Regioselectivity in the C-alkylation of triethyl 3-methyl-4-phosphonobut-2-enoate

The reaction of triethyl 3-methyl-4-phosphonobut-2-enoate (1) with three alkyl halides AlkX (Alk=Prⁱ, Me₂CHCH₂CH₂, andc-C₅H₉; X=Br, I) in the system KOH(solid)∩DMF∩Buⁿ ₄NBr at ≈20°C gives exclusively products of alkylation at C(2) with Δ² and/or Δ³ position of the double bond. Under the same conditions, the reaction of 1 with MeI gives a mixture of products with different substitution patterns. Only the use of an ion pair extraction technique affords 2-methyl-Δ²-products selectively, albeit in rather moderate yields. The Horner—Emmons olefination of PhCHO with the resulting phosphonates gives ethyl 2-alkyl-3-methyl-5-phenylpenta-2,4-dienoates in high yields. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1839–1844, October, 1997.