Enolates dianions

Enolate dianions from 3-benzoyl-2-fc-rt-butylT-methyl-5-oxo-4-imidazolidineacetic and -propanoic acids 4, obtained from aspartic and glutamic acid, have also been used in diastcreose-lective alkylation reactions (see Table 4)7. 

In the reduction of octalone of the type 212, the resulting enolate dianion 213 can adopt three different half-chair conformations 216, 217, and 218. Of these, only conformations 216 and 217 have the carbanion electron pair parallel to the a orbital of the enolate system allowing an electronic de-... 

Other factors (charge repulsion, solvation factors, etc.) could influence the position of the equilibrium in favor of enolate dianion 216. It is also possible that there is a kinetic preference for the formation of dianion 216 and that this species would undergo protonation more rapidly than equilibration. This rule of axial protonation" of 216 has been found to be widely applicable in many cases. However, in systems in which a significant amount of strain must be introduced in order for protonation to occur axially on 216, protonation of conformer 217 (and even conformer 218) becomes important (60). 

The above stereoelectronic arguments were proposed by Stork and Darling (61) to explain why the more stable isomer is not necessarily always obtained (62). For example, reduction of the octal one 221 with lithium-ammonia-ethanol followed by oxidation afforded the trans-2-decalone 222 even though the isomeric cis-2-decalone 223 is about 2 kcal/mol more stable than 222. Conformation 226 of the enolate dianion is the most favored sterically but it is electronically disfavored. Conformations 224 and 225 are both electronically favored but 225 is less favored sterically than 224. Therefore,... 

On the other hand, an electron-withdrawing substituent, particularly a carbonyl group 63, will attract the anion 64 and again we get a non-conjugated product. If the acid 63 R=H is used, the less stable, non-conjugated anion in the intermediate 64 R=H captures the proton from the acid giving the enolate dianion 66 as the immediate product. This can be combined with electrophiles such as alkyl halides as we shall see. 

The asymmetric hydroxylation of ester enolates with N-sulfonyloxaziridines has been less fully studied. Stereoselectivities are generally modest and less is known about the factors influencing the molecular recognition. For example, (/J)-methyl 2-hydroxy-3-phenylpropionate (10) is prepared in 85.5% ee by oxidizing the lithium enolate of methyl 3-phenylpropionate with (+)-( ) in the presence of HMPA (eq 13). Like esters, the hydroxylation of prochiral amide enolates with N-sulfonyloxaziridines affords the corresponding enantiomerically enriched a-hydroxy amides. Thus treatment of amide (11) with LDA followed by addition of (+)-( ) produces a-hydroxy amide (12) in 60% ee (eq 14). Improved stereoselectivities were achieved using double stereodifferentiation, e.g., the asymmetric oxidation of a chiral enolate. For example, oxidation of the lithium enolate of (13) with (—)-(1) (the matched pair) affords the a-hydroxy amide in 88-91% de (eq 15). (+)-(Camphorsulfonyl)oxaziridine (1) mediated hydroxylation of the enolate dianion of (/J)-(14) at —100 to —78 °C in the presence of 1.6 equiv of LiCl gave an 86 14 mixture of syn/anti-(15) (eq 16). The syn product is an intermediate for the C-13 side chain of taxol. 

The functional equivalent of an enolate dianion (103) was prepared by Stork et al. by treatment of en-amine (104) with r-butyllithium. ° This anion crystallized as the symmetrical dimer (105) with the car-banionic carbon nearly symmetrically bridging two lithium atoms as shown in (105). Doubly bridging carbons represent a characteristic feature of these lithiated vinylic anions and this structural feature is normally expected in these compounds as well as in aryl anions (see ref. 11). 

Dihydrofurans are also available by the addition of enolate dianion derivatives to (/r -diene) Co(CO)3 tetrafluoroborates (Equation (103)) <88JOC2li4>. 

The emitting molecule, decarboxyketoludferin, has been isolated and synthesized. When it is excited photochemically by photon absorption in basic solution (pH > 7.5-8.0), it fluoresces, giving a fluorescence emission specfrum that is identical to the emission spectrum produced by the interaction of firefly luciferin and firefly luciferase. The emitting form of decarboxykefoluciferin has thus been identified as the enol dianion. In neutral or acidic solution, the emission spectrum of decarboxyketo-luciferin does not match the emission spectrum of the bioluminescent system. 

The exact function of fhe enzyme firefly luciferase is not yet known, but it is clear that all these reactions occur while luciferin is bound to the enzyme as a substrate. Also, because the enzyme undoubtedly has several basic groups (—COO , —NHj, and so on), the buffering action of those groups would easily explain why the enol dianion is also the emitting form of decarboxyketoluciferin in the biological system. 

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