Dienediolates

In the presence of a very strong base, such as an alkyllithium, sodium or potassium hydride, sodium or potassium amide, or LDA, 1,3-dicarbonyl compounds can be converted to their dianions by two sequential deprotonations.79 For example, reaction of benzoylacetone with sodium amide leads first to the enolate generated by deprotonation at the more acidic methylene group between the two carbonyl groups. A second equivalent of base deprotonates the benzyl methylene group to give a dienediolate. 

For recent information of other dienediols prepared by this method and for the range of products prepared from these compounds see Roberts, S.M. J.C.S. Perkin Trans. L, 1998, 164 1999, 10 2000, 623. 

A related dienediol-phenol rearrangement which can occur by different pathways was reported as a new method for synthesis of the oxepine system180. Protonation of the starting diol 344 produces a cation 345 which can follow normal dienone-phenol rearrangement (path a) when the substituents R2 = Me, Ph and R1 = t-Bu are eliminated in the step 346 — 347. However, when R1 = t-Bu and R2 is a substituted phenyl which decreases the nucleophility, the cationoid intermediate 345 cyclizes to the oxonium ion 348 (path b) which then undergoes deprotonation to give the oxepine 349 (equation 124)180. 

Reaction with p-benzoquinones.6 Fischer and Henderson have found conditions that result in 1,2-addition of various RLi reagents to p-benzoquinonc. The reaction when conducted in ether at —78° results in 4-alkyl-4-hydroxycyclohexa-2,5-diene-l-ones. These products undergo further 1,2-addition with RLi in THF to give dialkyl-cyclohexa-2,5-diene-l,4-diols. Mixed dienediols can be obtained by use of two different RLi reagents. 

Aurell, M. J. Gil, S. Mestres, R. Parra, M. Parra, L. Alkylation of lithium dienediolates of butenoic adds. Regioselectivity effects of... 

S. Gil, M. Parra, Dienediolates of Carboxylic Acids in Synthesis. Recent Advances, Curr. Org. Chem. 2002, 6, 283-302. 

The nature and amount of the amine used for acid deprotonation determines the reaction yield. In most cases, dienediolates of unsaturated carboxylic acids can be generated, without Barbier s reduction or Michael adduct formation, by deprotonation of the corresponding acid with butyllithium in the presence of a catalytic amount of amine198. This renders dienediolates compatible with a large number of functional groups, as happens with nitriles where self-condensation is minimized under these conditions. Unfortunately, this cannot be considered a general rule and it is convenient to optimize the amine and its amount for each acid and nitrile. 

The T and X subscripts on G refer to T- and X-junctions. Note that Pi is crosslinked. In a similar polymerization (Vollmert s example 4) 1,4-buta-dienediol is introduced along with monomer 3, and the last step in the synthesis involves a crosslinking of polymer 2. In his example 7, four different polymers are mixed. To conserve space, details are omitted. 

Pt(II), bonded to two PhsP ligands, exhibits a profound difference in its bis-enolate complexes with the doubly deprotonated triones RC(0)CH2C(0)CH2C(0)R, R = Me and Ph. For R = Me, a highly puckered 2,4-diacetylated 3-platinacyclobutanone, 57, is formed with a weak transannular Pt-C bond. For R = Ph, the more usual 1 1 0,0 -chelate is formed and the product is isolated as the dienediolate complex 58. What would be the product if the two substituent groups were different, R = Me and R = Ph Would a 2-acetyl-4-benzoyl-3-platinacyclobutanone be formed would a 6-membered-ring fi-diketonate be formed with one Me and a PhCOCH substituent apiece or such a species with one Ph and an MeCOCH, or would an unprecedented l,3-dioxa-2-platinacyclooctadienone (59) result ... 

Acid lability. Several of the polyols although odorless, are acid labile and readily form volatile flavorants at ambient temperature and juice pH (18,28,29) Hotrienol, for example, appears to be formed wholly by acid catalyzed dehydration of dienediol 30a (26,30). Four naturally occurring hydroxyllnalool derivatives l.e. 29,30a,31 and 32 were heated for 15 min at 70°C and pH 3.2 and thirteen volatile monoterpene products (l.e. la,5,6a,9,10,11,12,15, 16,17,18,21 and 22) were identified by headspace analysis (18). 

Shimizu et al. (72) established that B. cinerea did not produce any monoterpenes in must under culture conditions but led to an oxidation of linalool. Although the authors identified the four linalool oxides among the products, they did not identify the major metabolite, which from their data, was probably dienediol 31. Thus a parasitic organism commonly found on grapes can metabolize one of the major monoterpene flavorants of the fruit to a flavorless polyol. Of note in this regard were the reported losses of both aroma and volatile terpenes in botrytised grapes (73). 

The allylic proton of the exo methylene derivative 175 was abstracted when treated with an organolithium reagent and subsequent elimination afforded dienediol 176, Eq. 113. The analogous ring opening reaction occurred for exo methylene [2.2.1] oxabicyclic substrates as well [120a]. 

C-q, a-bonded, 367 charge transfer complexes, 386 dienediolate ligands, 369 electrochemical synthesis, 375 standard enthalpies of formation, 366 reactions... 

---