Ketones dienol ethers

The mechanism of this novel photochemical rearrangement involves a photochemical conrotatory cyclization, a novel 1,9-hydrogen shift, lateral ring opening, and finally photochemical conversion of the dienol ether to the conjugated ketone (Scheme 22). 

Coupling of ketones with alkenesf This reaction can be effected with Sml2 catalyzed by HMPA (14, 280-281). The alkene can be an activated terminal one (CH2=CHOAc), a conjugated diene, or a silyl dienol ether. 

Regioselective Peterson reaction. Aldehydes react with the anion of an (a-al-koxy)allyltrimethylsilane (1) at both a- and y-positions. Addition of HMPT favors reaction at the y-position, whereas addition of Ti(0-/-Pr)4 (1 equiv.) results in exclusive reaction at the a-position to give an (E)-l, 3-dienol ether (2). These products are readily hydrolyzed to vinyl ketones (3).4... 

Other reagents have been utilized for this transformatian. For exanqile, lead tetiabenzoate (with fluoride-induced hydrolysis of (136) and (137).However, success is restricted to acyclic ketones. Where cyclic dienol ethers are employed, products derived from a-oxidation are obtained. 

Fluoroalkyl ketones may be employed as the electrophilic partners in condensation reactions with other carbonyl compounds. The highly electrophilic hexafluoroacetone has been used in selective hexafluoroisopropylidenation reactions with silyl enol ethers and silyl dienol ethers, e.g. formation of 1. ... 

Lewis acid catalyzed alkylations of cross-conjugated silyl dienol ethers provide routes to a -alkylated ketones. A short synthesis of the sesquiterpene ( )-ar-turmerone has been accomplished using the cross-conjugated TMS dienol ether of mesityl oxide (equation 9). ... 

Mono-enolisation of a 1,5-diketone, then the formation of a cyclic hemiacetal, and its dehydration, produces dienol ethers (4//-4-pyrans) which require only hydride abstraction to arrive at the pyrylium oxidation level. The diketones are often prepared in situ by the reaction of an aldehyde with two mols of a ketone (compare Hantzsch synthesis, section 5.15.1.2) or of a ketone with a previously prepared conjugated ketone - a chalcone in the case of aromatic ketones/aldehydes. It is the excess chalcone which serves as the hydride acceptor in this approach. 

Three-carbon homologation of aldehydes, and also of cyclic and acyclic ketones to form a/8-unsaturated aldehydes, can be performed by treating the carbonyl compounds with 3-methoxyallylidenetriphenylphosphorane (23) followed by in situ hydrolysis of the dienol ether (Scheme 53). ... 

In concert with the principle of vinylogy, the phosphoramide/SiCLt system could be applied to silyl dienol ethers derived from a,(3-unsaturated esters, ketones [65],... 

The conversion of an aj3-unsaturated ketone to its dienol ether is usually effected by treatment with an orthoformic ester [112, 113, 114] or with 2,2-dimethoxy-propane [115] in the presence of an acid catalyst and in the corresponding alcohol or in dioxan as solvent. For the formation of a dienethiol ether treatment with the more reactive corresponding thiol is sufficient benzyl thiol has been used most frequently [116, 117]. Under the conditions generally used (at room temperature) saturated ketone groups react much more slowly and it is usually possible to differentiate completely between the two types of ketone group [112, 117]. With unhindered a,/5-unsaturated ketones and under more basic conditions alternative 1,4-addition of a thiol to the conjugated system may occur [117]. 

The 5,6-double bond in dienol ethers derived from steroidal 4-ene-3-ketones can be hydrogenated selectively, giving an A/B trans-junction with isomerization of the original 3,4-double bond to the 2,3-position [120,121]. An interesting illustration of a case in which an enol ether has served a specific function additional to its being a protective group is shown in Fig. 9.5. 

Cyanohydrins are formed selectively from saturated as against oc, 3-unsaturated ketones [113], and in the steroid field a further distinction can be made between Cs-ketones and C2o ketones by choice of suitable conditions [198]. Base-catalyzed exchange with acetone cyanohydrin is conveniently employed for their formation [198,199]. In the presence of this grouping which is quite stable to mildly acidic conditions, firee (x, unsaturated ketone groups can be converted into dienol ethers, benzyl thioenol ethers, and dioxolans by conventional methods [113], and saturated ketones can be brominated [200]. Cleavage of this group occurs under very mildly basic conditions such as in ethanol in the presence of pyridine [113]. An additional modification is provided by further conversion into cyanohydrin tetrahydropyranyl ethers which are more base-stable [199]. 

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