A wide range of structures encountered in natural terpenoid and steroid derivatives can only be rationalized as originating from C5 isoprene units if some fundamental rearrangement process has occurred during biosynthesis. These rearrangements have, in many cases, been confirmed experimentally, and are almost always consistent with the participation of carbocation intermediates. Rearrangements in chemical reactions involving car-bocation intermediates, e.g. SN1 and E1 reactions, are not uncommon, and typically consist of 1,2-shifts of hydride, methyl, or alkyl groups. Occasionally, 1,3- or longer shifts are encountered. These shifts, termed Wagner-Meerwein rearrangements, are readily rationalized in terms of generating a more stable carbocation, or relaxing ring strain (Figure 2.6). Thus, tertiary carbo-cations are favoured over secondary carbocations,
Wagner-Meerwein rearrangements and the usual objective in these rearrangements is to achieve tertiary status at the positive centre. However, a tertiary to secondary transition might be favoured if the rearrangement allows a significant release of ring strain. These general concepts are occasionally ignored by nature, but it must be remembered that the reactions are enzyme-mediated and carbocations may not exist as discrete species in the transformations. An interesting feature of some biosynthetic pathways, e.g. that leading to steroids, is a remarkable series of concerted 1,2-migrations rationalized via carboca-tion chemistry, but entirely a consequence of the enzyme's participation (Figure 2.6).
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