Enols fragments

Figure 11.14 Predicted labeling of the enol and aziridine fragments, via oxaloacetate (99) and threonine (100) for the enol fragment, and via a-ketoglutarate (101) for the aziridine fragment. a) First pass through Krebs cycle, b) Second pass through Krebs cycle, c) Third and subsequent passes through Krebs cycle.

Entry 8 was used to create the central nine-membered ring system found in the diterpene jatrophatrione. Entry 9 is an example of a boronate fragmentation (see p. 899). Entry 10 illustrates enolate fragmentation. The reaction presumably proceeds... 

In principle, the enolic fragment ion may or may not tautomerize to the keto form before further fragmentation takes place ... 

Intramolecular alkylation of phenol with diazoketone [14] can be analyzed as the following The carbenoid center acts first as an acceptor and then a donor. Since the resulting cyclopropane derivative is vicinally substituted with an acceptor (C=0) and a donor (enol), fragmentation follows instantaneously (vide infra). 

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

A donor substituent at position 4 or 6 will favor the reaction and an attractor similarly placed will hinder it. Conversely, an attractor (donor) at position 1 stabilizes (destabilizes) the enolate fragment. As atoms 2 and 5 bear no charge, the electronic character of the substituent has little consequence. By analogy with the Cope reaction, we expect that a substituent at these positions also accelerates the Claisen reaction. 

Anodic oxidation couples the pendant arm and the silyl enolate fragments of 54 to give the annulated product 55 (equation 40) , a key intermediate in the synthesis of hamigerans, an unusual halogenated marine natural product . 

In addition, the synthesis of enolate complexes of nickel has been carried out by Campora and co-workers <2003CC1742, 2003JA1482>. Treatment of a THF solution of Ni(C6H4-o-C(0)CH3(Gl)(dippe) 106 with 1 equiv of KO/-Bu allowed the preparation of the nickel enolate 107 in good isolated yield (60%). O-Coordination of the enolate fragment could be proposed on the basis of the NMR spectra. Thus, the terminal methylene group gave rise... 

Another recent example for a similar enolate fragmentation to construct a 10-membered ring is represented by the last step of the sericenine (108 Scheme 36) synthesis. (108) is a constituent of the essential oil of the leaves of Neolitsea sericea (Lauraceae), which grows in the southern parts of Japan and China. The tosyloxy ester (107) gives directly the furanogermacradiene (108), presumably via die neo-sericenine with the c s-a,3-unsaturated ester group, which isomerizes to the thermodynamically favored product. 

McLafferty rearrangement. Electron-impact-induced cleavage of carbonyl compounds having a hydrogen in the y-position, to an enolic fragment and an olefin. 

Aldol reactions have become very much more sophisticated in the level of control that may be achieved. The types of control we are about to discuss do not depend upon double bond geometry. In each case from 1,3 195 to 1,5-related stereocentres 197 the newly forming alcohol is one of the stereocentres and the other may be either in the incoming enolate fragment 196, 197 or already present in the aldehyde 195. So the chiral centres are on either side of the ketone in 196 and 197 but both on the same side of the ketone in 195. 

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. 

In the Conia-ene reaction, a carbonyl compound enolizes and then undergoes an intramolecular ene reaction. The enol fragment serves as the ene component of the reaction, while the olefin fragment serves as the enophile. An example is the reaction of 8-methyl-7-nonen-2-one (92), which yields 2-isopropylcyclopentyl methyl ketone (93, equation 11.41). ... 

Figure 3.11 Predicted labeling of the enol and aziridine fragments via oxaloacetate 27 and threonine 28 for the enol fragment, and via a-ketoglutarate 26 for the aziridine fragment.

Azinomycin A 20, produced by 5. sahachiroi in small quantities, lacks the enol fragment, leaving a terminal acetonylamide. Feeding experiments carried out with... 

SCHEME 2.95 Easy migration of the enol fragment from a- to P-position of pyrrole ring. 

---