Esters from alkyne ethers

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... 

Pyrazole derivatives can be prepared analogously from alkynes e.g., reaction of propynal with ethereal diazomethane gives 84% of pyrazole-3-carbaldehyde and that with diazoacetic ester gives 86% of ethyl 5-formyl-pyrazole-3-carb-oxylate.18 Further, pyrazole itself is readily accessible from acetylene and diazomethane.19... 

Treatment of tetrahydrofiiran solutions of the hydride-chloro complex 20, at room temperature, with the stoichiometric amount of another methanol solution of KOH gives the five-coordinate hydride-hydroxo derivative OsH(OH)(CO)(P Pr3)2 (35). The addition of the stoichiometric amount of acetylendicarboxylic methyl ester to diethyl ether solutions of 35 affords a red solution, from which orange crystals of the P-diketonato complex 36 are isolated in 95% yield, after crystallization from pentane at -20 °C (Scheme 11). The formation of 36, which has been characterized by X-ray diffraction analysis, is the result of the trans-addition of the 0-H bond of 35 to the carbon-carbon triple bond of the alkyne [19]. 

DCBS is highly regioselective, exhibiting the directive effects similar to those observed for monochloroborane diethyl etherate. Alkyldichloroboranes are readily isolated by distillation under vacuum. Methanolysis provides the corresponding alkyl-boronic esters (eq 2). Alkenylboronic esters can be obtained from alkynes (eq 3). ... 

Digitoxigenin, structure of, 1097 Digitoxin, structure of, 989 Dihalide, alkynes from, 261 Dihedral angle, 94 Diiodomelhane. Simmons-Smith reaction with, 228-229 Diisobutylaluminum hydride, reaction with esters, 812 structure of, 699 Diisopropylamine, pK.d of, 923 1,3-Diketone, pfCa of, 852 Dimethyl disulfide, bond angles in, 20 structure of, 20 Dimethyl ether, electrostatic... 

---