9-Membered Carbocycles: Strategies and Tactics for their Synthesis

Many natural products comprising a nine-membered carbocyclic core structure exhibit interesting biological effects. However, only a minority have succumbed to their synthesis in the past. The synthesis of functionalized nine-membered carbocycles still remains a challenging goal for synthetic chemists, mainly due to their high ring strain. Different strategies to overcome the unfavorable enthalpic and entropic factors associated with their formation are highlighted in this Concept article. The presented methods are classified into two different categories: (1) the ring-expansion of smaller rings or the ring-contraction of larger rings and (2) the direct cyclization of acyclic precursors.

Heuristic chemistry--addition reactions

Organic chemistry is often considered a difficult subject requiring great effort to achieve expert status. Only once this level has been reached, judging, deciding, and recalling chemical information will be fast and guided by heuristic strategies. These principles, used intuitively, improve the efficiency and speed of problem-solving and decision-making procedures. Establishing these strategies in teaching and learning chemistry should significantly help students acquire well-structured procedural knowledge early in their education. In an attempt to improve conceptual thinking in teaching organic chemistry, in this contribution we develop a heuristic view of addition reactions and propose a new way of perceiving this class of organic reactions.

Quantum chemical investigation of the thermal rearrangement of cis- and trans-pinane

The thermal rearrangement reactions of cis-pinane, 1, and trans-pinane, 2, into beta-citronellene, 3, and isocitronellene, 4, have been investigated using ab initio multiconfigurational CASSCF and CASSCF MP2 calculations. Concerted as well as stepwise retro-[2+2]-cycloaddition conversion mechanisms are discussed and the corresponding stationary points along the relevant reaction paths from the bicyclic starting compounds into their acyclic isomers have been optimized. Our calculations show that the stepwise retro-[2+2]-cycloaddition via biradicals is energetically favoured with respect to the concerted mechanism. In the biradical pathways to 3 and 4, it was found that a gauche ring opening of the cyclobutane ring in 1 and 2, respectively, shows significantly lower activation barriers than the competing anti ring opening. With the predicted reaction paths, the calculated activation energies are in very good agreement with experimental values. The reaction mechanisms can explain the differences in the reactivity of 1 and 2, as well as the selectivity differences with respect to the formation of 3 and 4, reported in previous kinetic studies.