Acid-Catalyzed Rearrangements of (-)-Thujopsene

The transformations of (-)-thujopsene in liquid (HSO₃F-SO₂FCl) and over solid (TiO₂/SO₄ ^{2 -}) superacids, as well as by the action of peroxy acids, were studied. New tricyclic hydrocarbons were isolated. The experimental results were compared with the data of computer analysis of the most probable transformation pathways using molecular-mechanics and quantum-chemical methods.     

Rearrangements of free radicals x : C7H7 - radicals and related systems

7-Norbornadienyl radical rearranges in matrix to tropylium radical. Deuterated and cyano substituted bicyclo(3.2.0)heptadienyl radicals do not undergo 1.2-vinyl shifts prior to electrocyclic ring opening.     

Synthesis of (-)-7-epiaustraline and (-)-1-epicastanospermine

Highly efficient and selective syntheses of the title compounds are described. The cornerstone of the synthetic plan is the tandem inter [4 + 2]/inter [3 + 2] cycloaddition process. These syntheses differ from previous applications of this strategy in that they incorporate an alkylation in the hydrogenolysis step to close the second ring of the azabicyclic systems. Notable features of the sequence are (1) the highly regio- and stereoselective [3 + 2] cycloaddition of nitronate 15 with siloxymethyl (Z)-beta-silylvinyl ketone (Z)-22b and (2) the highly selective reduction of the resulting ketone 24a with L-Selectride. A single-crystal X-ray structure analysis of synthetic (-)-7-epiaustraline confirmed that the targeted structure was successfully synthesized. This stimulated a reexamination of the structural assignment of the natural product. (-)-1-Epicastanospermine was synthesized in four steps from the common intermediate 27a. The absolute configuration of (-)-1-epicastanospermine was assured by single-crystal X-ray structure analysis of intermediate (-)-27a. Thus, the sign of the optical rotation had to be revised. The overall efficiency of these syntheses were 9 steps and 23% yield for (-)-7-epiaustraline and 10 steps and 20% yield for (-)-1-epicastanospermine     

Carbene Rearrangements Unsurpassed: Details of the C(7)H(6) Potential Energy Surface Revealed

The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene (3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3LYP), CISD, CCSD, and CCSD(T) methods in conjunction with the 6-31G, 6-311+G, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary points were characterized by vibrational frequency analyses at CASSCF(8,8)/6-31G and B3LYP/6-31G. Phenylcarbene (1) has a triplet ground state ((3)A") with a singlet-triplet separation (DeltaE(ST)) of 3-5 kcal mol(-)(1). In agreement with experiment, chiral 3 is the lowest lying structure on this part of the C(7)H(6) potential energy surface. Bicyclo[4.1.0]hepta-2,4,6-triene (2) is an intermediate in the rearrangement of 1 into 3, but it is unlikely to be observable experimentally due to a barrier height of only 1-2 kcal mol(-)(1). The enantiomers of 3 interconvert via the (1)A(2) state of cycloheptatrienylidene (4) with an activation energy of 20 kcal mol(-)(1). The "aromatic" (1)A(1) state, previously believed to be the lowest singlet state of 4, is roughly 10 kcal mol(-)(1) higher in energy than the (1)A(2) state, and, in violation of Hund's rule, (3)A(2) is also calculated to lie above (1)A(2) in energy. Thus, even if (3)A(2) were populated, it is likely to undergo rapid intersystem crossing to (1)A(2). We suggest (3)B(1)-4 is the metastable triplet observed by EPR.