1.2- Diketones ring contraction rearrangement

The ring contraction rearrangement of alicyclic a-diketones is much more valuable from the synthetic viewpoint e.g. equation 4). There are several routes available for the synthesis of a-diketones from monoketones. These include direct oxidation using selenium dioxide permanganate or osmium tetrox-ide addition across the double bond of an enone followed by base-promoted elimination of water bromi-nation to give the dibromo ketone followed by hydrolysis condensation with 4-nitroso-/V,/V-dimethylaniline followed by acidic hydrolysis — but there are many others. The bromination-hydrolysis route has been extensively studied by Wallach." The conversion of menthone into 1-hydroxy-... 

The benzilic acid rearrangement of steroid a-diketones is often the preferred method for ring contraction. Yields are generally excellent, and in several instances the reaction has been shown to be stereospecific. Preparation of the starting a-diketones is discussed below and illustrated with several excellent experimental procedures. 

In practice, the scope of the benzilic rearrangement as a tool for the ring contraction of steroids is defined by the availability of a-diketones, or the corresponding diosphenols. 

Kubota and co-workers describe a novel oxidative rearrangement of the diosphenol (58) of 17iS-hydroxyandrost-4-ene-2,3-dione to the A-nor-A -1,2-diketone (59) in 33 % yield by the action of specially prep d manganese dioxide in boiling acetone. The rate of ring contraction is very sensitive to the source of the oxidant, and a trace of dilute sulfuric acid in the reaction mixture causes oxidative fission of ring A. 

Ring contraction by benzilic acid rearrangement of a-diketones... 

Ring contraction by oxidative rearrangements of A-ring a-diketones by manganese dioxide... 

Several examples are known of the contraction of steroid rings by benzilic acid rearrangements of a -diketones [2g2]. Acyloin rearrangements in the steroid field, particularly in the conversion of i7-hydroxypregnan-20 ones into D-homo-ketols,... 

Dioxane ring contraction via Wolff rearrangement is the basis of the synthesis of dioxolane 565 by photolysis of diazo diketone 566 in methanol (85KGS321). 

An interesting ring-contraction biogenetic pathway from puupuhenone (45) to haterumadienone (46) might proceed via benzylic acid rearrangement of the diketone form of 45, followed by oxidative decarbonylation. Haterumadienone (46) is the first ring-contracted... 

The cyclopentapyrazoles (45 R = H, Me) are obtained in 67% and 54% yields, respectively, by benzilic acid rearrangement of the corresponding indazolediones (44). This ring contraction has many analogs in the next section, and may occur during alkaline permanganate oxidations of suitable substrates. For example, oxidation of fuscinic acid dimethyl ether (46) affords (48) and (49), implicating the a-diketone (47) as a common intermediate. ... 

In the benzil-benzilic acid rearrangement, an a-diketone is treated with a base to give the sodium salt of an a-hydroxy carboxylic acid. In the Favorskii rearrangement, an a-halogenoketone is treated with an alkoxide anion to give the a-alkyl ester. This reaction may also be used to effect a ring contraction. 

The photo-induced rearrangement of 4,5-epoxy-6-oxo- and 5,6-epoxy-4-oxo-cholestanes has been examined.All four compounds gave the 4,6-diketone as the main product but, in addition, the )3-epoxides produced ring-contracted byproducts whilst the a-epoxides gave the 4,5-seco keto-acid (566). 

This reaction has general application for preparing benzilic acids and their esters which have important biological activities. The formed benzilic acid can be used as reducing reagent to transform Qf, -unsaturated ketones into saturated ketones.In addition, the benzilic acid rearrangement on cyclic of-diketones will lead to ring contracted products 2ii, 2kk, 2nn... 

Schaltegger et al reported the base-catalysed benzilic acid rearrangement of cyclic diketone 29, which, resulted in ring contraction affording the acid 30. 

Hydrogen peroxide sulfuric acid Oxidative ring contraction of isocyclics Oxidative rearrangements of -diketones... 

Spiro(adamantane-2,2 -adamantane) (223) has been synthesized from diketone (224) by selenium dioxide oxidation and ring-contraction by a benzilic acid rearrangement and also from (225), which is obtained by rearrangement of (226). Also noted are C-labelling studies which show that Wolff... 

The photo-induced rearrangements of a series of 4,5 epoxy-3-ketones (12 lead to contraction of ring A and expansion of ring B to give diketones (13) [60-62]. The reaction seems likely to involve an excited singlet state, for it was unaffected by triplet quenchers. Rearrangement proceeds with retention of stereochemistry at C(io> (lOa- or lOjS-series) and at C(4>... 

---