Aldehydes fragmentation

In the discussion of some mass spectra of nitrones (41), intermediate isomerization to oxaziridines was concluded from the occurrence of aldehyde fragments. 

Attack of the OH radical on carbohydrates of low molecular mass gives rise to a variety of products. Indeed, the reaction of radiolytically-generated OH radical with lower hexose sugars produces lower saccharides (for di- and higher saccharide species), uronic and aldonic acids, and 3-, 2- and 1-carbon aldehydic fragments, e.g. 

The formation of substitution products of aldehyde fragments was observed during the interaction of substituted 2,5-diphenyl-1,3,2,5-dioxabor-... 

The C—O fragments separating phosphorus and boron atoms in dioxa-borataphosphoniarinanes are substituted when treated by aldehydes or nitriles. In this case the displacement of aldehyde fragments occurs through the initial dissociation of betaines into compounds with Uncoordinated phosphorus and boron atoms [Eq. (109)] (86IZV2502, 86IZV2510). 

In this ESI-FTMS screening proof-of-concept experiment, a DCL was generated using the hydrazone exchange reaction from two hydrazide fragments (1 and 2) and five aldehyde fragments (A-E) (Scheme 7.2). 

Fach derivatized aldehyde fragments to give a major peak at m/z 181. 

The azomethine ylid 53, generated thermally from the aziridine 52, underwent cycloaddition in near quantitative yield. With DMAD, only one product is possible with EP the sole product was 54, and EPP gave 54 and 55 in 80 20 ratio. This latter result indicates that steric effects are important with the phenylpropiolic ester.52 Nitrones (57) add to benzylideneacenaphthenone (56) forming the spiroisoxazolidines (58), which lose aldehyde fragments on heating or photolysis to give the... 

It should be emphasized that this method of synthesis of 5,6-dihydro-47/-l,3-oxazines 40 gives heterocycles which are isomeric to that obtained by participation of N-acyliminium ions, i.e. the products 40 have inverted location of the heteroatoms relative to the three-carbon fragment of the cycle. Indeed, if the Af-acyliminium ions participate in the formation of the heterocycle, the carbon atom (Cj) of starting aldehyde occupies position 4 of the heterocycle 45 (Figure 1) while Af-terf-butylnitrilium salt 41 gives the heterocycles 46 with aldehyde fragment Cj at position 6, i.e. near the oxygen atom. 

Reacts with alkenes in the presence of periodic acid to cleave the carbon-carbon double bond, yielding ketone or aldehyde fragments (Section 7.9). 

Lastly, in the reaction of LiH with 2-chloropropanal, five TSs were located, 30a-e (Figure 6.16). 30a and 30b lead to the major product, while the other three lead to the minor product. The two lowest energy structures correspond with the Felkin-Anh major (30a) and minor (30c) TSs. Their energy difference corresponds with the energy difference for their aldehyde fragments when LiH is removed. 30b and 30d display the chelation effect, but unlike with 29, chelation is not enough to make up for the favorable orbital interactions in the Felkin-Anh approach. 

Photolysis of 2-benzoyl-1,2-dihydroisoquinaldonitrile (8) with 2537 A light in acetonitrile or benzene solution, using a high-pressure mercury source, the yield of nitrile was lower. The photolyses also produced a low yield of the aldehydic fragment corresponding to the aroyl group. 

Another primary process which is not so pronounced in oxygen is the symmetrical scission giving two three-carbon aldehydic fragments (G 0.5). The degradation under vacuum may, therefore, be represented as follows. 

The bone collagen cross-link (+)-deoxypyrrololine has potential clinical utility in the diagnosis of osteoporosis and other metabolic bone diseases. Intrigued by its novel structure and its promise to allow the early discovery of various bone diseases, the research team of M. Adamczyk developed a convergent total synthesis for this 1,3,4-trisubstituted pyrrole amino acid. The key step of the synthesis was the union of the nitroalkane and aldehyde fragments to obtain a diastereomeric mixture of the expected -nitro alcohol in good yield. This new functionality served as a handle to install the pyrrole ring. 

The synthetic plan to achieve the total synthesis of avermectin involves the preparation of the aldehyde fragment (A) from D-glucal tripivalate (114), and the ketone fragment (B) from D-ribose aldehyde(125). Coupling of (A) and (B) gives the macrolactone (C). The resulting lactone is reacted with disaccharide (D) to form avermectin A [119]. 

Indeed, the forwards reaction uses a boron triflate and a bulky base of the type we have seen in order to make the cis boron enolate and achieve exactly this control. There are, of course, two. wn-aldol products possible here, 58 and 60, by virtue of the chiral centres present in the aldehyde fragment, and both do indeed form (in a 16 84 ratio). Trying to achieve selective formation of one of these syn diastereomers rather than the other syn diastereomer is beyond the scope of this chapter, even though that too is relative stereocontrol. It is complicated because it involves enantio-merically pure reagents in combination with the enantiomerically pure aldehyde and a match/mis-match issue. These issues are explored more fully in Chapter 30. Examples include combinations of chiral or achiral aldehydes with both achiral and chiral boron reagents. 

The aldehyde fragment adopts a conformation in which gauche interactions as well as eclipsing interactions with the enolate C2-Me and Cj-Ctt bonds are minimized. In model 50, which does not correspond to the Felkin-Anh model [119,120], the Re face of the aldehyde is attacked, leading to 47. The cyclohexyl substituent of the enolate lies in front and the aldehyde is introduced in the back of the enolate fragment. The new bond is formed anti to the builder substituent R. Model 51 accounts for the major formation of 48. In this case, the Si face of the aldehyde being attacked is because of the location of the cyclohexyl substituent behind the plane of the enolate. However, the eclipsing interaction of the C -Me and Ca-Me bonds cannot be avoided, so a decrease in selectivity is observed. These examples show that the delicate balance between the various repulsive interactions can sometimes be difficult to estimate. 

Formation of the protective film from ozonized rubber and AOZ creates a barrier against penetration of 03 into rubber [3,4]. Reactions of AOZ with ozonides 3, zwitterions 4 and aldehydic fragments 8 are envisaged. The respective reactivities are lower than the direct ozonation and the relevant contribution to the antiozonant mechanism is therefore inferior to the ozone scavenging. The reactivity of the ozonized rubber with AOZ accounts for rubber chain repairing, classified also as chain relinking or selfhealing mechanisms [4,21,247-249]. This contributes to formation of relaxed surface films. 

Aldehydic fragments 8 formed in ozonized rubbers or by PD/acid assisted ringopening of ozonides react with 11 under formation of bound-in species 161 or various relinked systems, e.g. 162 able to form crosslinks [3,4] (Scheme 30). 

In addition to their usefulness for the asymmetric addition of achiral aldehydes, it will be seen in the section 5.2.3 that the Paterson strategy is particularly useful for the aldol addition of chiral fragments such as the large, polyfunctional ketone and aldehyde fragments needed for convergent macrolide synthesis. 

VI. Synthesis of a C1-C9 Aldehyde Fragment and Failure of Dithiane Coupling 187... 

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