1.3.5- Hexatriene electronic configuration

Figure 3 Comparison of the orbital correlation diagrams for the thennal dis (a) and con (b) 6e electrocyclic ring closures. Note that the electronic configuration is shown in each case only for the ground state of the hexatriene revening to the cyclohexadiene via the dis and con modes.

Next, let us look at the thermal cyclization of a disubstituted hexatriene, RCH CH—CH=CH—CH CHR, whose electronic configuration is shown in Fig. 29.17. The HOMO for the ground state of the hexatriene is 3. If we compare this with the HOMO for the ground state of butadiene ( 2 in Fig. 29.6), we see that the relative symmetry about the terminal carbons is opposite in the two cases. 

An energy diagram showing the electron configuration of the ground state and excited state of 1,3,5-hexatriene. 

The photochemically-exdted hexatriene system has one of its r-electrons promoted to the first excited r-orbitaL The terminal symmetry of this orbital (13) is the reverse of that depicted in (9), and requires bonding overlap to develop from a "con-rotatory displacement of end-groups, so that orbital envelopes on opposite faces of the molecule can interact [10]. This results in photochemical cyclisation of the B-seco-trienes (2) and (3) giving products with anti configurations about the C(9> C(io) bond, corresponding to ergosterol (9a,iOjS) and lumisterol (9/ ,ioa) respectively. The photochemical process... 

Figure 29.17. A 1,3,5-hexatriene. Configuration of tt electrons in ground state and first excited state.

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