Dienyl carbonium ions

This stabilization is presumably due to electron donation by the Fe(0). perhaps with significant contributions from an allyl-Fe(II) species, XII. Fe(0)-stabilized carbonium ions have been used very little in organic synthesis, but their potential value is illustrated in Eqs. (21) and (22) (Whitesides et al., 1973). The closely related Fe(0)-stabiIized dienyl carbonium ions... 

If the initial cyclization of trans trans- or cis, /raw -farnesyl pyrophosphates (52-OPP and 51-OPP) occurs upon the terminal (10, 11) double in an anti-Markovnikov orientation, eleven-membered cyclic dienyl carbonium ions are produced. Deprotonation of the trans, trans-intermediate affords humulene (97) while bis-homoallyl- cyclobutyl carbinyl cyclization furnishes, after elimination, caryophyllene (99). Although the all traw -configuration of the three double bonds in humulene would appear to require trans, rra -farnesyl pyrophosphate as precursor, the possibility exists that the immediate predecessor of caryophyllene is cis, /rtz 5-farnesyl (or cisoid nerolidyl) pyrophosphate. 

As mentioned in the Introduction, the group 3 biomimetic approach (see Scheme 1) has been the most popular route to the trichothecene skeleton. Two different moieties have served as the electrophilic site for biomimetic cyclization. When the cyclization proceeds via an allylic carbonium ion (127) (Path A, Scheme 9), the desired trisubstituted olefin (126) is obtained directly. On the other hand, the Michael acceptor (128) (PathB) yields, upon cyclization, a ketone (129) which must then be transformed into the olefin (126), a process which shows good but not complete regioselectivity. Hence, Path A, which can also be entered from the enone (128), is the superior route. Further analysis of Scheme 9 reveals that the primary stereochemical challenge of the biomimetic approach is to control the relative stereochemistry at the two quaternary centers C-5 and C-6. Within the context of trichothecene synthesis, a number of useful protocols have been devised for this purpose and include photocyclization (99), selective ring contraction (134), Diels-Alder cycloaddition (117,125) conjugate addition (27,120), and interconversion of dienyl iron complexes (114). 

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