Diels-Alder reactions with 1,1-diethoxybut-3-yn-2-one and some 1,1-diethoxy-5-hydroxyalk-3-yn-2-ones and their acetates

The title compounds were reacted with a few conjugated dienes at room temperature and above. The alcohols were unreactive, but the other ynones reacted at a reasonable rate. Conceivably, the expected cyclohexa-1,4-diene adducts were formed, but they were unstable and aromatized to the corresponding benzene derivatives, which were isolated in low to excellent yield.


Introduction
The Diels-Alder reaction is a valuable transformation in organic synthesis.There are two main versions of the reaction, but the most common by far involves the addition of an electron-rich 4 conjugated diene to an electron-poor 2 partner, the so-called dienophile.Most dienophiles are alkenes attached to one or several electron-withdrawing groups, but some conjugated ynones have also been utilized.5][6] The results presented in these publications have been applied and developed further in recent years and made Diels-Alder reactions with ynones an attractive tool in organic synthesis. 7he structural variation among the ynones used as dienophiles is rather small.The substituents close to the ynone moiety are limited to a selection of aryl groups, some alkyl groups, and the trimethylsilyl moiety, 7 and our easy access to conjugated alkynones of the alk-3-yn-2-one type, [8][9][10] with polar substituents attached to C-1 and C-5, therefore encouraged us to investigate their reaction with some conjugated dienes commonly used in Diels-Alder reactions.The results of these studies are reported here.

Results and Discussion
2][3] It was therefore natural to pick 1,1-diethoxybut-3-yn-2-one (1), easily available in high yield from ethyl vinyl ether, 9.10 as the first dienophile to get an impression of the influence of polar substituents.In order to estimate the reactivity, exploratory experiments were carried out with isoprene varying the solvent, reaction temperature, and reaction time.A general picture emerged: 1) diluted solutions stirred at room temperature gave no product(s) and the reactants were recovered almost quantitatively; 2) reactions performed in a surplus of diene at reflux for 2-3 days afforded one or two products and a variable amount of a very viscous material, probably a mixture of polydiene on the basis of IR and NMR spectra. 11Based on these observations, ynone 1 was reacted with five dienes (2) by heating mixtures of 1 and a 10 times molar excess of the diene at reflux.Two dienes, 4-methyl-1,3-pentadiene and 2,5-dimethyl-2,4-hexadiene, both with geminal methyl groups, did not react.The three remaining dienes, 2a-2c, however, gave a 2,2-diethoxyacetylated methylated benzene derivative (3) along with fair amounts of polydiene, and 2a and 2b furnished in addition a minor amount of the benzoic acid (4) corresponding to 3a and 3b, respectively (Scheme 1).3][14] The by-product in this oxidation is water, which reacts with 3 to form benzoic acids 4 and formaldehyde diethyl acetal.
The results summarized in Scheme 1 clearly show that the adduct formation from 1 is substrate dependent.Thus, the reactivity of 1 in the Diels-Alder reaction deviates from that of the ethoxy-free analogue but-3-yn-2-one with respect to both relative yield 12 and selectivity. 13This difference can be understood in terms of a combination of the dienophiles' electron density and the steric hindrance at the diene termini.The isopropenyl terminal carbon is the most electron rich in the dienes and should therefore become attached to the  carbon of ynone 1. Formation of only ketone 3 proves that this mode of attack occurred consistently; the adduct formation was therefore regiospecific.But unlike 2a and 2b, 2c is methylated at the other terminus and that would make its bond formation at the ynone  carbon less favourable for steric reasons.Thus, a lower yield when 2c is applied is expected, and that was indeed observed (Scheme 1).
The results presented here show that the reactivity of the ynones is somewhat influenced by the substituent(s) attached to the alkynone chain near the -unsaturated ynone moiety.Since the reaction conditions required for 1 and 6 to react are comparable to those used when but-3-yn-2-one is reacted with the same dienes, the electronic impact from the ethoxy and acetyloxy groups is judged to rather unimportant compared to the steric influence.This conclusion is supported by the yields of 3 and 7, which can be rationalized in terms of variation in steric congestion around the ynone section of the dienophiles.When propargylic alcohols 5 are reacted, however, additional interactions become significant and prevent cycloaddition from taking place.Two conceivable effects can be envisaged to be involved.By hydrogen bonding to the carbon-carbon, triple-bond electron cloud, [19][20][21] the OH group is pulled slightly closer to the triple bond, and this may facilitate a Michael-like interaction between the oxygen atom and the  carbon. 22As a result, the electrophilicity and availability of the  carbon are reduced, apparently enough to make cycloaddition so unfavorable that the reaction does not occur.Scheme 3. Some contributing forms describing the interaction between the propargylic OH group and the C-C triple bond.

Conclusions
In summary, ethoxy and acetyloxy groups in the vicinity of the -unsaturated ynone moiety in conjugated alkynones influence the reactivity in Diels-Alder reactions sterically only.However, a hydroxyl group in  position renders the ynones unreactive towards 1,3-butadienes, which instead suffer polymerization.