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]. 

Aldehydes, enals, dienals, ketones, and hydrocarbons, which are responsible for disagreeable odors, generally bok at lower temperatures than fatty acids. Analysis showkig a free fatty acid concentration of less than 0.05% is an kidication that deodorization is sufficientiy complete. Some of the dienals have very low odor thresholds and sensory evaluation of the finished ok is a judicious quaHty assurance step. 

Citral is prepared starting from isobutene and formaldehyde to yield the important C intermediate 3-methylbut-3-enol (29). Pd-cataly2ed isomeri2ation affords 3-methylbut-2-enol (30). The second C unit of citral is derived from oxidation of (30) to yield 3-methylbut-2-enal (31). Coupling of these two fragments produces the dienol ether (32) and this is followed by an elegant double Cope rearrangement (21) (Fig. 6). 

Work at Rhc ne-Poulenc has involved a different approach to retinal and is based on the paHadium-cataly2ed rearrangement of the mixed carbonate (41) to the aHenyl enal (42). Isomerization of the aHene (42) to the polyene (43) completes the constmction of the carbon framework. Acid-catalyzed isomerization yields retinal (5). A decided advantage of this route is that no by-products such as triphenylphosphine oxide or sodium phenylsulfinate are formed. However, significant yield improvements would be necessary for this process to compete with the current commercial syntheses (25—27) (Fig. 9). 

Thermal cycli2alion of dienones enals, ynones, diones keloesters, etc, to monocyclic spirocycfc bicyclic derivatives, (ene reaction of unsalurated enol)... 

From a mechanistic standpoint, ammonia serves two functions 1) it behaves as a base to catalyze an aldol reaction between 2 equivalents of 31 to generate the corresponding enal 33, and 2) it is the source of nitrogen for the resultant pyridyl ring. This occurs through formation of enamine 34 with a third equivalent of 31. The Michael addition of 34 to 33 followed by cyclization gives rise to 32. 

Kelly applied this chemistry to the synthesis of cyclosexipyridine 66. This is an example of an intramolecular variation to this method. Masked enal 65 was prepared and treated with the standard reagents. The acidic medium liberated the aldehyde from its acetal protection. This in situ formation of the reactive species, similar to the above example, then undergoes cyclization to the expected pyridine derivative 66. 

Cblorotriruethylsilane-accelerated divinylcuprate addibon to enal 61, followed by a Wibig oleflnation, provided enoate 62 as a single stereoisomer in excellent yield (Sdieme 6.10) [23]. Hie enoate 62 cnUld be transformed in ftillher steps into olivin i63), die aglygon of olivomycin. 

Although the C-3 stereocenter in 6 may be susceptible to epimer-ization in the presence of a basic organolithium reagent, enal 6 condenses smoothly in the desired and expected way with lithio sul-fone 5 at -78 °C to give, after quenching with acetic anhydride, a stereoisomeric mixture of acetoxy sulfones (see 35, Scheme 7). ( ,E,7f)-Triene 36 is then unveiled on reduction of the stereoisomeric acetoxy sulfones with 5 % sodium amalgam (77 % overall yield from 6).3... 

Only two operations remain. Reaction of the enolic form of aldehyde 162 with Eschenmoser s reagent (H2C=NMe2+I-)62 in the presence of triethylamine provides the desired enal after a simple / -elimination. Finally, cleavage of the remaining tcrt-butyldi-methylsilyl ether with HF pyridine completes the total synthesis of (+)-brevetoxin B (1). 

This provides a route to a/3-unsaturated aldehydes (25). 2-Methyl-3-phenylprop-2-enal (25)... 

Methoxytrimethylsilane, 123 Methyl acetoacetate, 92 Methyl bromoacetate, 107 Methyl 11-hydroxyundecanoate, 58 Methyl lithium, 27,28 Methyl 10-undecenoatc, 58 2-MethyM,3-dithiane, 81 (V f ,55)-Methyl-3-phenyldimethyl-silyl-3-phenylpropionic acid. 53-4 2-Methyl-3-Phenylprop-2-enal, 111 2-Methyl-2-trimethylsilyl-l,3-dithiane, 81 2-Methyl-l-(trimethylsilyloxy)cydo-hex-l-ene, 100,109 2-Methyl-l-trimethylsilyloxy-cyclo-hex-6-ene, 100... 

Olefin cross metathesis starts to compete with traditional C=C bondforming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

An endo-selective ionic Diels-Alder reaction of a,/f-enone and a,/f-enal acetals catalyzed by electrogenerated acid [99]... 

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

An oxygen analogue, 2,4,6-trimethylpyrilium (51) perchlorate, is also found to undergo a photoinduced hydration reaction, to give experimental data that this is the only, or even the preferred reaction route. 

The formation of the c -annulated hexahydrothioxanthene (34) from thiophenol and hept-6-enals may also result from a [ +2] cycloaddition, although the minor stereoisomers probably arise from a cationic non-concerted cyclisation <96SL396>. 

Answer Normal a,6 disconnection reveals another enal and hence another a,6 disconnection. 

Butanone, see Methyl ethyl ketone (MEK) trans-But-2-enal — ... 

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