Bicyclic intermediates, formation between

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

Scheme 13.33 shows broad retrosynthetic formulations of the longifolene syntheses that are discussed in this subsection. Four different patterns of bond formation are represented. In A, the C(7)-C(10) bond is formed from a bicyclic intermediate. This pattern corresponds to the syntheses in Schemes 13.24, 13.25, 12.26, and 13.29. In retrosynthesis B, there is concurrent formation of the C(l)-C(2) and C(10)-C(ll) bonds, as in the synthesis in scheme 13.28. This is also the pattern found in the synthesis in Scheme 13.32. The synthesis in Scheme 13.29 corresponds to retrosynthesis C, in which the C(l)-C(2) and C(6)-C(7) bonds are formed and an extraneous bond between C(2) and C(5) is broken. Finally, retrosynthesis D, corresponding to formation of the C(2)-C(3) bond, is represented by the synthesis in Scheme 13.31.

The internal cyclization-isomerization route (ICI), which involves the formation of a bicyclic species featuring a single bond between the ring-atoms in the 2- and 5-positions of the original heterocycle. Sigmatropic shifts then occur, leading to different bicyclic intermediates from which the final products will arise (see e.g., Scheme 12.2). 

Furthermore, the bicyclic intermediates, e.g. 44 t46, could interconvert directly 71,63) thus bypassing carbenes 45 and 37. It is therefore important to know the relative energies of the carbenes and the bicyclic intermediates. The heat of formation of can be estimated by various thermochemical schemes 24), yielding values between 115 and 128 kcal/mol. The lower value is obtained by adding the strain energy of benzocyclopropene (70 1 kcal/mol 23)) to the strain free value obtained... 

In the proposed scheme of the biosynthesis of cytochalasins A (25) and B (26), outlined in Fig. 14, the amide linkage of the nonaketide 53 to phenylalanine (46) may be regarded as the initial step. The subsequent condensation of the amide 54 leads to the five-membered ring of the y-lactam 55, which is a derivative of tetramic acid. One of the carbonyl groups of y-lactam 55 reacts with the penultimate carbon atom of the ketide after reduction to the intermediate 56, with formation of the bond between C(4) and C(5). Partial reductions and water eliminations yield the bicyclic intermediate 58, in which the bond between C(8) and C(9) is transformed by condensation. By an... 

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