Favorskii-type ring contraction

The study of the Diels-Alder adducts of levopimaric acid has continued with an examination of the structure and stereochemistry of the adducts with cyclopentenone and cyclopent-l-ene-3,5-dione. The major product with cyclo-pentenone is the endo,cis adduct (83). The enedione gives a mixture of enolic endo,cis adducts whose stereochemistry was determined by photocyclisation to give compounds such as (84). A correlation was also achieved with the benzo-quinone adduct through a Favorskii-type ring-contraction of the epoxide (85) to (86). The 13(14) double bond of these adducts is hindered to oxidation, excepting the adduct with acetylenedicarboxylic acid. Condensation of 12-hydroxymethylabiet-7(8)-en-18-oic acid with formaldehyde gives the cyclic ether (87). 

The reaction of o>-halogeno-l-(NJV-dialkylamino)cycloalkenes with amines is known to result in a Favorskii-type ring contraction, giving rise to cm-diamino-bicyclo[n,l,0]alkanes, but, under the same conditions, the acyclic analogue (28) yields predominantly acyclic products. However, by employing (28), AgBF, and MejNH in a molar ratio of 1 1 2, cyclization is almost quantitative. Starting from... 

The racemic mixture of cyclobutanones is converted into bisulphite adducts (note that diastereoselective attack of the bisulphite anion takes place exclusively anti to the two adjacent substituents). Fractional crystallisation of the salt formed with enantiomerically pure (5)-l-phenylethylamine allows separation of the (25,4/ )-diastereomer (1), which, after decomposition of the bisulphite adduct, undergoes a Favorskii-type ring contraction and elimination of HCl to give enantiomerically pure (2). The undesired (2R, 45) diastereomer can be converted back to the racemic mixture for recycling, rendering the process highly efficient. "... 

Sodium hydroxide Favorskii-type ring contraction... 

With cyclic a-halo ketones, e.g. 2-chloro cyclohexanone 6, the Favorskii rearrangement leads to a ring contraction by one carbon atom. This type of reaction has for example found application as a key step in the synthesis of cubane by Eaton and Cole for the construction of the cubic carbon skeleton ... 

RING CONTRACTION via A FAVORSKII-TYPE REARRANGEMENT CYCLOUNDECANONE... 

Two types of ring contractions have been used in the synthesis of pyrrolidines. The older one, an analogous Favorskii process, is the key step of Honjo s a-allokainic acid synthesis. The general lines of this approach are given in Scheme 29 (173). 

All of the examples described thus far have involved the use of bases to initiate the Favorskii rearrangements. However various Lewis acids have also been used for this purpose. An early example of this methodology is shown in Scheme A2P The Lewis acid promoted ring contractions depicted in Scheme 43 and Scheme 44 are also Favorskii-type rearrangements. 

Ring Contraction via a Favorskii-Type Rearrangement Cyclounde-... 

In these constrained molecules, the Favorskii rearrangement leads essentially to the ring-contracted acid or ester in good yields. However, the formation of ring-opened products has been reported when polyhalogenated strained ketones react according to the quasi-Favorskii mechanism. Results using this type of substrate are reported in Table 8. 

A version of this type of oxidation, which used a combination of lead tetraacetate and boron trifluoride etherate, allowed the transformation of enamines of cyclic ketones into esters of the corresponding contracted ring, as in the classical Favorskii rearrangement of a-haloketones under basic conditions72 (Scheme 51). 

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