3-Methylbut-2-enol

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). 

As with prenylphenols in the monohydric series mild acid-catalysed procedures have been developed for the prenylation of polyhydric phenols. Thus a variety of these compounds have been condensed with 3-methylbut-2-enol and with 2-methylbut-3-en-2-ol in dilute aqueous formic acid. Hydroquinone for example afforded isopentenylhydroquinone, 4-hydroxy-2-(3-methylbuten-2-yl)phenol (ref. 15). 

All these easy condensations of 2-methylbut-3-ene-2-ol or 3-methylbut-2-enol (y,y-dimethylallyl alcohol) with mono- and dihydric pheniols under very mild acidic conditions do support the biogenetic theory of C-isopentenylation with y,y-dimethylallyl pyrophosphate (ref.1). 

For the synthesis of (2/ ,2 / )-oscillol (II), the aglycone of oscillaxanthin (208), the AD was selected [1IJ. The starting material 3-methylbut-2-enol (12) was converted, with p-nitrobenzoyl chloride, into the ester 13 (yield 88%). After AD the diol 14 was converted, with p-toluene-sulphonic acid, into the acetal 15 and seven further steps gave the Cio-phosphonium salt 16 which, in a Wittig reaction with crocetindialdehyde (77) gave (2/ ,2 / )-oscillol (//) (Scheme 6). 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

Isopropylidene carbene (Me2C=C ) is generated efficiently from the enol triflate derived from iso-propyl trimethylsilyl ketone using KF/Aliquat [45], but full experimental details are not given. The carbene can be trapped by isocyanides to produce, after solvolysis, -substituted 3-methylbut-2-enoamides in 35-52% yield [46]. 

They resemble hyperforin in their chemical structure, having in addition a 2,2-dimcthyl-2//-pyran ring, most probably derived by the cyclization of a 3-methylbut-2-enyl side chain with an enolic hydroxyl group. Only a few reports describe a similar natural tricyclic system [59-60]. 

A synthetic example of this reaction is taken from Evan s synthesis of (+)-miyakolide.l 3 Ketone 279 was treated with B-chlorodicyclohexylhorane in the presence of triethylamine, and this generated boron enolate 280. 4-Methylbut-2-enal was also present in the reaction and they condensed to give a 98% yield of 281, with a diastereoselectivity of 88 12. 

The lithium enolate derived from angelica lactone (LDA, THF, - 78 °C) reacts as a Michael acceptor exclusively at the y-position thus the butenolide (65) is obtained from ethyl acrylate. Surprisingly, in view of this finding, reaction with methyl iodide results in mixtures of the a-methyl and aa-dimethyl derivatives. Similar results have been obtained with the enolates of 2- and 3-methylbut-2-enolides which undergo prenylation by dimethylallyl bromide at both the a- and y-positions. By contrast the tetronic acid derivative 3-methoxy-2-methylbut-2-enolide is alkylated only at the y-position to give (66) in high yields. 

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