Conjugated enynones

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 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]. 

Addition of organocuprates to cross-conjugated enynones leads to the dienones. Dimethyl sulfide stabilizes the cuprate adduct intermediates when it is employed as a reaction medium. ... 

The reaction is general for other (3-methoxy- , S-unsaturated ketones that can assume the cisoid conformation and provides a useful synthesis of conjugated enynones. 

B-Alkenyl- and B-l-alkynyl-9-borabicyclo[3,3,l]nonanes undergo smooth 1,4-addition to acyclic -unsaturated ketones. Cycloalk-2-enones do not yield any conjugate addition product. Furthermore the alkynylboranes react with acyclic /8-methoxy-a/8-unsaturated ketones to give conjugated enynones. ... 

Representative procedure for the gold-catalyzed synthesis of fiirans from cross-conjugated enynones. 4,5,6,7-Tetrahydro-4-methoxy-2-(4-methoxyphenyl)benzofiiran ... 

B-l-Alkynyls-9-BBN easily and quantitatively prepared from lithium methyl al-kynyldialkylborinate [12] undergo a facile condensation with readily available 4-methoxy-3-buten-2-one in hexane at room temperature to provide, in excellent yield, conjugated enynones (Eq. 7.9) [18]. 

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>. 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

The interesting dithia-analogue (317) of TXA has been prepared. This noteworthy synthesis commences with the readily available ketoester (313) and involves the conjugate addition of H S and Me02C(CH2)2S to the enynone (314) in the key steps. The monocyclic acetal (315) so formed was converted into the mesylate (316) which, on treatment with base and deprotection of the functional groups, gave the target molecule (317) (Scheme 44). 

Cyclic a-diazocarbonyl compounds (59) and enynones (61) have been used as Rh-and Zn-carbenoid precursors, respectively. Cyclic derivatives (59) have been found to favour intermolecular Rh-catalysed cyclopropanation reactions, relative to the formation of conjugated alkene (60) by intramolecular -hydride elimination as is usually observed in the case of a-alkyl-a-diazocarbonyl compounds this high level of chemoselectivity is reported for the first time. Rh-carbenoids derived from (59) have also promoted cyclo-propenation reactions as well as diverse X-H insertion reactions (i.e., X = C, N, O, S). In parallel, highly functionalized cyclopropylfiirans (62) have been successfiilly prepared from an alkene and an enynone (61) by a cyclization/cyclopropanation sequence conducted in the presence of catalytic amounts of ZnCl2, which is cheap and of low toxicity computations support the probable participation of intermediate Fisher-type Zn(II) carbene complexes (63). 

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