2.4- Enynones

Reduction of enynones to dienones is structure sensitive and is often unsatisfactory if the acetylenic bond is attached directly to the carbonyl 30J 1,52). Selectivity is improved if the acetylenic bond is terminal 52,70,71). 

Figure 3.45. Scope of Rh/(ff)-segphos-catalyzed asymmetric 1,6-addition of ArTi (Oi-Pr)4Li to conjugated enynones in the presence of Me3SiCl.

The same catalytic system (SeOi/TBHP) has also been used by Chabaud and Sharpless in the allylic oxidation of alkynes. The oxidation products resulting from the Se02-catalyzed allylic oxidation with TBHP are the allylic alcohol, the allylic diol, the allylic ketone, the ketol and the enynone (Scheme 127). The main product of the reaction is either the alcohol or the diol, depending on the substrate employed (together 76-100% of the whole yield). The yields of allylic oxidation products together range from 15 to 88%. From the observed results with unsymmetrical alkynes it could be concluded that the reactivity sequence for the carbon attached to the triple bond of alkynes is CH2 CH > CH3. 

The synthesis of pent-4-en-l-yn-3-ones from alkynes involves their deprotonation and condensation with ot,(3-unsaturated aldehydes and subsequent oxidation of the allylic alcohol. A double thia-Michael addition occurs when the enynone is treated with sulfide which leads to 2,3-dihydrothiopyran-4-ones bearing different substituents at the 2- and 6-positions (Scheme 217) <2006TL5095>. 

Dihydrothiopyran-4-ones have been efficiently obtained from terminal alkynes through conjugate addition to a,/3-unsaturated aldehydes followed by oxidation to the enynone and cyclization <2007MC13>. The [2+2] photocycloaddition of ethylidenemalononitrile to the 2,2-dimethyldihydrothiopyranone is both regio- and stereo- specific <2007S1426>. 

Cyclization of enynones to methylenecyclopentenones. This cyclization can be effected under thermal conditions, particularly in the case of activated (Z)-enynones, but in only moderate yield. Some acceleration is observed with reagents known to promote enolization (LiCl and collidine p-toluenesulfonate), but dramatic effects can be obtained with phenols such as 1. Vitamin E is marginally more... 

Cyclization of enynones to phenols. This sulfonate can effect an acid-catalyzed cyclization of enynones to phenols (equation I). Thus the enynone 2 can be cyclized to the phenol 3 by catalysis with 1. Note that a different phenol (4) obtains from acid-catalyzed cyclization of the enol acetate of 2. 

Addition to an enynone (1). Cuprates, particularly higher-order ones, undergo 1,6-addition to 1 to provide the less stable (Z)-dienone 2 as the major product. The (Z)-stereoselectivity increases with the bulk of the transferred R group. The addition may involve an allenyl enol. Addition of QHjSLi to 1, however, provides (E)-dienones, possibly via an intermediate 1,2-adduct. 

Assign the 400 MHz H NMR spectrum of this enynone as far as possible, justifying both chemical shifts and coupling patterns. 

This reaction has been shown to proceed stereospecifically in the conversion of the dr-substituted cyclopropylcarbinol (29) to the (Z)-enynone (30) shown in equation (30). The substrates with R = H, Me and TMS all gave comparable yields. 

A Pt-catalyzed cyclization of an enynone as illustrated in the following scheme using a nucleophilic solvent afforded substituted furans in good to high yields. In this reaction, the solvent serves as a nucleophile to attack the p-position of the double bond <06TL5307>. The same reaction can also be catalyzed by Bu N[AuClJ in [bmim]BF,. It is noteworthy that the catalyst can be recycled several time and keep the catalytic activity unchanged <06SL1962>. 

The alkynyl-9-BBN 15 undergoes a facile reaction with enol alkyl ethers 16 of jS-ketoaldehydes in hexane at room temperature to provide conjugated enynones 17 in good yield [Eq. (8)] [14]. 

Bis-acetylenic alcohol 1 undergoes a facile oxy-Cope rearrangement to afford mixtures of ( )-and (Z)-enynones. The latter compounds react further to give spiroannulated methylenecy-clopentenone 4 upon enolization and clectrocyclic ring closure1134. 

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