Acetals from alkynes

RXN63 Preparation of Allylic Acetates from Alkynes by Tandem Redox-Addition... 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

A series of 3-hydroxybut-4-ynoic esters (319) has been prepared from alkynic ketones and bromoacid esters in the presence of zinc (61AP234). Ring closure to the pyran-2-one occurs in a mixture of acetic and sulfuric acids. The preparation of 4,6- and 3,4,6-substituted pyranones is possible by this route, the substituents being different or identical, and as such is complementary to the alkynic route to the 3,6-disubstituted compound (57JA2602). 

Methylene-l-cyclopentenes.1 All attempts to effect cycloaddition of the acetate 1 (9,454 11,578) to alkynes are unsuccessful, but 1 does add to norbornadienes (prepared from alkynes) in the presence of a palladium(O) catalyst to form adducts that afford 4-methylene-1-cyclopentenes on flash vacuum thermolysis (equation I). 

By using a more sterically hindered and electron-rich catalyst, (C5Me5)RuCl (COD), y,<5-unsaturated acetals and aldehydes have been synthesized from alkynes and allyl alcohol [41] (Eq. 29). It is noteworthy that in this case the branched isomer is the major product. This is due to the bulkiness of the C5Me5, with respect to the C5H5 ligand, which favors the head-to-tail coupling. 

Iron(n) salts mediated the coupling of trialkylboranes with KSGN to give RSCN via a radical process.553,554 Stereochemically pure ( )-l-thiocyanato-alkenes or ( )-l-azide-alkenes were obtained from alkynes when hydro-boration with disiamylborane was followed by reaction with potassium thiocyanate or sodium azide in the presence of copper(n) nitrate, copper(n) acetate, and small amount of water in polar aprotic solvent (Equation (116)).555... 

The most obvious method for the formation of alkenes from alkynes is by partial reduction. This reaction can be effected in high yield with a palladium-calcium carbonate catalyst that has been partially deactivated by addition of lead(II) acetate or quinoline (Lindlar s catalyst). It is aided by the fact that the more electrophilic alkynes are adsorbed on the electron-rich catalyst surface more strongly than the corresponding alkenes. An important feature of these reductions is their high stereoselectivity. In most cases the product consists very largely of the thermodynamically... 

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