Vw^ynthesis of the higher terpenes in the sterol and carotenoid classes requires head-to-head condensation of two head-totail polyprenyl pyrophosphates. In squalene (sterol) synthesis the overall process is reductive, while in phytoene (carotenoid) synthesis it is not1 (see Scheme I). With the discovery of cyclopropylcarbinyl pyrophosphates as intermediates in these two pathways (Epstein and Rilling, 1970; Altman et al., 1972),
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