Chromium enone

The formation of an epoxyketone (1) is generally favoured when the expected product of oxidation of an allylic alcohol is a cisoid enone. This type of reaction is promoted by acid conditions and may be prevented by using the chromium trioxide-pyridine reagent which gives only the unsaturated ketone (2) corresponding to the starting alcohol. ... 

We had two possible routes in which alcohol 72 could be used (Scheme 8.19). Route A would involve rearrangement of tertiary alcohol 72 to enone 76. Deprotonation at C5 and generation of the enolate followed by exposure to an oxaziridine or other oxygen electrophile equivalents might directly afford the hydrated furan C-ring of phomactin A (see 82) via hydroxy enone 81. We had also hoped to make use of a chromium-mediated oxidative rearrangement of tertiary allylic alcohols. Unfortunately, treatment of 72 to PCC produced only unidentified baseline materials, thereby quickly eliminating this route. 

An interesting chromium system example is represented by complex 145. Addition of cyano-Gilman cuprates occurred with complete diastereoselectivity to give conjugate adducts 146 (Scheme 6.28). Interestingly, the opposite diastereomer was accessible by treatment of enone 145 with a titanium tetrachloride/Grignard reagent combination [71c]. 

Addition of chromium(ll ) [114] or tin(li) [115] ions to the electrolyte strongly favours the formation of head-to-head pinacols from the reduction of enones. It is believed that these ions direct tlie dimerization step by coordination through the oxygen centre... 

The enantioselective lithiation of anisolechromium tricarbonyl was used by Schmalz and Schellhaas in a route towards the natural product (+)-ptilocaulin . In situ hthi-ation and silylation of 410 with ent-h M gave ewf-411 in an optimized 91% ee (reaction carried ont at — 100°C over 10 min, see Scheme 169). A second, substrate-directed lithiation with BuLi alone, formation of the copper derivative and a quench with crotyl bromide gave 420. The planar chirality and reactivity of the chromium complex was then exploited in a nucleophilic addition of dithiane, which generated ptilocaulin precnrsor 421 (Scheme 172). The stereochemistry of componnd 421 has also been used to direct dearomatizing additions, yielding other classes of enones. ... 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

This oxidative transposition of tertiary allylic alcohols into enones or enals is carried out under mild conditions and has ample application in organic synthesis. Although, it can be carried out with other chromium-based reagents (see pages 16 and 35), PCC is the reagent of choice.272... 

Oxidation of n-allylpalladiam chloride complexes. Snatzke s reagent is superior lo other chromium(VI) reagents for oxidation of steroidal tt-allylpalladium chloride complexes to a,/l-enones.1 Examples ... 

Chromium (II) Reagents Reduction of a-Acetylenic Ketones to trans-Enones"... 

Reactions of 4-amino-l-metalla-l,3-dienes, generated from (1-alkynyl) carbene complexes 1 by addition of secondary amines (Scheme 32), with alkynes may lead to production of different compounds, depending on the substitution pattern as well as on the reaction conditions employed.3r Whereas successive addition of a secondary amine and one molecule of a terminal alkyne to a (l-alkynyl)carbene complex lj affords cyclopentadi-enes (Scheme 35), addition of two molecules of a terminal alkyne to complex lj yields cyclopenta[h]pyrans (Scheme 36). In an extension of this reaction sequence, (methylene)cyclopent-2-enones 103 are obtained from (1-alky-nyl)carbene chromium compounds 1 (R = rc-Pr, Ph, CH2CH2CH2OTBDS) by addition of secondary amines (NR) = NMe2, morpholino) and disubsti-tuted alkynes (Scheme 37).9e... 

Salts of other transition metals including vanadium, cerium, chromium and manganese have been used for a-oxygenation, although rarely applied in synthesis. Manganese triacetate has been used for the efficient a -oxidation of enones (Section 2.3.2.2.1.i), but appears not to have been used for the a-hydrox-ylation of saturated ketones des]Hte its known ability to form the corresponding a-keto radicals. Similarly the use of Lewis acid assisted enolization in the oxidative process appears to have been limited to the LTA-mediated examples. 

Other alkene positional isomers of the product enone can be obtained selectively, depending on the conditions of the acid treatment and the acid hydrolysis. The addition/protonation process does not change the oxidation state of the chromium, and a procedure has been defined for recovery of the Cr in order to allow direct recycling. ... 

Chromium(II) chloride reduction of 9a-bromo-A -3-oxosteroids 19 leads to 5,9-cyclosteroids 20. The formation of these eyclosteroids is explained by assuming a one-electron transfer in the cleavage of the C-Hal bond with chromium(II) chloride to give a carbon radical, followed by a second one-electron transfer from Cr to give a carbanionic intermediate that intra-molecularly undergoes addition to the enone moiety and thus forms the cyclopropane ring. 

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