Methoxybutenyne

To shed light on the reaction pathway, a cyclization with methylhydrazine was carried out (70ZOR439 73ZOR832), since pyrazoles with nonequivalent positions 3 and 5 are obtained in this case. From l-methoxybut-l-en-3-yne and methylhydrazine sulfate a 4 5 mixture of 1,3- (110) and 1,5-dimethylpyrazoles (111) was formed (GLC) (70ZOR439). In the reaction of methoxybutenyne homologs (R = Ft, -Pr) with methylhydrazine (72°C, H", H2O, 2 h), lengthening of the alkyl radical leads to an increase in the content of 1,5-isomer (111), with yields of 50-60%. Thus, when R = Ft, the ratio is 2 3 (73ZOR832). 

Since l-heterobut-l-en-3-ynes are readily alkylated and functionalized at the terminal acetylenic carbon atom, their reaction with hydrazines makes it possible to introduce diverse (including functional) substituents into the pyrazole ring. For instance, from benzylated methoxybutenyne 112, isomeric 2-phenylethylpyrazoles 113 were obtained in 74% yield (81H146). 

The reaction of methoxybutenyne with diethylcarbonate gives a mixture of substituted acrylates 119 and 120 (yield about 50%), which quantitatively form, with... 

For methoxybutenyne the 127 128 ratio is 1 2.6 (GLC). With lengthening of the alkyl radical at the acetylene bond the ratio of isomers is changed in favor of... 

Dipolar cycloaddition of 2-diazopropane and 1,3-diphenyldinitrilimine to E- and Z-methoxybutenynes occurs at the triple bond to form 3,3-dimethyl-5-(2-methoxyvinyl)pyrazole (168) and a mixture of , Z-l,3-diphenyl-4- (169) and -5-(2-methoxyvinyl)pyrazole (170) [70CR(C)80]. 

Cyclic carbonylylid.es 173 react with Z-methoxybutenyne according to the [2+3]-eyeloaddition seheme to form the furan derivatives 175 via nonisolable intermediate 174 (85CB785). 

A synthesis of 2-alkyl-2,3-dihydro-y-pyrones (187) from methoxybutenyne and aldehydes has been described (83TL4551). The condensation of lithiomethoxy-butenyne (184) with aldehydes at -78°C leads to the secondary alcohols 185, which form the dihydropyrones 187 via hydration of the acetylenic bond and hydrolysis of the methoxyethenyl group to the ketoenol 186 (0°C, p-TSA, THF, H2O or 30% HCIO4, 20 min) folowed by intramolecular cycloaddition. 

Methoxybut-3-en-2-one is formed on heating 4,4-dimethoxybut-2-one (160°C, 1 h, yield 90%) in the presenee of sodium hydroearbonate or by hydration of methoxybutenyne at 50°C in the presenee of mereury sulfate (1 h, yield 60-70%) (53CB793 60MI1 80MI1). 

Substituents on the double bond direct the addition to the triple bond,46 73"74 although when the latter is also substituted, the double bond is again preferred. Frequently only one sense of addition to a given bond is observed (Scheme 3),46,74,744 although with 1-methoxybutenyne this depends... 

Z-Methoxybutenyne has been converted into dimetallated butadiyne by using an excess of alkyllithium [151] ... 

Methoxybutenyne undergoes a similar elimination with BuLi in THF [115] ... 

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