“…On the other hand, the somewhat alleviated internal repulsion in alkoxide 4 allowed it to be trapped by protonation below −10 °C before the cyclization could interfere [ 3 ], so that the resultant alcohol 5 could be isolated (crude yield 90% from 1,1,3,3-tetramethylindan-2-one) and dehydrated to give 2-(dichloromethylidene)-1,1,3,3-tetramethylindane ( 6 ) as the only product (97% yield). (ii) In X-ray diffraction analyses [ 4 – 9 ], the 1,1,3,3-tetramethylindan-2-ylidene parts turned out to be rather rigid, except for an occasional folding along the C-1/C-3 axis, and they did not exhibit the structural disorder problems and vexing angular flexibility which can arise with the t- Bu 2 C groups exemplified in Scheme 1 . (iii) Depending on the substituents at the exocyclic C-α atom in 7 , all four methyl groups in the 1,1,3,3-tetramethylindan-2-ylidene parts [ 10 ] and in their truncated analogue 1,1,3,3-tetramethylcyclopent-2-ylidene [ 11 ] may be nonequivalent and provide useful stereochemical and stereodynamic NMR information that would not be available from models containing the t- Bu 2 C moiety with free rotation about the t- Bu–C bonds.…”