Ring-opening of the compound 5.

Entry	Solvent	Temperature	Reagents	Yield
1	THF	0 °C	LDA	Decom.
2	DME	r.t.	Li, EDAa	Decom.
3	DME	0 °C	Li, EDA	Decom.
4	CH3CN	r.t.	Anhydrideb	8, 98% (trans : cis = 3 : 2)
5	THF	−78 °C to 0 °C	LHMDS, MeOTf	Decom.
6	THF	−78 °C to 0 °C	LDA, MeOTf	Decom.

^aEthylenediamine.

^bAcetic toluene-p-sulfonic anhydride, prepared by acetyl chloride and PTSA.²⁵