Enolesters

This enolate can then react with a plethora of electrophiles, setting a new stereocenter by a diastereoface-selective reaction. The simplest electrophile to trap enolate 71 is H" ", which can, for example, originate from methanol [89] or diphenyl acetaldehyde (as a readily enolizable aldehyde) [90] leading to the acy-lated catalyst species (Fig. 38). The free catalyst is regenerated by acyl-group transfer to methanol(ate) or the aldehyde-derived enolate, producing methyl or enolesters 72/73 in good yields and enantioselectivities. 

Besonders eingehend untersucht ist die Umesterung cyclischer Enolester mit Alkoholen zu ( 3-Oxo-propyl)-phosphinsaure-estern642- 648 ... 

Dimethylketene reacts in the same way and gives 148, whereas methyl-ketene gives the enolester (149).212 The reaction of diphenylketene with 1,2-dithiole-3-thiones occurs with loss of the thiocarbonyl sulfur213 (e.g., 150). It is suggested that this reaction proceeds via the normal 1,3-dithiolane derivative.211... 

As an example of an enantiospecific acylation in organic solvent with the irreversible acyl donor vinyl laurate (see Fig. 3), only the (R)-enantiomer is acylated, while the (S)-2-octanol is obtained directly in 69% yield, a purity >99% and with an excellent enantiomeric ratio S R>99.5 0.5 (as determined by chiral GC-analysis). The selectivity of the forward reaction, catalyzed by a lipase in MTBE, is thereby maximized, because the use of the enolester precludes the reverse reaction. Batches of about 70 kg are produced routinely and the technology can be easily transferred to larger sizes. 

The bufatenolides (90) probably have a biosynthesis in plants similar to the cardenolides. Pregnenolone is incorporated into hellebrin [(90) with (89 Sp-OH, 19-oxo)] and scilliroside [(90) with (89 A )]. However, in the toad Bufo) cholanic acid derivatives are far better precursors than pregnane derivatives. This suggests that all five carbon atoms of the enolester ring (90) may be derived from cholesterol in the toad compared with only two in plants. In the toad, marinobufagin [(90) with (89 5 -OH, 14,15-oxide)] is the precursor of telecinobufagin [(90) with (89 5 -OH)]. ... 

Die Enolester von 3-Oxo-aldehyden (z. B. I) werden oft mit guten Ausbeuten unter Erhalt von Carbonyl-Gruppen in die entsprechenden a -Methyl-ketone iiber fiihrt. So gelingt es, auf einfache Weise l-Oxo-2-methyl-tetraline herzustellen2 ... 

Ebenso leicht lassen sich cyclische Enolester (Enollactone) hydrogenolytisch spalten, wobei z. B. beim Keto-enollacton der Dehydroabietinsaure gleichzeitig die Carbonyl-Gruppe hydrogenolysiert wird3 ... 

Markownikoff addition. 2-Methylpent-l-ene allowed to react with tertAmiyl hydroperoxide and mercuric acetate with initial cooling and shaking, after 2 hrs. the volatiles removed by distillation in vacuo, and the residue digested repeatedly with coned, aq. NaCl -> l-diloromercuri-2-tert-butylperoxy-2-methylpentane. Y 19%. F. e., also hydroperoxymercuration with H2 2 s. E. Schmitz, A. Riedie, and O. Brede, J. pr. 312, 30 (1970) a-alkoxyketones from enolesters s. ibid. 312, 43 peroxy- and methoxy-mercuration of a,i -unsatd. ketones and esters s. A. J. Bloodworth and R. J. Bunce, Soc. (C) 1971, 1453. 

Hydroxy-6-methyl-2-pyrone and NaH added to dioxane, refluxed 1 hr., acryloyl diloride in dioxane added dropwise with stirring, and refluxed 7 hrs. 3,4-di-hydro-7-methyl-2H,5H-pyrano[4,3-b]pyran-2,5-dione. Y 84%. F. e., also enolester formation without cyclization, s. B. M. Monroe, J. Heterocyclic Chem. 6, 917 (1969). 

---