Enol triflates, decarboxylative

The ring-opening of the cyclopropane nitrosourea 233 with silver triflate followed by stereospecific [4 -i- 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-cyclohexen-l-ones [131], allylic oxidation of oc, -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

When the anions of these acids are good leaving groups, elimination from their enol esters can be readily induced by comparatively weak bases. This is the case for p-bromobenzenesulphonates and triflates. Thus decarboxylative elimination from enol sulphonates 28 furnishes good yields of 2-alkynoic acids (equation 88), when R... 

A conceptually different [4-1-2] cycloaddition catalyzed by a chiral lanthanide complex has been disclosed. The inverse electron demand Diels Alder reaction of 3-methoxycarbonyl-2-pyrone (67) and enol ethers or sulfides [135] was catalyzed by a chiral ytterbium(III) triflate-binaphthol complex in the presence of diisopropylethylamine (Scheme 51) [136]. Thermal decarboxylations of bicyclic lactones such as 68 are known to yield dienes which may undergo subsequent pericyclic reactions [137] thus, the adducts of this process are potentially useful chiral building blocks. The nature of the substituent on the 2k component was found to be crucial for the realization of high enantioselectivity. 

Lhommet and coworkers prepared the chiral bicyclic carbamate 1189 from L-lysine by a method that involved an electrochemical methoxylation (Scheme 150). ° This precursor, in effect a disguised acyliminium ion, reacted with the sdylated enol 1190 and trimethylsilyl triflate to give a diaste-reomeric mixture of p-keto esters (+)-1191, hydrolysis and decarboxylation of which produced ketone (—)-1192 as the only discernible product. It was essential to reduce this ketone stereoselectively in order to set up the configuration of the target alkaloid s side chain correctly. Fortunately, this was achieved with either K-Selectride or with lithium triethylborohydride. 

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