2-Lithio-3-methoxy-1,3-dimethylcyclopentene: A synthetic equivalent of 2-lithio-1,3-dimethylcyclopentadiene and useful synthon for 6-alkyl-1,4- dimethylfulvenes and ansa-metallocene complexes

2-Bromo-3-methoxy-1,3-dimethylcyclopentene (3). is synthesized in 85% yield in 80 g scale by 1,2-addition of MeLi to 2-bromo-3-methyl-2-cyclopenten-1-one and subsequent in situ treatment of MeI. Addition of n-BuLi to the bromo-compound 3 in Et₂O affords 2-lithio-3-methoxy-1,3- dimethylcyclopentene (1). Successive addition of RCHO [R = CH₃, (CH₂)₂CH₃, CH(CH₃)₂, C(CH₃)₃, H] and MeI to the lithium compound and aqueous acidic work-up provide cyclopentadiene compounds, 2-[CHR(OMe)]-1,3-Me ₂C₅H₃ in 60-84% yields. Treatment of the cyclopentadienes with NaH or KH in pentane affords 1,4,6-trimethylfulvene, 6-n-propyl-1,4-dimethylfulvene, 6-isopropyl-1,4-dimethylfulvene, 6-tert-butyl-1,4-dimethylfulvene and 1,4-dimethylfulvenein 67-83% yields. Ligand for an ansa-metallocene complex, CH₂C(1,3-Me₂Cp) ₂ZrCl₂, can be shortly afforded in 64% yield by reacting the lithium compound 1 with the newly synthesized 1,4-dimethylfulvene.