of R1 to R6 is a hydrogen atom or a hydrocarbon residue having 1 to 3 carbon atoms...http://www.google.com/patents/US4891456?utm_source=gb-gplus-sharePatent US4891456 - Method for preparing norbornene structure compoundAdvanced Patent SearchPublication numberUS4891456 APublication typeGrantApplication numberUS 07/213,547Publication dateJan 2, 1990Filing dateJun 30, 1988Priority dateJul 6, 1987Fee