Alkoxy dienones

In 2004, Trauner and co-workers published a follow-up communication on their asymmetric catalytic system. Under optimized conditions, they were able to achieve good to excellent levels of enantioselectivity for a variety of substrates using complex 78 with lower catalyst loadings (10 mol %). It is important to note however, that the specific use of an alkoxy dienone substrate lacking a i-substituent on one of the alkenes (such as 76) was required for high yields and good enantioselectivities. Since the stereocenter formed during electrocyclization is subsequent destroyed on deprotonation of the allylic cation (see Section 3.4.3), the control of absolute stereochemistry in this case is solely due to facially selective reprotonation of the enolate. 

A Br0nsted acid-catalysed asymmetric Nazarov cyclization of acyclic a-alkoxy dienones to chiral cyclopentenones has been reported (Scheme 169). ... 

Cross-conjugated dienones are quite inert to nucleophilic reactions at C-3, and the susceptibility of these systems to dienone-phenol rearrangement precludes the use of strong acid conditions. In spite of previous statements, A " -3-ketones do not form ketals, thioketals or enamines, and therefore no convenient protecting groups are available for this chromophore. Enol ethers are not formed by the orthoformate procedure, but preparation of A -trienol ethers from A -3-ketones has been claimed. Another route to A -trien-3-ol ethers involves conjugate addition of alcohol, enol etherification and then alcohol removal from la-alkoxy compounds. 

Wlien an alkyl and an alkoxy moiety ate ptesenl al tlie 4-position of tlie dienone fSdieme 7.17fb)), desymmetrizalion during tlie 1,4-addition produces two stereo-centers in a single step. Hie diital coppet-pbospbotamidile calalyst derived from... 

Oxidations of variously-substituted 4-alkyl- and 4-alkoxyphenols with BAIB or BTIB in alcoholic solvents provide ready access to alkoxy(alkyl)- and dialkoxycyclohexadienones (Scheme 23) [70-72]. Dienone formation is generally attributed to the capture of aryloxyiodane and/or aryloxenium ion intermediates with the alcohol [73]. Related C-0 bond forming oxidations of phenols with BAIB and BTIB, including intramolecular cyclizations leading to spiro-dienones, are summarized in several reviews [1 - 3,74]. 

The importance of conformation in selective attack of the 1 l/ -alkoxy radical on C18 and C19 was nicely illustrated by the photolysis of the nitrite 18 derived from the dienone 19. This afforded only C18 attack.7 Treatment of the derived oxime with nitrous acid gave 1-dehydro-aldosterone acetate 20. This is a convenient precursor for labelled aldosterone acetate since selective hydrogenation with tritium gives 1,2-ditritiated aldosterone acetate. 

When an alkyl and an alkoxy moiety are present at the 4-position of the dienone (Scheme 7.17(b))j desymmetrization during the lj4-addition produces two stereocenters in a sin e step. The chiral copper-phosphoramidite catalyst derived from... 

The influence of bulky ortlro-substituents on the tautomerism of phenols can be illustrated by the recently reported generation and isolation of 4-alkoxy-2,6-di-tert-butylcyclo-hexa-2,5-dienones 13. They were generated efficiently by the Ag ion mediated reaction of 4-bromocyclohexa-2,5-dienone 12 with simple alcohols (equation 6). All the dienones... 

When the reaction is applied to j8-alkoxyenones, the j8-alkoxy group is eliminated, providing 2,4-dienones. ... 

More functionalized substrates have also been considered recently [94], giving, for instance, access to 5-alkoxy-2 ,4 -dienones that are regarded as useful 1,4-disubstituted push-pull dienes. 

Hydroxycyclopentenones related to 8 and 9 are obtained by irradiation of dilute aqueous acetic acid solutions of dienones with a medium- or high-pressure mercury lamp in a quartz reactor or screened by a Pyrex filter. Acetoxy or alkoxy ketones are obtained under these conditions using acetic acid or alcohols (or alcoholic acetic acid), respectively, as the solvent. However, deviations from this general trend are possible, depending on the nature of the substituents in or around the chromophore. 

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