Transfer enals

Acetylene is also protected as propargyl alcohol (300)[2H], which is depro-tected by hydrolysis with a base, or oxidation with MnOi and alkaline hydrolysis. Sometimes, propargyl alcohols are isomerized to enals. Propargyl alcohol (300) reacts with 3-chloropyridazine (301) and EtiNH to give 3-diethylami-noindolizine (303) in one step via the enal 302[2I2]. Similarly, propargyl alcohol reacts with 2-halopyridines and secondary amines. 2-Methyl-3-butyn-2-ol (304) is another masked acetylene, and is unmasked by treatment with KOH or NaOH in butanol[205,206,213-2l5] or in situ with a phase-transfer cata-lyst[2l6]. 

One of the first synthetic applications of organoboranes in radical chemistry is the conjugate addition to enones (Scheme 23, Eq. 23a) and enals reported by Brown [58-61]. Addition to -substituted enones and enals are not spontaneous and initiation with the oxygen [62], diacetyl peroxide [63], or under irradiation [63] is necessary (Eq. 23b). A serious drawback of this strategy is that only one of the three alkyl groups is efficiently transferred, so the method is restricted to trialkylboranes derived from the hydroboration of easily available and cheap alkenes. To overcome this limitation B-alkylboracyclanes have been used but this approach was not successful for the generation of tertiary alkyl radicals [64,65]. 

The proposed catalytic cycle is shown in Scheme 35 and begins with the imida-zolylidene carbene adding to the enal. Proton transfer provides acyl anion equivalent XLVII, which may be drawn as its homoenolate resonance form XLVIII. Addition of the homoenolate to aldehyde followed by tautomerization affords L the precursor for lactonization and regeneration of the carbene. 

Organocatalytic transfer hydrogenation of enals has been discussed as a route for synthesis of some lepidopteran pheromones. 

As shown in Scheme 5, respective enals [28a-c] were incubated in the presence of 1.0 equiv of piperidine and 1.0 equiv of Ac20 in EtOAc (added at -10 °C) at 85 °C in a sealed flask for 45 min to 1 h. The solution containing the iminium salt 29a-c was then transferred without cooling to a solution of pyrone 22 in EtOAc. After stirring at 85 °C in a sealed flask for an additional 24-48 h, the 2//-pyrans 23,30b, and 30c were obtained in good yields.35,37... 

The best selectivity in the Michael addition of 2-carboxycyclopentanones to an enone or enal were recently achieved by Maruoka et al. [9]. As shown in Scheme 4.5, as little as 2 mol% of the binaphthyl-derived phase-transfer catalyst 10 - in the presence of 10 mol% solid potassium carbonate - enabled the highly efficient... 

A primary alcohol and amines can be used as an aldehyde precursor, because it can be oxidized by transfer hydrogenation. For example, the reaction of benzyl alcohol with excess olefin afforded the corresponding ketone in good yield in the presence of Rh complex and 2-amino-4-picoline [18]. Similarly, primary amines, which were transformed into imines by dehydrogenation, were also employed as a substrate instead of aldehydes [19]. Although various terminal olefins, alkynes [20], and even dienes [21] have been commonly used as a reaction partner in hydroiminoacylation reactions, internal olefins were ineffective. Recently, methyl sulfide-substituted aldehydes were successfully applied to the intermolecu-lar hydroacylation reaction [22], Also in the intramolecular hydroacylation, extension of substrates such as cyclopropane-substituted 4-enal [23], 4-alkynal [24], and 4,6-dienal [25] has been developed (Table 1). 

General Procedure for Transfer Hydrogenation of Enals Using Symmetric Hantzsch Ester [97] (pp. 110 and 394)... 

Scheme 25. Asymmetric counteranion-directed catalysis transfer hydrogenation of enals...

The phase-transfer-catalyzed (PTC) reaction of 1,2-dicyanoethene-l,2-dithiolate with 3,3-dichloroprop-2-enal gave the 1,3-dithiole derivative 545 in 77% yield (Equation 69) <2003HCA2589>. 

Deoxygenation. a,P-Epoxy aldehydes and ketones undergo deoxygenation with basic thiourea dioxide to provide enals and enones, respectively, in good yields. Bu NBr is added to the reaction media as a phase-transfer catalyst. 

Conjugate addition. Allyl transfer from allyltributylstannane to enals, enones, and... 

Coupling of vinyl halides with allyl alcohols,6 Palladium-catalyzed coupling of vinyl halides with allylic alcohols under usual Heck conditions results in a complex mixture of products. Addition of silver carbonate and a phase-transfer catalyst can result in a palladium-catalyzed coupling of vinyl halides with a primary allylic alcohol to give a -y.S-enal, with retention of configuration of the vinyl halide. 

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