Base-sensitive aldehyde

These reagents exhibit a high selectivity for the attack of aldehydic over ketonic oxo groups26. The triethylammonium salt of 2-(bromomethyl)acrylic acid and base-sensitive aldehydes, under these conditions, can be used for the synthesis of lactones27. 

Applicable to base-sensitive aldehydes and phosphonates for the Horner-Wadsworth-Emmons reaction... 

This reaction is useful for base-sensitive aldehydes no epimerization. 

As shown above (see also Scheme 6.87B), formation of sulfur ylides by reaction of a carbenoid with a sulfide is an efficient alternative which has also been found to be applicable to enolizable and base-sensitive aldehydes. This route, developed by the Aggarwal group, is based on use of a metal catalyst to form a carbene which subsequently reacts with the sulfide generating the sulfur ylide [200, 212, 213, 226]. Because the catalytically active species of the asymmetric process is the sulfide, this concept can be also regarded as an organocatalytic reaction. 

Asymmetric sulfur-ylide-type epoxidation is an excellent tool for enantioselective and diastereoselective synthesis of epoxides. By use of Aggarwal-type methodology a broad range of aromatic, enolizable, and base-sensitive aldehydes can be converted into the desired epoxides. In addition to an excellent diastereomeric ratio, the optimized organocatalytic systems of this sulfur-ylide-type epoxidation also... 

The 1 -phenyl-1 J/-tetrazol-5-yl sulfones do not have a tendency to self-condense, so they can first be deprotonated with base and then reacted with the aldehyde. This makes possible a far milder reaction process, including reactions with base-sensitive aldehydes. 

The cooperative catalysis of CpRu(PPh3)2(CH3CN)PF6 (18) and DBU has permitted chemoselective nucleophilic activation of acetonitrile in the presence of base-sensitive aldehydes to afford corresponding /1-hydroxynitriles (19) in good yield (Scheme... 

Relatively strong bases are used for the deprotonation of phosphonate reagents, and the phosphonate-stabilized carbanions formed are more basic than the corresponding phosphorane reagents. Such conditions may be incompatible with base-sensitive aldehydes and ketones, causing epimerization of chiral compounds or... 

K2CO3. No side reactions like the Cannizzaro or Knoevenagel reactions are observed on treatment with C-200. In the reaction with benzaldehyde and furfural the products 71 (R = Ph and 2-furyl) are formed nearly quantitatively. Bulky aromatic aldehydes such as pyrene-l-carboxaldehyde and aliphatic aldehydes are also suitable substrates for this process. ( )-acyclic a,/ -unsaturated ketones 74 can be prepared by use of a similar HWE reaction of 2-oxoalkanephosphonates 72 with aliphatic aldehydes 73 under the influence of barium hydroxide C-200 (Scheme 5.14) [36]. The HWE procedure employing activated Ba(OH)2 is applicable to the structurally complex, base-sensitive aldehydes which are susceptible to elimination and/or epimerization under the traditional basic conditions of the HWE reaction using NaH. For instance, when phosphonate 75 and aldehyde 76 are exposed to 0.8 equiv. activated Ba(OH)2 in aqueous THF at 20°C, (//-enone 77 is obtained in 70% yield (Scheme 5.14) [37]. 

A simple protocol for the highly ( )-selective (E/Z =180 1 in some cases) Horner-Wittig reactions of alkyl diethylphosphonoacetates and a range of straight-chain and branched aliphatic, substituted aromatic, and base-sensitive aldehydes in the presence of MeMgBr, as a base generating the corresponding phosphonate carbanions, has been reported (Scheme 67) ... 

As the complexity of substrates being submitted to the Wittig reaction dramatically increased, gentler reactions conditions were required to accommodate greater inter-functional group compatibility. Thus the nature of the base necessary to form the ylide was studied. Roush and coworkers reported the successful use of lithium chloride with amine bases (DBU or DIPEA) for base sensitive aldehydes or phosphonates. One example cited involved the coupling of phosphonate 62 with aldehyde 63 to afford triene 64 in good yield. This stands in sharp contrast to when NaH was used and no desired product was observed. 

Vacuum pyrolysis of sodium (or lithium) salts of l-acyl-2-p-tosylhydrazines realizes a simple method for the preparation of aldehydes and represents a modification of the McFadyen-Stevens procedure that is applicable to base-sensitive aldehydes [equation (10)]/°... 

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