme 17. 

When by chance a catalyst consisting of Pd metal was contaminated with Pb (i.e., a Lindlar catalyst) and it was tested in this reaction, it was discovered that decarbonylation was effectively suppressed. Taking advantage of this fortuitous discovery, binary catalysts consisting of a combination of Pd metal with various elements were widely examined, and it was discovered that a catalyst system consisting of a combination of Pd with various sixth period elements of the periodic table - Hg, Tl, Pb, and Bi - was superior. Figure 13.1 shows an example of the reaction performance when metallic Pd is used as a catalyst in combination with Pb(II) acetate as a Pb species. 

The synthesis of bixin (533) requires the regiospecific introduction of a Z double bond. Attention was first focused on the total synthesis of (all- )-methylbixin (534) [35]. (3-Methylepichlorohydrin (180) was treated with sodium acetylide (68) to give the alcohol 181 which was transformed with dihydropyran and phosphorus oxychloride to the acetylenic compound 182. Condensation of two moles of 182 with oct-4-ene-2,7-dione (5) in the presence of PhLi resulted in the C2o-diol 183. Treatment with p-toluenesulphonic acid first in toluene gave the diether 184 and afterwards in ethanol led to the diol 185, which was oxidized with Mn02 to the dial 186. The Knoevenagel condensation with malonic acid (187) and methylation with diazomethane gave the diester 188, which was hydrogenated in the presence of Lindlar catalyst and isomerized with iodine to (all- )-methylbixin (534) in an overall yield of 0.04% referred to 180 (Scheme 41). 

The side chain imit was prepared as shown in Scheme 45. Carbamate 213 was carboxylated, reduced by the lindlar catalyst, and condensed with serine methyl ester to give )8-hydroxyamide amide 214, which was readily converted to oxazole 215 via the one-pot procedure with DAST-DBU-BrCCls [95]. The... 

Hydrogenation of alkynes with this new catalyst gives the Z-alkene as the major product, with only trace amounts of the )-alkene. This selective catalyst is called the Lindlar catalyst and hydrogenation of alkynes using it is called Lindlar hydrogenation or Lindlar reduction, named for its discoverer, H. Lindlar (Switzerland). This catalyst has been supplanted by another version that is easier to prepare and less prone to the oxidation of the lead carbonate and lead oxide constituents. It is now the standard and is formally called the Rosenmund catalyst, but it is commonly referred to as a Lindlar catalyst. The new catalyst is composed of palladium-on-barium sulfate (BaS04) and it... 

Alkenes can also be formed using the Wittig reaction (Section 8.3.4.3) and by reducing alkynes. Z-AIkenes are prepared on hydrogenation of alkynes in the presence of a Lindlar catalyst, and E-alkenes are formed from reaction of alkynes with Na/NHg (Section 6.3.2.4). 

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