1,4-Bishomo[6]prismane (garudane): molecular structure and strain analysis of reactions potentially leading to prismanes

Abstract

The proposed structure of garudane (1), the face-to-face D_2h dimer of norbornadiene, has been confirmed by the X-ray analysis of a single crystal of the 1,3-bis(methoxycarbonyl) derivative (2), monoclinic, space group C2/c, a= 16.570(6), b= 8.073(4), c= 12.151(4)Å,β = 114.89(2)°, final R= 0.055, R_w= 0.070. Detailed structural characteristics observed for (2) are compared with the isomeric D_2d dimer (isogarudane)(3). Compound (1) is calculated to be 44 kcal mol^-1 mere strained than (3). The demethylenation procedure on (1)-and the 1,4-bis homo[7] prismane (10) of similar structure-as a potential synthetic route to [6]- and [7]-prismanes (4) and (5) has advantages over the intramolecular [2+2]-photocyclization of polycyclic dienes [e.g.(12) and (13)] and the pinacolic coupling of polycyclic diketones [e.g.(14)]. These are discussed on the basis of strain analysis.