Enol tritiate

The most direct route towards functionalized aliphatic polyesters is based on the functionalization of polyester chains. This approach is a very appealing because a wide range of functionalized aliphatic polyesters could then be made available from a single precursor. This approach was implemented by Vert and coworkers using a two-step process. Eirst, PCL was metallated by lithium diisopropylamide with formation of a poly(enolate). Second, the poly(enolate) was reacted with an electrophile such as naphthoyl chloride [101], benzylchloroformate [101] acetophenone [101], benzaldehyde [101], carbon dioxide [102] tritiated water [103], ot-bromoacetoxy-co-methoxy-poly(ethylene oxide) [104], or iodine [105] (Fig. 26). The implementation of this strategy is, however, difficult because of a severe competition between chain metallation and chain degradation. Moreover, the content of functionalization is quite low (<30%), even under optimized conditions. 

The formation of 76a is essential, as the enol 76 would not be expected to be decarboxylated. When allowed to react in tritiated water, both L-ascorbic acid and D-amfomo-hexulosonic acid produced 2-furaldehyde having an activity approximately 60% of that of the solvent most of the activity was situated at the a-carbon atom. The failure to detect any pentose is probably ascribable to the high reactivity of the 1,2-enediol (77). 

In an attempt to learn whether collagen hydroxyproline synthesized in granuloma minces incubated with tritiated proline contained tritium at the 4 position, we converted such tritiated hydroxyproline to the corresponding N-carbobenzoxy derivative and then oxidized this derivative with chromic acid to N-carbobenzoxy-4-ketoproline according to the procedure of Patchett and Witkop(29). In most of these experiments very little tritiated water was formed although some tritiated water was detected in several experiments, the small amount formed could logically be attributed to adventitious oxidative reactions or perhaps to enolization of ketoproline. The evidence was therefore consistent with the absence of tritium at carbon atom 4 of the isolated hydroxyproline. 

Energetically, Reaction 2 would be far simpler for the enzyme to carry out. To study these two proposals, an experiment was set up in which the enzyme reaction was conducted in tritiated water (30). One could distinguish between the two reactions on the basis that Reaction 2 would incorporate tritium into the final product, pyruvate, during the addition reaction. In this case, Reaction 2 would yield tritiated pyruvate. When this experiment was conducted and the pyruvate was isolated from the tritiated water on a Dowex-l-Cl column, it was found that pyruvate was labeled with tritium. Unfortunately, however, when the boiled enzyme was incubated with pyruvate and tritium, the pyruvate was also tritiated. Apparently then at pH 8.5, pyruvate picked up tritium by enolization. 

Dictamnine labeled on the furan hydrogens with tritium or deuterium was prepared by cyclization of aldehyde 247 with tritiated or deuterated PPA (J). Treatment of [2,3-2H2]dictamnine with aqueous acid did not result in exchange of deuterium atoms. In the formation of [2,3-2H2]dictamnine, exchange of deuterium between the 2- and 3-positions of the furan ring may occur by a 1,2-shift, (251) (252) (Scheme 19), although an alternative mechanism for exchange of the aldehyde proton involves addition of D+ to the enol form of aldehyde 250 assisted by the... 

Cyclohepta-amylose has been converted into the phosphates (21)—(23), which were examined as catalysts for the hydrolysis of 4-nitrophenyl tetrahydropyranyl ether at low pH and for the exchange of 4-t-butylphenacyl alcohol (tritiated in the a-methylene group) at high pH. All three isomers, as the phosphate dianions, were effective catalysts for the latter reaction in which the functional group assists enolization of the bound ketone. Only the 3-phosphate (22) showed net catalysis of the hydrolysis of the ether, which was bound and hydrolysed with the assistance of a monoanionic phosphoric acid group. 

The reduction of lithium enolates of j8-keto esters with ZrCp2Cl H to give /8-tritiated a,j8-unsaturated esters has been demonstrated by the transmetallation of the lithium enolate of 301. followed by an intramolecular tritide transfer. Spontaneous elimination of O = ZrCp2 converted the intermediate zirconium enolate into 302. a key intermediate in the radiosynthesis of NVP GLC756 13031. The specific activity of 302 was 24Ci/mmoF ... 

Diazomethane is a valuable reagent for one-carbon extension of acyl halides and anhydrides, as well as for ring expansion reactions of cyclic ketones " . It is also widely used in small-scale organic synthesis for the esterification of carboxylic acids and the etherification of phenols, enols and alcohols " . In these reactions, however, CH2N2 installs the label in positions that are potentially metabolically labile and usually unsuitable for use in metabolism smdies. For in vitro studies where metabolism is not an issue, tritiated diazomethane is usually preferred because of its higher specific activity. 

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