Triptindane‐9,10,11‐triol: Threefold Hydride Transfer from Lithium Aluminum Hydride as the Key Step to C ₃ ‐Symmetrically Trifunctionalized Tribenzo[3.3.3]propellanes

The C 3v ‐symmetric framework of triptindane ( C 3v ‐tribenzo[3.3.3]propellane, 1 ) was equipped with three benzylic monovalent functional groups in a C 3 ‐symmetric orientation. Key reaction was the reduction of 9,10,11‐triptindanetrione ( 2 ) with lithium aluminum hydride or sodium borohydride giving the corresponding C 3 ‐symmetric triol 5 in high yield and perfect diastereoselectivity. The diastereoselective reduction of 9,10‐triptindanedione 9 with LiAlH 4 was also studied for comparison. Conversion of triol 5 to the respective tribromide 13 , trichloride 14, and triiodide 15 and subsequent refunctionalization of 13 to 9,10,11‐triaminotripindane 18 via the corresponding triazide 16 was achieved with preservation of C 3 ‐symmetry in each case. In contrast, the C 1 ‐triazide 17 is formed along with 16 under Lewis‐acid conditions, enabling an access to the C 1 ‐symmetric triamine 19 . The increased torsion about the propellane axis of tribromotriptindane 13 was determined by X‐ray crystallography. There is strong evidence that the reduction 2 → 5 occurs by fast intracomplex transfer of three hydrides from the same alanate ion. In turn, attempts to confirm this by use of an equimolar mixture of LiAlH 4 and LiAlD 4 were nonsupportive; rather, they point to a hitherto unknown fast hydride exchange occurring in etheral alanate solutions.