Esters, conjugated anion

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

The reactivity of 2-alkyloxazoles in base-catalyzed additions and condensations is due to the intermediacy of delocalized anions analogous to (142). 2-Methyl-4,5-diphenyloxazole forms the adduct (278) by the action of benzophenone in liquid ammonia containing lithium amide (equation 89), and 2-methylbenzoxazole condenses with diethyl oxalate in the presence of potassium ethoxide to yield the keto ester (279). Anions with more extended conjugation are involved in the condensation of 2-(propen-l-yl)benzoxazole with diethyl oxalate (equation 90) and in the formation of the stilbene derivative (280) from 5-phenyl-2-p-tolyloxazole and benzylideneaniline (78AHC(23)l7l). 

Assorted anions. These are generated by deprotonation of allylic halides," chloromethylphosphonic esters, conjugated hydrazones, chiral carbamates,unsaturated a-aminonitriles, phosphonamides," and sulfonamides. The dianions derived from tu-haloalkanecarboxylic acids cyclize, and this reaction forms the basis of a synthesis of V-Boc cyclic imino acids. The conjugate bases of 2-(arylmethoxy)-methyl-2-oxazolines are unstable as [2,3]sigmatropic rearrangement takes place even at — 75°C. 

For SrnI C-alkylation of the conjugate anions of 2-nitropropionate esters, on reaction with p-nitrobenzyl chloride, only moderate asymmetric induction is induced when the ester is derived from a chiral carbohydrate the 8-phenyl menthol moiety is more effective. 

Stabilized anions exhibit a pronounced tendency to undergo conjugate addition to a p unsaturated carbonyl compounds This reaction called the Michael reaction has been described for anions derived from p diketones m Section 18 13 The enolates of ethyl acetoacetate and diethyl malonate also undergo Michael addition to the p carbon atom of a p unsaturated aldehydes ketones and esters For example... 

These reversible reactions are cataly2ed by bases or acids, such as 2iac chloride and aluminum isopropoxide, or by anion-exchange resias. Ultrasonic vibrations improve the reaction rate and yield. Reaction of aromatic aldehydes or ketones with nitroparaffins yields either the nitro alcohol or the nitro olefin, depending on the catalyst. Conjugated unsaturated aldehydes or ketones and nitroparaffins (Michael addition) yield nitro-substituted carbonyl compounds rather than nitro alcohols. Condensation with keto esters gives the substituted nitro alcohols (37) keto aldehydes react preferentially at the aldehyde function. 

Although deprotonation of simple 1,3-dithiolanes at the 2 position is usually accompanied by cycloreversion to the alkene and dithiocarboxylate, this does not occur for the 2-ethoxycarbonyl compound 55. The anion of this is readily generated with LDA and undergoes conjugate addition to a,(3-unsaturated ketones, esters, and lactones to give, after deprotection, the a,8-diketoester products 56 (73TL2599). In this transformation 55 therefore acts as an equivalent of Et02C-C(0) . 

Quaternization of harman (235) with ethyl bromoacetate, followed by cyclization of the pyridinium salt 236 with 1,2-cyclohexane-dione in refluxing ethanol yielded an ester which on hydrolysis gave the pseudo-cross-conjugated mesomeric betaine 237. Decarboxylation resulted in the formation of the alkaloid Sempervirine (238). The PCCMB 237 is isoconjugate with the 11/7-benzo[u]fluorene anion—an odd nonalternant hydrocarbon anion—and belongs to class 14 of heterocyclic mesomeric betaines (Scheme 78). 

---