Conia rearrangement

Claisen or Cope rearrangement Ester cracking Conia ene reaction... 

Siloxy-l-vinylcyclopropanes rearrange thermally to give 1-siloxycyclopentene derivatives or may be hydrolyzed to cyclobutanones by acids (J.M. Conia, 1975 B.M. Trost, 1973 G.C. Girard, 1974). The cyclopropane ring may also be opened by base-catalyzed hydrolysis of the silyl ethers to form ethyl ketones. 

Conia has used this technique extensively with a,p-unsaturated ketones. As seen in equation (14), alkylation of 2-benzyl-3-methylcyclohexenone with allyl bromide gives diene (15), which again rearranges on heating to the isomeric conjugated ketone (16). 

Due to efforts of Conia s (equation 148) and Trost s groups access to more substituted vinylcyclopropanes has been gained operating with siloxy derivatives prepared either by Simmons-Smith cyclopropanation or using diphenylsulphonium cyclopropylide as a key substance (see Section VI.D.l). Oxidative rearrangement has led to a cyclopentanol... 

The pinacol rearrangements of several cyclobutanediols, mostly induced by BFs-OEta, have been examined by Conia. The reactions of symmetrically substituted materials occurred with high specificity and yield. Both isomers cis and trans) gave the same product in instances where this question was examined. As shown in equation (17), for the substituents R = H, alkyl or phenyl, only ring contraction was observed, while equally regiospecific formation of the cyclobutanone product was found for the substituents R = allyl or vinyl. 

Rearrangements of epoxides catalyzed by LiBr HMPA or Lil have figured in a number of natural product syntheses in which ring expansions to cyclopentanones are required. Both salts were earlier shown to effect the especially facile rearrangements of the small ring spiro epoxides which serve as models for these processes. Thus (289 equation 123) was found by Salaun and Conia to rearrange rapidly and in excellent yield to cyclobutanone, simply on exposure to commercial hydrated Lil. 

Favorskii rearrangements have been reported with alicyclic and heterocyclic bromocycloalkanones containing 4-13 (but not S) atoms in the rings. The important mechanistic studies with 2-bromocyclobu-tanone have already been mentioned (Scheme 10), and Scheme 23 contains a summary of other results obtained by Conia and coworkers.Of particular note is the fact that the rearrangement will even proceed with water as reagent, and that the stereospecificity observed is consistent with a mechanism involving (as expected) inversion of configuration at the cartmn center initially attached to the bromine atom. 

These mechanisms can be identified by analysis of deuterium incorporation (Scheme 13) as in the experiments of Conia and coworkers. According to Bredt s rule the deuterium exchange of a hydrogen before rearrangement (see also experiment 5 in Scheme 13) is difficult. Consequently, deuterium incorporation is a direct reflection of the reaction process involved. 

---