Aldrichimica Acta Volume 30 No 4 (pdf) from Sigma-Aldrich These reactions, which are improved by conducting the experiments under high-pressure conditions, can be extended to the formation of bisadducts by using the biscyclobutene 6; e.g., adamantylidenylfulvene37 affords the symmetrical product 39 as outlined in Scheme 8.
Source: wiktionary