2.4- Dienols, cyclic

Scheme 69. MIMIRC reactions of cyclic dienolates of different ring size with the chloro ester 1-Me [ 104 h, 107]...

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

It is important to note that, under the same conditions, isophorone or other /3-alkyl cyclic enones predominantly afford the exocyclic conjugated silyl dienol ether (Scheme 3). 

The endocyclic silyl dienol ether is also obtained as the major product when the conjugated cyclic enone is successively treated with the Fe complex prepared according to Scheme 2, then with one equivalent of methylmagnesium bromide (Table 2). This is interesting since these products are difficult to obtain otherwise. The mechanism of this unusual reaction remains obscure. 

Bicyckmnneiation. The first step of a new method for this reaction involves conjugate addition of a cyclic dienolate to a nitro olefin followed by an intramolecular displacement with loss of the nitro group. An example is the reaction of the a -enolale of isophorone (1) with 1-nitropropene. When refluxed with HMPT the initial adduct (2), which can be isolated, is converted into the tricyclooctanones 3 uml 4 iu 63 yield.4... 

Vinyl epoxides have been found to eliminate a 0-H on treatment with Pd° in the absence of nucleophiles. Acyclic vinyl epoxides give dienols, whereas cyclic vinyl epoxides yield fj/y-unsaturated ketones (equations 127 and 128).326 Additional examples of 0-H elimination of allylpalladium complexes to generate dienes have been reported.327-332... 

Sequential double Michael addition of cyclic dienolates, generated from (107) or (108), to fulvene (109) has been developed as a method for rapid access to the tricyclo[5.3.0. 2,5]alkane system (Scheme 24).90... 

The transition state shown in Scheme 9.6 was also postulated by the Pete group [16], Both functional groups of the /(-amino alcohol catalyst play an important role in proton transfer by coordinating to the dienol. The resulting cyclic transition state is shown in Scheme 9.6. It is postulated that in this transition state abstraction of the hydroxyl proton from the dienol occurs in concert with protonation of the carbon in a-position by the hydroxyl group of the /(-amino alcohol. 

Trimethylsilyl dienol ethers. Reaction ot cyclic enones with 1 equiv. of this activated Fe(0) and ClSi(CH,),/N(C2H,)i results in predominant formation of the exocyclic through-conjugated dienol ether, rather than the. ross-conjugated (kinetic) isomer, which is formed preferentially by deprotonation with I DA. 

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. 

One of these useful characteristics is that of stereoselectivity. The stereoselectivities observed in the acetoxylation of methylcyclohexenes are good examples of reactions taking place at the interface (see Section 7.1.3.2.1), Further examples of this selectivity have been observed in the anodic acetoxylation of some cyclic dienol acetates carried out in acetic acid containing potassium acetate as the supporting electrolyte (equation 3). 

As described in Section 6.3.4, activation of coordinated dienols (by protonation) or coordinated dienol acetates (by treatment with Lewis acid) affords the corresponding (pentadienyl)iron cations (248) (Scheme 69). Since the acyclic pentadienyl cationic complexes do not have the geometrical constraints of their cyclic counterparts, they can potentially adopt either a cisoid ( U ) or transoid ( S , sickle) conformation. Nearly all of the (pentadienyl)iron cations prepared appear to be in the cisoid conformation by H NMR spectroscopy. Only a single, sterically biased transoid pentadienyl cation (269) has been spectroscopically observed, but not isolated (equation 66). Owing to their potential to undergo U to S conformational inversion in solution, and owing to the considerably higher reactivity of the less stable S conformer, the (pentadienyl)iron cations are considerably more sensitive to moisture than their cyclic counterparts. 

Enantioselective protonadon of lithium dienolates obtained from Schiff bases of methyl a-aminobutenoates was carried out using (2/ ,3/ )-DPTA, in order to synthesize vinylglycine by deconjugation (ee 36%). Protonadon of a cyclic lithium enolate derived from mandelic acid with (2/ ,3/ )-DPTA was reported to occur with a low ee. ... 

Analogous boion-ene addition to enol ethers (36) requires 110-140 C. Under these harsh conditions die initially formed 2-alkoxyboranes (37).undergo a spontaneous -eliInination to furnish 1,4-dienes (38) in good yields (Scheme 9 Table 5) Application of this cu-metallo-allylation/syR-elimination sequence to cyclic enol ethers provides a stereocontrolled access to 1,4-dienols containing a trisubstituted alkenic bond, as illustrated by the transformation (39) + (40) - (42) (Scheme 9). 

On treatment with a palladium(O) catalyst, vinyl epoxides undergo facile unimolecular rearrangement to give dienols or enones depending on the substitution pattern of the substrate. In the presence of an active methylene compound in the reaction system, however, a single alkylation product is formed. Cyclic and acyclic vinyl epoxides participate equally well. The reaction proceeds with clean alkylation from the same face as the oxygen of the epoxide, and proceeds with allyl inversion (Scheme 22). ... 

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