And vinylogy

The reactions of vinylogous amides and vinylogous urethanes with excess ketene were found to give a-pyrones (421), which could also be obtained from further reactions of acylated enamines with ketene (383,421). 

Using chiral catalysts, not only various enantioselective Mukaiyama and vinylogous Mukaiyama aldol reactions have been developed but also asymmetric reactions of a,a-difluoro silyl enol ethers (1) with carbonyl compounds have been reported ... 

Mukaiyama-type aldol reactions and vinylogous aldol reactions... 

Fluoropregn-5-en-20-one, 437 2a-Fluorocholestanone, 475 2a-Fluorocortisone BMD, 485 2-Fluoroindanone, 477 Fluoro ketones and vinylogous fluoro ketones, 474... 

Because sulfonamides are generally more stable towards enzymatic degradation than amides, the preparation of oligosulfonamides as peptide mimetics has been thoroughly investigated. The synthesis of a-aminosulfonamides has not been successful, probably because of the instability of this class of compound [261], For the more stable P-amino sulfonamides and vinylogous a-aminosulfonamides (3-amino-l-pro-pene-l-sulfonamides), however, suitable synthetic routes have been developed. Unfortunately, the preparations of the required monomers are not as easy as those of related amino acid derived monomers (Figure 16.27). 

Complementary reactive centers are needed in the chain synthones in the a- and co-positions. In particular, derivatives of formic acid are used to prepare monomethine dyes for dyes with longer chromophores, the application of vinylogous aminals or CO-methylpolyenals are preferred. In the most famous reactions, the orthoformate was originally proposed (2), and vinylogous amidines that can be regarded as dianils of conjugated dialdehydes are widely used. A general reaction is shown in equation 7 (3), where X = OR, SR, halide, or NIT and Y = O, S, NR, or N + R2 ... 

Other examples of deuterium incorporation at the epimeric centre have also been reported. When treated with a deuterated acid (DCl/MeOD and TFA-d, respectively), both P-carboline derivative 18 [20] and vinylogous urethane 19 [21] resulted in deuterium incorporation at C-l and C-12b (both correspond to C-3), respectively, Fig. (2). Mechanisms similar to that of Mechanism 1 were suggested to explain the epimerization. It should be noted, however, that, as pointed out above, mere deuterium incorporation is not sufficient evidence for Mechanism 1. Were Mechanism 1 alone responsible for the epimerization, both deuterium incorporation and epimerization would have to happen at the same rate. 

Hydrogen bond-promoted asymmetric aldol reactions and related processes represent an emerging facet of asymmetric proton-catalyzed reactions, with the first examples appearing in 2005. Nonetheless, given their importance, these reactions have been the subject of investigation in several laboratories, and numerous advances have already been recorded. The substrate scope of such reactions already encompasses the use of enamines, silyl ketene acetals and vinylogous silyl ketene acetals as nucleophiles, and nitrosobenzene and aldehydes as electrophiles. 

Due to sharp and selective reduction in bond dissociation energy of radical cations [106,107], the cleavage of strong -C-C- or C heteroatom bonds has been observed during PET reactions. In this context, examples of -C-C- bond cleavage from jS-phenylethers [108], bibenzyls and pinacols [25,109,110] may be mentioned. In an extensive study, Whitten et al. [Ill, 112] have reported clean -C-C- bond cleavage from the reaction of /J-aminoalcohols and vinylog-ous amino alcohols. 

Figure 11) and vinylogous porphyrins [246,264], (e.g. 25 and 27 Figure 12), which appear attractive because they too appear to overcome many of the deficiencies seen with Photofrin. In particular, many of these agents are monomeric species that absorb in the far-red spectral region and which show favorable (i.e, low) toxicity characteristics in vivo. 

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