Crotonic aldehyde, preparation

Cyclic g-haloacetals and -ketals have been prepared by variations on two basic methods. The most frequently used method involves the combination of an a,B-unsaturated carbonyl compound (acrolein, methyl vinyl ketone, croton-aldehyde, etc.) a diol, and the anhydrous hydrogen halide. All possible sequences of combining these three have been used. In most cases the... 

The methoxy-substituted acetal C is isolated and is subjected to another two reactions (1) the acetal function is hydrolyzed under acidic conditions to provide the methoxylated aldehyde E. (2) With a catalytic amount of sodium methoxide, the methoxylated aldehyde E reacts via an Elcb elimination, which leads to the formation of the poly-unsaturated aldehyde D. The substrates A and B of the three-step reaction sequence in Figure 12.23 derive from crotonic aldehyde (E13C-CE[=CE[-CH=0) the respective methods of preparation are outlined in the figure. The final product, which is the highly unsaturated aldehyde D, displays the structure of an aldol condensation product, namely of the crotonic aldehyde mentioned above. 

Now the aldehyde which yields crotonic acid on oxidation, i.e., crotonic aldehyde, may be prepared by a synthesis which shows clearly that it must have the constitution of the first of these isomeric aldehydes, A 2-buten-al, CH3—CH = CH—CHO. 

Ethyl-3-methylpyridine (also known as aldehyde-collidine ) has been prepared by heating aldehyde-ammonia aldehyde-ammonia and acetaldehyde or paraldehyde aldol-ammonia and ammonia paraldehyde and ammonia <> 11,12 acetamide,1 or acetamide and phosphorus pentoxide ethylene glycol and ammonium chloride ethylidene chloride or bromide and ammonia ethylidene chloride and acetamide, ethylamine, or n-amylamine crotonic acid and a calcium chloride-ammonia complex 1 and by passage of acetylene or acetaldehyde and ammonia over alumina and other catalysts. 

Examples given in Expt 5.216 include the preparation of non-2-enoic acid starting from hexanal, but-2-enoic acid (crotonic acid) from acetaldehyde, and also hexa-2,4-dienoic acid (sorbic acid) starting from the conjugated aldehyde crotonaldehyde. 

The first enantioselective organocatalytic 1,3-dipolar cycloaddition of acyclic nitrones with acrolein and crotonal-dehyde has been reported <2000JA9874>. In particular, the reversible formation of iminium ions from a,/3-unsatu-rated aldehydes and the enantiopure imidazolidinone 535 provided ( A-4-formylisoxazolidines in high yields and ees (Equation 86). A polymer-supported version of catalyst 535 was also prepared <2004EJ0567>. The catalytic performance of various chiral pyrrolidinium salts in the cycloaddition of 1-cycloalkene-l-carboxaldehydes was also evaluated <2003EJO2782>. 

Similarly, the magnesium enolate of t-butyl (/ )-(p-tolylsulfmyl)acetate adds to aldehydes and ketones. The subsequent reductive desulfurization yields the corresponding optically active -hydroxy esters (Scheme 27). This process can be utiliz for preparation of a synthetic intermediate (95) of maytan-sine with a selectivity of 93 7 (equation 23). Michael- q)e addition of r-butyl (/7)-(p-tolylsulfinyl)ace-tate to crotonic ester with NaH in DMF has also been reported, but the enantiomeric excess of the desulfurized product is very low (12%). ... 

As shown in Scheme 3, (Z)-enolate (57), prepared by conjugate addition of lithium bis(phenyldi-methylsilyl)cuprate to methyl crotonate or methyl cinnamate, reacts with acetaldehyde or benzaldehyde to give a mixture of two diastereomeric aldols, (58) and (59), with excellent diastereomeric excess favoring (58) (ratios of 85 15 to 94 6). On the other hand, deprotonation of ester (60) by LDA provides the ( )-enolate (61), which reacts with the same two aldehydes to give the aldol (59) as the major product... 

Aldehyde is a colorless, mobile liquid has a strong, suffocating odoE sp. gr. 0.790 at 18° (64°.4 P.) boils at 31° (C9°.8 P.) soluble in all proportions in water, alcohol and ether. If perfectly pure, it may be kept unchanged but if an excess of acid have been used in its preparation, it gradually decomposes. AVhen lieated to 100° (313° P.), it is decomposed into water and crotonic alde- hyde. ... 

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